wg-quick: auto MTU discovery

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/wg-quick.8

@@ -22,7 +22,7 @@ suitable for a few common use cases.
 
 Use \fIup\fP to add and set up an interface, and use \fIdown\fP to tear down and remove
 an interface. Running \fIup\fP adds a WireGuard interface, brings up the interface with the
-supplied IP addresses, sets up routes, and optionally runs pre/post up scripts. Running \fIdown\fP
+supplied IP addresses, sets up mtu and routes, and optionally runs pre/post up scripts. Running \fIdown\fP
 optionally saves the current configuration, removes the WireGuard interface, and optionally
 runs pre/post down scripts.
 
@@ -65,6 +65,10 @@ which are handled by this tool:
 Address \(em a comma-separated list of ip (v4 or v6) addresses (optionally with CIDR masks)
 to be assigned to the interface. May be specified multiple times.
 .IP \(bu
+MTU \(em if not specified, the MTU is automatically determined from the endpoint addresses
+or the system default route, which is usually a sane choice. However, to manually specify
+an MTU to override this automatic discovery, this value may be specified explicitly.
+.IP \(bu
 PreUp, PostUp, PreDown, PostDown \(em script snippets which will be executed by
 .BR bash (1)
 before/after setting up/tearing down the interface, most commonly used

src/wg-quick.bash

@@ -13,6 +13,7 @@ export PATH="${SELF%/*}:$PATH"
 WG_CONFIG=""
 INTERFACE=""
 ADDRESSES=( )
+MTU=""
 PRE_UP=""
 POST_UP=""
 PRE_DOWN=""
@@ -39,6 +40,7 @@ parse_options() {
 		if [[ $interface_section -eq 1 ]]; then
 			case "$key" in
 			Address) ADDRESSES+=( ${value//,/ } ); continue ;;
+			MTU) MTU="$value"; continue ;;
 			PreUp) PRE_UP="$value"; continue ;;
 			PreDown) PRE_DOWN="$value"; continue ;;
 			PostUp) POST_UP="$value"; continue ;;
@@ -108,6 +110,25 @@ add_addr() {
 	cmd ip address add "$1" dev "$INTERFACE"
 }
 
+set_mtu() {
+	local mtu=0 endpoint output
+	if [[ -n $MTU ]]; then
+		cmd ip link set mtu "$MTU" 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]+) ) ) && ${BASH_REMATCH[1]} -gt $mtu ]] && mtu="${BASH_REMATCH[1]}"
+	done < <(wg show "$INTERFACE" endpoints)
+	if [[ $mtu -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]+) ) ) && ${BASH_REMATCH[1]} -gt $mtu ]] && mtu="${BASH_REMATCH[1]}"
+	fi
+	[[ $mtu -gt 0 ]] || mtu=1500
+	cmd ip link set mtu $(( mtu - 80 )) dev "$INTERFACE"
+}
+
 add_route() {
 	if [[ $1 == 0.0.0.0/0 || $1 == ::/0 ]]; then
 		add_default "$1"
@@ -146,6 +167,7 @@ save_config() {
 	for address in ${BASH_REMATCH[1]}; do
 		new_config+="Address = $address"$'\n'
 	done
+	[[ -n $MTU && $(ip link show dev "$INTERFACE") =~ mtu\ ([0-9]+) ]] && new_config+="MTU = ${BASH_REMATCH[1]}"$'\n'
 	[[ $SAVE_CONFIG -eq 0 ]] || new_config+=$'SaveConfig = true\n'
 	[[ -z $PRE_UP ]] || new_config+="PreUp = $PRE_UP"$'\n'
 	[[ -z $POST_UP ]] || new_config+="PostUp = $POST_UP"$'\n'
@@ -200,6 +222,7 @@ cmd_up() {
 	for i in "${ADDRESSES[@]}"; do
 		add_addr "$i"
 	done
+	set_mtu
 	up_if
 	for i in $(wg show "$INTERFACE" allowed-ips | grep -Po '(?<=[\t ])[0-9.:/a-f]+' | sort -nr -k 2 -t /); do
 		[[ $(ip route get "$i" 2>/dev/null) == *dev\ $INTERFACE\ * ]] || add_route "$i"