[CentOS] Reply to ICMP echo request (type 8) on different (ethernet) interface

Giovanni Tirloni tirloni at gmail.com
Thu Oct 1 18:19:50 UTC 2009

On Thu, Oct 1, 2009 at 2:02 PM, Timo Schoeler
<timo.schoeler at riscworks.net>wrote:

> Hash: SHA1
> Hi list,
> I have a weird (?) problem here on a setup running CentOS 5.3 x86_64
> (and OpenVZ, and some home-brew L2TP daemons, RIPd, BGPd, etc).
> There's a (VE in OpenVZ speak) virtual machine that has two ethernet
> interfaces, seen as eth0 and eth1, respectively. Those live in VLANs,
> but it's not important here.
> The thing is that on eth1 the default route lives, while on eth0 all
> traffic comes in.
> So, sending a ping to the IP address of eth0 tcpdump shows that the echo
> request (type 8) packet arrives on the machine. However, the machine
> does _not_ send an echo reply (type 0) back to the machine that pings
> eth0, maybe because it would have to emerge from eth1.
> One exception (an obvious one) is that IPs on the /29 where eth0 lives
> on _can_ ping eth0 and receive an answer -- this is because the packets
> don't have to take 'the default route', which lives on the other
> interface, eth1.
> This seems to me like decent behaviour.
> However, I really need eth0 to be able to be pinged from the outside
> world, it's totally okay for me that eth1 would 'answer' and send the
> echo replies instead of eth0.
> Is there anything I can tweak (via sysctl or whatever)?

You need a way to tell that packets originating from eth0 destined outside
should be routed to eth0. This thread should help:


Giovanni P. Tirloni
tirloni at gmail.com
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