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Date:      Tue, 15 Aug 2000 13:51:18 -0400 (EDT)
From:      Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
To:        "Eugene Mogutov" <eugene_m@mail.ru>
Cc:        freebsd-net@FreeBSD.ORG
Subject:   Broadcast address and multihomed host
Message-ID:  <200008151751.NAA54480@khavrinen.lcs.mit.edu>
In-Reply-To: <E13OXww-0008DB-00@f10.mail.ru>
References:  <E13OXww-0008DB-00@f10.mail.ru>

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<<On Tue, 15 Aug 2000 08:03:38 +0400, "Eugene Mogutov" <eugene_m@mail.ru> said:

> Recently a friend of mine noticed that his FreeBSD router treats
> broadcast addresses of local subnets as local (i.e. as if they were
> addresses of router interfaces).

[Reformatted.  It would help if you pressed ``return'' at the end of a
line.]

The behavior is arguable.  The current behavior is relatively easy to
explain:

1) The remote host sends its SYN segment as a unicast, since it does
not have the information to determine that x.y.z.255 is a broadcast --
indeed, it might not be if x.y.z.255 is on a /23 or larger network.

2) A packet arrives on a.b.c.1/24 with destination address x.y.z.255.
ip_input() looks to see if x.y.z.255 is configured on any interface.
It is -- it's configured on the x.y.z.1/24 interface -- so it accepts
the packet.

3) The code in tcp_input() which checks for broadcasts and multicasts
looks only at the delivery flags (M_MCAST and M_BCAST); it doesn't
re-examine the addresses.

-GAWollman

--
Garrett A. Wollman   | O Siem / We are all family / O Siem / We're all the same
wollman@lcs.mit.edu  | O Siem / The fires of freedom 
Opinions not those of| Dance in the burning flame
MIT, LCS, CRS, or NSA|                     - Susan Aglukark and Chad Irschick


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