I am using a PPPoE connection to access the internet. Behind the router is my Wireguard server to which I connect from a remote location and use the VPN connection.

I noticed that for some situations, the connection doesn’t work. Googling for the solution gave me a hint on tweaking the MTU of the connection (on both ends), but this also didn’t seem to fix it.

Given the below network diagram, I have set an iptables rule to clamp the MSS (Maximum Segment Size) to the PMTU (Path Maximum Transfer Unit).

This was needed because, most probably, my ISP on the location is blocking ICMP fragmentation packets needed for the connection.

iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

If you need to manually set the MTU for the wg0 connection, you need to edit /etc/wireguard/wg0.conf and add the following line:

MTU = 1420

Note that 1420 is the default MTU calculated by Wireguard, therefore this edit is not needed if everything else works as intended.