[Bridge] bridge problem with with ICMP fragment reassembly

[Bridge] bridge problem with with ICMP fragment reassembly

[Shawn Delaney]

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Hi,

I have a bridge (br0) with two enslaved interfaces (int0 and ext0). From the ext0 side, with a windows device, I ping with packet size 1932:

ping -n 1 -l 1932 10.64.160.94

ping request and reply is bridged.

Now if I do the same with packet size 1933:

ping -n 1 -l 1933 10.64.160.94

the request never seems to make it to the bridge interface

If I sniff the ping from the windows device on the ext0 side, I see two frames sent, request + fragment, as expected.
In the working case, if I sniff ext0 on the bridge device I see the same two frames received.
In the working case, if I sniff br0 on the bridge I see one frame (I assume the bridge reassembles the request + fragment).
In the non working case,  if I sniff ext0 on the bridge device I see the same two frames received.
In the non working case, if I sniff br0 I see nothing.

It seems there is a bug with bridging and reassembly of  packets or fragments of a certain size, and am wondering If there is a patch.

The MTU of all interfaces is 1500.
I am using connection tracking.
The problem is seen on kernels 2.6.11 and 2.6.24

Any help would be much appreciated.

Regards,
Shawn






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Re: [Bridge] bridge problem with with ICMP fragment reassembly

[Nicolas de Pesloüan]

Le 02/12/2010 23:19, Shawn Delaney a écrit :
> Hi,
> I have a bridge (br0) with two enslaved interfaces (int0 and ext0). From
> the ext0 side, with a windows device, I ping with packet size 1932:
> ping –n 1 –l 1932 10.64.160.94
> ping request and reply is bridged.
> Now if I do the same with packet size 1933:
> ping –n 1 –l 1933 10.64.160.94
> the request never seems to make it to the bridge interface
> If I sniff the ping from the windows device on the ext0 side, I see two
> frames sent, request + fragment, as expected.
> In the working case, if I sniff ext0 on the bridge device I see the same
> two frames received.
> In the working case, if I sniff br0 on the bridge I see one frame (I
> assume the bridge reassembles the request + fragment).
> In the non working case, if I sniff ext0 on the bridge device I see the
> same two frames received.
> In the non working case, if I sniff br0 I see nothing.
> It seems there is a bug with bridging and reassembly of packets or
> fragments of a certain size, and am wondering If there is a patch.

Bridges are not expected to change frames in any way. This is not specific to the bridging function 
of Linux.

No reassembly occurs at the bridge level. If one host send two frames, the bridge will forward two 
frames.

By the way, it means that the MTU for all interfaces of the bridge should be identical.

	Nicolas.

> The MTU of all interfaces is 1500.
> I am using connection tracking.
> The problem is seen on kernels 2.6.11 and 2.6.24
> Any help would be much appreciated.
> Regards,
> Shawn
>
>
>
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