FTP-PROXY(8) System Manager's Manual FTP-PROXY(8)

ftp-proxy — Internet File Transfer Protocol proxy server

ftp-proxy -i [-AnrVw] [-a address] [-D debuglevel] [-g group] [-M maxport] [-m minport] [-R address[:port]] [-S address] [-t timeout] [-u user]

ftp-proxy -p [-AnrVw] [-a address] [-D debuglevel] [-g group] [-M maxport] [-m minport] [-R address[:port]] [-S address] [-t timeout] [-u user]

ftp-proxy is a proxy for the Internet File Transfer Protocol. The proxy uses pf(4) and expects to have the FTP control connection as described in services(5) redirected to it via a pf(4) rdr command. An example of how to do that is further down in this document.

The options are as follows:

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.
address
Specify the local IP address to use in bind(2) as the source for connections made by 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 ftp-proxy is attempting connections to, or this address is different from the one connections are being NATed to. In the usual case this means that address should be a publicly visible IP address assigned to one of the interfaces on the machine running ftp-proxy and should be the same address to which you are translating traffic if you are using the -n option.
debuglevel
Specify a debug level, where the proxy emits verbose debug output into 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.
group
Specify the named group to drop group privileges to, after doing pf(4) lookups which require root. By default, ftp-proxy uses the default group of the user it drops privilege to.
Set ftp-proxy for use with IP-Filter.
maxport
Specify the upper end of the port range the proxy will use for the data connections it establishes. The default is IPPORT_HILASTAUTO defined in ⟨netinet/in.h⟩ as 65535.
minport
Specify the lower end of the port range the proxy will use for all data connections it establishes. The default is IPPORT_HIFIRSTAUTO defined in ⟨netinet/in.h⟩ as 49152.
Activate network address translation (NAT) 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 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, ftp-proxy does not require any IP forwarding or NAT beyond the rdr necessary to capture the FTP control connection.
Set ftp-proxy for use with pf.
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 inetd(8) on the port that external clients connect to (usually 21). Control connections and passive data connections are forwarded to the server.
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.
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.
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.
user
Specify the named user to drop privilege to, after doing pf(4) lookups which require root privilege. By default, ftp-proxy drops privilege to the user .

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 ftp-proxy will be chosen randomly from the range minport to maxport as described above.

Be verbose. With this option the proxy logs the control commands sent by clients and the replies sent by the servers to syslogd(8).
Use the tcp wrapper access control library hosts_access(3), allowing connections to be allowed or denied based on the tcp wrapper's hosts.allow(5) and 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.

ftp-proxy is run from inetd(8) and requires that FTP connections are redirected to it using a rdr rule. A typical way to do this would be to use either an ipnat rule such as

int_if = "xl0";
rdr $int_if 0/0 port 21 -> 127.0.0.1 port 8021 tcp

or a pf.conf(5) rule such as

int_if = "xl0"
rdr pass on $int_if proto tcp from any to any port 21 -> 127.0.0.1 port 8021

inetd(8) must then be configured to run ftp-proxy on the port from above using

127.0.0.1:8021 stream tcp nowait root /usr/libexec/ftp-proxy ftp-proxy -[ip]

in inetd.conf(5).

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 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. If you're using IP-Filter, the ipf.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:

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

If you're using pf, then the 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:

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

Alternatively, pf.conf(5) rules can make use of the fact that by default, ftp-proxy runs as user "proxy" to allow the backchannel connections, as in the following example:

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

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.

com.apple/100.InternetSharing/ftp-proxy PF anchor is required for this daemon to correctly function.

ftp(1), pf(4), hosts.allow(5), hosts.deny(5), inetd.conf(5), ipf.conf(5), ipnat.conf(5), pf.conf(5), inetd(8), ipf(8), ipnat(8), pfctl(8), syslogd(8)

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 configuration setup which allows the EPSV connections through to their destinations.

IPv6 is not yet supported.

March 16, 2011 macOS 15.0