arp replies from two adapters of same type in a machine

Hi all,

I have two RTL-8139 network adapters in a machine, they have these IP addresses:
eth1: 192.168.20.1
eth2: 192.168.21.1

When I take tcpdump on these interfaces and ping to 192.168.21.1 from 192.168.21.50, I see that both the adapters reply to ARP requst sent by 192.168.21.50 machine:

/usr/sbin/tcpdump -i eth1 -vv
.
.
arp reply 192.168.21.1 is-at <MAC add of eth1>

/usr/sbin/tcpdump -i eth2 -vv
.
.
arp reply 192.168.21.1 is-at <MAC add of eth2>

Ideally eth1 should not reply to the ARP request.

Any idea why this happens?

Regards,
Adhiraj.

I doubt that this is a netfilter issue, however....

I suspect proxy arp is enabled, on a 2.6 kernel (possibly on 2.4 also but I'm not sure) you can see if this is so
cat /proc/sys/net/ipv4/conf/eth1/proxy_arp
If you get a 1 then it is on, if you get a 0 then it is off. It can be controlled with the sysctl program (See man sysctl).

Proxy arp would tell an interface to answer arps with it's own mac address, the idea is to allow nodes on the network to route through this system without actually having a route entry. It's almost always a bad thing in practice.

Hi,

I will rephrase what you have:
You have a machine with the ip addresses 192.168.20.1 and 192.168.21.1 configured in the ip stack.
You have 2 nics in the machine which the following routing:
192.168.20.0/24 dev eth1 with default source ip 192.168.20.1 192.168.21.0/24 dev eth2 with default source ip 192.168.21.1 Both nics are on the same network.

Both eth1 and eth2 get an ARP (l2) for 192.168.21.1 (l3), which according to rfc's should be replied to no matter the source ip.

It should.

If you don't like this behaviour you should use rp-filter for arp, or use lvs patches to hide the ip address.

PS: this is has nothing to do with proxy-arp. Proxy-arp means you answer for addresses you can route to. Not for addresses you already have.

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