Years ago, I had to deal with some
aggravation around IPv6. My connection to my ISP is such that I'm
using PPPoE which means I have to use an MTU of 1492 instead
of the ubiquitous 1500 that everyone else is using:
# ifconfig tun1 tun1: flags=1008051<UP,POINTOPOINT,RUNNING,MULTICAST,LOWER_UP> metric 0 mtu 1492
I'm using Wireguard in various places to link
multiple networks. Wireguard packets have 80 bytes of overhead, so the
virtual interfaces it creates have an MTU of 1420 by default.
You can probably guess where this is going.
On a connection such as mine, a packet of size 1420 plus the 80 bytes of
overhead is 1500. This means we'll run into all of the same problems that
occurred with IPv6 back in 2017, with all of the same symptoms.
The solution? Set an MTU of 1410 on the Wireguard interfaces.
On FreeBSD:
# ifconfig wg0 mtu 1410
On Linux, you can either use an MTU=1410 directive in the Wireguard
configuration file, or:
# ip link set mtu 1410 up dev wg0