RFC 438 (rfc438) - Page 2 of 3
FTP server-server interaction
Alternative Format: Original Text Document
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.