I have a working Wireguard tunnel to Mullvad - mostly. I have not set an MTU anywhere but the Wireguard interface shows an MTU of 496 which is bizarrely low. I cannot connect to most sites due to this low MTU and I have confirmed that that is indeed the MTU by running ping -i 0.1 -D -g 300 -G 1500 8.8.8.8. Sure enough when I hit 497, pings die. I have tried setting the interface and the Wireguard Local MTU to 1420 (the usual default) and then no traffiic passes. I am stumped. Any thoughts on what's going on and how to get a normal/usable MTU?
This is an embarrassing one. Somehow my WAN interface had an MTU of 576 - and for all I know it's been like that forever. I had never set an MTU and that must be what the modem or who knows negotiates with the firewall. I hard set the WAN to 1500 and everything works. I'll be over here with my working VPN and my shame.