Fix setting up of MTU when value is outside of acceptable bounds, also account for ethernet headers in calculations, notify about clipping to stdout

src/tuntap/tun.go

@@ -63,9 +63,12 @@ type TunOptions struct {
 
 // Gets the maximum supported MTU for the platform based on the defaults in
 // defaults.GetDefaults().
-func getSupportedMTU(mtu int) int {
+func getSupportedMTU(mtu int, istapmode bool) int {
-	if mtu > defaults.GetDefaults().MaximumIfMTU {
+	if mtu < 1280 {
-		return defaults.GetDefaults().MaximumIfMTU
+		return 1280
+	}
+	if mtu > MaximumMTU(istapmode) {
+		return MaximumMTU(istapmode)
 	}
 	return mtu
 }
@@ -80,7 +83,7 @@ func (tun *TunAdapter) Name() string {
 // the maximum value is determined by your platform. The returned value will
 // never exceed that of MaximumMTU().
 func (tun *TunAdapter) MTU() int {
-	return getSupportedMTU(tun.mtu)
+	return getSupportedMTU(tun.mtu, tun.IsTAP())
 }
 
 // IsTAP returns true if the adapter is a TAP adapter (Layer 2) or false if it
@@ -97,7 +100,11 @@ func DefaultName() string {
 // DefaultMTU gets the default TUN/TAP interface MTU for your platform. This can
 // be as high as MaximumMTU(), depending on platform, but is never lower than 1280.
 func DefaultMTU() int {
-	return defaults.GetDefaults().DefaultIfMTU
+	ehbytes := 0
+	if DefaultIsTAP() {
+		ehbytes = 14
+	}
+	return defaults.GetDefaults().DefaultIfMTU - ehbytes
 }
 
 // DefaultIsTAP returns true if the default adapter mode for the current
@@ -109,8 +116,12 @@ func DefaultIsTAP() bool {
 // MaximumMTU returns the maximum supported TUN/TAP interface MTU for your
 // platform. This can be as high as 65535, depending on platform, but is never
 // lower than 1280.
-func MaximumMTU() int {
+func MaximumMTU(iftapmode bool) int {
-	return defaults.GetDefaults().MaximumIfMTU
+	ehbytes := 0
+	if iftapmode {
+		ehbytes = 14
+	}
+	return defaults.GetDefaults().MaximumIfMTU - ehbytes
 }
 
 // Init initialises the TUN/TAP module. You must have acquired a Listener from
@@ -167,6 +178,9 @@ func (tun *TunAdapter) _start() error {
 		if err := tun.setup(ifname, iftapmode, addr, tun.mtu); err != nil {
 			return err
 		}
+		if tun.MTU() != current.IfMTU {
+			tun.log.Warnf("Warning: Interface MTU %d automatically adjusted to %d (supported range is 1280-%d)", current.IfMTU, tun.MTU(), MaximumMTU(tun.IsTAP()))
+		}
 	}
 	if ifname == "none" || ifname == "dummy" {
 		tun.log.Debugln("Not starting TUN/TAP as ifname is none or dummy")
@@ -176,7 +190,7 @@ func (tun *TunAdapter) _start() error {
 	go tun.handler()
 	tun.reader.Act(nil, tun.reader._read) // Start the reader
 	tun.icmpv6.Init(tun)
-	if iftapmode {
+	if tun.IsTAP() {
 		go tun.icmpv6.Solicit(tun.addr)
 	}
 	tun.ckr.init(tun)

src/tuntap/tun_bsd.go

@@ -99,7 +99,7 @@ func (tun *TunAdapter) setup(ifname string, iftapmode bool, addr string, mtu int
 		panic(err)
 	}
 	tun.iface = iface
-	tun.mtu = getSupportedMTU(mtu)
+	tun.mtu = getSupportedMTU(mtu, iftapmode)
 	return tun.setupAddress(addr)
 }
 

src/tuntap/tun_darwin.go

@@ -18,7 +18,8 @@ import (
 // Configures the "utun" adapter with the correct IPv6 address and MTU.
 func (tun *TunAdapter) setup(ifname string, iftapmode bool, addr string, mtu int) error {
 	if iftapmode {
-		tun.log.Warnln("TAP mode is not supported on this platform, defaulting to TUN")
+		tun.log.Warnln("Warning: TAP mode is not supported on this platform, defaulting to TUN")
+		iftapmode = false
 	}
 	config := water.Config{DeviceType: water.TUN}
 	iface, err := water.New(config)
@@ -26,7 +27,7 @@ func (tun *TunAdapter) setup(ifname string, iftapmode bool, addr string, mtu int
 		panic(err)
 	}
 	tun.iface = iface
-	tun.mtu = getSupportedMTU(mtu)
+	tun.mtu = getSupportedMTU(mtu, iftapmode)
 	return tun.setupAddress(addr)
 }
 

src/tuntap/tun_linux.go

@@ -26,7 +26,7 @@ func (tun *TunAdapter) setup(ifname string, iftapmode bool, addr string, mtu int
 		panic(err)
 	}
 	tun.iface = iface
-	tun.mtu = getSupportedMTU(mtu)
+	tun.mtu = getSupportedMTU(mtu, iftapmode)
 	// The following check is specific to Linux, as the TAP driver only supports
 	// an MTU of 65535-14 to make room for the ethernet headers. This makes sure
 	// that the MTU gets rounded down to 65521 instead of causing a panic.

src/tuntap/tun_other.go

@@ -21,13 +21,13 @@ func (tun *TunAdapter) setup(ifname string, iftapmode bool, addr string, mtu int
 		panic(err)
 	}
 	tun.iface = iface
-	tun.mtu = getSupportedMTU(mtu)
+	tun.mtu = getSupportedMTU(mtu, iftapmode)
 	return tun.setupAddress(addr)
 }
 
 // We don't know how to set the IPv6 address on an unknown platform, therefore
 // write about it to stdout and don't try to do anything further.
 func (tun *TunAdapter) setupAddress(addr string) error {
-	tun.log.Warnln("Platform not supported, you must set the address of", tun.iface.Name(), "to", addr)
+	tun.log.Warnln("Warning: Platform not supported, you must set the address of", tun.iface.Name(), "to", addr)
 	return nil
 }

src/tuntap/tun_windows.go

@@ -17,7 +17,8 @@ import (
 // delegate the hard work to "netsh".
 func (tun *TunAdapter) setup(ifname string, iftapmode bool, addr string, mtu int) error {
 	if !iftapmode {
-		tun.log.Warnln("TUN mode is not supported on this platform, defaulting to TAP")
+		tun.log.Warnln("Warning: TUN mode is not supported on this platform, defaulting to TAP")
+		iftapmode = true
 	}
 	config := water.Config{DeviceType: water.TAP}
 	config.PlatformSpecificParams.ComponentID = "tap0901"
@@ -60,7 +61,7 @@ func (tun *TunAdapter) setup(ifname string, iftapmode bool, addr string, mtu int
 		panic(err)
 	}
 	tun.iface = iface
-	tun.mtu = getSupportedMTU(mtu)
+	tun.mtu = getSupportedMTU(mtu, iftapmode)
 	err = tun.setupMTU(tun.mtu)
 	if err != nil {
 		panic(err)