wg-quick set_mtu_up - largest or smallest MTU?

[M P Robert <mprobert@gmail.com>]

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I was looking at the auto MTU detection in wg-quick.

It appears that wg-quick is taking the LARGEST of all endpoint MTUs

https://github.com/WireGuard/wireguard-tools/blob/13f4ac4cb74b5a833fa7f825ba785b1e5774e84f/src/wg-quick/linux.bash#L134

In a scenario where you have different peers on different network devices or routes with different MTUs, I would think you would want to take the SMALLEST mtu from all peers in order to avoid having fragmentation talking to the peers on networks with smaller MTUs.

Or perhaps fragmentation for some peers is faster than selecting a smaller packet size for all peers?  Or I am missing something (more likely).  Happy to be educated on this point.

I don't have git-send-email setup at this point, but just in case this is a valid issue, I've attached a sample fix for set_mtu_up that will take the smallest of the discovered peer MTUs rather than the largest.  I'm not a bash guy, so just take it for illustration purposes.

Thanks!

-Matt



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set_mtu_up() {
	local mtu=0 mtufound=0 endpoint output
	if [[ -n $MTU ]]; then
		cmd ip link set mtu "$MTU" up dev "$INTERFACE"
		return
	fi
	while read -r _ endpoint; do
		[[ $endpoint =~ ^\[?([a-z0-9:.]+)\]?:[0-9]+$ ]] || continue
		output="$(ip route get "${BASH_REMATCH[1]}" || true)"
		[[ ( $output =~ mtu\ ([0-9]+) || ( $output =~ dev\ ([^ ]+) && $(ip link show dev "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) ) ) && ( $mtufound -eq 0 || ${BASH_REMATCH[1]} -lt $mtu ) ]] && mtu="${BASH_REMATCH[1]}" && mtufound=1
	done < <(wg show "$INTERFACE" endpoints)
	if [[ $mtufound -eq 0 ]]; then
		read -r output < <(ip route show default || true) || true
		[[ $output =~ mtu\ ([0-9]+) || ( $output =~ dev\ ([^ ]+) && $(ip link show dev "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) ) ]] && mtu="${BASH_REMATCH[1]}" && mtufound=1
	fi
	[[ $mtufound -eq 1 ]] || mtu=1500
	cmd ip link set mtu $(( mtu - 80 )) up dev "$INTERFACE"
}

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