MTU issue when using GRE over tinc

[Loic Dachary]

Hi Guus,

Thanks for the solution :-) I did

    ip link set mtu 1400 dev eth0

inside the virtual machine ( kvm ) that is sending packets that are ultimately transfered to the GRE over tinc and it works. I'll research a solution to convince OpenVSwitch to do this without requiring each virtual machine to change its MTU, if possible. But that's out of topic on this list. 

Cheers

On 10/12/2013 10:38, Guus Sliepen wrote:
> On Tue, Dec 10, 2013 at 10:08:37AM +0100, Loic Dachary wrote:
> 
>> In a setup where OpenVSwitch is used with GRE tunels on top of an interface provided by tinc, I'm experiencing MTU problems and I'm not sure how to fix them. The manifestation of the problem is, from the user point of view, communication hang. And using "tcpdump -i there" displays lines such as :
>>
>>     18:54:00.345666 IP 10.111.1.1 > 10.111.1.2: GREv0, key=0x5, length 1438: IP ceph.com.https > 10.0.3.15.57429: Flags [P.], seq 4917:6293, ack 658, win 71, length 1376
>>     18:54:00.345746 IP 10.111.1.2 > 10.111.1.1: ICMP 10.111.1.2 unreachable - need to frag (mtu 1445), length 556
>>
>> My actual use case very much ressembles what is described here http://blog.csdn.net/lynn_kong/article/details/9140659
>>
>> and I tried to change the MTU on hte tinc provided interfaces on the machines that have 10.111.1.2 and 10.111.1.1 bound to them:
>>
>>     ip link set mtu 1546 dev there
> 
> The problem is that the GRE tunnel sets the DF bit in its headers, meaning that
> tinc should not fragment the GRE packets, but instead reply with an ICMP error
> message when the packets are too large. This is exactly what tinc does. The
> problem is that GRE doesn't handle those ICMP packets at all. To make sure that
> the packets sent through the GRE tunnel do not exceed the maximum packet size,
> you should lower the MTU of the GRE interface, for example like so:
> 
> ip link set mtu 1400 dev gre0
> 
> Raising the MTU of the VPN interface does not help in any way, and lowering the
> MTU of the VPN interface will make the problem worse.
> 
> 
> 
> _______________________________________________
> tinc-devel mailing list
> tinc-devel at tinc-vpn.org
> http://www.tinc-vpn.org/cgi-bin/mailman/listinfo/tinc-devel
> 

-- 
Loïc Dachary, Artisan Logiciel Libre

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 263 bytes
Desc: OpenPGP digital signature
URL: <http://www.tinc-vpn.org/pipermail/tinc-devel/attachments/20131210/c6093a8e/attachment.sig>