path: root/libexec/ftp-proxy/ftp-proxy.8

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.Dd August 17, 2001
.Dt FTP-PROXY 8
.Os
.Sh NAME
.Nm ftp-proxy
.Nd
Internet File Transfer Protocol proxy server.
.Sh SYNOPSIS
.Nm ftp-proxy
.Op Fl AnVwr
.Op Fl D Ar debuglevel
.Op Fl g Ar group
.Op Fl m Ar minport
.Op Fl M Ar maxport
.Op Fl t Ar timeout
.Op Fl u Ar user
.Sh DESCRIPTION
.Nm
is a proxy for the Internet File Transfer Protocol.
The proxy uses
.Xr pf 4
and expects to have the FTP control connection as described in
.Xr services 5
redirected to it via a
.Xr pf 4
.Em rdr
command.
An example of how to do that is further down in this document.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl A
Permit only anonymous FTP connections.
The proxy will allow connections to log in to other sites as the user
"ftp" or "anonymous" only.
Any attempt to log in as another user will be blocked by the proxy.
.It Fl g Ar groupname
Specify the named group to drop group privileges to, after doing
.Xr pf 4
lookups which require root.
By default
.Nm ftp-proxy
uses the default group of the user it drops privilege to.
.It Fl u Ar username
Specify the named user to drop privilege to, after doing
.Xr pf 4
lookups which require root privilege.
By default
.Nm ftp-proxy
drops privilege to the user
.Em proxy .
.Pp
Running as root means that the source of data connections the proxy makes
for PORT and EPRT will be the RFC mandated port 20.
When running as a non-root user the source of the data connections from
.Nm ftp-proxy
will be chosen randomly from the range
.Nm minport
to
.Nm maxport
as described below.
.It Fl n
Activate network address translation mode.
In this mode, the proxy will not attempt to proxy passive mode
(PASV or EPSV) data connections.
In order for this to work, the machine running the proxy will need to
be forwarding packets and doing network address translation to allow
the outbound passive connections from the client to reach the server.
See
.Xr pf.conf 5
for more details on nat.
The proxy only ignores passive mode data connections when using this flag,
it will still proxy PORT and EPRT mode data connections.
Without this flag,
.Nm ftp-proxy
does not require any IP forwarding or NAT beyond the
.Em rdr
necessary to capture the FTP control connection.
.It Fl V
Be verbose.
With this option the proxy logs the control commands
sent by clients and the replies sent by the servers to
.Xr syslogd 8
.It Fl w
Use the tcp wrapper access control library
.Xr hosts_access 3
allowing connections to be allowed or denied based on the tcp wrapper's
.Xr hosts.allow 5
and
.Xr hosts.deny 5
files.
The proxy does libwrap operations after determining the destination
of the captured control connection, so that tcp wrapper rules may
be written based on the destination as well as the source of FTP connections.
.It Fl r
Use reverse host (reverse DNS) lookups for logging and libwrap use.
By default the proxy does not look up hostnames for libwrap or logging
purposes.
.It Fl m Ar minport
Specify the lower end of the port range the proxy will use for all
data connections it establishes.
The default is
.Ev IPPORT_HIFIRSTAUTO
defined in <netinet/in.h>
as 49152.
.It Fl M Ar maxport
Specify the upper end of the port range the proxy will use for the
data connections it establishes.
The default is
.Ev IPPORT_HILASTAUTO
defined in <netinet/in.h>
as 65535.
.It Fl t Ar timeout
Specifies a timeout, in seconds.
The proxy will exit and close open connections if it sees no data
for the duration of the timeout.
The default is 0, which means the proxy will not time out.
.It Fl D Ar debuglevel
Specify a debug level, where the proxy emits verbose debug output
into
.Xr syslogd 8
at level LOG_DEBUG.
Meaningful values of debuglevel are 0-3, where 0 is no debug output and
3 is lots of debug output, the default being 0.
.El
.Pp
.Nm ftp-proxy
is run from
.Xr inetd 8
and requires that FTP connections are redirected to it using a
.Em rdr
rule.
A typical way to do this would be to use a
.Xr pf.conf 5
rule such as
.Pp
.Bd -literal
  int_if = xl0
  rdr on $int_if proto tcp from any to any port 21 -> 127.0.0.1 port 8021
.Ed
.Pp
.Xr inetd 8
must then be configured to run
.Nm ftp-proxy
on the port from above using
.Pp
  127.0.0.1:8021 stream tcp nowait root /usr/libexec/ftp-proxy ftp-proxy
.Pp
in
.Xr inetd.conf 5 .
.Pp
.Nm ftp-proxy
accepts the redirected control connections and forwards them
to the server.
The proxy replaces the address and port number that the client
sends through the control connection to the server with his own
address and proxy port, where it listens for the data connection.
When the server opens the data connection back to this port, the
proxy forwards it to the client.
The
.Xr pf.conf 5
rules need to let pass connections to these proxy ports
(see options -u, -m and -M above) in on the external interface.
The following example allows only ports 49152 to 65535 to pass in
statefully:
.Bd -literal
  block in on $ext_if proto tcp all
  pass  in on $ext_if inet proto tcp from any to $ext_if \\
      port > 49151 keep state
.Ed
.Pp
Alternatively, rules can make use of the fact that by default,
.Nm ftp-proxy
runs as user "proxy" to allow the backchannel connections, as in the
following example:
.Bd -literal
  block in on $ext_if proto tcp all
  pass  in on $ext_if inet proto tcp from any to $ext_if \\
      user proxy keep state
.Ed
.Pp
These examples do not cover the connections from the proxy to the
foreign ftp server.
If one does not pass outgoing connections by default additional rules
are needed.
.Sh SEE ALSO
.Xr ftp 1 ,
.Xr pf 4 ,
.Xr hosts.allow 5 ,
.Xr hosts.deny 5 ,
.Xr inetd.conf 5 ,
.Xr pf.conf 5 ,
.Xr inetd 8 ,
.Xr pfctl 8 ,
.Xr syslogd 8 .
.Sh BUGS
Extended Passive mode (EPSV) is not supported by the proxy and will
not work unless the proxy is run in network address translation mode.
When not in network address translation mode, the proxy returns an error
to the client, hopefully forcing the client to revert to Passive mode (PASV)
which is supported.
EPSV will work in network address translation mode, assuming a
.Xr pf.conf 5
setup which allows the EPSV connections through to their destinations.
.Pp
IPv6 is not yet supported.