NMAP(1) Insecure.Org Zero Day NMAP(1)
Nmap Network Scanning Nmap Network Scanning
09/12/2008
NAME
nmap - Network exploration tool and security / port scanner
SYNOPSIS
nmap [Scan Type...] [Options] {target specification}
DESCRIPTION
Nmap (Network Mapper) is an open source tool for network exploration
and security auditing. It was designed to rapidly scan large networks,
although it works fine against single hosts. Nmap uses raw IP packets
in novel ways to determine what hosts are available on the network,
what services (application name and version) those hosts are offering,
what operating systems (and OS versions) they are running, what type
of packet filters/firewalls are in use, and dozens of other
characteristics. While Nmap is commonly used for security audits, many
systems and network administrators find it useful for routine tasks
such as network inventory, managing service upgrade schedules, and
monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental
information on each depending on the options used. Key among that
information is the interesting ports table. That table lists the port
number and protocol, service name, and state. The state is either
open, filtered, closed, or unfiltered. Open means that an application
on the target machine is listening for connections/packets on that
port. Filtered means that a firewall, filter, or other network
obstacle is blocking the port so that Nmap cannot tell whether it is
open or closed. Closed ports have no application listening on them,
though they could open up at any time. Ports are classified as
unfiltered when they are responsive to Nmap's probes, but Nmap cannot
determine whether they are open or closed. Nmap reports the state
combinations open|filtered and closed|filtered when it cannot
determine which of the two states describe a port. The port table may
also include software version details when version detection has been
requested. When an IP protocol scan is requested (-sO), Nmap provides
information on supported IP protocols rather than listening ports.
In addition to the interesting ports table, Nmap can provide further
information on targets, including reverse DNS names, operating system
guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 15.1. The only Nmap arguments
used in this example are -A, to enable OS and version detection,
script scanning, and traceroute; -T4 for faster execution; and then
the two target hostnames.
Example 15.1. A representative Nmap scan
# nmap -A -T4 scanme.nmap.org playground
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Starting Nmap ( http://nmap.org )
Interesting ports on scanme.nmap.org (64.13.134.52):
(The 1663 ports scanned but not shown below are in state: filtered)
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 3.9p1 (protocol 1.99)
53/tcp open domain
70/tcp closed gopher
80/tcp open http Apache httpd 2.0.52 ((Fedora))
113/tcp closed auth
Device type: general purpose
Running: Linux 2.4.X|2.5.X|2.6.X
OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
Interesting ports on playground.nmap.org (192.168.0.40):
(The 1659 ports scanned but not shown below are in state: closed)
PORT STATE SERVICE VERSION
135/tcp open msrpc Microsoft Windows RPC
139/tcp open netbios-ssn
389/tcp open ldap?
445/tcp open microsoft-ds Microsoft Windows XP microsoft-ds
1002/tcp open windows-icfw?
1025/tcp open msrpc Microsoft Windows RPC
1720/tcp open H.323/Q.931 CompTek AquaGateKeeper
5800/tcp open vnc-http RealVNC 4.0 (Resolution 400x250; VNC port: 5900)
5900/tcp open vnc VNC (protocol 3.8)
MAC Address: 00:A0:CC:63:85:4B (Lite-on Communications)
Device type: general purpose
Running: Microsoft Windows NT/2K/XP
OS details: Microsoft Windows XP Pro RC1+ through final release
Service Info: OSs: Windows, Windows XP
Nmap finished: 2 IP addresses (2 hosts up) scanned in 88.392 seconds
The newest version of Nmap can be obtained from http://nmap.org. The
newest version of the man page is available at
http://nmap.org/book/man.html.
OPTIONS SUMMARY
This options summary is printed when Nmap is run with no arguments,
and the latest version is always available at
http://nmap.org/data/nmap.usage.txt. It helps people remember the most
common options, but is no substitute for the in-depth documentation in
the rest of this manual. Some obscure options aren't even included
here.
Nmap 4.76 ( http://nmap.org )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
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Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
--exclude <host1[,host2][,host3],...>: Exclude hosts/networks
--excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sP: Ping Scan - go no further than determining if host is online
-PN: Treat all hosts as online -- skip host discovery
-PS/PA/PU [portlist]: TCP SYN/ACK or UDP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-PO [protocol list]: IP Protocol Ping
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers <serv1[,serv2],...>: Specify custom DNS servers
--system-dns: Use OS's DNS resolver
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
--traceroute: Trace hop path to each host
--reason: Display the reason a port is in a particular state
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
-F: Fast mode - Scan fewer ports than the default scan
-r: Scan ports consecutively - don't randomize
--top-ports <number>: Scan <number> most common ports
--port-ratio <ratio>: Scan ports more common than <ratio>
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity <level>: Set from 0 (light) to 9 (try all probes)
--version-light: Limit to most likely probes (intensity 2)
--version-all: Try every single probe (intensity 9)
--version-trace: Show detailed version scan activity (for debugging)
SCRIPT SCAN:
-sC: equivalent to --script=default
--script=<Lua scripts>: <Lua scripts> is a comma separated list of
directories, script-files or script-categories
--script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts
--script-trace: Show all data sent and received
--script-updatedb: Update the script database.
OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
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TIMING AND PERFORMANCE:
Options which take <time> are in milliseconds, unless you append 's'
(seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
-T[0-5]: Set timing template (higher is faster)
--min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
--min-parallelism/max-parallelism <time>: Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
--max-retries <tries>: Caps number of port scan probe retransmissions.
--host-timeout <time>: Give up on target after this long
--scan-delay/--max-scan-delay <time>: Adjust delay between probes
--min-rate <number>: Send packets no slower than <number> per second
--max-rate <number>: Send packets no faster than <number> per second
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/--source-port <portnum>: Use given port number
--data-length <num>: Append random data to sent packets
--ip-options <options>: Send packets with specified ip options
--ttl <val>: Set IP time-to-live field
--spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP checksum
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use twice or more for greater effect)
-d[level]: Set or increase debugging level (Up to 9 is meaningful)
--open: Only show open (or possibly open) ports
--packet-trace: Show all packets sent and received
--iflist: Print host interfaces and routes (for debugging)
--log-errors: Log errors/warnings to the normal-format output file
--append-output: Append to rather than clobber specified output files
--resume <filename>: Resume an aborted scan
--stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Nmap.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enables OS detection and Version detection, Script scanning and Traceroute
--datadir <dirname>: Specify custom Nmap data file location
--send-eth/--send-ip: Send using raw ethernet frames or IP packets
--privileged: Assume that the user is fully privileged
--unprivileged: Assume the user lacks raw socket privileges
-V: Print version number
-h: Print this help summary page.
EXAMPLES:
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nmap -v -A scanme.nmap.org
nmap -v -sP 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -PN -p 80
SEE THE MAN PAGE FOR MANY MORE OPTIONS, DESCRIPTIONS, AND EXAMPLES
TARGET SPECIFICATION
Everything on the Nmap command-line that isn't an option (or option
argument) is treated as a target host specification. The simplest case
is to specify a target IP address or hostname for scanning.
Sometimes you wish to scan a whole network of adjacent hosts. For
this, Nmap supports CIDR-style addressing. You can append /numbits to
an IP address or hostname and Nmap will scan every IP address for
which the first numbits are the same as for the reference IP or
hostname given. For example, 192.168.10.0/24 would scan the 256 hosts
between 192.168.10.0 (binary: 11000000 10101000 00001010 00000000) and
192.168.10.255 (binary: 11000000 10101000 00001010 11111111),
inclusive. 192.168.10.40/24 would do exactly the same thing. Given
that the host scanme.nmap.org is at the IP address 64.13.134.52, the
specification scanme.nmap.org/16 would scan the 65,536 IP addresses
between 64.13.0.0 and 64.13.255.255. The smallest allowed value is /0,
which scans the whole Internet. The largest value is /32, which scans
just the named host or IP address because all address bits are fixed.
CIDR notation is short but not always flexible enough. For example,
you might want to scan 192.168.0.0/16 but skip any IPs ending with .0
or .255 because they are commonly broadcast addresses. Nmap supports
this through octet range addressing. Rather than specify a normal IP
address, you can specify a comma separated list of numbers or ranges
for each octet. For example, 192.168.0-255.1-254 will skip all
addresses in the range that end in .0 and or .255. Ranges need not be
limited to the final octets: the specifier 0-255.0-255.13.37 will
perform an Internet-wide scan for all IP addresses ending in 13.37.
This sort of broad sampling can be useful for Internet surveys and
research.
IPv6 addresses can only be specified by their fully qualified IPv6
address or hostname. CIDR and octet ranges aren't supported for IPv6
because they are rarely useful.
Nmap accepts multiple host specifications on the command line, and
they don't need to be the same type. The command nmap scanme.nmap.org
192.168.0.0/16 10.0.0,1,3-7.0-255 does what you would expect.
While targets are usually specified on the command lines, the
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following options are also available to control target selection:
-iL inputfilename (Input from list)
Reads target specifications from inputfilename. Passing a huge
list of hosts is often awkward on the command line, yet it is a
common desire. For example, your DHCP server might export a list
of 10,000 current leases that you wish to scan. Or maybe you want
to scan all IP addresses except for those to locate hosts using
unauthorized static IP addresses. Simply generate the list of
hosts to scan and pass that filename to Nmap as an argument to the
-iL option. Entries can be in any of the formats accepted by Nmap
on the command line (IP address, hostname, CIDR, IPv6, or octet
ranges). Each entry must be separated by one or more spaces, tabs,
or newlines. You can specify a hyphen (-) as the filename if you
want Nmap to read hosts from standard input rather than an actual
file.
-iR num hosts (Choose random targets)
For Internet-wide surveys and other research, you may want to
choose targets at random. The num hosts argument tells Nmap how
many IPs to generate. Undesirable IPs such as those in certain
private, multicast, or unallocated address ranges are
automatically skipped. The argument 0 can be specified for a
never-ending scan. Keep in mind that some network administrators
bristle at unauthorized scans of their networks and may complain.
Use this option at your own risk! If you find yourself really
bored one rainy afternoon, try the command nmap -sS -PS80 -iR 0 -p
80
to locate random web servers for browsing.
--exclude host1[,host2[,...]] (Exclude hosts/networks)
Specifies a comma-separated list of targets to be excluded from
the scan even if they are part of the overall network range you
specify. The list you pass in uses normal Nmap syntax, so it can
include hostnames, CIDR netblocks, octet ranges, etc. This can be
useful when the network you wish to scan includes untouchable
mission-critical servers, systems that are known to react
adversely to port scans, or subnets administered by other people.
--excludefile exclude_file (Exclude list from file)
This offers the same functionality as the --exclude option, except
that the excluded targets are provided in a newline, space, or tab
delimited exclude_file rather than on the command line.
HOST DISCOVERY
One of the very first steps in any network reconnaissance mission is
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to reduce a (sometimes huge) set of IP ranges into a list of active or
interesting hosts. Scanning every port of every single IP address is
slow and usually unnecessary. Of course what makes a host interesting
depends greatly on the scan purposes. Network administrators may only
be interested in hosts running a certain service, while security
auditors may care about every single device with an IP address. An
administrator may be comfortable using just an ICMP ping to locate
hosts on his internal network, while an external penetration tester
may use a diverse set of dozens of probes in an attempt to evade
firewall restrictions.
Because host discovery needs are so diverse, Nmap offers a wide
variety of options for customizing the techniques used. Host discovery
is sometimes called ping scan, but it goes well beyond the simple ICMP
echo request packets associated with the ubiquitous ping tool. Users
can skip the ping step entirely with a list scan (-sL) or by disabling
ping (-PN), or engage the network with arbitrary combinations of
multi-port TCP SYN/ACK, UDP, and ICMP probes. The goal of these probes
is to solicit responses which demonstrate that an IP address is
actually active (is being used by a host or network device). On many
networks, only a small percentage of IP addresses are active at any
given time. This is particularly common with private address space
such as 10.0.0.0/8. That network has 16 million IPs, but I have seen
it used by companies with less than a thousand machines. Host
discovery can find those machines in a sparsely allocated sea of IP
addresses.
If no host discovery options are given, Nmap sends a TCP ACK packet
destined for port 80 and an ICMP echo request query to each target
machine. An exception to this is that an ARP scan is used for any
targets which are on a local ethernet network. For unprivileged Unix
shell users, a SYN packet is sent instead of the ACK using the connect
system call. These defaults are equivalent to the -PA -PE options.
This host discovery is often sufficient when scanning local networks,
but a more comprehensive set of discovery probes is recommended for
security auditing.
The -P* options (which select ping types) can be combined. You can
increase your odds of penetrating strict firewalls by sending many
probe types using different TCP ports/flags and ICMP codes. Also note
that ARP discovery (-PR) is done by default against targets on a local
ethernet network even if you specify other -P* options, because it is
almost always faster and more effective.
By default, Nmap does host discovery and then performs a port scan
against each host it determines is online. This is true even if you
specify non-default host discovery types such as UDP probes (-PU).
Read about the -sP option to learn how to perform only host discovery,
or use -PN to skip host discovery and port scan all target hosts. The
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following options control host discovery:
-sL (List Scan)
The list scan is a degenerate form of host discovery that simply
lists each host of the network(s) specified, without sending any
packets to the target hosts. By default, Nmap still does
reverse-DNS resolution on the hosts to learn their names. It is
often surprising how much useful information simple hostnames give
out. For example, fw.chi is the name of one company's Chicago
firewall.
Nmap also reports the total number of IP addresses at the end. The
list scan is a good sanity check to ensure that you have proper IP
addresses for your targets. If the hosts sport domain names you do
not recognize, it is worth investigating further to prevent
scanning the wrong company's network.
Since the idea is to simply print a list of target hosts, options
for higher level functionality such as port scanning, OS
detection, or ping scanning cannot be combined with this. If you
wish to disable ping scanning while still performing such higher
level functionality, read up on the -PN option.
-sP (Ping Scan)
This option tells Nmap to only perform a ping scan (host
discovery), then print out the available hosts that responded to
the scan. Traceroute and NSE host scripts are also run if
requested, but no further testing (such as port scanning or OS
detection) is performed. This is by default one step more
intrusive than the list scan, and can often be used for the same
purposes. It allows light reconnaissance of a target network
without attracting much attention. Knowing how many hosts are up
is more valuable to attackers than the list provided by list scan
of every single IP and host name.
Systems administrators often find this option valuable as well. It
can easily be used to count available machines on a network or
monitor server availability. This is often called a ping sweep,
and is more reliable than pinging the broadcast address because
many hosts do not reply to broadcast queries.
The -sP option sends an ICMP echo request and a TCP ACK packet to
port 80 by default. When executed by an unprivileged user, only a
SYN packet is sent (using a connect call) to port 80 on the
target. When a privileged user tries to scan targets on a local
ethernet network, ARP requests are used unless --send-ip was
specified. The -sP option can be combined with any of the
discovery probe types (the -P* options, excluding -PN) for greater
flexibility. If any of those probe type and port number options
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are used, the default probes (ACK and echo request) are
overridden. When strict firewalls are in place between the source
host running Nmap and the target network, using those advanced
techniques is recommended. Otherwise hosts could be missed when
the firewall drops probes or their responses.
-PN (No ping)
This option skips the Nmap discovery stage altogether. Normally,
Nmap uses this stage to determine active machines for heavier
scanning. By default, Nmap only performs heavy probing such as
port scans, version detection, or OS detection against hosts that
are found to be up. Disabling host discovery with -PN causes Nmap
to attempt the requested scanning functions against every target
IP address specified. So if a class B sized target address space
(/16) is specified on the command line, all 65,536 IP addresses
are scanned. Proper host discovery is skipped as with the list
scan, but instead of stopping and printing the target list, Nmap
continues to perform requested functions as if each target IP is
active. For machines on a local ethernet network, ARP scanning
will still be performed (unless --send-ip is specified) because
Nmap needs MAC addresses to further scan target hosts. This option
flag used to be P0 (uses zero), but was renamed to avoid confusion
with protocol ping's PO (uses the letter O) flag.
-PS portlist (TCP SYN Ping)
This option sends an empty TCP packet with the SYN flag set. The
default destination port is 80 (configurable at compile time by
changing DEFAULT_TCP_PROBE_PORT_SPEC
in nmap.h).
Alternate ports can be specified as a parameter. The syntax is the
same as for the -p except that port type specifiers like T: are
not allowed. Examples are -PS22 and -PS22-25,80,113,1050,35000.
Note that there can be no space between -PS and the port list. If
multiple probes are specified they will be sent in parallel.
The SYN flag suggests to the remote system that you are attempting
to establish a connection. Normally the destination port will be
closed, and a RST (reset) packet sent back. If the port happens to
be open, the target will take the second step of a TCP
3-way-handshake by responding with a SYN/ACK TCP packet. The
machine running Nmap then tears down the nascent connection by
responding with a RST rather than sending an ACK packet which
would complete the 3-way-handshake and establish a full
connection. The RST packet is sent by the kernel of the machine
running Nmap in response to the unexpected SYN/ACK, not by Nmap
itself.
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Nmap does not care whether the port is open or closed. Either the
RST or SYN/ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root is generally able to
send and receive raw TCP packets. For unprivileged users, a
workaround is automatically employed whereby the connect system
call is initiated against each target port. This has the effect of
sending a SYN packet to the target host, in an attempt to
establish a connection. If connect returns with a quick success or
an ECONNREFUSED failure, the underlying TCP stack must have
received a SYN/ACK or RST and the host is marked available. If the
connection attempt is left hanging until a timeout is reached, the
host is marked as down. This workaround is also used for IPv6
connections, as raw IPv6 packet building support is not yet
available in Nmap.
-PA portlist (TCP ACK Ping)
The TCP ACK ping is quite similar to the just-discussed SYN ping.
The difference, as you could likely guess, is that the TCP ACK
flag is set instead of the SYN flag. Such an ACK packet purports
to be acknowledging data over an established TCP connection, but
no such connection exists. So remote hosts should always respond
with a RST packet, disclosing their existence in the process.
The -PA option uses the same default port as the SYN probe (80)
and can also take a list of destination ports in the same format.
If an unprivileged user tries this, or an IPv6 target is
specified, the connect workaround discussed previously is used.
This workaround is imperfect because connect is actually sending a
SYN packet rather than an ACK.
The reason for offering both SYN and ACK ping probes is to
maximize the chances of bypassing firewalls. Many administrators
configure routers and other simple firewalls to block incoming SYN
packets except for those destined for public services like the
company web site or mail server. This prevents other incoming
connections to the organization, while allowing users to make
unobstructed outgoing connections to the Internet. This
non-stateful approach takes up few resources on the
firewall/router and is widely supported by hardware and software
filters. The Linux Netfilter/iptables firewall software offers the
--syn convenience option to implement this stateless approach.
When stateless firewall rules such as this are in place, SYN ping
probes (-PS) are likely to be blocked when sent to closed target
ports. In such cases, the ACK probe shines as it cuts right
through these rules.
Another common type of firewall uses stateful rules that drop
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unexpected packets. This feature was initially found mostly on
high-end firewalls, though it has become much more common over the
years. The Linux Netfilter/iptables system supports this through
the --state option, which categorizes packets based on connection
state. A SYN probe is more likely to work against such a system,
as unexpected ACK packets are generally recognized as bogus and
dropped. A solution to this quandary is to send both SYN and ACK
probes by specifying -PS and -PA.
-PU portlist (UDP Ping)
Another host discovery option is the UDP ping, which sends an
empty (unless --data-length is specified) UDP packet to the given
ports. The portlist takes the same format as with the previously
discussed -PS and -PA options. If no ports are specified, the
default is 31338. This default can be configured at compile-time
by changing DEFAULT_UDP_PROBE_PORT_SPEC in nmap.h. A highly
uncommon port is used by default because sending to open ports is
often undesirable for this particular scan type.
Upon hitting a closed port on the target machine, the UDP probe
should elicit an ICMP port unreachable packet in return. This
signifies to Nmap that the machine is up and available. Many other
types of ICMP errors, such as host/network unreachables or TTL
exceeded are indicative of a down or unreachable host. A lack of
response is also interpreted this way. If an open port is reached,
most services simply ignore the empty packet and fail to return
any response. This is why the default probe port is 31338, which
is highly unlikely to be in use. A few services, such as the
Character Generator (chargen) protocol, will respond to an empty
UDP packet, and thus disclose to Nmap that the machine is
available.
The primary advantage of this scan type is that it bypasses
firewalls and filters that only screen TCP. For example, I once
owned a Linksys BEFW11S4 wireless broadband router. The external
interface of this device filtered all TCP ports by default, but
UDP probes would still elicit port unreachable messages and thus
give away the device.
-PE; -PP; -PM (ICMP Ping Types)
In addition to the unusual TCP and UDP host discovery types
discussed previously, Nmap can send the standard packets sent by
the ubiquitous ping program. Nmap sends an ICMP type 8 (echo
request) packet to the target IP addresses, expecting a type 0
(echo reply) in return from available hosts. Unfortunately for
network explorers, many hosts and firewalls now block these
packets, rather than responding as required by RFC 1122[1]. For
this reason, ICMP-only scans are rarely reliable enough against
unknown targets over the Internet. But for system administrators
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monitoring an internal network, they can be a practical and
efficient approach. Use the -PE option to enable this echo request
behavior.
While echo request is the standard ICMP ping query, Nmap does not
stop there. The ICMP standard (RFC 792[2]) also specifies
timestamp request, information request, and address mask request
packets as codes 13, 15, and 17, respectively. While the
ostensible purpose for these queries is to learn information such
as address masks and current times, they can easily be used for
host discovery. A system that replies is up and available. Nmap
does not currently implement information request packets, as they
are not widely supported. RFC 1122 insists that a host SHOULD NOT
implement these messages. Timestamp and address mask queries can
be sent with the -PP and -PM options, respectively. A timestamp
reply (ICMP code 14) or address mask reply (code 18) discloses
that the host is available. These two queries can be valuable when
administrators specifically block echo request packets while
forgetting that other ICMP queries can be used for the same
purpose.
-PO protolist (IP Protocol Ping)
The newest host discovery option is the IP protocol ping, which
sends IP packets with the specified protocol number set in their
IP header. The protocol list takes the same format as do port
lists in the previously discussed TCP and UDP host discovery
options. If no protocols are specified, the default is to send
multiple IP packets for ICMP (protocol 1), IGMP (protocol 2), and
IP-in-IP (protocol 4). The default protocols can be configured at
compile-time by changing DEFAULT_PROTO_PROBE_PORT_SPEC in nmap.h.
Note that for the ICMP, IGMP, TCP (protocol 6), and UDP (protocol
17), the packets are sent with the proper protocol headers while
other protocols are sent with no additional data beyond the IP
header (unless the --data-length option is specified).
This host discovery method looks for either responses using the
same protocol as a probe, or ICMP protocol unreachable messages
which signify that the given protocol isn't supported on the
destination host. Either type of response signifies that the
target host is alive.
-PR (ARP Ping)
One of the most common Nmap usage scenarios is to scan an ethernet
LAN. On most LANs, especially those using private address ranges
specified by RFC 1918[3], the vast majority of IP addresses are
unused at any given time. When Nmap tries to send a raw IP packet
such as an ICMP echo request, the operating system must determine
the destination hardware (ARP) address corresponding to the target
IP so that it can properly address the ethernet frame. This is
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often slow and problematic, since operating systems weren't
written with the expectation that they would need to do millions
of ARP requests against unavailable hosts in a short time period.
ARP scan puts Nmap and its optimized algorithms in charge of ARP
requests. And if it gets a response back, Nmap doesn't even need
to worry about the IP-based ping packets since it already knows
the host is up. This makes ARP scan much faster and more reliable
than IP-based scans. So it is done by default when scanning
ethernet hosts that Nmap detects are on a local ethernet network.
Even if different ping types (such as -PE or -PS) are specified,
Nmap uses ARP instead for any of the targets which are on the same
LAN. If you absolutely don't want to do an ARP scan, specify
--send-ip.
--traceroute (Trace path to host)
Traceroutes are performed post-scan using information from the
scan results to determine the port and protocol most likely to
reach the target. It works with all scan types except connect
scans (-sT) and idle scans (-sI). All traces use Nmap's dynamic
timing model and are performed in parallel.
Traceroute works by sending packets with a low TTL (time-to-live)
in an attempt to elicit ICMP Time Exceeded messages from
intermediate hops between the scanner and the target host.
Standard traceroute implementations start with a TTL of 1 and
increment the TTL until the destination host is reached. Nmap's
traceroute starts with a high TTL and then decrements the TTL
until it reaches 0. Doing it backwards lets Nmap employ clever
caching algorithms to speed up traces over multiple hosts. On
average Nmap sends 5-10 fewer packets per host, depending on
network conditions. If a single subnet is being scanned (i.e.
192.168.0.0/24) Nmap may only have to send a single packet to most
hosts.
--reason (Host and port state reasons)
Shows the reason each port is set to a specific state and the
reason each host is up or down. This option displays the type of
the packet that determined a port or hosts state. For example, A
RST packet from a closed port or an echo reply from an alive host.
The information Nmap can provide is determined by the type of scan
or ping. The SYN scan and SYN ping (-sS and -PS) are very
detailed, but the TCP connect scan (-sT) is limited by the
implementation of the connect system call. This feature is
automatically enabled by the debug option (-d) and the results are
stored in XML log files even if this option is not specified.
-n (No DNS resolution)
Tells Nmap to never do reverse DNS
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resolution on the active IP addresses it finds. Since DNS can be
slow even with Nmap's built-in parallel stub resolver, this option
can slash scanning times.
-R (DNS resolution for all targets)
Tells Nmap to always do reverse DNS resolution on the target IP
addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--system-dns (Use system DNS resolver)
By default, Nmap resolves IP addresses by sending queries directly
to the name servers configured on your host and then listening for
responses. Many requests (often dozens) are performed in parallel
to improve performance. Specify this option to use your system
resolver instead (one IP at a time via the getnameinfo call). This
is slower and rarely useful unless you find a bug in the Nmap
parallel resolver (please let us know if you do). The system
resolver is always used for IPv6 scans.
--dns-servers server1[,server2[,...]] (Servers to use for reverse DNS
queries)
By default, Nmap determines your DNS servers (for rDNS resolution)
from your resolv.conf file (Unix) or the Registry (Win32).
Alternatively, you may use this option to specify alternate
servers. This option is not honored if you are using --system-dns
or an IPv6 scan. Using multiple DNS servers is often faster,
especially if you choose authoritative servers for your target IP
space. This option can also improve stealth, as your requests can
be bounced off just about any recursive DNS server on the
internet.
This option also comes in handy when scanning private networks.
Sometimes only a few name servers provide proper rDNS information,
and you may not even know where they are. You can scan the network
for port 53 (perhaps with version detection), then try Nmap list
scans (-sL) specifying each name server one at a time with
--dns-servers until you find one which works.
PORT SCANNING BASICS
While Nmap has grown in functionality over the years, it began as an
efficient port scanner, and that remains its core function. The simple
command nmap target scans more than 1660 TCP ports on the host target.
While many port scanners have traditionally lumped all ports into the
open or closed states, Nmap is much more granular. It divides ports
into six states: open, closed, filtered, unfiltered, open|filtered, or
closed|filtered.
These states are not intrinsic properties of the port itself, but
describe how Nmap sees them. For example, an Nmap scan from the same
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network as the target may show port 135/tcp as open, while a scan at
the same time with the same options from across the Internet might
show that port as filtered.
The six port states recognized by Nmap
open
An application is actively accepting TCP connections or UDP
packets on this port. Finding these is often the primary goal of
port scanning. Security-minded people know that each open port is
an avenue for attack. Attackers and pen-testers want to exploit
the open ports, while administrators try to close or protect them
with firewalls without thwarting legitimate users. Open ports are
also interesting for non-security scans because they show services
available for use on the network.
closed
A closed port is accessible (it receives and responds to Nmap
probe packets), but there is no application listening on it. They
can be helpful in showing that a host is up on an IP address (host
discovery, or ping scanning), and as part of OS detection. Because
closed ports are reachable, it may be worth scanning later in case
some open up. Administrators may want to consider blocking such
ports with a firewall. Then they would appear in the filtered
state, discussed next.
filtered
Nmap cannot determine whether the port is open because packet
filtering prevents its probes from reaching the port. The
filtering could be from a dedicated firewall device, router rules,
or host-based firewall software. These ports frustrate attackers
because they provide so little information. Sometimes they respond
with ICMP error messages such as type 3 code 13 (destination
unreachable: communication administratively prohibited), but
filters that simply drop probes without responding are far more
common. This forces Nmap to retry several times just in case the
probe was dropped due to network congestion rather than filtering.
This slows down the scan dramatically.
unfiltered
The unfiltered state means that a port is accessible, but Nmap is
unable to determine whether it is open or closed. Only the ACK
scan, which is used to map firewall rulesets, classifies ports
into this state. Scanning unfiltered ports with other scan types
such as Window scan, SYN scan, or FIN scan, may help resolve
whether the port is open.
open|filtered
Nmap places ports in this state when it is unable to determine
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whether a port is open or filtered. This occurs for scan types in
which open ports give no response. The lack of response could also
mean that a packet filter dropped the probe or any response it
elicited. So Nmap does not know for sure whether the port is open
or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas scans
classify ports this way.
closed|filtered
This state is used when Nmap is unable to determine whether a port
is closed or filtered. It is only used for the IP ID idle scan.
PORT SCANNING TECHNIQUES
As a novice performing automotive repair, I can struggle for hours
trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.)
to the task at hand. When I fail miserably and tow my jalopy to a real
mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless. The
art of port scanning is similar. Experts understand the dozens of scan
techniques and choose the appropriate one (or combination) for a given
task. Inexperienced users and script kiddies, on the other hand, try
to solve every problem with the default SYN scan. Since Nmap is free,
the only barrier to port scanning mastery is knowledge. That certainly
beats the automotive world, where it may take great skill to determine
that you need a strut spring compressor, then you still have to pay
thousands of dollars for it.
Most of the scan types are only available to privileged users. This
is because they send and receive raw packets, which requires root
access on Unix systems. Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on
that platform when WinPcap has already been loaded into the OS.
Requiring root privileges was a serious limitation when Nmap was
released in 1997, as many users only had access to shared shell
accounts. Now, the world is different. Computers are cheaper, far more
people have always-on direct Internet access, and desktop Unix systems
(including Linux and Mac OS X) are prevalent. A Windows version of
Nmap is now available, allowing it to run on even more desktops. For
all these reasons, users have less need to run Nmap from limited
shared shell accounts. This is fortunate, as the privileged options
make Nmap far more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind that all
of its insights are based on packets returned by the target machines
(or firewalls in front of them). Such hosts may be untrustworthy and
send responses intended to confuse or mislead Nmap. Much more common
are non-RFC-compliant hosts that do not respond as they should to Nmap
probes. FIN, NULL, and Xmas scans are particularly susceptible to this
problem. Such issues are specific to certain scan types and so are
discussed in the individual scan type entries.
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This section documents the dozen or so port scan techniques supported
by Nmap. Only one method may be used at a time, except that UDP scan
(-sU) may be combined with any one of the TCP scan types. As a memory
aid, port scan type options are of the form -sC, where C is a
prominent character in the scan name, usually the first. The one
exception to this is the deprecated FTP bounce scan (-b). By default,
Nmap performs a SYN Scan, though it substitutes a connect scan if the
user does not have proper privileges to send raw packets (requires
root access on Unix) or if IPv6 targets were specified. Of the scans
listed in this section, unprivileged users can only execute connect
and FTP bounce scans.
-sS (TCP SYN scan)
SYN scan is the default and most popular scan option for good
reasons. It can be performed quickly, scanning thousands of ports
per second on a fast network not hampered by restrictive
firewalls. SYN scan is relatively unobtrusive and stealthy, since
it never completes TCP connections. It also works against any
compliant TCP stack rather than depending on idiosyncrasies of
specific platforms as Nmap's FIN/NULL/Xmas, Maimon and idle scans
do. It also allows clear, reliable differentiation between the
open, closed, and filtered states.
This technique is often referred to as half-open scanning, because
you don't open a full TCP connection. You send a SYN packet, as if
you are going to open a real connection and then wait for a
response. A SYN/ACK indicates the port is listening (open), while
a RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as
filtered. The port is also marked filtered if an ICMP unreachable
error (type 3, code 1,2, 3, 9, 10, or 13) is received.
-sT (TCP connect scan)
TCP connect scan is the default TCP scan type when SYN scan is not
an option. This is the case when a user does not have raw packet
privileges or is scanning IPv6 networks. Instead of writing raw
packets as most other scan types do, Nmap asks the underlying
operating system to establish a connection with the target machine
and port by issuing the connect system call. This is the same
high-level system call that web browsers, P2P clients, and most
other network-enabled applications use to establish a connection.
It is part of a programming interface known as the Berkeley
Sockets API. Rather than read raw packet responses off the wire,
Nmap uses this API to obtain status information on each connection
attempt.
When SYN scan is available, it is usually a better choice. Nmap
has less control over the high level connect call than with raw
packets, making it less efficient. The system call completes
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connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take longer
and require more packets to obtain the same information, but
target machines are more likely to log the connection. A decent
IDS will catch either, but most machines have no such alarm
system. Many services on your average Unix system will add a note
to syslog, and sometimes a cryptic error message, when Nmap
connects and then closes the connection without sending data.
Truly pathetic services crash when this happens, though that is
uncommon. An administrator who sees a bunch of connection attempts
in her logs from a single system should know that she has been
connect scanned.
-sU (UDP scans)
While most popular services on the Internet run over the TCP
protocol, UDP[4] services are widely deployed. DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most
common. Because UDP scanning is generally slower and more
difficult than TCP, some security auditors ignore these ports.
This is a mistake, as exploitable UDP services are quite common
and attackers certainly don't ignore the whole protocol.
Fortunately, Nmap can help inventory UDP ports.
UDP scan is activated with the -sU option. It can be combined with
a TCP scan type such as SYN scan (-sS) to check both protocols
during the same run.
UDP scan works by sending an empty (no data) UDP header to every
targeted port. If an ICMP port unreachable error (type 3, code 3)
is returned, the port is closed. Other ICMP unreachable errors
(type 3, codes 1, 2, 9, 10, or 13) mark the port as filtered.
Occasionally, a service will respond with a UDP packet, proving
that it is open. If no response is received after retransmissions,
the port is classified as open|filtered. This means that the port
could be open, or perhaps packet filters are blocking the
communication. Version detection (-sV) can be used to help
differentiate the truly open ports from the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and
filtered ports rarely send any response, leaving Nmap to time out
and then conduct retransmissions just in case the probe or
response were lost. Closed ports are often an even bigger problem.
They usually send back an ICMP port unreachable error. But unlike
the RST packets sent by closed TCP ports in response to a SYN or
connect scan, many hosts rate limit ICMP port unreachable messages
by default. Linux and Solaris are particularly strict about this.
For example, the Linux 2.4.20 kernel limits destination
unreachable messages to one per second (in net/ipv4/icmp.c).
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Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine
will drop. Unfortunately, a Linux-style limit of one packet per
second makes a 65,536-port scan take more than 18 hours. Ideas for
speeding your UDP scans up include scanning more hosts in
parallel, doing a quick scan of just the popular ports first,
scanning from behind the firewall, and using --host-timeout to
skip slow hosts.
-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans)
These three scan types (even more are possible with the
--scanflags option described in the next section) exploit a subtle
loophole in the TCP RFC[5] to differentiate between open and
closed ports. Page 65 of RFC 793 says that if the [destination]
port state is CLOSED .... an incoming segment not containing a RST
causes a RST to be sent in response. Then the next page discusses
packets sent to open ports without the SYN, RST, or ACK bits set,
stating that: you are unlikely to get here, but if you do, drop
the segment, and return.
When scanning systems compliant with this RFC text, any packet not
containing SYN, RST, or ACK bits will result in a returned RST if
the port is closed and no response at all if the port is open. As
long as none of those three bits are included, any combination of
the other three (FIN, PSH, and URG) are OK. Nmap exploits this
with three scan types:
Null scan (-sN)
Does not set any bits (TCP flag header is 0)
FIN scan (-sF)
Sets just the TCP FIN bit.
Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up like
a Christmas tree.
These three scan types are exactly the same in behavior except for
the TCP flags set in probe packets. If a RST packet is received,
the port is considered closed, while no response means it is
open|filtered. The port is marked filtered if an ICMP unreachable
error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little
more stealthy than even a SYN scan. Don't count on this
though-most modern IDS products can be configured to detect them.
The big downside is that not all systems follow RFC 793 to the
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letter. A number of systems send RST responses to the probes
regardless of whether the port is open or not. This causes all of
the ports to be labeled closed. Major operating systems that do
this are Microsoft Windows, many Cisco devices, BSDI, and IBM
OS/400. This scan does work against most Unix-based systems
though. Another downside of these scans is that they can't
distinguish open ports from certain filtered ones, leaving you
with the response open|filtered.
-sA (TCP ACK scan)
This scan is different than the others discussed so far in that it
never determines open (or even open|filtered) ports. It is used to
map out firewall rulesets, determining whether they are stateful
or not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you
use --scanflags). When scanning unfiltered systems, open and
closed ports will both return a RST packet. Nmap then labels them
as unfiltered, meaning that they are reachable by the ACK packet,
but whether they are open or closed is undetermined. Ports that
don't respond, or send certain ICMP error messages back (type 3,
code 1, 2, 3, 9, 10, or 13), are labeled filtered.
-sW (TCP Window scan)
Window scan is exactly the same as ACK scan except that it
exploits an implementation detail of certain systems to
differentiate open ports from closed ones, rather than always
printing unfiltered when a RST is returned. It does this by
examining the TCP Window field of the RST packets returned. On
some systems, open ports use a positive window size (even for RST
packets) while closed ones have a zero window. So instead of
always listing a port as unfiltered when it receives a RST back,
Window scan lists the port as open or closed if the TCP Window
value in that reset is positive or zero, respectively.
This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it. Systems
that don't support it will usually return all ports closed. Of
course, it is possible that the machine really has no open ports.
If most scanned ports are closed but a few common port numbers
(such as 22, 25, 53) are filtered, the system is most likely
susceptible. Occasionally, systems will even show the exact
opposite behavior. If your scan shows 1000 open ports and 3 closed
or filtered ports, then those three may very well be the truly
open ones.
-sM (TCP Maimon scan)
The Maimon scan is named after its discoverer, Uriel Maimon. He
described the technique in Phrack Magazine issue #49 (November
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1996). Nmap, which included this technique, was released two
issues later. This technique is exactly the same as NULL, FIN, and
Xmas scans, except that the probe is FIN/ACK. According to RFC
793[5] (TCP), a RST packet should be generated in response to such
a probe whether the port is open or closed. However, Uriel noticed
that many BSD-derived systems simply drop the packet if the port
is open.
--scanflags (Custom TCP scan)
Truly advanced Nmap users need not limit themselves to the canned
scan types offered. The --scanflags option allows you to design
your own scan by specifying arbitrary TCP flags. Let your
creative juices flow, while evading intrusion detection systems
whose vendors simply paged through the Nmap man page adding
specific rules!
The --scanflags argument can be a numerical flag value such as 9
(PSH and FIN), but using symbolic names is easier. Just mash
together any combination of URG, ACK, PSH, RST, SYN, and FIN. For
example, --scanflags URGACKPSHRSTSYNFIN sets everything, though
it's not very useful for scanning. The order these are specified
in is irrelevant.
In addition to specifying the desired flags, you can specify a TCP
scan type (such as -sA or -sF). That base type tells Nmap how to
interpret responses. For example, a SYN scan considers no-response
to indicate a filtered port, while a FIN scan treats the same as
open|filtered. Nmap will behave the same way it does for the base
scan type, except that it will use the TCP flags you specify
instead. If you don't specify a base type, SYN scan is used.
-sI zombie host[:probeport] (idle scan)
This advanced scan method allows for a truly blind TCP port scan
of the target (meaning no packets are sent to the target from your
real IP address). Instead, a unique side-channel attack exploits
predictable IP fragmentation ID sequence generation on the zombie
host to glean information about the open ports on the target. IDS
systems will display the scan as coming from the zombie machine
you specify (which must be up and meet certain criteria). This
fascinating scan type is too complex to fully describe in this
reference guide, so I wrote and posted an informal paper with full
details at http://nmap.org/book/idlescan.html.
Besides being extraordinarily stealthy (due to its blind nature),
this scan type permits mapping out IP-based trust relationships
between machines. The port listing shows open ports from the
perspective of the zombie host. So you can try scanning a target
using various zombies that you think might be trusted (via
router/packet filter rules).
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You can add a colon followed by a port number to the zombie host
if you wish to probe a particular port on the zombie for IP ID
changes. Otherwise Nmap will use the port it uses by default for
TCP pings (80).
Ports can also be specified by name according to what the port is
referred to in the nmap-services. You can even use the wildcards *
and ? with the names. For example, to scan ftp and all ports whose
names begin with http, use -p ftp,http*. Be careful about shell
expansions and quote the argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to indicate
ports inside that range that appear in nmap-services. For example,
the following will scan all ports in nmap-services equal to or
below 1024: -p [-1024]. Be careful with shell expansions and quote
the argument to -p if unsure.
-sO (IP protocol scan)
IP protocol scan allows you to determine which IP protocols (TCP,
ICMP, IGMP, etc.) are supported by target machines. This isn't
technically a port scan, since it cycles through IP protocol
numbers rather than TCP or UDP port numbers. Yet it still uses the
-p option to select scanned protocol numbers, reports its results
within the normal port table format, and even uses the same
underlying scan engine as the true port scanning methods. So it is
close enough to a port scan that it belongs here.
Besides being useful in its own right, protocol scan demonstrates
the power of open-source software. While the fundamental idea is
pretty simple, I had not thought to add it nor received any
requests for such functionality. Then in the summer of 2000,
Gerhard Rieger conceived the idea, wrote an excellent patch
implementing it, and sent it to the nmap-hackers mailing list. I
incorporated that patch into the Nmap tree and released a new
version the next day. Few pieces of commercial software have users
enthusiastic enough to design and contribute their own
improvements!
Protocol scan works in a similar fashion to UDP scan. Instead of
iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the 8-bit IP protocol
field. The headers are usually empty, containing no data and not
even the proper header for the claimed protocol. The three
exceptions are TCP, UDP, and ICMP. A proper protocol header for
those is included since some systems won't send them otherwise and
because Nmap already has functions to create them. Instead of
watching for ICMP port unreachable messages, protocol scan is on
the lookout for ICMP protocol unreachable messages. If Nmap
receives any response in any protocol from the target host, Nmap
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marks that protocol as open. An ICMP protocol unreachable error
(type 3, code 2) causes the protocol to be marked as closed Other
ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13) cause
the protocol to be marked filtered (though they prove that ICMP is
open at the same time). If no response is received after
retransmissions, the protocol is marked open|filtered
-b FTP relay host (FTP bounce scan)
An interesting feature of the FTP protocol (RFC 959[6]) is support
for so-called proxy FTP connections. This allows a user to connect
to one FTP server, then ask that files be sent to a third-party
server. Such a feature is ripe for abuse on many levels, so most
servers have ceased supporting it. One of the abuses this feature
allows is causing the FTP server to port scan other hosts. Simply
ask the FTP server to send a file to each interesting port of a
target host in turn. The error message will describe whether the
port is open or not. This is a good way to bypass firewalls
because organizational FTP servers are often placed where they
have more access to other internal hosts than any old Internet
host would. Nmap supports FTP bounce scan with the -b option. It
takes an argument of the form username:password@server:port.
Server is the name or IP address of a vulnerable FTP server. As
with a normal URL, you may omit username:password, in which case
anonymous login credentials (user: anonymous password:-wwwuser@)
are used. The port number (and preceding colon) may be omitted as
well, in which case the default FTP port (21) on server is used.
This vulnerability was widespread in 1997 when Nmap was released,
but has largely been fixed. Vulnerable servers are still around,
so it is worth trying when all else fails. If bypassing a firewall
is your goal, scan the target network for open port 21 (or even
for any FTP services if you scan all ports with version
detection), then try a bounce scan using each. Nmap will tell you
whether the host is vulnerable or not. If you are just trying to
cover your tracks, you don't need to (and, in fact, shouldn't)
limit yourself to hosts on the target network. Before you go
scanning random Internet addresses for vulnerable FTP servers,
consider that sysadmins may not appreciate you abusing their
servers in this way.
PORT SPECIFICATION AND SCAN ORDER
In addition to all of the scan methods discussed previously, Nmap
offers options for specifying which ports are scanned and whether the
scan order is randomized or sequential. By default, Nmap scans all
ports up to and including 1024 as well as higher numbered ports listed
in the nmap-services file for the protocol(s) being scanned.
-p port ranges (Only scan specified ports)
This option specifies which ports you want to scan and overrides
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the default. Individual port numbers are OK, as are ranges
separated by a hyphen (e.g. 1-1023). The beginning and/or end
values of a range may be omitted, causing Nmap to use 1 and 65535,
respectively. So you can specify -p- to scan ports from 1 through
65535. Scanning port zero is allowed if you specify it explicitly.
For IP protocol scanning (-sO), this option specifies the protocol
numbers you wish to scan for (0-255).
When scanning both TCP and UDP ports, you can specify a particular
protocol by preceding the port numbers by T: or U:. The qualifier
lasts until you specify another qualifier. For example, the
argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP ports
53,111,and 137, as well as the listed TCP ports. Note that to scan
both UDP and TCP, you have to specify -sU and at least one TCP
scan type (such as -sS, -sF, or -sT). If no protocol qualifier is
given, the port numbers are added to all protocol lists.
Ports can also be specified by name according to what the port is
referred to in the nmap-services. You can even use the wildcards *
and ? with the names. For example, to scan FTP and all ports whose
names begin with http, use -p ftp,http*. Be careful about shell
expansions and quote the argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to indicate
ports inside that range that appear in nmap-services. For example,
the following will scan all ports in nmap-services equal to or
below 1024: -p [-1024]. Be careful with shell expansions and quote
the argument to -p if unsure.
-F (Fast (limited port) scan)
Specifies that you only wish to scan for ports listed in the
nmap-services file which comes with Nmap (or the protocols file
for -sO). This is much faster than scanning all 65535 ports on a
host. Because this list contains so many TCP ports (more than
1200), the speed difference from a default TCP scan (about 1650
ports) isn't dramatic. The difference can be enormous if you
specify your own tiny nmap-services file using the --servicedb or
--datadir options.
-r (Don't randomize ports)
By default, Nmap randomizes the scanned port order (except that
certain commonly accessible ports are moved near the beginning for
efficiency reasons). This randomization is normally desirable, but
you can specify -r for sequential port scanning instead.
--port-ratio <decimal number between 0 and 1>
Scans all ports in nmap-services file with a ratio greater than
the number specified as the argument. (new format nmap-services
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only.)
--top-ports <integer of 1 or greater>
Scans the N highest-ratio ports found in nmap-services file. (new
format nmap-services only.)
SERVICE AND VERSION DETECTION
Point Nmap at a remote machine and it might tell you that ports
25/tcp, 80/tcp, and 53/udp are open. Using its nmap-services database
of about 2,200 well-known services, Nmap would report that those ports
probably correspond to a mail server (SMTP), web server (HTTP), and
name server (DNS) respectively. This lookup is usually accurate-the
vast majority of daemons listening on TCP port 25 are, in fact, mail
servers. However, you should not bet your security on this! People can
and do run services on strange ports.
Even if Nmap is right, and the hypothetical server above is running
SMTP, HTTP, and DNS servers, that is not a lot of information. When
doing vulnerability assessments (or even simple network inventories)
of your companies or clients, you really want to know which mail and
DNS servers and versions are running. Having an accurate version
number helps dramatically in determining which exploits a server is
vulnerable to. Version detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other scan
methods, version detection interrogates those ports to determine more
about what is actually running. The nmap-service-probes database
contains probes for querying various services and match expressions to
recognize and parse responses. Nmap tries to determine the service
protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC
BIND, Apache httpd, Solaris telnetd), the version number, hostname,
device type (e.g. printer, router), the OS family (e.g. Windows,
Linux) and sometimes miscellaneous details like whether an X server is
open to connections, the SSH protocol version, or the KaZaA user
name). Of course, most services don't provide all of this information.
If Nmap was compiled with OpenSSL support, it will connect to SSL
servers to deduce the service listening behind that encryption layer.
When RPC services are discovered, the Nmap RPC grinder (-sR) is
automatically used to determine the RPC program and version numbers.
Some UDP ports are left in the open|filtered state after a UDP port
scan is unable to determine whether the port is open or filtered.
Version detection will try to elicit a response from these ports (just
as it does with open ports), and change the state to open if it
succeeds. open|filtered TCP ports are treated the same way. Note that
the Nmap -A option enables version detection among other things. A
paper documenting the workings, usage, and customization of version
detection is available at http://nmap.org/book/vscan.html.
When Nmap receives responses from a service but cannot match them to
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its database, it prints out a special fingerprint and a URL for you to
submit if to if you know for sure what is running on the port. Please
take a couple minutes to make the submission so that your find can
benefit everyone. Thanks to these submissions, Nmap has about 3,000
pattern matches for more than 350 protocols such as SMTP, FTP, HTTP,
etc.
Version detection is enabled and controlled with the following
options:
-sV (Version detection)
Enables version detection, as discussed above. Alternatively, you
can use -A, which enables version detection among other things.
--allports (Don't exclude any ports from version detection)
By default, Nmap version detection skips TCP port 9100 because
some printers simply print anything sent to that port, leading to
dozens of pages of HTTP GET requests, binary SSL session requests,
etc. This behavior can be changed by modifying or removing the
Exclude directive in nmap-service-probes, or you can specify
--allports to scan all ports regardless of any Exclude directive.
--version-intensity intensity (Set version scan intensity)
When performing a version scan (-sV), Nmap sends a series of
probes, each of which is assigned a rarity value between 1 and 9.
The lower-numbered probes are effective against a wide variety of
common services, while the higher numbered ones are rarely useful.
The intensity level specifies which probes should be applied. The
higher the number, the more likely it is the service will be
correctly identified. However, high intensity scans take longer.
The intensity must be between 0 and 9.
The default is 7.
When a probe is registered to the target port via the
nmap-service-probes ports directive, that probe is tried
regardless of intensity level. This ensures that the DNS probes
will always be attempted against any open port 53, the SSL probe
will be done against 443, etc.
--version-light (Enable light mode)
This is a convenience alias for --version-intensity 2. This light
mode makes version scanning much faster, but it is slightly less
likely to identify services.
--version-all (Try every single probe)
An alias for --version-intensity 9, ensuring that every single
probe is attempted against each port.
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--version-trace (Trace version scan activity)
This causes Nmap to print out extensive debugging info about what
version scanning is doing. It is a subset of what you get with
--packet-trace.
-sR (RPC scan)
This method works in conjunction with the various port scan
methods of Nmap. It takes all the TCP/UDP ports found open and
floods them with SunRPC program NULL commands in an attempt to
determine whether they are RPC ports, and if so, what program and
version number they serve up. Thus you can effectively obtain the
same info as rpcinfo -p even if the target's portmapper is behind
a firewall (or protected by TCP wrappers). Decoys do not currently
work with RPC scan. This is automatically enabled as part of
version scan (-sV) if you request that. As version detection
includes this and is much more comprehensive, -sR is rarely
needed.
OS DETECTION
One of Nmap's best-known features is remote OS detection using TCP/IP
stack fingerprinting. Nmap sends a series of TCP and UDP packets to
the remote host and examines practically every bit in the responses.
After performing dozens of tests such as TCP ISN sampling, TCP options
support and ordering, IP ID sampling, and the initial window size
check, Nmap compares the results to its nmap-os-db database of more
than a thousand known OS fingerprints and prints out the OS details if
there is a match. Each fingerprint includes a freeform textual
description of the OS, and a classification which provides the vendor
name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g.
10), and device type (general purpose, router, switch, game console,
etc).
If Nmap is unable to guess the OS of a machine, and conditions are
good (e.g. at least one open port and one closed port were found),
Nmap will provide a URL you can use to submit the fingerprint if you
know (for sure) the OS running on the machine. By doing this you
contribute to the pool of operating systems known to Nmap and thus it
will be more accurate for everyone.
OS detection enables some other tests which make use of information
that is gathered during the process anyway. One of these is TCP
Sequence Predictability Classification. This measures approximately
how hard it is to establish a forged TCP connection against the remote
host. It is useful for exploiting source-IP based trust relationships
(rlogin, firewall filters, etc) or for hiding the source of an attack.
This sort of spoofing is rarely performed any more, but many machines
are still vulnerable to it. The actual difficulty number is based on
statistical sampling and may fluctuate. It is generally better to use
the English classification such as worthy challenge or trivial joke.
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This is only reported in normal output in verbose (-v) mode. When
verbose mode is enabled along with -O, IP ID sequence generation is
also reported. Most machines are in the incremental class, which means
that they increment the ID field in the IP header for each packet they
send. This makes them vulnerable to several advanced information
gathering and spoofing attacks.
Another bit of extra information enabled by OS detection is a guess at
a target's uptime. This uses the TCP timestamp option (RFC 1323[7]) to
guess when a machine was last rebooted. The guess can be inaccurate
due to the timestamp counter not being initialized to zero or the
counter overflowing and wrapping around, so it is printed only in
verbose mode.
A paper documenting the workings, usage, and customization of OS
detection is available at http://nmap.org/book/osdetect.html.
OS detection is enabled and controlled with the following options:
-O (Enable OS detection)
Enables OS detection, as discussed above. Alternatively, you can
use -A to enable OS detection along with other things.
--osscan-limit (Limit OS detection to promising targets)
OS detection is far more effective if at least one open and one
closed TCP port are found. Set this option and Nmap will not even
try OS detection against hosts that do not meet this criteria.
This can save substantial time, particularly on -PN scans against
many hosts. It only matters when OS detection is requested with -O
or -A.
--osscan-guess; --fuzzy (Guess OS detection results)
When Nmap is unable to detect a perfect OS match, it sometimes
offers up near-matches as possibilities. The match has to be very
close for Nmap to do this by default. Either of these (equivalent)
options make Nmap guess more aggressively. Nmap will still tell
you when an imperfect match is printed and display its confidence
level (percentage) for each guess.
--max-os-tries (Set the maximum number of OS detection tries against a
target)
When Nmap performs OS detection against a target and fails to find
a perfect match, it usually repeats the attempt. By default, Nmap
tries five times if conditions are favorable for OS fingerprint
submission, and twice when conditions aren't so good. Specifying a
lower --max-os-tries value (such as 1) speeds Nmap up, though you
miss out on retries which could potentially identify the OS.
Alternatively, a high value may be set to allow even more retries
when conditions are favorable. This is rarely done, except to
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generate better fingerprints for submission and integration into
the Nmap OS database.
NMAP SCRIPTING ENGINE (NSE)
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and
flexible features. It allows users to write (and share) simple scripts
(using the Lua programming language[8], ) to automate a wide variety
of networking tasks. Those scripts are executed in parallel with the
speed and efficiency you expect from Nmap. Users can rely on the
growing and diverse set of scripts distributed with Nmap, or write
their own to meet custom needs.
Tasks we had in mind when creating the system include network
discovery, more sophisticated version detection, vulnerability
detection. NSE can even be used for vulnerability exploitation.
To reflect those different uses and to simplify the choice of which
scripts to run, each script contains a field associating it with one
or more categories. Currently defined categories are safe, intrusive,
malware, version, discovery, vuln, auth, and default. These are all
described at http://nmap.org/book/nse-usage.html#nse-categories.
The Nmap Scripting Engine is described in detail at
http://nmap.org/book/nse.html
and is controlled by the following options:
-sC
Performs a script scan using the default set of scripts. It is
equivalent to --script=default. Some of the scripts in this
category are considered intrusive and should not be run against a
target network without permission.
--script script-categories|directory|filename|all
Runs a script scan (like -sC) using the comma-separated list of
script categories, individual scripts, or directories containing
scripts, rather than the default set. Nmap first tries to
interpret the arguments as categories, then (if that fails) as
files or directories. A script or directory of scripts may be
specified as an absolute or relative path. Absolute paths are used
as supplied. Relative paths are searched for in the following
places until found: --datadir/; $NMAPDIR/; ~/.nmap/ (not searched
on Windows); NMAPDATADIR/ or scripts/ subdirectory is also tried
in each of these.
If a directory is specified and found, Nmap loads all NSE scripts
(any filenames ending with nse extension are ignored. Nmap does
not search recursively into subdirectories to find scripts. If
individual file names are specified, the file extension does not
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have to be nse.
Nmap scripts are stored in a scripts subdirectory of the Nmap data
directory by default (see Chapter 14, Understanding and
Customizing Nmap Data Files). For efficiency, scripts are indexed
in a database stored in scripts/script.db. which lists the
category or categories in which each script belongs. Give the
argument all to execute all scripts in the Nmap script database.
Malicious scripts are not run in a sandbox and thus could damage
your system or invade your privacy. Never run scripts from third
parties unless you trust the authors or have carefully audited the
scripts yourself.
--script-args name1=value1,name2={name3=value3},name4=value4
lets you provide arguments to NSE scripts. Arguments are passed as
name=value pairs. The provided argument is processed and stored
inside a Lua table, to which all scripts have access. The names
are taken as strings (which must be alphanumeric values) and used
as keys inside the argument-table. Values are either strings or
tables themselves (surrounded by { and }). Subtables make it
possible to override arguments for specific scripts (e.g. when you
want to provide different login/password pairs for different
scripts). For example, you could pass the comma-separated
arguments: user=bar,password=foo, and
anonFTP={password=nobody@foobar.com}. If you want to override an
option to a script, you should index the subtable with the
script's id, since this is the only way the script knows about its
special argument.
--script-trace
This option does what --packet-trace does, just one ISO layer
higher. If this option is specified all incoming and outgoing
communication performed by a script is printed. The displayed
information includes the communication protocol, the source, the
target and the transmitted data. If more than 5% of all
transmitted data is not printable, then the trace output is in a
hex dump format.
--script-updatedb
This option updates the script database found in scripts/script.db
which is used by Nmap to determine the available default scripts
and categories. It is only necessary to update the database if you
have added or removed NSE scripts from the default scripts
directory or if you have changed the categories of any script.
This option is generally used by itself: nmap --script-updatedb.
TIMING AND PERFORMANCE
One of my highest Nmap development priorities has always been
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performance. A default scan (nmap hostname) of a host on my local
network takes a fifth of a second. That is barely enough time to
blink, but adds up when you are scanning tens or hundreds of thousands
of hosts. Moreover, certain scan options such as UDP scanning and
version detection can increase scan times substantially. So can
certain firewall configurations, particularly response rate limiting.
While Nmap utilizes parallelism and many advanced algorithms to
accelerate these scans, the user has ultimate control over how Nmap
runs. Expert users carefully craft Nmap commands to obtain only the
information they care about while meeting their time constraints.
Techniques for improving scan times include omitting non-critical
tests, and upgrading to the latest version of Nmap (performance
enhancements are made frequently). Optimizing timing parameters can
also make a substantial difference. Those options are listed below.
Some options accept a time parameter. This is specified in
milliseconds by default, though you can append s, m, or h to the value
to specify seconds, minutes, or hours. So the --host-timeout arguments
900000, 900s, and 15m all do the same thing.
--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel
scan group sizes)
Nmap has the ability to port scan or version scan multiple hosts
in parallel. Nmap does this by dividing the target IP space into
groups and then scanning one group at a time. In general, larger
groups are more efficient. The downside is that host results can't
be provided until the whole group is finished. So if Nmap started
out with a group size of 50, the user would not receive any
reports (except for the updates offered in verbose mode) until the
first 50 hosts are completed.
By default, Nmap takes a compromise approach to this conflict. It
starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024.
The exact default numbers depend on the options given. For
efficiency reasons, Nmap uses larger group sizes for UDP or
few-port TCP scans.
When a maximum group size is specified with --max-hostgroup, Nmap
will never exceed that size. Specify a minimum size with
--min-hostgroup and Nmap will try to keep group sizes above that
level. Nmap may have to use smaller groups than you specify if
there are not enough target hosts left on a given interface to
fulfill the specified minimum. Both may be set to keep the group
size within a specific range, though this is rarely desired.
These options do not have an effect during the host discovery
phase of a scan. This includes plain ping scans (-sP). Host
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discovery always works in large groups of hosts to improve speed
and accuracy.
The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly. A common
choice is 256 to scan a network in Class C sized chunks. For a
scan with many ports, exceeding that number is unlikely to help
much. For scans of just a few port numbers, host group sizes of
2048 or more may be helpful.
--min-parallelism numprobes; --max-parallelism numprobes (Adjust probe
parallelization)
These options control the total number of probes that may be
outstanding for a host group. They are used for port scanning and
host discovery. By default, Nmap calculates an ever-changing ideal
parallelism based on network performance. If packets are being
dropped, Nmap slows down and allows fewer outstanding probes. The
ideal probe number slowly rises as the network proves itself
worthy. These options place minimum or maximum bounds on that
variable. By default, the ideal parallelism can drop to 1 if the
network proves unreliable and rise to several hundred in perfect
conditions.
The most common usage is to set --min-parallelism to a number
higher than one to speed up scans of poorly performing hosts or
networks. This is a risky option to play with, as setting it too
high may affect accuracy. Setting this also reduces Nmap's ability
to control parallelism dynamically based on network conditions. A
value of ten might be reasonable, though I only adjust this value
as a last resort.
The --max-parallelism option is sometimes set to one to prevent
Nmap from sending more than one probe at a time to hosts. This can
be useful in combination with --scan-delay (discussed later),
although the latter usually serves the purpose well enough by
itself.
--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout
time (Adjust probe timeouts)
Nmap maintains a running timeout value for determining how long it
will wait for a probe response before giving up or retransmitting
the probe. This is calculated based on the response times of
previous probes. If the network latency shows itself to be
significant and variable, this timeout can grow to several
seconds. It also starts at a conservative (high) level and may
stay that way for a while when Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and --initial-rtt-timeout
than the defaults can cut scan times significantly. This is
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particularly true for pingless (-PN) scans, and those against
heavily filtered networks. Don't get too aggressive though. The
scan can end up taking longer if you specify such a low value that
many probes are timing out and retransmitting while the response
is in transit.
If all the hosts are on a local network, 100 milliseconds is a
reasonable aggressive --max-rtt-timeout value. If routing is
involved, ping a host on the network first with the ICMP ping
utility, or with a custom packet crafter such as hping2 that is
more likely to get through a firewall. Look at the maximum round
trip time out of ten packets or so. You might want to double that
for the --initial-rtt-timeout and triple or quadruple it for the
--max-rtt-timeout. I generally do not set the maximum RTT below
100 ms, no matter what the ping times are. Nor do I exceed
1000 ms.
--min-rtt-timeout is a rarely used option that could be useful
when a network is so unreliable that even Nmap's default is too
aggressive. Since Nmap only reduces the timeout down to the
minimum when the network seems to be reliable, this need is
unusual and should be reported as a bug to the nmap-dev mailing
list.
--max-retries numtries (Specify the maximum number of port scan probe
retransmissions)
When Nmap receives no response to a port scan probe, it could mean
the port is filtered. Or maybe the probe or response was simply
lost on the network. It is also possible that the target host has
rate limiting enabled that temporarily blocked the response. So
Nmap tries again by retransmitting the initial probe. If Nmap
detects poor network reliability, it may try many more times
before giving up on a port. While this benefits accuracy, it also
lengthen scan times. When performance is critical, scans may be
sped up by limiting the number of retransmissions allowed. You can
even specify --max-retries 0 to prevent any retransmissions,
though that is rarely recommended.
The default (with no -T template) is to allow ten retransmissions.
If a network seems reliable and the target hosts aren't rate
limiting, Nmap usually only does one retransmission. So most
target scans aren't even affected by dropping --max-retries to a
low value such as three. Such values can substantially speed scans
of slow (rate limited) hosts. You usually lose some information
when Nmap gives up on ports early, though that may be preferable
to letting the --host-timeout expire and losing all information
about the target.
--host-timeout time (Give up on slow target hosts)
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Some hosts simply take a long time to scan. This may be due to
poorly performing or unreliable networking hardware or software,
packet rate limiting, or a restrictive firewall. The slowest few
percent of the scanned hosts can eat up a majority of the scan
time. Sometimes it is best to cut your losses and skip those hosts
initially. Specify --host-timeout with the maximum amount of time
you are willing to wait. I often specify 30m to ensure that Nmap
doesn't waste more than half an hour on a single host. Note that
Nmap may be scanning other hosts at the same time during that half
an hour as well, so it isn't a complete loss. A host that times
out is skipped. No port table, OS detection, or version detection
results are printed for that host.
--scan-delay time; --max-scan-delay time (Adjust delay between probes)
This option causes Nmap to wait at least the given amount of time
between each probe it sends to a given host. This is particularly
useful in the case of rate limiting. Solaris machines (among many
others) will usually respond to UDP scan probe packets with only
one ICMP message per second. Any more than that sent by Nmap will
be wasteful. A --scan-delay of 1s will keep Nmap at that slow
rate. Nmap tries to detect rate limiting and adjust the scan delay
accordingly, but it doesn't hurt to specify it explicitly if you
already know what rate works best.
When Nmap adjusts the scan delay upward to cope with rate
limiting, the scan slows down dramatically. The --max-scan-delay
option specifies the largest delay that Nmap will allow. Setting
this value too low can lead to wasteful packet retransmissions and
possible missed ports when the target implements strict rate
limiting.
Another use of --scan-delay is to evade threshold based intrusion
detection and prevention systems (IDS/IPS).
--min-rate number (Specify a minimum scanning rate)
Nmap's dynamic timing does a good job of finding an appropriate
speed at which to scan. Sometimes, however, you may happen to know
an appropriate scanning rate for a network, or you may have to
guarantee that a scan will be finished by a certain time. When the
--min-rate option is given Nmap will do its best to send packets
as fast or faster than the given rate. The argument is a positive
real number representing a packet rate in packets per second. For
example, specifying --min-rate 300 means that Nmap will try to
keep the sending rate at or above 300 packets per second.
Specifying a minimum rate does not keep Nmap from going faster if
conditions warrant.
There are two conditions when the actual scanning rate may fall
below the specified minimum. The first is if the minimum is faster
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than the fastest rate at which Nmap can send, which is dependent
on hardware. In this case Nmap will send packets as fast as
possible, but be aware that such high rates are likely to cause a
loss of accuracy. The second case is when Nmap has nothing to
send, for example at the end of a scan when the last probes have
been sent and Nmap is waiting for them to time out or be responded
to. It's normal to see the scanning rate drop at the end of a scan
or in between groups of hosts.
Specifying a minimum rate should be done with care. Scanning
faster than a network can support may lead to a loss of accuracy.
In some cases, using a faster rate can make a scan take longer
than it would with a slower rate. This is because Nmap's adaptive
retransmission will detect the network congestion caused by an
excessive scanning rate and increase the number of retransmissions
in order to improve accuracy. So even though packets are sent at a
higher rate, more packets are sent overall. Cap the number of
retransmissions with the --max-retries option if you need to set
an upper limit on total scan time.
The --min-rate option is global, affecting an entire scan, not
individual hosts. It only affects port and host discovery scans.
Other features like OS detection implement their own timing.
--max-rate number (Specify a maximum scanning rate)
Complementary to --min-rate is --max-rate, which limits a scan's
sending rate to a given maximum. Use --max-rate 100, for example,
to limit sending to 100 packets per second on a fast network. Use
--max-rate 0.1 for a slow scan of one packet every ten seconds.
--max-rate, like --min-rate, is a global option affecting an
entire scan. It affects only port and host discovery scans.
The sending rate may temporarily exceed the maximum to make up for
unpredictable delays, but on average the rate will stay at or
below the maximum. Nmap may go slower than the maximum rate if
conditions require it. To keep the sending rate within a specified
range, use --min-rate and --max-rate together.
--defeat-rst-ratelimit
Many hosts have long used rate limiting to reduce the number of
ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset)
packets they generate. This can slow Nmap down dramatically as it
adjusts its timing to reflect those rate limits. You can tell Nmap
to ignore those rate limits (for port scans such as SYN scan which
don't treat non-responsive ports as open) by specifying
--defeat-rst-ratelimit.
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Using this option can reduce accuracy, as some ports will appear
non-responsive because Nmap didn't wait long enough for a
rate-limited RST response. With a SYN scan, the non-response
results in the port being labeled filtered rather than the closed
state we see when RST packets are received. This optional is
useful when you only care about open ports, and distinguishing
between closed and filtered ports isn't worth the extra time.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing
template)
While the fine-grained timing controls discussed in the previous
section are powerful and effective, some people find them
confusing. Moreover, choosing the appropriate values can sometimes
take more time than the scan you are trying to optimize. So Nmap
offers a simpler approach, with six timing templates. You can
specify them with the -T option and their number (05) or their
name. The template names are paranoid (0), sneaky (1), polite (2),
normal (3), aggressive (4), and insane (5). The first two are for
IDS evasion. Polite mode slows down the scan to use less bandwidth
and target machine resources. Normal mode is the default and so
-T3 does nothing. Aggressive mode speeds scans up by making the
assumption that you are on a reasonably fast and reliable network.
Finally insane mode assumes that you are on an extraordinarily
fast network or are willing to sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they wish
to be, while leaving Nmap to pick the exact timing values. The
templates also make some minor speed adjustments for which
fine-grained control options do not currently exist. For example,
-T4 prohibits the dynamic scan delay from exceeding 10 ms for TCP
ports and -T5 caps that value at 5 ms. Templates can be used in
combination with fine-grained controls, and the fine-grained
controls will you specify will take precedence over the timing
template default for that parameter. I recommend using -T4 when
scanning reasonably modern and reliable networks. Keep that option
even when you add fine-grained controls so that you benefit from
those extra minor optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would
recommend always using -T4. Some people love -T5 though it is too
aggressive for my taste. People sometimes specify -T2 because they
think it is less likely to crash hosts or because they consider
themselves to be polite in general. They often don't realize just
how slow -T polite really is. Their scan may take ten times longer
than a default scan. Machine crashes and bandwidth problems are
rare with the default timing options (-T3) and so I normally
recommend that for cautious scanners. Omitting version detection
is far more effective than playing with timing values at reducing
these problems.
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While -T0 and -T1 may be useful for avoiding IDS alerts, they will
take an extraordinarily long time to scan thousands of machines or
ports. For such a long scan, you may prefer to set the exact
timing values you need rather than rely on the canned -T0 and -T1
values.
The main effects of T0 are serializing the scan so only one port
is scanned at a time, and waiting five minutes between sending
each probe. T1 and T2 are similar but they only wait 15 seconds
and 0.4 seconds, respectively, between probes. T3 is Nmap's
default behavior, which includes parallelization. -T4 does the
equivalent of --max-rtt-timeout 1250 --initial-rtt-timeout 500
--max-retries 6 and sets the maximum TCP scan delay to 10
milliseconds. T5 does the equivalent of --max-rtt-timeout 300
--min-rtt-timeout 50 --initial-rtt-timeout 250 --max-retries 2
--host-timeout 15m as well as setting the maximum TCP scan delay
to 5 ms.
FIREWALL/IDS EVASION AND SPOOFING
Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any
two nodes. This allows hosts to act as true peers, serving and
retrieving information from each other. People could access all of
their home systems from work, changing the climate control settings or
unlocking the doors for early guests. This vision of universal
connectivity has been stifled by address space shortages and security
concerns. In the early 1990s, organizations began deploying firewalls
for the express purpose of reducing connectivity. Huge networks were
cordoned off from the unfiltered Internet by application proxies,
network address translation, and packet filters. The unrestricted flow
of information gave way to tight regulation of approved communication
channels and the content that passes over them.
Network obstructions such as firewalls can make mapping a network
exceedingly difficult. It will not get any easier, as stifling casual
reconnaissance is often a key goal of implementing the devices.
Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented defenses.
One of the best methods of understanding your network security posture
is to try to defeat it. Place yourself in the mind-set of an attacker,
and deploy techniques from this section against your networks. Launch
an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel
through one of your own proxies.
In addition to restricting network activity, companies are
increasingly monitoring traffic with intrusion detection systems
(IDS). All of the major IDSs ship with rules designed to detect Nmap
scans because scans are sometimes a precursor to attacks. Many of
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these products have recently morphed into intrusion prevention systems
(IPS) that actively block traffic deemed malicious. Unfortunately for
network administrators and IDS vendors, reliably detecting bad
intentions by analyzing packet data is a tough problem. Attackers with
patience, skill, and the help of certain Nmap options can usually pass
by IDSs undetected. Meanwhile, administrators must cope with large
numbers of false positive results where innocent activity is
misdiagnosed and alerted on or blocked.
Occasionally people suggest that Nmap should not offer features for
evading firewall rules or sneaking past IDSs. They argue that these
features are just as likely to be misused by attackers as used by
administrators to enhance security. The problem with this logic is
that these methods would still be used by attackers, who would just
find other tools or patch the functionality into Nmap. Meanwhile,
administrators would find it that much harder to do their jobs.
Deploying only modern, patched FTP servers is a far more powerful
defense than trying to prevent the distribution of tools implementing
the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and subverting
firewalls and IDS systems. It takes skill and experience. A tutorial
is beyond the scope of this reference guide, which only lists the
relevant options and describes what they do.
-f (fragment packets); --mtu (using the specified MTU)
The -f option causes the requested scan (including ping scans) to
use tiny fragmented IP packets. The idea is to split up the TCP
header over several packets to make it harder for packet filters,
intrusion detection systems, and other annoyances to detect what
you are doing. Be careful with this! Some programs have trouble
handling these tiny packets. The old-school sniffer named Sniffit
segmentation faulted immediately upon receiving the first
fragment. Specify this option once, and Nmap splits the packets
into 8 bytes or less after the IP header. So a 20-byte TCP header
would be split into 3 packets. Two with eight bytes of the TCP
header, and one with the final four. Of course each fragment also
has an IP header. Specify -f again to use 16 bytes per fragment
(reducing the number of fragments). Or you can specify your own
offset size with the --mtu option. Don't also specify -f if you
use --mtu. The offset must be a multiple of 8. While fragmented
packets won't get by packet filters and firewalls that queue all
IP fragments, such as the CONFIG_IP_ALWAYS_DEFRAG option in the
Linux kernel, some networks can't afford the performance hit this
causes and thus leave it disabled. Others can't enable this
because fragments may take different routes into their networks.
Some source systems defragment outgoing packets in the kernel.
Linux with the iptables connection tracking module is one such
example. Do a scan while a sniffer such as Wireshark is running to
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ensure that sent packets are fragmented. If your host OS is
causing problems, try the --send-eth option to bypass the IP layer
and send raw ethernet frames.
-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys)
Causes a decoy scan to be performed, which makes it appear to the
remote host that the host(s) you specify as decoys are scanning
the target network too. Thus their IDS might report 5-10 port
scans from unique IP addresses, but they won't know which IP was
scanning them and which were innocent decoys. While this can be
defeated through router path tracing, response-dropping, and other
active mechanisms, it is generally an effective technique for
hiding your IP address.
Separate each decoy host with commas, and you can optionally use
ME as one of the decoys to represent the position for your real IP
address. If you put ME in the sixth position or later, some common
port scan detectors (such as Solar Designer's excellent Scanlogd)
are unlikely to show your IP address at all. If you don't use ME,
Nmap will put you in a random position. You can also use RND to
generate a random, non-reserved IP address, or RND:number to
generate number addresses.
Note that the hosts you use as decoys should be up or you might
accidentally SYN flood your targets. Also it will be pretty easy
to determine which host is scanning if only one is actually up on
the network. You might want to use IP addresses instead of names
(so the decoy networks don't see you in their nameserver logs).
Decoys are used both in the initial ping scan (using ICMP, SYN,
ACK, or whatever) and during the actual port scanning phase.
Decoys are also used during remote OS detection (-O). Decoys do
not work with version detection or TCP connect scan. When a scan
delay is in effect, the delay is enforced between each batch of
spoofed probes, not between each individual probe. Because decoys
are sent as a batch all at once, they may temporarily violate
congestion control limits.
It is worth noting that using too many decoys may slow your scan
and potentially even make it less accurate. Also, some ISPs will
filter out your spoofed packets, but many do not restrict spoofed
IP packets at all.
-S IP_Address (Spoof source address)
In some circumstances, Nmap may not be able to determine your
source address (Nmap will tell you if this is the case). In this
situation, use -S with the IP address of the interface you wish to
send packets through.
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Another possible use of this flag is to spoof the scan to make the
targets think that someone else is scanning them. Imagine a
company being repeatedly port scanned by a competitor! The -e
option and -PN are generally required for this sort of usage. Note
that you usually won't receive reply packets back (they will be
addressed to the IP you are spoofing), so Nmap won't produce
useful reports.
-e interface (Use specified interface)
Tells Nmap what interface to send and receive packets on. Nmap
should be able to detect this automatically, but it will tell you
if it cannot.
--source-port portnumber; -g portnumber (Spoof source port number)
One surprisingly common misconfiguration is to trust traffic based
only on the source port number. It is easy to understand how this
comes about. An administrator will set up a shiny new firewall,
only to be flooded with complains from ungrateful users whose
applications stopped working. In particular, DNS may be broken
because the UDP DNS replies from external servers can no longer
enter the network. FTP is another common example. In active FTP
transfers, the remote server tries to establish a connection back
to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions. Noting
that DNS replies come from port 53 and active FTP from port 20,
many administrators have fallen into the trap of simply allowing
incoming traffic from those ports. They often assume that no
attacker would notice and exploit such firewall holes. In other
cases, administrators consider this a short-term stop-gap measure
until they can implement a more secure solution. Then they forget
the security upgrade.
Overworked network administrators are not the only ones to fall
into this trap. Numerous products have shipped with these insecure
rules. Even Microsoft has been guilty. The IPsec filters that
shipped with Windows 2000 and Windows XP contain an implicit rule
that allows all TCP or UDP traffic from port 88 (Kerberos). In
another well-known case, versions of the Zone Alarm personal
firewall up to 2.1.25 allowed any incoming UDP packets with the
source port 53 (DNS) or 67 (DHCP).
Nmap offers the -g and --source-port options (they are equivalent)
to exploit these weaknesses. Simply provide a port number and Nmap
will send packets from that port where possible. Nmap must use
different port numbers for certain OS detection tests to work
properly, and DNS requests ignore the --source-port flag because
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Nmap relies on system libraries to handle those. Most TCP scans,
including SYN scan, support the option completely, as does UDP
scan.
--data-length number (Append random data to sent packets)
Normally Nmap sends minimalist packets containing only a header.
So its TCP packets are generally 40 bytes and ICMP echo requests
are just 28. This option tells Nmap to append the given number of
random bytes to most of the packets it sends. OS detection (-O)
packets are not affected because accuracy there requires probe
consistency, but most pinging and portscan packets support this.
It slows things down a little, but can make a scan slightly less
conspicuous.
--ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string
(Send packets with specified ip options)
The IP protocol[9] offers several options which may be placed in
packet headers. Unlike the ubiquitous TCP options, IP options are
rarely seen due to practicality and security concerns. In fact,
many Internet routers block the most dangerous options such as
source routing. Yet options can still be useful in some cases for
determining and manipulating the network route to target machines.
For example, you may be able to use the record route option to
determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are being
dropped by a certain firewall, you may be able to specify a
different route with the strict or loose source routing options.
The most powerful way to specify IP options is to simply pass in
values as the argument to --ip-options. Precede each hex number
with \x then the two digits. You may repeat certain characters by
following them with an asterisk and then the number of times you
wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is a hex
string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options.
Simply pass the letter R, T, or U to request record-route,
record-timestamp, or both options together, respectively. Loose or
strict source routing may be specified with an L or S followed by
a space and then a space-separated list of IP addresses.
If you wish to see the options in packets sent and received,
specify --packet-trace. For more information and examples of using
IP options with Nmap, see
http://seclists.org/nmap-dev/2006/q3/0052.html.
--ttl value (Set IP time-to-live field)
Sets the IPv4 time-to-live field in sent packets to the given
value.
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--randomize-hosts (Randomize target host order)
Tells Nmap to shuffle each group of up to 16384 hosts before it
scans them. This can make the scans less obvious to various
network monitoring systems, especially when you combine it with
slow timing options. If you want to randomize over larger group
sizes, increase PING_GROUP_SZ in nmap.h and recompile. An
alternative solution is to generate the target IP list with a list
scan (-sL -n -oN filename), randomize it with a Perl script, then
provide the whole list to Nmap with -iL.
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC address)
Asks Nmap to use the given MAC address
for all of the raw ethernet frames it sends. This option implies
--send-eth to ensure that Nmap actually sends ethernet-level
packets. The MAC given can take several formats. If it is simply
the number 0, Nmap chooses a completely random MAC address for the
session. If the given string is an even number of hex digits (with
the pairs optionally separated by a colon), Nmap will use those as
the MAC. If fewer than 12 hex digits are provided, Nmap fills in
the remainder of the 6 bytes with random values. If the argument
isn't a 0 or hex string, Nmap looks through nmap-mac-prefixes to
find a vendor name containing the given string (it is case
insensitive). If a match is found, Nmap uses the vendor's OUI
(3-byte prefix) and fills out the remaining 3 bytes randomly.
Valid --spoof-mac argument examples are Apple, 0,
01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco. This option
only affects raw packet scans such as SYN scan or OS detection,
not connection-oriented features such as version detection or the
Nmap Scripting Engine.
--badsum (Send packets with bogus TCP/UDP checksums)
Asks Nmap to use an invalid TCP or UDP checksum for packets sent
to target hosts. Since virtually all host IP stacks properly drop
these packets, any responses received are likely coming from a
firewall or IDS that didn't bother to verify the checksum. For
more details on this technique, see http://nmap.org/p60-12.html
OUTPUT
Any security tools is only as useful as the output it generates.
Complex tests and algorithms are of little value if they aren't
presented in an organized and comprehensible fashion. Given the number
of ways Nmap is used by people and other software, no single format
can please everyone. So Nmap offers several formats, including the
interactive mode for humans to read directly and XML for easy parsing
by software.
In addition to offering different output formats, Nmap provides
options for controlling the verbosity of output as well as debugging
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messages. Output types may be sent to standard output or to named
files, which Nmap can append to or clobber. Output files may also be
used to resume aborted scans.
Nmap makes output available in five different formats. The default is
called interactive output, and it is sent to standard output (stdout).
There is also normal output, which is similar to interactive except
that it displays less runtime information and warnings since it is
expected to be analyzed after the scan completes rather than
interactively.
XML output is one of the most important output types, as it can be
converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.
The two remaining output types are the simple grepable output which
includes most information for a target host on a single line, and
sCRiPt KiDDi3 0utPUt for users who consider themselves |<-r4d.
While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that results
should be stored in. Multiple formats may be specified, but each
format may only be specified once. For example, you may wish to save
normal output for your own review while saving XML of the same scan
for programmatic analysis. You might do this with the options -oX
myscan.xml -oN myscan.nmap. While this chapter uses the simple names
like myscan.xml for brevity, more descriptive names are generally
recommended. The names chosen are a matter of personal preference,
though I use long ones that incorporate the scan date and a word or
two describing the scan, placed in a directory named after the company
I'm scanning.
While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command nmap
-oX myscan.xml target prints XML to myscan.xml and fills standard
output with the same interactive results it would have printed if -oX
wasn't specified at all. You can change this by passing a hyphen
character as the argument to one of the format types. This causes Nmap
to deactivate interactive output, and instead print results in the
format you specified to the standard output stream. So the command
nmap -oX - target will send only XML output to stdout. Serious errors
may still be printed to the normal error stream, stderr.
Unlike some Nmap arguments, the space between the logfile option flag
(such as -oX) and the filename or hyphen is mandatory. If you omit the
flags and give arguments such as -oG- or -oXscan.xml, a backwards
compatibility feature of Nmap will cause the creation of normal format
output files named G- and Xscan.xml respectively.
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All of these arguments support strftime-like conversions in the
filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as
in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D
is the same as %m%d%y. A % followed by any other character just yields
that character (%% gives you a percent symbol). So -oX
'scan-%T-%D.xml' will use an XML file in the form of
scan-144840-121307.xml.
Nmap also offers options to control scan verbosity and to append to
output files rather than clobbering them. All of these options are
described below.
Nmap Output Formats
-oN filespec (normal output)
Requests that normal output be directed to the given filename. As
discussed above, this differs slightly from interactive output.
-oX filespec (XML output)
Requests that XML output be directed to the given filename. Nmap
includes a document type definition (DTD) which allows XML parsers
to validate Nmap XML output. While it is primarily intended for
programmatic use, it can also help humans interpret Nmap XML
output. The DTD defines the legal elements of the format, and
often enumerates the attributes and values they can take on. The
latest version is always available from
http://nmap.org/data/nmap.dtd.
XML offers a stable format that is easily parsed by software. Free
XML parsers are available for all major computer languages,
including C/C++, Perl, Python, and Java. People have even written
bindings for most of these languages to handle Nmap output and
execution specifically. Examples are Nmap::Scanner[10]
and Nmap::Parser[11]
in Perl CPAN. In almost all cases that a non-trivial application
interfaces with Nmap, XML is the preferred format.
The XML output references an XSL stylesheet which can be used to
format the results as HTML. The easiest way to use this is simply
to load the XML output in a web browser such as Firefox or IE. By
default, this will only work on the machine you ran Nmap on (or a
similarly configured one) due to the hard-coded nmap.xsl
filesystem path. Use the --webxml or --stylesheet options to
create portable XML files that render as HTML on any web-connected
machine.
-oS filespec (ScRipT KIdd|3 oUTpuT)
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Script kiddie output is like interactive output, except that it is
post-processed to better suit the l33t HaXXorZ who previously
looked down on Nmap due to its consistent capitalization and
spelling. Humor impaired people should note that this option is
making fun of the script kiddies before flaming me for supposedly
helping them.
-oG filespec (grepable output)
This output format is covered last because it is deprecated. The
XML output format is far more powerful, and is nearly as
convenient for experienced users. XML is a standard for which
dozens of excellent parsers are available, while grepable output
is my own simple hack. XML is extensible to support new Nmap
features as they are released, while I often must omit those
features from grepable output for lack of a place to put them.
Nevertheless, grepable output is still quite popular. It is a
simple format that lists each host on one line and can be
trivially searched and parsed with standard Unix tools such as
grep, awk, cut, sed, diff, and Perl. Even I usually use it for
one-off tests done at the command line. Finding all the hosts with
the SSH port open or that are running Solaris takes only a simple
grep to identify the hosts, piped to an awk or cut command to
print the desired fields.
Grepable output consists of comments (lines starting with a pound
(#)) and target lines. A target line includes a combination of 6
labeled fields, separated by tabs and followed with a colon. The
fields are Host, Ports, Protocols, Ignored State, OS, Seq Index,
IP ID, and Status.
The most important of these fields is generally Ports, which gives
details on each interesting port. It is a comma separated list of
port entries. Each port entry represents one interesting port, and
takes the form of seven slash (/) separated subfields. Those
subfields are: Port number, State, Protocol, Owner, Service,
SunRPC info, and Version info.
As with XML output, this man page does not allow for documenting
the entire format. A more detailed look at the Nmap grepable
output format is available from
http://nmap.org/book/output-formats-grepable-output.html.
-oA basename (Output to all formats)
As a convenience, you may specify -oA basename to store scan
results in normal, XML, and grepable formats at once. They are
stored in basename.nmap, basename.xml, and basename.gnmap,
respectively. As with most programs, you can prefix the filenames
with a directory path, such as ~/nmaplogs/foocorp/ on Unix or
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c:\hacking\sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level)
Increases the verbosity level, causing Nmap to print more
information about the scan in progress. Open ports are shown as
they are found and completion time estimates are provided when
Nmap thinks a scan will take more than a few minutes. Use it twice
or more for even greater verbosity.
Most changes only affect interactive output, and some also affect
normal and script kiddie output. The other output types are meant
to be processed by machines, so Nmap can give substantial detail
by default in those formats without fatiguing a human user.
However, there are a few changes in other modes where output size
can be reduced substantially by omitting some detail. For example,
a comment line in the grepable output that provides a list of all
ports scanned is only printed in verbose mode because it can be
quite long.
-d [level] (Increase or set debugging level)
When even verbose mode doesn't provide sufficient data for you,
debugging is available to flood you with much more! As with the
verbosity option (-v), debugging is enabled with a command-line
flag (-d) and the debug level can be increased by specifying it
multiple times. Alternatively, you can set a debug level by
giving an argument to -d. For example, -d9 sets level nine. That
is the highest effective level and will produce thousands of lines
unless you run a very simple scan with very few ports and targets.
Debugging output is useful when a bug is suspected in Nmap, or if
you are simply confused as to what Nmap is doing and why. As this
feature is mostly intended for developers, debug lines aren't
always self-explanatory. You may get something like: Timeout vals:
srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987
rttvar: 14987 to: 100000. If you don't understand a line, your
only recourses are to ignore it, look it up in the source code, or
request help from the development list (nmap-dev). Some lines are
self explanatory, but the messages become more obscure as the
debug level is increased.
--packet-trace (Trace packets and data sent and received)
Causes Nmap to print a summary of every packet sent or received.
This is often used for debugging, but is also a valuable way for
new users to understand exactly what Nmap is doing under the
covers. To avoid printing thousands of lines, you may want to
specify a limited number of ports to scan, such as -p20-30. If you
only care about the goings on of the version detection subsystem,
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use --version-trace instead.
--open (Show only open (or possibly open) ports)
Sometimes you only care about ports you can actually connect to
(open ones), and don't want results cluttered with closed,
filtered, and closed|filtered ports. Output customization is
normally done after the scan using tools such as grep, awk, and
Perl, but this feature was added due to overwhelming requests.
Specify --open to only see open, open|filtered, and unfiltered
ports. These three ports are treated just as they normally are,
which means that open|filtered and unfiltered may be condensed
into counts if there are an overwhelming number of them.
--iflist (List interfaces and routes)
Prints the interface list and system routes as detected by Nmap.
This is useful for debugging routing problems or device
mischaracterization (such as Nmap treating a PPP connection as
ethernet).
--log-errors (Log errors/warnings to normal mode output file)
Warnings and errors printed by Nmap usually go only to the screen
(interactive output), leaving any normal-format output files
(usually specified with -oN) uncluttered. When you do want to see
those messages in the normal output file you specified, add this
option. It is useful when you aren't watching the interactive
output or when you want to record errors while debugging a
problem. The error and warning messages will still appear in
interactive mode too. This won't work for most errors related to
bad command-line arguments because Nmap may not have initialized
its output files yet. In addition, some Nmap error and warning
messages use a different system which does not yet support this
option.
An alternative to --log-errors is redirecting interactive output
(including the standard error stream) to a file. Most Unix shells
make this approach easy, though it can be difficult on Windows.
Miscellaneous output options
--append-output (Append to rather than clobber output files)
When you specify a filename to an output format flag such as -oX
or -oN, that file is overwritten by default. If you prefer to keep
the existing content of the file and append the new results,
specify the --append-output option. All output filenames specified
in that Nmap execution will then be appended to rather than
clobbered. This doesn't work well for XML (-oX) scan data as the
resultant file generally won't parse properly until you fix it up
by hand.
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--resume filename (Resume aborted scan)
Some extensive Nmap runs take a very long time-on the order of
days. Such scans don't always run to completion. Restrictions may
prevent Nmap from being run during working hours, the network
could go down, the machine Nmap is running on might suffer a
planned or unplanned reboot, or Nmap itself could crash. The
administrator running Nmap could cancel it for any other reason as
well, by pressing ctrl-C. Restarting the whole scan from the
beginning may be undesirable. Fortunately, if normal (-oN) or
grepable (-oG) logs were kept, the user can ask Nmap to resume
scanning with the target it was working on when execution ceased.
Simply specify the --resume option and pass the normal/grepable
output file as its argument. No other arguments are permitted, as
Nmap parses the output file to use the same ones specified
previously. Simply call Nmap as nmap --resume logfilename. Nmap
will append new results to the data files specified in the
previous execution. Resumption does not support the XML output
format because combining the two runs into one valid XML file
would be difficult.
--stylesheet path or URL (Set XSL stylesheet to transform XML output)
Nmap ships with an XSL
stylesheet
named nmap.xsl
for viewing or translating XML output to HTML.
The XML output includes an xml-stylesheet directive which points
to nmap.xml where it was initially installed by Nmap (or in the
current working directory on Windows). Simply load Nmap's XML
output in a modern web browser and it should retrieve nmap.xsl
from the filesystem and use it to render results. If you wish to
use a different stylesheet, specify it as the argument to
--stylesheet. You must pass the full pathname or URL. One common
invocation is --stylesheet http://nmap.org/data/nmap.xsl. This
tells a browser to load the latest version of the stylesheet from
Nmap.Org. The --webxml option does the same thing with less typing
and memorization. Loading the XSL from Nmap.Org makes it easier to
view results on a machine that doesn't have Nmap (and thus
nmap.xsl) installed. So the URL is often more useful, but the
local filesystem location of nmap.xsl is used by default for
privacy reasons.
--webxml (Load stylesheet from Nmap.Org)
This convenience option is simply an alias for --stylesheet
http://nmap.org/data/nmap.xsl.
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--no-stylesheet (Omit XSL stylesheet declaration from XML)
Specify this option to prevent Nmap from associating any XSL
stylesheet with its XML output. The xml-stylesheet directive is
omitted.
MISCELLANEOUS OPTIONS
This section describes some important (and not-so-important) options
that don't really fit anywhere else.
-6 (Enable IPv6 scanning)
Since 2002, Nmap has offered IPv6 support for its most popular
features. In particular, ping scanning (TCP-only), connect
scanning, and version detection all support IPv6. The command
syntax is the same as usual except that you also add the -6
option. Of course, you must use IPv6 syntax if you specify an
address rather than a hostname. An address might look like
3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are
recommended. The output looks the same as usual, with the IPv6
address on the interesting ports line being the only IPv6 give
away.
While IPv6 hasn't exactly taken the world by storm, it gets
significant use in some (usually Asian) countries and most modern
operating systems support it. To use Nmap with IPv6, both the
source and target of your scan must be configured for IPv6. If
your ISP (like most of them) does not allocate IPv6 addresses to
you, free tunnel brokers are widely available and work fine with
Nmap. I use the free IPv6 tunnel broker service at
http://www.tunnelbroker.net. Other tunnel brokers are listed at
Wikipedia[12]. 6to4 tunnels are another popular, free approach.
-A (Aggressive scan options)
This option enables additional advanced and aggressive options. I
haven't decided exactly which it stands for yet. Presently this
enables OS detection (-O), version scanning (-sV), script scanning
(-sC) and traceroute (--traceroute).
More features may be added in the future. The point is to enable a
comprehensive set of scan options without people having to
remember a large set of flags. However, because script scanning
with the default set is considered intrusive, you should not use
-A against target networks without permission. This option only
enables features, and not timing options (such as -T4) or
verbosity options (-v) that you might want as well.
--datadir directoryname (Specify custom Nmap data file location)
Nmap obtains some special data at runtime in files named
nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
nmap-mac-prefixes, and nmap-os-db. If the location of any of these
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files has been specified (using the --servicedb or --versiondb
options), that location is used for that file. After that, Nmap
searches these files in the directory specified with the --datadir
option (if any). Any files not found there, are searched for in
the directory specified by the NMAPDIR environmental variable.
Next comes ~/.nmap for real and effective UIDs (POSIX systems
only) or location of the Nmap executable (Win32 only), and then a
compiled-in location such as /usr/local/share/nmap or
/usr/share/nmap
--servicedb services file (Specify custom services file)
Asks Nmap to use the specified services file rather than the
nmap-services data file that comes with Nmap. Using this option
also causes a fast scan (-F) to be used. See the description for
--datadir for more information on Nmap's data files.
--versiondb service probes file (Specify custom service probes file)
Asks Nmap to use the specified service probes file rather than the
nmap-service-probes data file that comes with Nmap. See the
description for --datadir for more information on Nmap's data
files.
--send-eth (Use raw ethernet sending)
Asks Nmap to send packets at the raw ethernet (data link) layer
rather than the higher IP (network) layer. By default, Nmap
chooses the one which is generally best for the platform it is
running on. Raw sockets (IP layer) are generally most efficient
for Unix machines, while ethernet frames are required for Windows
operation since Microsoft disabled raw socket support. Nmap still
uses raw IP packets on Unix despite this option when there is no
other choice (such as non-ethernet connections).
--send-ip (Send at raw IP level)
Asks Nmap to send packets via raw IP sockets rather than sending
lower level ethernet frames. It is the complement to the
--send-eth option discussed previously.
--privileged (Assume that the user is fully privileged)
Tells Nmap to simply assume that it is privileged enough to
perform raw socket sends, packet sniffing, and similar operations
that usually require root privileges on Unix systems. By default
Nmap quits if such operations are requested but geteuid is not
zero. --privileged is useful with Linux kernel capabilities and
similar systems that may be configured to allow unprivileged users
to perform raw-packet scans. Be sure to provide this option flag
before any flags for options that require privileges (SYN scan, OS
detection, etc.). The NMAP_PRIVILEGED environmental variable may
be set as an equivalent alternative to --privileged.
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--unprivileged (Assume that the user lacks raw socket privileges)
This option is the opposite of --privileged. It tells Nmap to
treat the user as lacking network raw socket and sniffing
privileges. This is useful for testing, debugging, or when the raw
network functionality of your operating system is somehow broken.
The NMAP_UNPRIVILEGED environmental variable may be set as an
equivalent alternative to --unprivileged.
--release-memory (Release memory before quitting)
This option is only useful for memory-leak debugging. It causes
Nmap to release allocated memory just before it quits so that
actual memory leaks are easier to spot. Normally Nmap skips this
as the OS does this anyway upon process termination.
--interactive (Start in interactive mode)
Starts Nmap in interactive mode, which offers an interactive Nmap
prompt allowing easy launching of multiple scans (either
synchronously or in the background). This is useful for people who
scan from multi-user systems as they often want to test their
security without letting everyone else on the system know exactly
which systems they are scanning. Use --interactive to activate
this mode and then type h for help. This option is rarely used
because proper shells are usually more familiar and
feature-complete. This option includes a bang (!) operator for
executing shell commands, which is one of many reasons not to
install Nmap setuid root.
-V; --version (Print version number)
Prints the Nmap version number and exits.
-h; --help (Print help summary page)
Prints a short help screen with the most common command flags.
Running Nmap without any arguments does the same thing.
RUNTIME INTERACTION
During the execution of Nmap, all key presses are captured. This
allows you to interact with the program without aborting and
restarting it. Certain special keys will change options, while any
other keys will print out a status message telling you about the scan.
The convention is that lowercase letters increase the amount of
printing, and uppercase letters decrease the printing. You may also
press ? for help.
v / V
Increase / decrease the verbosity level
d / D
Increase / decrease the debugging Level
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p / P
Turn on / off packet tracing
?
Print a runtime interaction help screen
Anything else
Print out a status message like this:
Stats: 0:00:08 elapsed; 111 hosts completed (5 up), 5 undergoing
Service Scan
Service scan Timing: About 28.00% done; ETC: 16:18 (0:00:15
remaining)
EXAMPLES
Here are some Nmap usage examples, from the simple and routine to a
little more complex and esoteric. Some actual IP addresses and domain
names are used to make things more concrete. In their place you should
substitute addresses/names from your own network.. While I don't think
port scanning other networks is or should be illegal, some network
administrators don't appreciate unsolicited scanning of their networks
and may complain. Getting permission first is the best approach.
For testing purposes, you have permission to scan the host
scanme.nmap.org. This permission only includes scanning via Nmap and
not testing exploits or denial of service attacks. To conserve
bandwidth, please do not initiate more than a dozen scans against that
host per day. If this free scanning target service is abused, it will
be taken down and Nmap will report Failed to resolve given
hostname/IP: scanme.nmap.org. These permissions also apply to the
hosts scanme2.nmap.org, scanme3.nmap.org, and so on, though those
hosts do not currently exist.
nmap -v scanme.nmap.org
This option scans all reserved TCP ports on the machine
scanme.nmap.org -v option enables verbose mode.
nmap -sS -O scanme.nmap.org/24
Launches a stealth SYN scan against each machine that is up out of the
255 machines on class C network where Scanme resides. It also tries to
determine what operating system is running on each host that is up and
running. This requires root privileges because of the SYN scan and OS
detection.
nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
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Launches host enumeration and a TCP scan at the first half of each of
the 255 possible 8 bit subnets in the 198.116 class B address space.
This tests whether the systems run SSH, DNS, POP3, or IMAP on their
standard ports, or anything on port 4564. For any of these ports found
open, version detection is used to determine what application is
running.
nmap -v -iR 100000 -PN -p 80
Asks Nmap to choose 100,000 hosts at random and scan them for web
servers (port 80). Host enumeration is disabled with -PN since first
sending a couple probes to determine whether a host is up is wasteful
when you are only probing one port on each target host anyway.
nmap -PN -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap
216.163.128.20/20
This scans 4096 IPs for any web servers (without pinging them) and
saves the output in grepable and XML formats.
BUGS
Like its author, Nmap isn't perfect. But you can help make it better
by sending bug reports or even writing patches. If Nmap doesn't behave
the way you expect, first upgrade to the latest version available from
http://nmap.org. If the problem persists, do some research to
determine whether it has already been discovered and addressed. Try
searching for the error message on our search page at
http://insecure.org/search.html or at Google. Also try browsing the
nmap-dev archives at http://seclists.org/. Read this full manual page
as well. If nothing comes of this, mail a bug report to
<nmap-dev@insecure.org>. Please include everything you have learned
about the problem, as well as what version of Nmap you are running and
what operating system version it is running on. Problem reports and
Nmap usage questions sent to <nmap-dev@insecure.org> are far more
likely to be answered than those sent to Fyodor directly. If you
subscribe to the nmap-dev list before posting, your message will
bypass moderation and get through more quickly. Subscribe at
http://cgi.insecure.org/mailman/listinfo/nmap-dev.
Code patches to fix bugs are even better than bug reports. Basic
instructions for creating patch files with your changes are available
at http://nmap.org/data/HACKING. Patches may be sent to nmap-dev
(recommended) or to Fyodor directly.
AUTHOR
Fyodor <fyodor@insecure.org> (http://insecure.org)
Hundreds of people have made valuable contributions to Nmap over the
years. These are detailed in the CHANGELOG file which is distributed
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with Nmap and also available from http://nmap.org/changelog.html.
LEGAL NOTICES
Nmap Copyright and Licensing
The Nmap Security Scanner is (C) 1996-2008 Insecure.Com LLC. Nmap is
also a registered trademark of Insecure.Com LLC. This program is free
software; you may redistribute and/or modify it under the terms of the
GNU General Public License as published by the Free Software
Foundation; Version 2 with the clarifications and exceptions described
below. This guarantees your right to use, modify, and redistribute
this software under certain conditions. If you wish to embed Nmap
technology into proprietary software, we sell alternative licenses
(contact <sales@insecure.com>). Dozens of software vendors already
license Nmap technology such as host discovery, port scanning, OS
detection, and version detection.
Note that the GPL places important restrictions on derived works, yet
it does not provide a detailed definition of that term. To avoid
misunderstandings, we consider an application to constitute a
derivative work for the purpose of this license if it does any of the
following:
+ Integrates source code from Nmap
+ Reads or includes Nmap copyrighted data files, such as nmap-os-db
or nmap-service-probes.
+ Executes Nmap and parses the results (as opposed to typical shell
or execution-menu apps, which simply display raw Nmap output and
so are not derivative works.)
+ Integrates/includes/aggregates Nmap into a proprietary executable
installer, such as those produced by InstallShield.
+ Links to a library or executes a program that does any of the
above.
The term Nmap should be taken to also include any portions or derived
works of Nmap. This list is not exclusive, but is just meant to
clarify our interpretation of derived works with some common examples.
These restrictions only apply when you actually redistribute Nmap. For
example, nothing stops you from writing and selling a proprietary
front-end to Nmap. Just distribute it by itself, and point people to
http://nmap.org to download Nmap.
We don't consider these to be added restrictions on top of the GPL,
but just a clarification of how we interpret derived works as it
applies to our GPL-licensed Nmap product. This is similar to the way
Linus Torvalds has announced his interpretation of how derived works
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applies to Linux kernel modules. Our interpretation refers only to
Nmap-we don't speak for any other GPL products.
If you have any questions about the GPL licensing restrictions on
using Nmap in non-GPL works, we would be happy to help. As mentioned
above, we also offer alternative license to integrate Nmap into
proprietary applications and appliances. These contracts have been
sold to many security vendors, and generally include a perpetual
license as well as providing for priority support and updates as well
as helping to fund the continued development of Nmap technology.
Please email <sales@insecure.com> for further information.
As a special exception to the GPL terms, Insecure.Com LLC grants
permission to link the code of this program with any version of the
OpenSSL library which is distributed under a license identical to that
listed in the included COPYING.OpenSSL file, and distribute linked
combinations including the two. You must obey the GNU GPL in all
respects for all of the code used other than OpenSSL. If you modify
this file, you may extend this exception to your version of the file,
but you are not obligated to do so.
If you received these files with a written license agreement or
contract stating terms other than the terms above, then that
alternative license agreement takes precedence over these comments.
Creative Commons License for this Nmap Guide
This Nmap Reference Guide is (C) 2005-2008 Insecure.Com LLC. It is
hereby placed under version 2.5 of the Creative Commons Attribution
License[13]. This allows you redistribute and modify the work as you
desire, as long as you credit the original source. Alternatively, you
may choose to treat this document as falling under the same license as
Nmap itself (discussed previously).
Source Code Availability and Community Contributions
Source is provided to this software because we believe users have a
right to know exactly what a program is going to do before they run
it. This also allows you to audit the software for security holes
(none have been found so far).
Source code also allows you to port Nmap to new platforms, fix bugs,
and add new features. You are highly encouraged to send your changes
to <fyodor@insecure.org> for possible incorporation into the main
distribution. By sending these changes to Fyodor or one of the
Insecure.Org development mailing lists, it is assumed that you are
offering Fyodor and Insecure.Com LLC the unlimited, non-exclusive
right to reuse, modify, and relicense the code. Nmap will always be
available Open Source, but this is important because the inability to
relicense code has caused devastating problems for other Free Software
projects (such as KDE and NASM). We also occasionally relicense the
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code to third parties as discussed above. If you wish to specify
special license conditions of your contributions, just say so when you
send them.
No Warranty
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License v2.0 for more details at
http://www.gnu.org/licenses/gpl-2.0.html, or in the COPYING file
included with Nmap.
It should also be noted that Nmap has occasionally been known to crash
poorly written applications, TCP/IP stacks, and even operating
systems. While this is extremely rare, it is important to keep in
mind. Nmap should never be run against mission critical systems
unless you are prepared to suffer downtime. We acknowledge here that
Nmap may crash your systems or networks and we disclaim all liability
for any damage or problems Nmap could cause.
Inappropriate Usage
Because of the slight risk of crashes and because a few black hats
like to use Nmap for reconnaissance prior to attacking systems, there
are administrators who become upset and may complain when their system
is scanned. Thus, it is often advisable to request permission before
doing even a light scan of a network.
Nmap should never be installed with special privileges (e.g. suid
root) for security reasons.
Third-Party Software
This product includes software developed by the Apache Software
Foundation[14]. A modified version of the Libpcap portable packet
capture library[15] is distributed along with Nmap. The Windows
version of Nmap utilized the Libpcap-derived WinPcap library[16]
instead. Regular expression support is provided by the PCRE
library[17], which is open-source software, written by Philip Hazel.
Certain raw networking functions use the Libdnet[18] networking
library, which was written by Dug Song. A modified version is
distributed with Nmap. Nmap can optionally link with the OpenSSL
cryptography toolkit[19] for SSL version detection support. The Nmap
Scripting Engine uses an embedded version of the Lua programming
language[20]. All of the third-party software described in this
paragraph is freely redistributable under BSD-style software licenses.
United States Export Control Classification
U.S. Export Control: Insecure.Com LLC believes that Nmap falls under
U.S. ECCN (export control classification number) 5D992. This category
is called Information Security software not controlled by 5D002. The
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only restriction of this classification is AT (anti-terrorism), which
applies to almost all goods and denies export to a handful of rogue
nations such as Iran and North Korea. Thus exporting Nmap does not
require any special license, permit, or other governmental
authorization.
AUTHOR
Gordon Fyodor Lyon
Insecure.Org
Author.
COPYRIGHT c 2008 Nmap Project
Copyright
NOTES
1. RFC 1122
http://www.rfc-editor.org/rfc/rfc1122.txt
2. RFC 792
http://www.rfc-editor.org/rfc/rfc792.txt
3. RFC 1918
http://www.rfc-editor.org/rfc/rfc1918.txt
4. UDP
http://www.rfc-editor.org/rfc/rfc768.txt
5. TCP RFC
http://www.rfc-editor.org/rfc/rfc793.txt
6. RFC 959
http://www.rfc-editor.org/rfc/rfc959.txt
7. RFC 1323
http://www.rfc-editor.org/rfc/rfc1323.txt
8. Lua programming language
http://lua.org
9. IP protocol
http://www.rfc-editor.org/rfc/rfc791.txt
10. Nmap::Scanner
http://sourceforge.net/projects/nmap-scanner/
11. Nmap::Parser
http://nmapparser.wordpress.com/
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12. listed at Wikipedia
http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers
13. Creative Commons Attribution License
http://creativecommons.org/licenses/by/2.5/
14. Apache Software Foundation
http://www.apache.org
15. Libpcap portable packet capture library
http://www.tcpdump.org
16. WinPcap library
http://www.winpcap.org
17. PCRE library
http://www.pcre.org
18. Libdnet
http://libdnet.sourceforge.net
19. OpenSSL cryptography toolkit
http://www.openssl.org
20. Lua programming language
http://www.lua.org
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