Re: [Openvpn-users] WARNING: 'tun-mtu' is used inconsistently, local='tun-mtu 1532', remote='tun-mtu 1500'

[Gert D. <ge...@gr...>]

Hi,

On Thu, Jul 24, 2025 at 08:38:44AM +0200, Marc SCHAEFER wrote:
>    WARNING: 'tun-mtu' is used inconsistently, local='tun-mtu 1532', remote='tun-mtu 1500'
[..]
> Any idea why this happens, or should I just ignore this warning?

Just ignore the warning.  As long as all interfaces end up with the same
configured MTU, things are working.

These option warnings do not work very well across major versions (2.5
to 2.6) - I think, in this case, because we actually *fixed* the reporting
in the "OCC" handshake ("OpenVPN Config Check"), but 2.5 is still using
"something plus some guesswork" to arrive at "1532".

You can try to set "occ-mtu 1532" on the server to silence 2.5 :-)

              The OCC MTU can be used to avoid warnings about mismatched MTU
              from clients. If occ-mtu is not specified, it will to default to
              the tun-mtu.

... workarounds for old code, old versions, long history...  (and we can't
"just remove" an option from the OCC string, because then the clients would
complain "warning: 'tun-mtu' configured locally and missing on remote" or so)

gert
-- 
"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             ge...@gr...