In my environment most of the servers have 2 network cards. A primary/user network card used by the application, and a secondary/backup adapter used for backups, bulk copies etc.

I just set up a new server and have a problem pinging its secondary adapter from a clients that only have a primary/user adapter.

From my test client I am able to ping my other servers on their primary/secondary adapters.

The Networks:

  • == user network with my servers

  • == user network with my client

  • == backup network

The Computers:

newServer (rhel 6) [ a.k.a. the troublemaker]

  • eth0 : (user network)

  • eth1 : (backup network)

oldServer (rhel 5)

  • eth0 : (user network)

  • eth1 : (backup network)

pubClient (rhel5) [client system with only a user interface]

  • eth0 : (user)

From pubClient I can ping oldServer on either its eth0 or eth1 ip addresses. I noticed (tcpdump) that when I ping eth1, the return path of the ping is the eth0 interface of oldServer.

From pubClient I can ping newServer's eth0 interface, but when I ping newServer's eth1 adapter nothing happens. Using tcpdump I see the traffic coming in to the backup network interface, but no traffic coming out. The eth1 adapter is working fine - from newServer I can ping other devices on the backup network using the 245 interface, or pubClient without any indication of a problem.

The routing tables on both servers are simple and functionally identical.

newServer # ip route list dev eth0  proto kernel  scope link  src dev eth1  proto kernel  scope link  src
default via dev eth0

oldServer # ip route list dev eth0  proto kernel  scope link  src dev eth1  proto kernel  scope link  src
default via dev eth2

There are no firewalls running on either box. net.ipv4.ip_forward = 0 on both servers. The newServer is running RHEL6 I wonder if a default for some kind of network traversal (like ip_forward) has been disabled by default?

My network guys insist nothing is wrong with the routing between these machines.

Any guesses to what the problem might be would be appreciated.



  • I am not sure, but this might be a duplicate of serverfault.com/questions/595386/… – kasperd May 17 '14 at 0:16
  • do you have access to the default gateway at ? how does it route to ? what routes does pubClient have ? does any of these machines have iptables running (and/or blocking icmp)? – Niko S P May 17 '14 at 3:18
  • Yes. From the machines in question, I can ping the gateways and ip tables is not running. I think I need to take this back to our network team again to verify their configuration. – scott-- May 19 '14 at 19:02


Thanks Wayne.

On the new (rhel6 server)

echo "0" >/proc/sys/net/ipv4/conf/eth0/rp_filter
echo "0" >/proc/sys/net/ipv4/conf/eth1/rp_filter  

"uRPF check does is looks at the source address of the packets as they come in. If the route to that source address would not go out that interface then the packet is dropped"


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