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From: smb@cs.purdue.EDU (Scott Ballew)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Question on forwarding broadcasts
Message-ID: <11858@medusa.cs.purdue.edu>
Date: 28 Sep 90 17:02:45 GMT
References: <1990Sep28.143945.7440@ultra.com>
Sender: news@cs.purdue.EDU
Reply-To: smb@tristram.cs.purdue.edu (Scott M Ballew)
Distribution: comp
Organization: Department of Computer Science, Purdue University
Lines: 39

In article <1990Sep28.143945.7440@ultra.com> wayne@ultra.com (Wayne Hathaway) writes:
<I have a little "IP forwarding of broadcasts" question. 
<
<Anyway, consider:
<
<    +------------+    +------------------------+    +------------+
<    |   Host A   |    |     Default Router     |    |   Host B   |
<    |            |    |                        |    |            |
<    | 192.40.1.1 |    | 192.40.1.2  192.40.2.2 |    | 192.40.2.1 |
<    +------+-----+    +------+----------+------+    +-----+------+
<           |                 |          |                 |
<===========#=================#==========#=================#===========
<                               Ethernet
<
<where both Host A and Host B are configured to forward things they
<cannot handle to the Default Router.  Note that there are TWO
<DIFFERENT NETWORKS active on a SINGLE ETHERNET CABLE (192.40.1 and
<192.40.2).  (This is of course key to the problem, but it is a
<perfectly legal configuration, no?) 
<
<Now assume a user on Host A does a broadcast to network 192.40.1 (its
<own network).  No problem, Host A simply puts the message out with the
<Ethernet MAC layer broadcast address, and it is picked up by everybody
<on network 192.40.1.  But Host B will also pick up this message, and
<will find that it is not destined for him.  Host B will then forward
<it to the Default Router over network 192.40.2.

Hold it!  In your configuration, Host B should NOT be forwarding ANY
packets at all, unless it is configured as a gateway.  Now, assuming
that Host B IS a gateway and has another network connected behind it,
the answer is found in (I believe) the gateway's requirements RFC (RFC
1009).  Somewhere it says that if a datagram is received via a media
broadcast that is not destined for a local broadcast address (ie, one
of Host B's own broadcast addresses), then it should be quietly
discarded.

Scott Ballew
Cypress Network Operations Center
Purdue University Department of Computer Sciences