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Man Pages


Manual Reference Pages  -  FTP.PROXY (1)

NAME

ftp.proxy - FTP proxy server

CONTENTS

Synopsis
Description
Options

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

remote-user@remote-ftp.server

The password that is send to the proxy server is the password required for logging 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.

    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 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).
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 one 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, 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.

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 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).
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).
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.
Notice that the 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.

    Interface specific configurations

ftp.proxy’s configuration file supports interface specific configuration sections. Such section begin with a line that starts with
[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.
-d enter debug mode, the communication between server and client is written to stderr.
-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.
-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’!
-v prefix
  set prefix as variable prefix for the variable passwd to the access and command control program.
-x ctp set a connection translation program to overwrite the server and login information supplied by the user.
-X file
  write xferlog loggin to file.
-V show version number
-y allow any data ports on any remote interfaces (dangerous!).
-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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--> FTP.PROXY (1) 23 JANUARY 2003

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