Subject: Re: arp.
To: andrea <freebsd-stable@FreeBSD.ORG>
From: Robin Melville <>
List: tech-net
Date: 04/05/1999 12:50:03
At 12:22 pm +0100 5/4/99, andrea wrote:
>I have to add a gateway to my net for experimental reasons.
>Actually there are : a main-router that works as interface to the Internet,
>and some hosts on my sub net.
>        Internet-----MyRouter----MySubNet
>NOw i need to configure one host of MYSubNet to act as a gatway for the
>secondary subnet.
>Both the 1SubNet and 2 SubNEt share the same ip-range.
>     Internet-----MyRouter----MySubNet-----My2SubNet
>All the have to be seen from the Internet so I'll need to add a
>route to MainRouter in order to route the Secondary Subnet.
>The problem is that i cannot change configuration of the mainroute,so i
>wonder if is possible to configure the new gateway to do a sort of "proxy
>arp" for my secondary Subnet.
>But arp-tables are system-wide so if i change arp entry to cacth request on
>PrimaryNet the 2subnet dont'works anymore.
>Is possible to catch arp request only on a single subnet,without broke any
>other subnet connected to the same host.?
>thank you!

Assuming that these are IP routers and not ethernet switches, the arp
tables aren't particularly relevent. If the main router is running 'routed'
or some other RIP routing daemon, all you need to do is run a similar
daemon and your new subnet route will be propagated. Alternatively, or
essentially if you are using unregistered IP, you could use 'natd' on
'MyRouter' to masquerade the internal addresses onto the external interface.

The handbook pages on natd make it very easy to set up.

Good luck


Robin Melville, Addiction Information Services
Nottingham Alcohol & Drug Team
Tel:  +44 (0)115 952 9478       Fax:  +44 (0)115 952 9421
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