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RE: Problem with ARP response

To: "Brad Kemp" <>,
Subject: RE: Problem with ARP response
From: "Rajesh B. V." <>
Date: Wed, 15 Sep 2004 21:49:18 +0530
In-reply-to: <>
Original-recipient: rfc822;
References: <>
At 10:45 AM 9/15/2004 -0400, Brad Kemp wrote:

>-- Original Message --
>Date:  Wed, 15 Sep 2004 10:45:06 +0530
>From:  "Rajesh B. V." <>
>Subject: Problem with ARP response
>Hi All,
>I have a network setup where both the LAN interface (eth0) and WAN
>interface (eth1) of a Router running Linux will be put into same Switch.
>The setup is :
>LAN network is :  ( Router LAN interface IP is
> with xx:01 as MAC)
>Router WAN interface IP is a static public IP with xx:02 as MAC.
>Now when a PC from the LAN networks tries to reach (ping) LAN interface

>(eth0) of Router i observe vague ARP entry in the PC for the IP.
>I see some times WAN interface (eth1) MAC as the ARP entry in PC and some
>times LAN interface (eth0) MAC as the ARP entry in the PC.
>For every ARP request i see two responses one with WAN interface MAC and
>one with LAN interface MAC.
>I observed this is happening because, both the interfaces are receiving
>ARP broadcast request from the PC and sending up the stack to ARP module
>and which responds with the corresponding interface MAC upon which it
>received the packet.
>My requirement is to make ARP module not to respond for the packet received
>on WAN interface (eth1) with requested IP as LAN interface (eth0) IP.
>Will the arp_filter () in net/ipv4/arp.c can do this ?
>Also is there any draw back by doing so ?

If you disable the arp response from eth1, no traffic will go to it. Is this
really what you want to do?
You might want to look in to 802.1q VLAN tagging if you want to run to logical
networks on one physical network

Hi Brad,

Thanks for the response.
I got the issue resolved. ARP is handling this kinda scenarios by doing arp_filter (). But to use arp_filter the flag arp_filter should be enabled i.e. keep 1 in proc/sys/net/ipv4/conf/<interface name>/arp_filter on all the interfaces that are put in same switch/hub.


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