FTP-PROXY(8) OpenBSD 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] [-m maxsessions]
[-P port] [-p port] [-q queue] [-R address] [-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 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.
Assuming the FTP control connection is from $client to $server, the proxy
connected to the server using the $proxy source address, and $port is ne-
gotiated, 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 flags S/SAFR keep state
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 flags S/SAFR keep state
pass out quick inet proto tcp \
from $proxy to $server port $port flags S/SAFR keep state
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 al-
lowed 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.
-m maxsessions
Maximum number of concurrent FTP sessions. When the proxy reach-
es this limit, new connections are denied. The default is 100.
-P port
Fixed server port. Only used in combination with -R. The de-
fault 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 timeout
Number of seconds that the control connection can be idle, before
the proxy will disconnect. The default is 24 hours. 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, pf.conf(5) needs the following rules. All an-
chors 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 keep state
SEE ALSO
ftp(1), pf(4), pf.conf(5)
CAVEATS
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/empty" and changes to user "proxy" to drop
privileges.
OpenBSD 3.9 November 28, 2004 2