[SIPForum-discussion] Why CANT a SIP (UDP) packet go through a IP gateway whereas a ICMP packet CAN ?!

Badri Ranganathan badri at arcatech.com
Thu Jun 27 13:37:53 UTC 2013

Hi all,

Could someone help me with a strange problem I'm facing. (or is it strange only to me ?! ).    I have a setup as below -

Linux Box1 (eth2) --------------------- D E L L - S E R V E R ------------------------(eth3)Linux Box 2
(                            (                         (                               (

I have a sip client running on both linux box 1 and linux box 2.
So linux box1 is on network and linux box2 belongs to network (because of the /28 !)

On linux Box 1, I have a route setup to reach as follows -
route add -net netmask dev eth2

On linux Box 2, I have a route setup to reach as follows -
route add -net netmask dev eth3

I am able to ping to from and vice versa.

But when I try to make a SIP call to client from client, I can't see the INVITE message on Linux Box 2 ! I can see the INVITE getting sent to the DELL SERVER (as I run wireshark on all these boxes including the dell server !)

Why does the DELL SERVER NOT forward the INVITE request to the Linux Box 2 in the same way that it sent the ICMP packet earlier when I did a ping !! Both are IP packets right.
Does the DELL Server need to be replaced by a router? Or otherwise - Do I need to do any IP Forwarding configuration etc on the DELL server to enable the packets to go from one network to the other?

(The reason I neither used a Router nor tried to do any IP Forwarding/iptables etc., on the DELL Server is because I am able to reach Linux Box2 from Box 1 through a ping, and I believe  it should be ok for any IP packet.)

Sincerely appreciate any help on solving my problem.


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