Re: [uCsimm] proxy arp seems broken to me

From: Robert Tagliarini (
Date: Mon Feb 28 2000 - 08:21:28 EST

On Sat, 26 Feb 2000, you wrote:
> On Fri, 25 Feb 2000, elijah wright wrote:
> > Do the other machines in the network have a route to the device? if "all
> > systems are on the same subnet (taking that to include the embedded
> > system)" it should NOT work. you'll need a static route to the embedded
> > system's IP address either from the defaultrouter of the local network or
> no - the whole point with proxy arp is that you don't need routes set-up
> for the proxied hosts. the host with the proxy arp entry will answer arp
> queries on behalf of the hosts "on the other side" (like on a ppp link)
> as long as the hosts are on the same subnet, it should work (because arp
> queries are local) as long as the IP addressing is consequent.
> as to if it works in uClinux - yes it does (why shouldn't it ? :) but you
> should of course remember that you need the "IP Forwarding" kernel option
> enabled in 'make menuconfig' to actually make linux bridge packets between
> the proxied hosts and the ethernet. maybe you forgot that ?
> also check /proc/sys/net/ipv4/ip_forward (by catting it) it should be 1.
> otherwise you do 'echo 1 >/proc/sys/net/ipv4/ip_forward' to enable it, but
> in my own setups, it has always been 1 by default..
> i think this is in the Documentation/networking docs as well.
> -bjorn
Thank you all for your help. Bjorn's description is exactly my understanding
of how proxy arp should work. I did not have forwarding turned on. Once I
turned it on (used redhat's control panel), the proxy arp seems to work. i.e.
an entry for the uClinux system shows up in the arp cache of a third host on
the subnet when I TRY to ping the uClinux system from this third host.
Evidently, the packets are not being forwarded properly because there is no
response to the ping.

 /etc/sysconfig/network has:

and cat /proc/sys/net/ipv4/ip_forward shows:

I didn't try Stu's suggestion of setting promisc for eth0 adapter since it
appears (now) to be responding to an arp for the uClinux system.

 cat /proc/version on the host that is directly connected to the uClinux system
 Linux version 2.2.12-20 ( (gcc version
egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)) #1 Mon Sep 27 10:40:35 EDT

I'd really appreciate any further insights you all can provide.


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