Subject: Re: [Bugme-new] [Bug 14837] New: gretap does not fragment IP packets

From: Andrew Morton <[email-masked]>
Date: Fri, 18 Dec 2009 15:32:09 -0800

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Fri, 18 Dec 2009 23:10:01 GMT
[email-masked] wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=14837
> 
>            Summary: gretap does not fragment IP packets
>            Product: Networking
>            Version: 2.5
>     Kernel Version: 2.6.32
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: IPV4
>         AssignedTo: [email-masked]
>         ReportedBy: [email-masked]
>         Regression: No
> 
> 
> When gretap is used to encapsulate Ethernet packets into IP packets, the
> encapsulated IP packets are larger than the original Ethernet packet, as
> expected.
> 
> Let's say you create a gre0 interface with a 1500 bytes MTU (since this
> interface will latter be inserted in a bridge interface, its MTU must be 1500).
> And Let's say the GRE encapsulated packet (now larger than 1500 bytes) is going
> to be routed over an IP interface with a 1500 bytes MTU.
> 
> The expected behavior would be that the encapsulated packet be fragmented. The
> observed behavior is that any encapsulated packets over 1500 bytes are simply
> dropped and an ICMP "fragmentation needed" message is sent to ... who knows.
> 
> My feeling is that DF bit is not playing nice here.
>