Understanding IP forwarding
g.butler at hud.ac.uk
Fri Aug 3 13:08:35 UTC 2012
I have a duel NIC server (Ubuntu 12.04) connected to two different subnets and I am trying to understand a possible ip forwarding problem I have. I would appreciate a nudge in the right direction.
If I ping the first interface (220.127.116.11) from another server, not on the same subnet as second interface, I get a reply i.e. the ping and reply are on the same interface.
If I ping the second interface (172.17.193.146) from another server on its own subnet (say 172.17.193.62), I get a reply. Once again the ping and the reply are on the same sub net.
The problem is when a ping is received on one interface, and the reply wants to go down the other interface, it all fails. On a failed request, I can see the traffic coming in on one interface (using tcpdump), but I cannot see any replies on either interface. From my experience with Solaris, if 'IP forwarding' is enabled, it allows a reply on another interface. I am very new to Ubuntu.
Can I assume from this that setting ip_forward in Ubuntu to allow communication between the two interfaces is not sufficient, and that I may have to use iptables?
The server is not intended to be a router as such, but a development server for testing as a transparent proxy server. I have not got to the transparent proxy bit yet, as I am still trying to understand this problem.
~# netstat -r
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
default 18.104.22.168 0.0.0.0 UG 0 0 0 eth0
localnet * 255.255.255.0 U 0 0 0 eth0
172.17.193.0 * 255.255.255.0 U 0 0 0 eth1
ip_forward is set to 1 and UFW is disabled.
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