Re: [Openvpn-users] How to determine the correct MTU/fragment value in OpenVPN 2.6

[tincantech <tin...@pr...>]

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Hi,

This seems to have been forgotten.

------- Original Message -------
On Thursday, July 20th, 2023 at 11:24, Niccolò Belli <dar...@li...> wrote:

> I'm using Debian 12 Bookworm with OpenVPN 2.6.3 on the server and Arch
> Linux with OpenVPN 2.6.5 [git:makepkg/cbc9e0ce412e7b42+] on the client.

<snip>

> I really don't understand what value I should put in fragment and the
> fact that things change a on day per day basis with apparently no reason
> makes everything even more difficult to understand.

If your PMTU is changing "on a daily basis" then you should probably report
that as a fault to your Internet Service Provider(s).

The only possible way to configure OpenVPN to deal with such an unreliable
network is to set options such as --link-mtu, --max-packet-size, --fragment
and --mssfix to a value lower than the lowest expected PMTU.

With such a low PMTU as you describe, it may not be possible to use OpenVPN
at all, because TLS does not allow for packet fragmentation, to my knowledge.

Regards.
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