netdev - Packet Sniffing on Intermediate Nodes

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Date:   Mon, 10 May 2021 22:02:53 +0530
From:   Ajay Garg <ajaygargnsit@...il.com>
To:     netdev@...r.kernel.org
Subject: Packet Sniffing on Intermediate Nodes

Hi All,

I have been playing around with sniffing packets, using raw sockets.

Following is my setup :

* Machine-1 (Sender), with IP 1.2.3.4
* Machine-2 (Intermediate), with IP 5.6.7.8
* Machine-3 (Receiver), with IP 9.10.11.12

The packet-path is Machine-1 <=> Machine-2 <=> Machine-3.
I have an agent installed, one each on all the 3 machines,
communicating on port 12345.

Following is what I have accomplished so far :

1.
I have been able to do communication over raw-sockets.
Most importantly, Machine-3 (Receiver) is able to receive the packets
from Machine-1, via raw-socket paradigm.

2.
Now, raw sockets in general accept all packets.
So, the next step was to apply socket-flitering, so that only packets
intended for port 12345 are received on Machine-3 agent.

This was accomplished via the help from
https://www.kernel.org/doc/Documentation/networking/filter.txt

3.
Now, I intend to sniff packets on Machine-2 (the intermediate node).

Is there a normal / legal way to do this?
Would be grateful for any pointers.

Thanks and Regards,
Ajay