We have one server that is connected to two independent networks through two NICs.

Network A and Network B

The server gets two IPs - but those IPs are in completely different networks. So far everything is fine - and the server can be accessed from both networks without any problem.

Now - in network A we are also providing a public IP (handled by a Firewall - no configuration of public IP on server) to the server, so that the server can be seen from the outside. That does not happen in network B. Also this works initially - so our server can be seen from outside and from the two internal networks - and everything works fine...

Internally our DNS servers make sure that we browse internally - but we could also access the network from all networks through the internet.

Just after a while, the public IP stops working - no ping nothing. Then - if I take the NIC of network B down and up again, it works.

What could be the problem?

interfaces config:

## This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet static
address xxx.xxx.7.41
broadcast xxx.xxx.7.255
gateway xxx.xxx.7.254
dns-nameservers xxx.xxx.7.17

# The secondary network interface
allow-hotplug eth1
iface eth1 inet static
address xxx.xxx.20.9
broadcast xxx.xxx.20.255
gateway xxx.xxx.20.1
dns-nameservers xxx.xxx.10.12`
  • 1
    What about your routing tables?
    – NickW
    Mar 27, 2013 at 14:04
  • Do you have both logical networks plugged in one physical network segment (or same vlan)?
    – dsznajder
    Mar 27, 2013 at 14:11
  • No, the two networks are different physical network segments. There do not share any resource.
    – ChrisZZ
    Mar 27, 2013 at 14:15
  • And public IP stop working at all (no ping reply from outside and from inside)?
    – dsznajder
    Mar 27, 2013 at 14:57
  • Yes, it stops working - but not immediately - it will work fine for some time - then the public ip will not be accessible anymore. When I start a ping on the public IP - I can see, that - the moment I take eth1 down and up again - that it will start to work again.
    – ChrisZZ
    Mar 27, 2013 at 15:13

1 Answer 1


Make sure that the entry on your routing table is statically set to use the public interface. On top of that, make sure that all local subnets are set to use interface B

  • As in remove your default gateway from your inside interface and static route internal networks to the old internal default gateway IP.
    – cpt_fink
    Mar 28, 2013 at 3:35

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