FTP.PROXY(1) FTP.PROXY(1)
10 JUNE 2005
NAME
ftp.proxy - FTP proxy server
SYNOPSIS
ftp.proxy [options] [server]
DESCRIPTION
ftp.proxy is a proxy server for a subset of the file tranfer protocol
described in RFC 959. It forwards traffic between a client and a
server without looking too much if both hosts do real FTP. The FTP
server can be either given on the command line or supplied by the
client.
ftp.proxy can be started from a TCP superserver like inetd(1) or
tcpproxy(1). but can also bind to a TCP/IP port on it's own and run
in standalone (or daemon) mode.
Protocol Support
ftp.proxy supports the following FTP commands:
ABOR, ACCT, APPE, CDUP, CWD, DELE, FEAT, LIST,
MDTM, MKD, MODE, NLIST, NOOP, PASS, PASV, PORT,
PWD, QUIT, RETR, REST, RNFR, RNTO, RMD, SITE,
SIZE, SMNT, STAT, STOR, SYST, TYPE, USER, XCUP,
XCWD, XMKD, XPWD, XRMD
Transfer of structured data is not supported.
Command Parameters
By default ftp.proxy does not accept blanks in command parameters.
This is to protect your UNIX server against users who work on
computers where these things are usual.
To allow blanks the option -b must be given on the command line.
Notice that blanks at the beginning or end of the parameter are still
not supported.
The `SITE' is in neither case affected by this limitation, ftp.proxy
accepts always blanks in `SITE' parameters.
The option -y enables ftp.proxy to accept data connections from
different remote interfaces. Try to avoid using this option, because
it can cause security problems (see HISTORY for details).
Server Selection
If client-side server selection it turned on with the -e option the
user must select the FTP server he wants to use with the `@' notation.
Instead of specifying the real ftp server on the command line the user
has to connect to the gateway machine where ftp.proxy is running and
to enter the username in the form
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remote-user@remote-ftp.server
The password that is send to the proxy server is the password required
for log into remote-ftp-server with the account remote-user.
In situations where the FTP client doesn't support usernames
containing an `@' the percent sign `%' might be used for that.
Session Logging
ftp.proxy logs to syslog using `ftp' as facility under Linux or BSD
and `daemon' else, `ftp.proxy' is the logname. Both settings can be
changed with the facility and logname configuration option or the -L
command line option. Log options are only processed globally not
after a client connected; log options appearing in an interface
specific section are ignored.
Each file transfer is logged with the filename, file size in bytes and
transfer times with `t1' as time between FTP transfer command and
finish of data transfer and `td' as time between connecting the data
channel. `td' is the best approximation for the data transfer time.
In case monitor mode is enabled (-m option) ftp.proxy logs the file's
full pathname on the FTP server and the FTP command parameter
otherwise.
Access Control
If an access control program is given with the -a option on the
command line the connection data is passed to the acp before the
server is contacted. The acp should return 0 as exit code to grant
access and another value to deny.
The access controller receives the following variables:
PROXY_INTERFACE, PROXY_PORT
interface and port where the client is connected to the proxy.
PROXY_CLIENT, PROXY_CLIENTNAME
IP number an name of the connected client.
PROXY_SERVER, PROXY_SERVERPORT, PROXY_SERVERNAME
IP number, port and name of the FTP server the client wants to
contact.
PROXY_SERVERLOGIN
the supplied username for the FTP server.
PROXY_USERNAME, PROXY_PASSWD
supplied username and password for usage of the proxy server.
The values for PROXY_USERNAME and PROXY_PASSWD are taken from the
supplied remote username and password if they contain a colon `:'. In
this case the local authentication data is taken from the left side of
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the colon and the remaining right side is passed on to the server.
Furthermore the acp's stdout is connected to the FTP client and it's
stderr is read by ftp.proxy which writes the acp's stderr output to
syslog.
Notice also that a non-zero acp exit code signals ftp.proxy that
something's wrong and that ftp.proxy should terminate.
Connection Translation
Beginning with version 1.1.6 ftp.proxy supports connection translation
programs (ctp's). A ctp can completly overwrite the user's server
selection and login. If configured the ctp is called before the acp.
It receives the same environment variables like the acp and returns
server and login information that should ftp.proxy for the server
connection on it's stdout. The format of the ctp output lines is
variable [<whitespace>]= [<whitespace>] value
where variable is one of
SERVERNAME, SERVERLOGIN, SERVERPASSWD, SERVERPORT
and value the corresponding value. Alternativly to these four
variables you can use the shorter forms
SERVER, LOGIN, PASSWD, PORT
as variable names. Furthermore the case of the variable names doesn't
matter and any whitespace around value is ignored.
The ctp can deny the proxy request by exiting with an non-zero exit
code, In which case ftp.proxy drops the connection immediately.
Alternativly the ctp can also print a line starting with -ERR, which
is written to syslog before the connection is closed.
Command Control
If a command control program (ccp) is given with the -c option this
program is called for the FTP commands
APPE, CDUP, CWD, DELE, LIST, MDTM, MKD,
NLST, RETR, RNFR, RNTO, RMD, SIZE, STAT,
STOR, STOU, XCUP, XCWD, XMKD, XRMD
The ccp returns an exit code of 0 to grant and any other to deny
access (the exit code to the `QUIT' command is ignored). For the ccp
the same variables as for acp's are set with the addition of
PROXY_COMMAND, PROXY_PARAMETER
FTP command and parameter (if set).
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PROXY_SESSION
a unique identifier for the proxy session.
PROXY_CCPCOLL,
the client's number of collisions with the ccp's permission rules
(number of `permission denied' responses).
The ccp's stdout and stderr are connected to ftp.proxy. A one line
message written to stdout by the ccp goes to syslog, while a message
on stderr is sent to the client. If this message does not contain a
status ftp.proxy substitutes a `553' code. If the message is empty
the client gets a simle `553 permission denied'. Notice that the
stderr message is only used if the ccp returns an exit code other the
zero.
On normal program termination (`QUIT' command or timeout) the ccp is
called with the command `+EXIT' to do some final clean up. It is not
reliable that the ccp receives the `+EXIT' event. There are lots of
possiblities that the proxy terminates without generating it, e.g.
client timeout, server error or signal reciption by the proxy.
Monitor Mode
The -m option puts ftp.proxy into the monitor mode. ftp.proxy will
then try to keep track of the client's current directory on the server
side. With this information the file parameter for the commands
APPE, CDUP, CWD, DELE, FEAT, LIST, MDTM, MKD
NLST, RETR, RNFR, RNTO, RMD, SIZE, STOR,
XCUP, XCWD, XMKD, XRMD
is converted into an absolute path. This value is then used in syslog
messages and given to a ccp in the PROXY_FTPPATH variable.
Furthermore the variable PROXY_FTPHOME contains the user's initial
directory which is assumed to be his home directory.
The `LIST' and `NLIST' command may have a parameter or not. If it is
absent ftp.proxy sets the parameter to `*' but this affects only the
PROXY_FTPPATH variable, not the command that is sent to the server.
For the `CDUP' command PROXY_FTPPATH contains the full path of the
target directory.
Monitoring may not work with all server systems since the output of
the `PWD' command which is used by ftp.proxy to get the current
directory in not completely defined. If the directory can not be
clearly determined ftp.proxy will terminate.
Filecopy Mode
ftp.proxy can create copies of all transferred files on the proxy
server. Files are grouped by date (every day gets its own directory
named YYYY/MM/DD) under a base directory (/var/tmp by default).
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Filenames have the form
date+time,Ppid,Ncount,Ffilename.data
with
date, time
current date and time
pid ftp.proxy process id
count
number of transfered file in that session
filename
filename (without path) of the transfered file; characters other
than alphanumerics and `_', `-', `.' and `+' are %-encoded. Each
`.data' file contains a single file and is accompanied by a
`.info' file which stores corresponding session data.
Filecopy mode is activated and configured in the configuration file
only; there are no command line options. For logging purposes
filecopy mode switches monitor mode on.
Filecopy mode must be compiled into the proxy, run `ftp.proxy -V' to
check if it is or not.
Transparent Redirection
Under Linux ftp.proxy is able to run as transparent proxy if the -r
option is set. That is if the operating system packet filter
redirects TCP sessions ftp.proxy can detect and handle this depending
on the redirection mode which is set with the -r option. Possible
modes are
none, off
turns redirection support off.
redirect, accept
accepts transparent redirection but does nothing special with it.
forward
accepts transparent redirection and proxies them to the
originally requested server.
forward-only
like forward but rejects all non-redirected connections.
The redirection mode can also be set in the configuration file using
the redirection option.
An appropriate iptables rule is
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iptables -t nat -A PREROUTING --protocol TCP \
--dport 21 \
-j REDIRECT --to-port 21
which redirects all incoming traffic on port 21 to the local port 21.
CONFIGURATION FILE
ftp.proxy can take most of its command line options also from a
configuration file which can be set with the -f option.
The configuration file can contain comments and blank lines (usual
UN*X-style) but ftp.proxy terminates immediately with an error code if
an unknown or invalid configuration option is found.
The following options can be set:
acp /path/to/acp
sets the path to the access control program (-a option).
allow-anyremote yes|no
if enabled ftp.proxy does not check the remote's end in data
connection, required for some bad multi-homed servers and FXP (-y
option).
allow-blanks yes|no
allows blanks in FTP command parameters (-b option).
allow-passwdblanks yes|no
allows blanks in the FTP login password (-B option).
bind portnum
sets the port number to which ftp.proxy should bind to, activates
daemon mode (-D option).
ccp /path/to/ccp
sets the path to the command control command (-c option).
ctp /path/to/ctp
sets the path to the connection translation program (-x option).
debug yes|no
turns debugging mode on or off (-d option).
monitormode yes|no
enables monitor mode (-m option).
proxy-routing yes|no
if enabled ftp.proxy uses the last `@' in the username to
determine to which server it should connect. This make proxy
hopping (or routing) possible (-u option).
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selectserver yes|no
enables client side server selection, disables the server option
(-e option).
server ftpserver
sets the connection's FTP server, disables selectserver.
serverlist list-of-allowed-server
specifies a command separated list of servers to which the
clients are allowed to connect (-s option).
serverdelimiter charset
define charset for the username/server delimeter, default is `@'.
sourceip ip-number
defines the IP address for the outgoing control connection to the
remote server, which also determines the local IP address for
data transmissions.
timeout timeout
set the timeout in seconds.
xferlog filename
sets the location of the xferlog file and enables xferlog
logging.
Filecopy Configuration
The following options configure ftp.proxy's filecopy mode. They are
only valid if ftp.proxy is compiled with filecopy support (run
`ftp.proxy -V').
fc.basedir directory
the full path to the directory under which the file copies are
stored, the default is /var/tmp.
fc.create-copies yes|no
activate creation of file copyies or not.
fc.error-mode continue|terminate|server-error
sets the proxy behaviour in case the copy- of infofile cannot be
created. In case of `continue' the proxy does nothing special,
`terminate' terminates the current proxy session and `server-
error' sends "500 server error" to the client. The `server-
error' mode works only properly if the error is detected directly
after the FTP transfer command before it is acknowledged to the
client.
Interface specific configurations
ftp.proxy's configuration file supports interface specific
configuration sections. Such section begin with a line that starts
with
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[interface-ip]
followed by the configuration options for connections on this specific
interface. ftp.proxy checks for such sections immidiately after the
client connection is accepted. If it finds at least one interface
specific section in the configuration file but none for the current
interface it considers itself to be not configured for it and drops
the connection sending a `421 not available' message to the client.
ftp.proxy accepts all global configuration options from above
(allthough not all make sense, e.g. bind) in interface specific
section. That is, ftp.proxy can have completely different
configurations on different interfaces. But to deactivate a non-
boolean option, e.g. ctp you can not simply give the option without a
value, this would be considered as `bad configuration option'.
Instead you must supply a single dash `-' to clear an option.
Configuration checking
ftp.proxy prints an error message and terminates immediately if it
finds an unknown or bad configuration option. More worse, these error
messages are printed to ftp.proxy's stderr and not to syslog which
makes it a little bit difficult to observe. ftp.proxy addresses this
issue by supporting the -F option.
The -F option sets the configuration file and the `check-and-print'
option, that is ftp.proxy will only read, check and print it's
configuration options as they are set after reading the configuration.
An interface IP-number may be given as optional command line parameter
to make ftp.proxy print the configuration for this particular
interface.
OPTIONS
The following options are available:
-a acp
specify an access control program that grants or denies access
via ftp.proxy.
-b allows blanks in filenames.
-B allows blanks and other special charackters in passwords.
-c ccp
sets a command control program that grants or denies the usage of
FTP commands through ftp.proxy.
-C charset
define charset for the username/server delimeter, default is `@'.
-d enter debug mode, the communication between server and client is
written to stderr.
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-D port
run ftp.proxy in daemon mode.
-f configfile
sets ftp.proxy's configuration file.
-F configfile [interface]
read and print the proxy configuration for interface from
configfile. If interface is missing the global configuration is
printed. This is a check-only option, after the configuration
has been printed ftp.proxy terminates, no connection handling is
done.
-e enable client-side server selection. With this option the server
argument isn't accepted.
-l sets logging of most of the FTP commands.
-L facility[,logname]
sets syslog facility and name.
-m sets the monitor mode.
-p port
tell ftp.proxy to use port as source port for data transfers
(using port number 20 is FTP standard). Keep in mind that port
numbers below 1024 require root permissions.
-q sourceip
sets the IP number for the outgoing control connection.
-s list
the FTP server selected by the client must match one of the
pattern from the comma separated list. The wildcards `*' and `?'
can be used.
-t timeout
specify a different FTP timeout in seconds than the default of
900 (15 minutes).
-u search for the last appearance of an '@' in the username. This
allows the use of usernames with a '@' in it. Be careful with
this option, this can be abused to do 'proxy hopping'!
In case server selection is not enabled -u allows `user@hostname'
style usernames.
-v prefix
set prefix as variable prefix for the variable passwd to the
access and command control program. The default value is FTP_.
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-V show version number
-x ctp
set a connection translation program to overwrite the server and
login information supplied by the user.
-X file
write xferlog logging to file.
-y allow any data ports on any remote interfaces, required to make
ftp.proxy support FXP-mode.
-z size
sets the amount of data in bytes ftp.proxy tries to read with one
system call from either the client or the server. The default is
1024 bytes, valid values range from 1 to 4096. Playing around
with larger values than the default may increase the proxy's data
troughput.
SYSLOG
ftp.proxy reports to FTP log facility on linux and BSD systems and
Daemon log facility on other.
AUTHOR
Andreas Schoenberg <asg@ftpproxy.org>
SEE ALSO
inetd(1), tcpproxy(1), syslogd(8), syslog.conf(5).
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