[PATCH] wg-quick: Pick smallest MTU of any endpoint route

[Daniel Gröber]

If the goal is to avoid fragmentation picking the largest MTU of any
endpoint route doesn't make sense. Users with networks supporting jumbo
frames (MTU>1500) can still set MTU=, but most users will want to avoid
fratmentation by default.

Incidentally android.c already had the correct behaviour:

    next_mtu = get_route_mtu(endpoint);
    if (next_mtu > 0 && next_mtu < endpoint_mtu)
    	endpoint_mtu = next_mtu;

only the shell implementations were problematic.

Signed-off-by: Daniel Gröber <dxld at darkboxed.org>
---
 src/man/wg-quick.8        | 14 +++++++++++---
 src/wg-quick/freebsd.bash |  4 ++--
 src/wg-quick/linux.bash   |  4 ++--
 src/wg-quick/openbsd.bash |  4 ++--
 4 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/src/man/wg-quick.8 b/src/man/wg-quick.8
index bc9e145..391a095 100644
--- a/src/man/wg-quick.8
+++ b/src/man/wg-quick.8
@@ -83,9 +83,17 @@ specified multiple times. Upon bringing the interface up, this runs
 .BR resolvconf (8)
 are undesirable, the PostUp and PostDown keys below may be used instead.
 .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.
+MTU \(em wg tunnel interface MTU. If not specified, the MTU is set to the
+lowest route MTU of any peer endpoint or if route lookups don't return an
+explicit MTU the system default route's MTU less encapsulation overhead.
+
+Note that matching the tunnel MTU to the underlying network MTU is normally
+only a performance concern. WireGuard allows encapsulated (UDP) packets to
+be fragmented through PMTUD (IPv4/IPv6) or in-network fragmentation (IPv4)
+depending on system behaviour.
+ 
+However, to forgoe this automatic behaviour, a static value may may be
+specified here.
 .IP \(bu
 Table \(em Controls the routing table to which routes are added. There are two
 special values: `off' disables the creation of routes altogether, and `auto'
diff --git a/src/wg-quick/freebsd.bash b/src/wg-quick/freebsd.bash
index f72daf6..3886f47 100755
--- a/src/wg-quick/freebsd.bash
+++ b/src/wg-quick/freebsd.bash
@@ -191,11 +191,11 @@ set_mtu() {
 		family=inet
 		[[ ${BASH_REMATCH[1]} == *:* ]] && family=inet6
 		output="$(route -n get "-$family" "${BASH_REMATCH[1]}" || true)"
-		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -gt $mtu ]] && mtu="${BASH_REMATCH[1]}"
+		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -lt $mtu ]] && mtu="${BASH_REMATCH[1]}"
 	done < <(wg show "$INTERFACE" endpoints)
 	if [[ $mtu -eq 0 ]]; then
 		read -r output < <(route -n get default || true) || true
-		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -gt $mtu ]] && mtu="${BASH_REMATCH[1]}"
+		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -lt $mtu ]] && mtu="${BASH_REMATCH[1]}"
 	fi
 	[[ $mtu -gt 0 ]] || mtu=1500
 	cmd ifconfig "$INTERFACE" mtu $(( mtu - 80 ))
diff --git a/src/wg-quick/linux.bash b/src/wg-quick/linux.bash
index 4193ce5..eab411c 100755
--- a/src/wg-quick/linux.bash
+++ b/src/wg-quick/linux.bash
@@ -131,11 +131,11 @@ set_mtu_up() {
 	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]}"
+		[[ ( $output =~ mtu\ ([0-9]+) || ( $output =~ dev\ ([^ ]+) && $(ip link show dev "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) ) ) && ${BASH_REMATCH[1]} -lt $mtu ]] && mtu="${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]}"
+		[[ ( $output =~ mtu\ ([0-9]+) || ( $output =~ dev\ ([^ ]+) && $(ip link show dev "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) ) ) && ${BASH_REMATCH[1]} -lt $mtu ]] && mtu="${BASH_REMATCH[1]}"
 	fi
 	[[ $mtu -gt 0 ]] || mtu=1500
 	cmd ip link set mtu $(( mtu - 80 )) up dev "$INTERFACE"
diff --git a/src/wg-quick/openbsd.bash b/src/wg-quick/openbsd.bash
index b58ecf5..14f26ff 100755
--- a/src/wg-quick/openbsd.bash
+++ b/src/wg-quick/openbsd.bash
@@ -174,11 +174,11 @@ set_mtu() {
 		family=inet
 		[[ ${BASH_REMATCH[1]} == *:* ]] && family=inet6
 		output="$(route -n get "-$family" "${BASH_REMATCH[1]}" || true)"
-		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -gt $mtu ]] && mtu="${BASH_REMATCH[1]}"
+		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -lt $mtu ]] && mtu="${BASH_REMATCH[1]}"
 	done < <(wg show "$REAL_INTERFACE" endpoints)
 	if [[ $mtu -eq 0 ]]; then
 		read -r output < <(route -n get default || true) || true
-		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -gt $mtu ]] && mtu="${BASH_REMATCH[1]}"
+		[[ $output =~ interface:\ ([^ ]+)$'\n' && $(ifconfig "${BASH_REMATCH[1]}") =~ mtu\ ([0-9]+) && ${BASH_REMATCH[1]} -lt $mtu ]] && mtu="${BASH_REMATCH[1]}"
 	fi
 	[[ $mtu -gt 0 ]] || mtu=1500
 	cmd ifconfig "$REAL_INTERFACE" mtu $(( mtu - 80 ))
-- 
2.39.2