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mtu-test: Empirical MTU test completed - how to use result?

Posted: Wed Sep 19, 2012 3:59 pm
by Apokrif
I’m trying to troubleshoot my non-working bridge VPN still.

Run mtu-test today, got:
Wed Sep 19 11:09:20 2012 us=183000 NOTE: Empirical MTU test completed
[Tried,Actual] local->remote=[1573,1573] remote->local=[1469,1469]

Few questions:
1. Based on result, how do I need to adjust both server and client files?
Put "fragment 1469" into both server and client config?

2. Even if mtu setting is wrong, it shouldn’t affect small ping size (32 bytes), right?
I.e. ping will work, but some other stuff won’t - depend on used packet size.

3. About mtu-test itself. It’s kinda obvious, but want to clarify still:
During measurement, OpenVPN does send ping packets.
The packed are subject to VPN compression.
Naturally, to minimize compression effect, OpenVPN does send "uncompressible packets" (sorta "encrypted garbage"). Am I right about it?

--mtu-test
To empirically measure MTU on connection startup, add the --mtu-test option to your configuration.
OpenVPN will send ping packets of various sizes to the remote peer and measure the largest packets which were successfully received.
The --mtu-test process normally takes about 3 minutes to complete.

Re: mtu-test: Empirical MTU test completed - how to use resu

Posted: Thu Sep 27, 2012 7:04 am
by Mimiko
mtu-test implies OpenVPN to find the maximum packat size which can be used other the existing phisical link from client to server, so it will set the mtu internally, not on the tadp adapter or eth adapter, for use. OpenVPN uses its own algorithm to find this value.