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From: jerry@wg3b20.UUCP
Newsgroups: net.unix-wizards
Subject: Re: Gateway generates wrong IP returns address (4.2, Vax)
Message-ID: <617@wg3b20.UUCP>
Date: Wed, 21-May-86 19:04:45 EDT
Article-I.D.: wg3b20.617
Posted: Wed May 21 19:04:45 1986
Date-Received: Sat, 24-May-86 20:44:46 EDT
References: <29@uscvax.UUCP>
Organization: The Wollongong Group., Palo Alto, CA 94303.
Lines: 27

> 
> We have a VAX 750 running 4.2 gatewaying between two ethernets.  For some
> reason, connections running over the second ethernet have the return
> (source) IP address of the first ethernet.  This causes minor problems like
> machines on the second net generating arp requests that are not responded
> to.  Also the address of the first ethernet happens to end in .255 (class C
> host number is all 1's) which some systems don't like (we're planning to
> change this).

To: pesnta!hplabs!sdcrdcf!usc-oberon!bacall!uscvax!pv
Subject: Re: Gateway generates wrong IP returns address (4.2, Vax)
In-reply-to: your article <29@uscvax.UUCP>

This is a bug in the 4.2 code for TCP sockets.  If you look at
in_pcb.c in routine in_pcbconnect you will see where the source IP
address is chosen for the IP header.  Instead of choosing the
source address based on the interface the packet will be sent to,
it chooses the address of the FIRST interface in the ifnet list
that has an address family of AF_INET.  Of course this is the first
DEUNA and thus the IP source address is that of this first DEUNA.

There has been a fix for this problem (when I saw it it was combined
with a TCP maxsegsize change down by Mike Muus) and I will post it
to the net or to your mailbox directly.

Regards,
Jerry Scott