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
.Bk -words
.Op Fl AnrVw
.Op Fl a Ar address
.Op Fl D Ar debuglevel
.Op Fl g Ar group
.Op Fl M Ar maxport
.Op Fl m Ar minport
.Op Fl R Ar address[:port]
.Op Fl S Ar address
.Op Fl t Ar timeout
.Op Fl u Ar user
.Ek
.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
.Qq ftp
or
.Qq anonymous
only.
Any attempt to log in as another user will be blocked by the proxy.
.It Fl a Ar address
Specify the local IP address to use in
.Xr bind 2
as the source for connections made by
.Nm ftp-proxy
when connecting to destination FTP servers.
This may be necessary if the interface address of
your default route is not reachable from the destinations
.Nm
is attempting connections to, or this address is different from the one
connections are being NATed to.
In the usual case this means that
.Ar address
should be a publicly visible IP address assigned to one of
the interfaces on the machine running
.Nm
and should be the same address to which you are translating traffic
if you are using the
.Fl n
option.
.It Fl D Ar debuglevel
Specify a debug level, where the proxy emits verbose debug output
into
.Xr syslogd 8
at level
.Dv 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.
.It Fl g Ar group
Specify the named group to drop group privileges to, after doing
.Xr pf 4
lookups which require root.
By default,
.Nm
uses the default group of the user it drops privilege to.
.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
.Dv IPPORT_HILASTAUTO
defined in
.Aq Pa netinet/in.h
as 65535.
.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
.Dv IPPORT_HIFIRSTAUTO
defined in
.Aq Pa netinet/in.h
as 49152.
.It Fl n
Activate network address translation
.Pq NAT
mode.
In this mode, the proxy will not attempt to proxy passive mode
.Pq 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
does not require any IP forwarding or NAT beyond the
.Em rdr
necessary to capture the FTP control connection.
.It Fl R Ar address:[port]
Reverse proxy mode for FTP servers running behind a NAT gateway.
In this mode, no redirection is needed.
The proxy is run from
.Xr inetd 8
on the port that external clients connect to (usually 21).
Control connections and passive data connections are forwarded
to the server.
.It Fl r
Use reverse host
.Pq reverse DNS
lookups for logging and libwrap use.
By default,
the proxy does not look up hostnames for libwrap or logging purposes.
.It Fl S Ar address
Source address to use for data connections made by the proxy.
Useful when there are multiple addresses (aliases) available
to the proxy.
Clients may expect data connections to have the same source
address as the control connections, and reject or drop other
connections.
.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 u Ar user
Specify the named user to drop privilege to, after doing
.Xr pf 4
lookups which require root privilege.
By default,
.Nm
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
will be chosen randomly from the range
.Ar minport
to
.Ar maxport
as described above.
.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.
.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
.Bd -literal -offset 2n
int_if = \&"xl0\&"
rdr pass on $int_if proto tcp from any to any port 21 -> 127.0.0.1 \e
	port 8021
.Ed
.Pp
.Xr inetd 8
must then be configured to run
.Nm
on the port from above using
.Bd -literal -offset 2n
127.0.0.1:8021 stream tcp nowait root /usr/libexec/ftp-proxy ftp-proxy
.Ed
.Pp
in
.Xr inetd.conf 5 .
.Pp
.Nm
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 its 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
.Fl u , m ,
and
.Fl M
above) in on the external interface.
The following example allows only ports 49152 to 65535 to pass in
statefully:
.Bd -literal -offset indent
block in on $ext_if proto tcp all
pass  in on $ext_if inet proto tcp from any to $ext_if \e
    port > 49151 keep state
.Ed
.Pp
Alternatively, rules can make use of the fact that by default,
.Nm
runs as user
.Qq proxy
to allow the backchannel connections, as in the following example:
.Bd -literal -offset indent
block in on $ext_if proto tcp all
pass  in on $ext_if inet proto tcp from any to $ext_if \e
    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
.Pq 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
.Pq 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.