RFC 438 (rfc438) - Page 2 of 3

FTP server-server interaction

RFC 438              FTP Server-Server Interaction          January 1973


   FTP could be modified to support server-server interaction by:

      1. making the establishment of data connections symmetric, or;

      2. providing a mechanism for instructing a server to establish its
         end of a data connection as if it were a user.

   The second alternative probably requires fewer changes to the
   existing protocol.

   The following proposed extension to FTP uses the second method.   It
   involves the addition of a single new command (LSTN) and minor
   modifications to several existing commands (SOCK, APPE, RETR, STOR):

   a. The LSTN (Listen) command requests the FTP server to allocate a
      socket for use as a data connection.  To establish the
      corresponding data connection the server is to "listen" on the
      socket allocated when an appropriate transfer command is given.


      syntax: LSTN  CRLF

         where  is either "S" for send or "R" for receive.

      The server responds to LSTN by:

         1. refusing to allocate such a socket, or:

         2. sending the user the number of the socket allocated (the 255
            FTP server data socket reply could be used for this
            purpose).

   b. Receipt of an appropriate STOR, RETR or APPE command following a
      successful LSTN command causes the server to "listen" for an RFC
      for the socket allocated.   Data transfer may proceed after the
      server receives an RFC for the socket and responds with a matching
      RFC.   Once established, a data connection corresponding to a
      successful LSTN command has the same duration as one established
      in the usual way.

   c. The user may insure the security of his data transfer by using the
      SOCK command to instruct the server to accept an RFC for the
      listening socket only if it is from a specified host and socket.

   d. The SOCK command is modified in two ways:






Thomas, et. al.