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2001 Dec 30 17:08

The following is an archive of a post made to our 'vox-tech mailing list' by one of its subscribers.

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Re: [vox-tech] Linux as gateway
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Re: [vox-tech] Linux as gateway



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On Mon, 27 Aug 2001, Peter Jay Salzman wrote:

> make sure you understand that tcpdump tells you which direction the
> packets are going.  basically, what you're looking for is:
>
> 1. ping packets coming from the internal machine being received by the gw's
>    internal nic.
>
> if that works, you're looking for...
>
> 2. the ping packets leaving the gw's external nic bound for the internet.
>
> if that works, you're looking for...
>
> 3. the echo packets coming back to the gw's external nic
>
> if that works, you're looking for...
>
> 4. the echo packets leaving the gw's internal nic.
>
> if that works, you're looking for...
>
> 5. echo packets being received by the internal machine.
>
>
> which of these steps is broken?

It seems the 3rd step is broken. I run tcpdump on both gateway
and an external machine.

On gateway, tcpdump capture the echo request package, but no
reply packages.

On external machine, tcpdump capture both request and reply
packages. The src ip of request packages is the internal ip.

If I ping gateway from the external machine directly, both
tcpdump capture all request and reply packages.

Maybe it's because the internal ip of the reply packages make
it be dropped on some router?

Jimmy
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