Jason A. Donenfeld Jason at
Sun Dec 18 19:04:39 CET 2016

Hey John,

The old WireGuard gave you a default MTU of 1423. The new one gives
you a default MTU of 1420. That's your minus 3. From your email, it
looks like you subtracted twice!

If you're not using IPv6 as the outer endpoint IP address, then you
can make this 1440. The breakdown of the wireguard packet is:

20 byte IPv4 header or 40 byte IPv6 header
8 byte UDP header
1 byte type
3 byte reserved
4 byte index
8 byte nonce
16 byte authtag

So either you wind up with 20+8+1+3+4+8+16 bytes of overhead, or you
wind up with 40+8+1+3+4+8+16 bytes of overhead.


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