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From: chris@GYRE.UMD.EDU (Chris Torek)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: ethernet interface perversity
Message-ID: <8708252304.AA04887@gyre.umd.edu>
Date: Tue, 25-Aug-87 19:04:10 EDT
Article-I.D.: gyre.8708252304.AA04887
Posted: Tue Aug 25 19:04:10 1987
Date-Received: Thu, 27-Aug-87 02:56:08 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 22

(Apparently, someone wants a single Ethernet interface to respond to
ARP requests for several different Internet addresses.)  Jim O'Toole
did something like this under 4.2BSD, with an `IP magic' hook whereby
all IP packets for one IP address were forwarded to a user-level
program.  It takes about 15 minutes to figure out where and what to
write; we then had a user program that provided echo service a la
goonhilly-echo.arpa.

If all you want is for a 4.3 host to respond to multiple IP addresses,
this is even easier.  Configure some unused interface with the alternate
address:

	# ifconfig lo1 a.b.c.d		# or ifconfig sl1 a.b.c.d

Add it to your arp table:

	# arp -s a.b.c.d <your_ethernet_addr> pub

You should be set once the router picks up the address a.b.c.d from
interface sl0, or immediately if a.b.c.d is on the same `network'
as the 4.3 host as far as the sender is concerned.  I tested this,
and gyre (128.8.128.77) did respond to packets for 128.8.128.200.