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 control 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 connections. 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, logging 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 control 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 system 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 reasons. This makes third party file transfers impossible.

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