I am having problems getting my network to play nice, despite it working previously. Here are the main system details;

  • CentOS box as a router / firewall (firewall currently disabled while I get the network operating)
  • eth0 used for internal network with ip, gateway set to
  • eth1 used for external network with ip, gateway set to
  • ip forwarding enabled in sysctl.conf
  • external gateway to internet available at - tested and working.
  • Windows machine, Windows server and eth0 connected to a hub. eth1 connected to another hub which connects out to the external gateway.

From my CentOS box I can ping the external gateway but not the internal network, if I disable the eth1 it can then find everything again. Now this was all functioning fully until I began using IPTABLES to try and configure rules, when I had some issues I flushed all rules, disabled everything and stopped the service.

Any ideas or things to try? Thanks for any help.


Remove the gateway for the internal network. There should be no gateway for the internal network. Also you don't mention the network masks for the internal and external subnets. You may have overlapping subnets configured.

  • Thanks for the reply. While not exactly the problem you shunted me towards it. I was using only one subnet so I had the same subnet for internal and external - how could the system decide which interface to ping from? Short of setting a static route anyway. So I changed the internal network to a network - all is well again. – Draineh Apr 30 '11 at 16:24

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