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From: karn@FLASH.BELLCORE.COM.UUCP
Newsgroups: comp.protocols.tcp-ip
Subject: question about FTP data channels
Message-ID: <8705070357.AA28765@flash.bellcore.com>
Date: Wed, 6-May-87 23:57:49 EDT
Article-I.D.: flash.8705070357.AA28765
Posted: Wed May  6 23:57:49 1987
Date-Received: Fri, 8-May-87 05:37:01 EDT
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Question: what assumptions may an FTP client make about the address and
port (i.e., socket) that will be used when the server opens a data connection?

When my implementation sends a RETRieve command, it posts a listen for
the data connection. Rather than accepting anyone who connects, it
listens specifically for a SYN carrying the IP address of the server,
and TCP port 20. This works fine except in the case of a multi-homed FTP
server. If I have established my control connection to the "far side"
IP address, many such hosts will use the IP address associated with the
"nearer" interface when initiating the data channel, and my TCP will
refuse it.

I guess I'm answering my own question here, namely that I have to be
prepared to accept any IP address in the incoming connection, since I have
no way of knowing if the server will use an address different than the
one I used on the control connection.  Comments?

Phil