Path MTU discovery fixes and improvements (#3109)

- Existing option `WIREGUARD_MTU` , if set, disables PMTUD and is used
- New option `PMTUD_ICMP_ADDRESSES=1.1.1.1,8.8.8.8` and `PMTUD_TCP_ADDRESSES=1.1.1.1:443,8.8.8.8:443`
- ICMP PMTUD now targets external-by-default IP addresses
- New TCP PMTUD (binary search only) as a second MTU confirmation and fallback mechanism.
- Force set TCP MSS to MTU - IP header - TCP base header - "magic 20 bytes" 🎆
- Fix #3108

internal/provider/utils/wireguard.go

@@ -16,7 +16,17 @@ func BuildWireguardSettings(connection models.Connection,
 	settings.PreSharedKey = *userSettings.PreSharedKey
 	settings.InterfaceName = userSettings.Interface
 	settings.Implementation = userSettings.Implementation
-	settings.MTU = userSettings.MTU
+	if *userSettings.MTU > 0 {
+		settings.MTU = *userSettings.MTU
+	} else {
+		// The default is 1320 which is NOT the wireguard-go default
+		// of 1420 because this impacts bandwidth a lot on some
+		// VPN providers, see https://github.com/qdm12/gluetun/issues/1650.
+		// It has been lowered to 1320 following quite a bit of
+		// investigation in the issue: https://github.com/qdm12/gluetun/issues/2533.
+		const defaultMTU = 1320
+		settings.MTU = defaultMTU
+	}
 	settings.IPv6 = &ipv6Supported
 
 	const rulePriority = 101 // 100 is to receive external connections

internal/provider/utils/wireguard_test.go

@@ -22,7 +22,7 @@ func Test_BuildWireguardSettings(t *testing.T) {
 		ipv6Supported bool
 		settings      wireguard.Settings
 	}{
-		"some settings": {
+		"some_settings": {
 			connection: models.Connection{
 				IP:     netip.AddrFrom4([4]byte{1, 2, 3, 4}),
 				Port:   51821,
@@ -41,6 +41,7 @@ func Test_BuildWireguardSettings(t *testing.T) {
 				},
 				PersistentKeepaliveInterval: ptrTo(time.Hour),
 				Interface:                   "wg1",
+				MTU:                         ptrTo(uint32(1000)),
 			},
 			ipv6Supported: false,
 			settings: wireguard.Settings{
@@ -58,6 +59,7 @@ func Test_BuildWireguardSettings(t *testing.T) {
 				PersistentKeepaliveInterval: time.Hour,
 				RulePriority:                101,
 				IPv6:                        boolPtr(false),
+				MTU:                         1000,
 			},
 		},
 	}