PROBLEM: Fragmentation issue with 1521 bytes ip packets

PROBLEM: Fragmentation issue with 1521 bytes ip packets

[Omar Alhassane]

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?

Thanks

Re: PROBLEM: Fragmentation issue with 1521 bytes ip packets

[Eric Dumazet]

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.

Re: PROBLEM: Fragmentation issue with 1521 bytes ip packets

[Omar Alhassane]

Thank you for explaining.

Omar

On Tue, May 15, 2012 at 11:04 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
> 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.
>
>
>
>
>