How to specify the MTU of ocserv adapters

[Yick Xie]

Hello,

As the title indicated, ocserv just dynamically and frequently adjust
the MTU value of virtual adapters according to peer-endings, which
sometimes even was set to 576. But actually the network is still
stable, only with certain kind of packet loss, yet totally acceptable.
So I disabled the mtu-discovery in ocserv and specified the MSS in
iptables (iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS
--set-mss 1400), while the MTU of adapters seemed not to be impacted
at all (via "ifconfig"). In my case packets of ocserv shall be warped
by external programs, i.e. far too lower MTU definitely makes the VPN
worse even unusable. Is there some way to explicitly fix it like
OpenVPN? or some other way to work around it? Since I am sure ISP will
never ever change the MTU less than certain value as 1450 or 1400. I
have been stuck into this for long time, and still no clue of its
mechanism.

Regards,
Yick