The fact you have set MTU on the hosts you control means nothing
as the packets between them — in theory — might fly through any media with any MTU.
To deal with this issue (besides the IP fragmentation mechanism),
the so-called "Path MTU discovery" mechanism exists.
Note that it relies on the ICMP protocol to work properly.
Beside other things, it means that if you have tight firewall settings,
you must have something like
iptables -t nat -A $CHAIN -m state --state RELATED -j ACCEPT
in your rules for your
INPUT chain (and in the
FORWARD chain if your
firewall also does SNAT for its clients).
The "related" state means that when an ICMP packet of the type 3,
code 4 (destination unreachable / fragmentation required)
is received, the conntrack subsystem decides which connection
it is related to and then passes it further down the netfilter stack.
The state of such packet is "RELATED" as known to
so if you have passing of such packets denied — as often happens when
people disable everything wholesale and then punch the minimal holes —
P-MTU discovery won't work properly.
Note also that even if you "unbreak" proper support for P-MTU on your sides,
it might still be broken somewhere in between.
In order to deal with it, there exists at least two knobs:
- Clamping MSS to MTU in the IP stack.
- Limiting these things on the OpenVPN level by its
Note that these settings does affect performance, so use as the last resort.
¹ Because while not a part of an IP exchange, it's clearly related to it.