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

Timo Schoeler timo.schoeler at riscworks.net
Fri Oct 2 08:16:49 UTC 2009

Hash: SHA1

thus Giovanni Tirloni spake:
| 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:
| http://lists.centos.org/pipermail/centos/2009-January/070828.html
| Giovanni P. Tirloni
| tirloni at gmail.com

Thank you very much, Giovanni -- seems exactly to be what I need.



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