Re: ICMP Connections ...

[Antony Stone <Antony@Soft-Solutions.co.uk>]

On Tuesday 24 August 2004 2:36 pm, Chetan Nagaraja wrote:

> Dear All,
> First of all, let me introduce as a total newbie in filtering and iptables.
>
> To ping a host in a different network, I have configured a dual homed[ two
> network interfaces] linux system to act as NAT router. I have add a rule in
> the NAT table of iptables, to achieve the following. If the Linux system
> recieves a icmp packet from a particular host in NETWORK A addressed to a
> particular HOST in NETWORK B , perform SNAT of the ICMP packet to that of
> the Linux System, so that the icmp replies reaches the Linux system.

I think you probably mean DNAT?

> The above is working very fine. The ICMP requests are reaching the expected
> destination. But I'am unable to understand the fact that the ICMP replies
> are reaching the host in NETWORK A which had generated the requests,
> without adding a DNAT rule for the same. How is this possible, does
> iptables automatically redirect ICMP replies ?

Yes, and not just ICMP.   Netfilter automatically applies reverse-nat to reply 
packets in accordance with the forward nat rules you specify for the original 
packets.

> And How to avoid the same.

What do you mean?

Regards,

Antony.

-- 
How I want a drink, alcoholic of course, after the heavy chapters involving 
quantum mechanics.

 - 3.14159265358979

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