netdev - Re: PROBLEM: Fragmentation issue with 1521 bytes ip packets

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Date:	Tue, 15 May 2012 17:04:19 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Omar Alhassane <oalhassane@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: PROBLEM: Fragmentation issue with 1521 bytes ip packets

On Tue, 2012-05-15 at 10:00 -0400, Omar Alhassane wrote:
> Hello Folks,
> 
> I think i may have found a problem with the linux networking stack.
> Below is a description of the problem.
> 
> [1.] One line summary of the problem:
> No response to pings of certain sizes.
> 
> [2.] Full description of the problem/report:
> Using hping3, when i ping a linux machine with 1521 bytes ip packets i
> get only one response.
> But when i use 1482 bytes, everything works fine. I've tried this with
> both tcp and udp. The MTU of my interface is 1500.
> [3.] Keywords (i.e., modules, networking, kernel):
> ip, udp, tcp, networking, fragmentation
> [4.] Kernel version (from /proc/version):
> 3.3.1
> [5.] Output of Oops.. message (if applicable) with symbolic information
> [6.] A small shell script or example program which triggers the
> problem (if possible)
> The following commands works only if the target has tcp port 22 open
> 
> hping3 -d 1481 -S -P 22 10.0.30.225 (only one response)
> hping3 -d 1482 -S -P 22 10.0.30.225 (works fine)
> 
> Can somebody confirm if this is a problem?

hping3 bug : All the fragments it sends have the same ID field.

First 2 frags are reassembled by remote. Remote sends a SYNACK.


Following frags are 'ignored' because they have same ID than previous
packet.





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