Threat Level: green Handler on Duty: Jan Kopriva

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TShark & Multiple IP Addresses

In diary entry "TShark & jq", I mention that a single packet can have more than one ip.src entry.

I will illustrate this here. We are looking at a capture of network traffic where TCP SYN packets can not be delivered: "Destination unreachable".

Here is the view with TShark:

Notice TCP packets going from 192.168.10.10 to 192.168.10.1, and ICMP packets going from 192.168.10.1 to 192.168.10.10.

If we run a TShark command to extract all ip.src values, we get this:

For ICMP packets, we have 2 IPv4 addresses for the ip.src field: 192.168.10.1 and 192.168.10.10.

This is happening, because the ICMP packet embeds the IP packet to which it is replying. You can see this here in Wireshark:

The ICMP packet, encapsulated in an IP packet, has ip.src value of 192.168.10.1.

But it also embeds the IP packet (with TCP) to which it is replying. That one has a src.ip value equal to 192.168.10.10.

These are different fields, but both are accessed via ip.src and yield different values.

 

Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com

DidierStevens

652 Posts
ISC Handler
Feb 28th 2022

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