arp responses by dual port eepro100

Julian Anastasov ja at
Tue Jun 11 10:20:23 BST 2002


On Mon, 10 Jun 2002, Michael McConnell wrote:

> The problem I am face is that which using both ports of this dual
> interface network card (plugged into the same switch) I find that the
> second interface is answering arp requests (on rare occasions) that the
> first interface should be answering.

	Classic problem of attaching multiple Linux interfaces to
shared medium. You can set arp_filter on all your ARP devices or
why not to restrict even the IP protocol by setting rp_filter.

> I have used tcpdump and clearly seen eth1 answering arps requests that
> eth0 should be answering... how odd.... It's rare, but when it happens
> of course that address is offline. (Note this only seems to happen on
> alias IP address, it has never happened on the primary interface)

	Such answering can not be never a problem. If the Linux box
answers via many interfaces then it is willing to accept traffic
through these ifaces. Of course, the achieved failover when attaching
two interfaces to same hub is not perfect because the remote LAN boxes 
will use the alive Linux interface but Linux routing still uses the first 
interface (even if it is failed on Layer 2) for the used subnet.
If your goal is to restrict the talks for each subnet through one
interface then you have to use arp_filter=1 but still to use rp_filter=0
to allow cross-subnet talks. One day rp_filter will be aware of the
medium_id values for each interface and will allow the Linux box
to interconnect multiple hubs securely (and still to use many interfaces
to these hubs).

> I am using the open source drivers provided with the 2.2.19 kernel, I'm
> wondering if the drivers provided by Intel would help this problem? Does

	The drivers are not guilty.

> anyone have any other suggestions, or for that matter has anyone else
> run into this problem? Could this possibly be a problem with aliasing?
> Thanks for all your thoughts,
> Michael McConnell


Julian Anastasov <ja at>

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