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From: chris@columbia.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: rwho problems between 4.2 and 4.3 hosts
Message-ID: <4630@columbia.UUCP>
Date: Tue, 26-May-87 08:39:38 EDT
Article-I.D.: columbia.4630
Posted: Tue May 26 08:39:38 1987
Date-Received: Wed, 27-May-87 03:14:05 EDT
References: <7523@brl-adm.ARPA> <12249@topaz.rutgers.edu> <1158@sigi.Colorado.EDU>
Reply-To: chris@columbia.edu (Chris Maio)
Distribution: world
Organization: Columbia University
Lines: 30
Keywords: gateway, ifconfig, broadcast

In article <1158@sigi.Colorado.EDU> forys@boulder.Colorado.EDU (Jeff Forys) writes:
>In article <12249@topaz.rutgers.edu> hedrick@topaz.rutgers.edu
>(Charles Hedrick) writes:
>> However you can set the broadcast address in 4.3 using ifconfig.
>> Reasonable things to try are 128.x.0.0 and 0.0.0.0 [...]

>From what I've determined, this causes a problem iff the machine is a gateway
>on a class B net where only the subnet addr is different.  Given an address of
>`128.net.0.0', and two interfaces `128.net.sub1' and `128.net.sub2', it cant
>decide which one to use, and broadcast packets never get out.  Packets get out
>if you set the broadcast address to `128.net.sub.{0,1}', leaving a nasty
>kernel hack as the only way to get `128.net.0.0' out of a gateway.

>Can anyone confirm/deny the above behavior?

You're mostly right; with 4.3bsd networking code, if you have subnets enabled,
128.x.0.0 is interpreted as a host on subnet zero, and the kernel won't
recognize that as a broadcast address regardless of what the per-interface
broadcast address is set to.

You can set up a fake route so that packets addressed to 128.x.0.0 would
normally be sent to one or another of your subnet interfaces, but most of the
programs that send broadcasts (rwhod, routed, etc) explicitly bypass the route
table.

If each interface has a unique broadcast address, or you don't care mind having
all the 128.x.0.0 broadcasts going only to one subnet, you can get around the
problem with some simple fixes to in_broadcast() and ip_output(), but in
general, trying to use non-subnet broadcast addresses throughout a subnetted
network will open up a real can of worms.
							Chris