FTP-PROXY(8)            NetBSD System Manager's Manual            FTP-PROXY(8)

NAME
     ftp-proxy -- Internet File Transfer Protocol proxy daemon

SYNOPSIS
     ftp-proxy [-6Adrv] [-a address] [-b address] [-D level] [-i netif]
               [-m maxsessions] [-P port] [-p port] [-q queue] [-R address]
               [-T tag] [-t timeout]

DESCRIPTION
     ftp-proxy is a proxy for the Internet File Transfer Protocol.  FTP con-
     trol connections should be redirected into the proxy using the ipnat(4)
     or pf(4) rdr command, after which the proxy connects to the server on
     behalf of the client.

     The proxy allows data connections to pass, rewriting and redirecting them
     so that the right addresses are used.  All connections from the client to
     the server have their source address rewritten so they appear to come
     from the proxy.  Consequently, all connections from the server to the
     proxy have their destination address rewritten, so they are redirected to
     the client.  The proxy uses the pf(4) anchor facility for this, unless
     the option -i is specified, it will then use the ipnat(4) interface.

     Assuming the FTP control connection is from $client to $server, the proxy
     connected to the server using the $proxy source address, and $port is
     negotiated, then ftp-proxy adds the following rules to the various
     anchors.  (These example rules use inet, but the proxy also supports
     inet6.)

     In case of active mode (PORT or EPRT):

       rdr from $server to $proxy port $port -> $client
       pass quick inet proto tcp \
           from $server to $client port $port

     In case of passive mode (PASV or EPSV):

       nat from $client to $server port $port -> $proxy
       pass in quick inet proto tcp \
           from $client to $server port $port
       pass out quick inet proto tcp \
           from $proxy to $server port $port

     The options are as follows:

     -6      IPv6 mode.  The proxy will expect and use IPv6 addresses for all
             communication.  Only the extended FTP modes EPSV and EPRT are
             allowed with IPv6.  The proxy is in IPv4 mode by default.

     -A      Only permit anonymous FTP connections.  Either user "ftp" or user
             "anonymous" is allowed.

     -a address
             The proxy will use this as the source address for the control
             connection to a server.

     -b address
             Address where the proxy will listen for redirected control con-
             nections.  The default is 127.0.0.1, or ::1 in IPv6 mode.

     -D level
             Debug level, ranging from 0 to 7.  Higher is more verbose.  The
             default is 5.  (These levels correspond to the syslog(3) levels.)

     -d      Do not daemonize.  The process will stay in the foreground, log-
             ging to standard error.

     -i netif
             Set ftp-proxy for use with IP-Filter.  The argument netif should
             be set to the name of the network interface where rdr is applied
             on.

     -m maxsessions
             Maximum number of concurrent FTP sessions.  When the proxy
             reaches this limit, new connections are denied.  The default is
             100 sessions.  The limit can be lowered to a minimum of 1, or
             raised to a maximum of 500.

     -P port
             Fixed server port.  Only used in combination with -R.  The
             default is port 21.

     -p port
             Port where the proxy will listen for redirected connections.  The
             default is port 8021.

     -q queue
             Create rules with queue queue appended, so that data connections
             can be queued.

     -R address
             Fixed server address, also known as reverse mode.  The proxy will
             always connect to the same server, regardless of where the client
             wanted to connect to (before it was redirected).  Use this option
             to proxy for a server behind NAT, or to forward all connections
             to another proxy.

     -r      Rewrite sourceport to 20 in active mode to suit ancient clients
             that insist on this RFC property.

     -T tag  Automatically tag packets passing through the pf(4) rule with the
             name supplied.

     -t timeout
             Number of seconds that the control connection can be idle, before
             the proxy will disconnect.  The maximum is 86400 seconds, which
             is also the default.  Do not set this too low, because the con-
             trol connection is usually idle when large data transfers are
             taking place.

     -v      Set the 'log' flag on pf rules committed by ftp-proxy.  Use twice
             to set the 'log-all' flag.  The pf rules do not log by default.

CONFIGURATION
     To make use of the proxy using pf(4), pf.conf(5) needs the following
     rules.  All anchors are mandatory.  Adjust the rules as needed.

     In the NAT section:

       nat-anchor "ftp-proxy/*"
       rdr-anchor "ftp-proxy/*"
       rdr pass on $int_if proto tcp from $lan to any port 21 -> \
           127.0.0.1 port 8021

     In the rule section:

       anchor "ftp-proxy/*"
       pass out proto tcp from $proxy to any port 21

     To make use of the proxy using ipnat(4), ipnat.conf(5) need the following
     rule:

       rdr $int_if any port 21 -> 127.0.0.1 port 8021 tcp

SEE ALSO
     ftp(1), ipnat(4), pf(4), ipnat.conf(5), pf.conf(5)

CAVEATS
     ipnat(4) and pf(4) does not allow the ruleset to be modified if the sys-
     tem is running at a securelevel higher than 1.  At that level ftp-proxy
     cannot add rules to the anchors and FTP data connections may get blocked.

     Negotiated data connection ports below 1024 are not allowed.

     The negotiated IP address for active modes is ignored for security rea-
     sons.  This makes third party file transfers impossible.

     ftp-proxy chroots to "/var/chroot/ftp-proxy" and changes to user "_proxy"
     to drop privileges.

NetBSD 5.0_STABLE             September 14, 2009             NetBSD 5.0_STABLE
