Re: [BUG?] unwanted proxy arp in 2.4.19-pre10

From: Julian Anastasov (
Date: Wed Jul 17 2002 - 18:41:13 EST


Daniel Gryniewicz wrote:

> Let me explain my setup, and then you can hopefully explain why I'm
> wrong. I work for a company that writes routing software. We support
> and test on a mixture of Unix OSs, one being Linux, and RTOSs. Here's
> an example setup:
> ---+----+--- (192.168.1/24)
> | |
> A B
> | |
> ---+----+--- (10.10/16)
> A:,
> B:,
> A and B usually have default routes pointed to, as this is
> the router connecting the testbed to the workstations. In this example,
> A is Linux, B is any other OS we test against ([Free|Net|Open]BSD, BSDi,
> Solaris, AIX, True64). Both are running ISIS, which does *not* use IP
> as a transport (this is important, I'll explain later). ISIS installs a
> new default route on A that points at If, at this point,
> there is no ARP entry in A's ARP cache for, it will send an
> ARP reqest to the 10.10/16 network as follows:
> whohas tell
> This happens every time in this situation. If we were running, say,
> OSPF (which does use IP as a transport), then A's ARP cache would always
> have an entry for, and this situation would not show up.
> However, we have confirmed that, if you clear the ARP cache on A, and
> then change it's default route from one network to another, it will
> always generate these bizarre ARP requests. Since all the other OSs we

        Yes, such behaviour can be observed also when cross-subnet
talks occur.

> run will not answer that request, this results in A being completely
> unreachable from off of it's directly connected networks. Those ARP
> reqests are generated even if the packets being sent are pings to
> To me, this seems broken, and has resulted in us not
> recommending Linux to any of our customers. (The company is historically

        No, I don't prefer the word "broken". Note that the two
ways to answer (the Linux's way and the any other way) are
valid for specific setup. What we have is a limited behaviour.
We are not trying to fix bugs or weirdness in other OSes but
look: why these "other" hosts do not work with such ARP packets?
This is a real bug or at least wrong routing. Is it allowed IP
packet with saddr= and daddr=UNIVERSE to reach the
default gateway? Yes. Then why ARP is not answered? Because the
GW does not have valid host routes for and
Are the routes to these IPs through gateway and if yes, then which
is this gateway? You need valid host routes. At least, this is
in Linux. If you investigate additionally, you will see that
you need alternative routes for these hosts because you can
reach the both hosts through 2 devices. I.e. such complex setup
does not work by magic as you expect.

> largely a BSD house. I'm trying to change this bias somewhat). 2.4.16
> does this, as well as 2.2.*. I've gotten a patch for 2.2.19 that fixes
> the problem, but all I've heard about 2.4 is "That's how it's supposed
> to work". If this is not broken, how do I get it to work?

        As we see there are different setups that need different
ARP behaviour. This is the reason that I use the word "tuning"
for the ARP traffic/behaviour. If you want the behaviour not to
announce one IP through multiple devices (we assume it does not
cause other problems for your setup) then you can use such
iparp command:

ip arp add output from 0/0 src 0

        This will cause all ARP requests to announce the preferred
source IP to the target (what you really want). You can additionally
tune the behaviour by specifying subnets, interfaces, etc. if you
have problems as mentioned in my previous posting. See iparp.txt
for syntax.

> Daniel


Julian Anastasov <>

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