LVS/TUN MTU problem, missing hints in documentation J.Libak at
Thu Dec 7 23:35:26 GMT 2006

Joseph Mack NA3T wrote:
> On Thu, 7 Dec 2006, jarol1 at wrote:
>> Today I ran into problem of MTU in LVS/Tunneling.
>> Small packets were forwarded to real servers without problems, but 
>> the bigger ones weren't and TCP retransmissions occurred. I noticed 
>> the problem dissapeared when I switched to LVS/DR so this gave me 
>> hint to where the problem might be. MTU 1480 had to be set on the 
>> outgoing interface of realservers with tunl0 having standard 1500. 
>> Directors have 1500 on all interfaces. This way TCP syn ack contained 
>> correct MTU and client didn't send big packets that were discarded on 
>> director anymore. IP header is 20 bytes long so 1480 is the maximum 
>> value that works.
>> It isn't mentioned in documentation that this problem will occur if 
>> tunneling is used
> This was a hard problem to figure out and solve. People worked on this 
> for about 2yrs before finding a satisfactory solution. The problem is 
> that the ipip code in Linux doesn't handle fragmentation (or handle it 
> properly).
> You did well to come up with a solution in one go.
> Joe
I was thinking a sort of warning could be added either to or to the 
ultramonkey website. There could be a link to the 
on how to deal with it.


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