[Openvpn-users] Howto manualy configure the fragment and mssfix size

[Gerald Villemure <GVillemure@xxxxx>]

I had a problem with MTU sizes when setting up an OpenVPN 2.0 links, so 
here is my little howto. I may be completely off the bat on this info, 
but it works for me.

Setup the VPN link, then try it with this command: (IMHO, OpenVPN should 
do this systematically on linkup and generate a warning is it fails.)

For Win32:
ping -f -l 1472 192.168.X.X
               (IP of the other side of the link)
For Linux:
ping -M do -s 1472 192.168.X.X
                  (IP of the other side of the link)

If it works then your done you don't need to fix anything. If not! Here 
is what you do.

You lower the value of 1472 to something from the chart bellow until the 
ping works. Once it works set the value of "fragment" in your config to 
the value in the chart. Also set the value of "mssfix" to 0 (IMHO it 
should be set by default when fragment is set)

PING -> Fragment MTU
1467 -> 1532
1459 -> 1524
1451 -> 1516
1443 -> 1508
1435 -> 1500
1427 -> 1492
1419 -> 1484
1411 -> 1476
1403 -> 1468
1395 -> 1460
1387 -> 1452
1379 -> 1444
1371 -> 1436
1363 -> 1428
1355 -> 1420
1347 -> 1412
1339 -> 1404
1331 -> 1396

Hope this helps,
Gérald Villemure

PS, Feel free to correct these numbers if I am wrong.


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