ICMP response not sent to a local network ip

vidyasakaran · 08-19-2010, 09:32 AM

My problem is simple, but i am not sure if i can explain clearly here. Let me try

icmp request from an ip that is in the same network as one of the local interfaces is not responded to, if the ping request is received via an interface in a different network. Is this some security feature?

Consider the below network

x.1|RTR1|-y.1---------y.2-|RTR2|-z.2------z.3-|LNX|-x.3

RTR - Router
x.1 -> 192.168.x.1
LNX - Linux machine

RTR1's x.1 and LNX's x.3 are connected together.

Static routes added:
RTR1 - net z.0 gw y.2

I send a ping from RTR1 to z.3 with source IP x.1
I expect the ping to be responded by the LNX (response to be sent viz x.3<->x.1 link)
This doesn't happen

If I change the route in RTR1, making x.3 as the gw, it starts working fine.

Looks like some security feature in Linux prevents this ping response from being sent.

More light into this will help!

Thanks!

anomie · 08-19-2010, 02:27 PM

First, please don't use that special font. It makes reading your post harder, not easier. (For network diagrams and command output, etc., do use code tags.)

So your Linux host has two separate routes by which it can reach Router1, correct?

If so, let's see the output from the following two commands:

Code:

# netstat -rn

and

Code:

# ip rule show