PMTU = 1518 over local network at 1500 MTU

[Florent Bautista]

Hi all,

I have two nodes, connected to a switch, using Tinc 1.1 from git.

They connect each other with sptps, and to other nodes in the Internet
with old protocol because they have Tinc 1.0.

There is no problem with remote nodes, but between my 2 local nodes,
they see 1518 PMTU. But local network is 1500 MTU !!! So nodes can ping
each other but larger data does not go.

test1=sllm1
test2=sllm2

test1:/usr/src/tinc# tinc -n coppint info sllm2
Node:         sllm2
Address:      192.168.0.202 port 655
Online since: 2014-02-25 13:36:39
Status:       validkey visited reachable sptps udp_confirmed
Options:      pmtu_discovery clamp_mss
Protocol:     17.3
Reachability: directly with UDP
PMTU:         1518
Edges:        host1 host3 host4 host6 sllm1
Subnets:      1a:26:ce:e3:fb:13 6e:de:af:13:55:bf


root at test2:~# tinc -n coppint info sllm1
Node:         sllm1
Address:      192.168.0.200 port 655
Online since: 2014-02-25 13:36:39
Status:       validkey visited reachable sptps udp_confirmed
Options:      pmtu_discovery clamp_mss
Protocol:     17.3
Reachability: directly with UDP
PMTU:         1518
Edges:        host1 host3 host4 host6 sllm2
Subnets:      12:a5:76:ae:f6:d1 16:5c:d3:3:89:e6 aa:43:ea:86:94:19
c2:3a:f:4a:bd:2c c2:a9:10:51:bd:3e de:ad:ac:cf:a4:ad f2:7e:b:6f:98:48



I did -d5 in Tinc daemon but I see nothing interesting:

root at test2:~# cat /var/log/tinc-coppint.log | grep PMTU
2014-02-25 13:31:18 tinc.coppint[413140]: Increase in PMTU to sllm1
(192.168.0.200 port 655) detected, restarting PMTU discovery


What do I have to look for in this file ?

Host config contains only Address, Cipher, Digest and MACLength; but I
think that 3 last are rewritten by sptps.

Tinc config is :

AddressFamily = ipv4
#BindToInterface = vmbr1
Broadcast = mst
ConnectTo = host1
ConnectTo = host2
ConnectTo = host3
ConnectTo = host4
ConnectTo = host5
ConnectTo = host6
ConnectTo = sllm1
Forwarding = internal
GraphDumpFile = /tmp/tinc-graph
Mode = switch
Name = sllm2
PrivateKeyFile = /etc/tinc/coppint/rsa_key.priv
ProcessPriority = high


What do you think my problem is  ?

Thank you a lot for your help.