Subject: Re: arp.
To: Curt Sampson <>
From: Chuck Robey <>
List: tech-net
Date: 04/06/1999 23:06:47
On Tue, 6 Apr 1999, Curt Sampson wrote:

Curt, I sent out a message to the guys involved, and one FreeBSD list,
because of the egregious level of cross posting.  I didn't do it to the
lone NetBSD list, hoping that maybe (since your list is even better
behaved than ours) maybe it wouldn't be needed.  Likewise, I didn't send
it out to the other 2 FreeBSD lists involved, but there is so much
overlap in our lists, I think that was correct.

Any more replies to this, can we please keep it to one list at a time?
Or, maybe one FreeBSD list at a time, if you would tolerate the
crossposting more than we would.

This isn't any kind of comment on your reply's content, which was
exactly what was needed (I could not have done better).  I didn't cc the
originators, who already understood this (and agreed), and didn't need
to hear it again.

> On Mon, 5 Apr 1999, andrea wrote:
> > [etc.]
> If I read you correctly, what you want to do is something like this:
>          internet
>             |
>         main router
>             |
>             |-- other hosts on subnet
>             |
>         sub-router 
>             |
>             |-- other hosts on subnet
>             |
> In other words, you have split your network into two subnets, but
> because you have no control over the `main router' above, you cannot
> inform it of the new subnet mask, so it believes that all the hosts
> on the subnet are local.
> This is not hard to solve; you just turn on routing in the sub-router
> box and enable proxy-arp. This will cause the subrouter box, when
> it receives an arp request for the 128/25 subnet on the 0/25
> interface, to reply to that ARP with its own address. The host that
> requested the arp then sends all packets to the sub-router, and
> normal routing gets it to its destination.
> The question is, does NetBSD do this properly? I think it does,
> but I'm lacking the AUI/10base-T transceiver I need to test this
> out right now. However, in theory, if you have a host
> that needs to talk to the main router, you type the following
> command on the sub-router:
>     arp -s <sub-router's MAC address> pub
> (The sub-router's MAC address can be gotten from an `ifconfig -a'
> or `netstat -i'; it will be a sequence of six hex numbers separated
> by colons, such as `8:0:20:1f:77:e0'.)
> The unfortunate part about this is that you have to add a separate
> arp entry for each host you want to proxy-arp for. On a cisco
> router, the proxy-arp option allows you to arp for anything it
> knows how to route to. This feature wouldn't be too hard to add to
> NetBSD, actually; you'd just have to modify arplookup to generate
> and add a new (pub, temp) arp entry for any IP address it can find
> a route for in its routing tables. (This would be controlled by a
> sysctl that would default to off, of course.) I may look at doing
> this after the 1.4 release. Or someone else could do it and save
> me the trouble. :-)
> cjs
> -- 
> Curt Sampson  <>   604 801 5335   De gustibus, aut bene aut nihil.
> The most widely ported operating system in the world:
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Chuck Robey                 | Interests include any kind of voice or data       | communications topic, C programming, and Unix.
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