Internet File Transfer Protocol proxy
is a proxy for the Internet File Transfer
Protocol. FTP control connections should be redirected into the proxy using
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)
facility for this.
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 anchor. $server and $orig_server are the same unless
is used to force a different $server address
for all connections. (These example rules use inet, but the proxy also
In case of active mode (PORT or EPRT):
pass in from $server to $proxy port $proxy_port \
rdr-to $client port $port
pass out from $server to $client port $port \
nat-to $orig_server port $natport
In case of passive mode (PASV or EPSV):
pass in from $client to $orig_server port $proxy_port \
rdr-to $server port $port
pass out from $client to $server port $port nat-to $proxy
chroots to "/var/empty" and
changes to user "_ftp_proxy" to drop privileges.
The options are as follows:
- 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.
- Only permit anonymous FTP connections. Either user
"ftp" or user "anonymous" is allowed.
- The proxy will use this as the source address for the
control connection to a server.
- Address where the proxy will listen for redirected control
connections. The default is 127.0.0.1, or ::1 in IPv6 mode.
- Debug level, ranging from 0 to 7. Higher is more verbose.
The default is 5. (These levels correspond to the
- Do not daemonize. The process will stay in the foreground,
logging to standard error.
- 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.
- Fixed server port. Only used in combination with
-R. The default is port 21.
- Port where the proxy will listen for redirected
connections. The default is port 8021.
- Create rules with queue
queue appended, so that data connections
can be queued.
- 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
- Rewrite sourceport to 20 in active mode to suit ancient
clients that insist on this RFC property.
- The filter rules will add tag
tag to data connections, and will use
match rules instead of pass ones. This way alternative rules that use the
tagged keyword can be implemented
following the ftp-proxy anchor. These rules
can use special pf(4) features
like route-to, reply-to, label, rtable, overload, etc. that
ftp-proxy does not implement itself. There
must be a matching pass rule after the
ftp-proxy anchor or the data connections will
- 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.
- 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.
To make use of the proxy,
needs the following
rules. Adjust the rules as needed; depending on the rest of the ruleset, the
last rule explicitly allowing FTP sessions from the proxy may not be
pass in quick proto tcp to port ftp divert-to 127.0.0.1 port 8021
pass out inet proto tcp from (self) to any port ftp
does not allow the ruleset to be
modified if the system is running at a
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.
acts as a man-in-the-middle it
breaks explicit FTP TLS connections (RFC 4217).