RFC 1579 (rfc1579) - Page 2 of 4

Firewall-Friendly FTP

RFC 1579                 Firewall-Friendly FTP             February 1994


   the "server" area, i.e., port numbers below 1024.  But that strategy
   is risky; dangerous services such as X Windows live at higher-
   numbered ports.

   Outgoing calls, on the other hand, present fewer problems, either for
   the firewall administrator or for the packet filter.  Any TCP packet
   with the ACK bit set cannot be the packet used to initiate a TCP
   connection; filters can be configured to pass such packets in the
   outbound direction only.  We thus want to change the behavior of FTP
   so that the data channel is implemented as a call from the client to
   the server.

   Fortunately, the necessary mechanisms already exist in the protocol.
   If the client sends a PASV command, the server will do a passive TCP
   open on some random port, and inform the client of the port number.
   The client can then do an active open to establish the connection.

   There are a few FTP servers in existence that do not honor the PASV
   command.  While this is unfortunate (and in violation of STD 3, RFC
   1123 [2]), it does not pose a problem.  Non-conforming
   implementations will return a "500 Command not understood" message;
   it is a simple matter to fall back to current behavior.  While it may
   not be possible to talk to such sites through a firewall, that would
   have been the case had PASV not been adopted.

Recommendation

   We recommend that vendors convert their FTP client programs
   (including FTP proxy agents such as Gopher [3] daemons) to use PASV
   instead of PORT.  There is no reason not to use it even for non-
   firewall transfers, and adopting it as standard behavior will make
   the client more useful in a firewall environment.

   STD 3, RFC 1123 notes that the format of the response to a PASV
   command is not well-defined.  We therefore recommend that FTP clients
   and servers follow the recommendations of that RFC for solving this
   problem.

Discussion

   Given the behavior of most current FTP clients, the use of PASV does
   not cause any additional messages to be sent.  In all cases, a
   transfer operation is preceded by an extra exchange between the
   client and the server; it does not matter if that exchange involves a
   PORT command or a PASV command.

   There is some extra overhead with Gopher-style clients; since they
   transfer exactly one file per control channel connection, they do not



Bellovin