Merge pull request #201 from neilalexander/ckr

Tunnel traffic using crypto-key routing

src/yggdrasil/ckr.go

@@ -0,0 +1,256 @@
+package yggdrasil
+
+import (
+	"bytes"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"net"
+	"sort"
+)
+
+// This module implements crypto-key routing, similar to Wireguard, where we
+// allow traffic for non-Yggdrasil ranges to be routed over Yggdrasil.
+
+type cryptokey struct {
+	core        *Core
+	enabled     bool
+	ipv4routes  []cryptokey_route
+	ipv6routes  []cryptokey_route
+	ipv4cache   map[address]cryptokey_route
+	ipv6cache   map[address]cryptokey_route
+	ipv4sources []net.IPNet
+	ipv6sources []net.IPNet
+}
+
+type cryptokey_route struct {
+	subnet      net.IPNet
+	destination []byte
+}
+
+// Initialise crypto-key routing. This must be done before any other CKR calls.
+func (c *cryptokey) init(core *Core) {
+	c.core = core
+	c.ipv4routes = make([]cryptokey_route, 0)
+	c.ipv6routes = make([]cryptokey_route, 0)
+	c.ipv4cache = make(map[address]cryptokey_route, 0)
+	c.ipv6cache = make(map[address]cryptokey_route, 0)
+	c.ipv4sources = make([]net.IPNet, 0)
+	c.ipv6sources = make([]net.IPNet, 0)
+}
+
+// Enable or disable crypto-key routing.
+func (c *cryptokey) setEnabled(enabled bool) {
+	c.enabled = enabled
+}
+
+// Check if crypto-key routing is enabled.
+func (c *cryptokey) isEnabled() bool {
+	return c.enabled
+}
+
+// Check whether the given address (with the address length specified in bytes)
+// matches either the current node's address, the node's routed subnet or the
+// list of subnets specified in IPv4Sources/IPv6Sources.
+func (c *cryptokey) isValidSource(addr address, addrlen int) bool {
+	ip := net.IP(addr[:addrlen])
+
+	if addrlen == net.IPv6len {
+		// Does this match our node's address?
+		if bytes.Equal(addr[:16], c.core.router.addr[:16]) {
+			return true
+		}
+
+		// Does this match our node's subnet?
+		if bytes.Equal(addr[:8], c.core.router.subnet[:8]) {
+			return true
+		}
+	}
+
+	// Does it match a configured CKR source?
+	if c.isEnabled() {
+		// Build our references to the routing sources
+		var routingsources *[]net.IPNet
+
+		// Check if the prefix is IPv4 or IPv6
+		if addrlen == net.IPv6len {
+			routingsources = &c.ipv6sources
+		} else if addrlen == net.IPv4len {
+			routingsources = &c.ipv4sources
+		} else {
+			return false
+		}
+
+		for _, subnet := range *routingsources {
+			if subnet.Contains(ip) {
+				return true
+			}
+		}
+	}
+
+	// Doesn't match any of the above
+	return false
+}
+
+// Adds a source subnet, which allows traffic with these source addresses to
+// be tunnelled using crypto-key routing.
+func (c *cryptokey) addSourceSubnet(cidr string) error {
+	// Is the CIDR we've been given valid?
+	_, ipnet, err := net.ParseCIDR(cidr)
+	if err != nil {
+		return err
+	}
+
+	// Get the prefix length and size
+	_, prefixsize := ipnet.Mask.Size()
+
+	// Build our references to the routing sources
+	var routingsources *[]net.IPNet
+
+	// Check if the prefix is IPv4 or IPv6
+	if prefixsize == net.IPv6len*8 {
+		routingsources = &c.ipv6sources
+	} else if prefixsize == net.IPv4len*8 {
+		routingsources = &c.ipv4sources
+	} else {
+		return errors.New("Unexpected prefix size")
+	}
+
+	// Check if we already have this CIDR
+	for _, subnet := range *routingsources {
+		if subnet.String() == ipnet.String() {
+			return errors.New("Source subnet already configured")
+		}
+	}
+
+	// Add the source subnet
+	*routingsources = append(*routingsources, *ipnet)
+	c.core.log.Println("Added CKR source subnet", cidr)
+	return nil
+}
+
+// Adds a destination route for the given CIDR to be tunnelled to the node
+// with the given BoxPubKey.
+func (c *cryptokey) addRoute(cidr string, dest string) error {
+	// Is the CIDR we've been given valid?
+	ipaddr, ipnet, err := net.ParseCIDR(cidr)
+	if err != nil {
+		return err
+	}
+
+	// Get the prefix length and size
+	_, prefixsize := ipnet.Mask.Size()
+
+	// Build our references to the routing table and cache
+	var routingtable *[]cryptokey_route
+	var routingcache *map[address]cryptokey_route
+
+	// Check if the prefix is IPv4 or IPv6
+	if prefixsize == net.IPv6len*8 {
+		routingtable = &c.ipv6routes
+		routingcache = &c.ipv6cache
+	} else if prefixsize == net.IPv4len*8 {
+		routingtable = &c.ipv4routes
+		routingcache = &c.ipv4cache
+	} else {
+		return errors.New("Unexpected prefix size")
+	}
+
+	// Is the route an Yggdrasil destination?
+	var addr address
+	var snet subnet
+	copy(addr[:], ipaddr)
+	copy(snet[:], ipnet.IP)
+	if addr.isValid() || snet.isValid() {
+		return errors.New("Can't specify Yggdrasil destination as crypto-key route")
+	}
+	// Do we already have a route for this subnet?
+	for _, route := range *routingtable {
+		if route.subnet.String() == ipnet.String() {
+			return errors.New(fmt.Sprintf("Route already exists for %s", cidr))
+		}
+	}
+	// Decode the public key
+	if boxPubKey, err := hex.DecodeString(dest); err != nil {
+		return err
+	} else {
+		// Add the new crypto-key route
+		*routingtable = append(*routingtable, cryptokey_route{
+			subnet:      *ipnet,
+			destination: boxPubKey,
+		})
+
+		// Sort so most specific routes are first
+		sort.Slice(*routingtable, func(i, j int) bool {
+			im, _ := (*routingtable)[i].subnet.Mask.Size()
+			jm, _ := (*routingtable)[j].subnet.Mask.Size()
+			return im > jm
+		})
+
+		// Clear the cache as this route might change future routing
+		// Setting an empty slice keeps the memory whereas nil invokes GC
+		for k := range *routingcache {
+			delete(*routingcache, k)
+		}
+
+		c.core.log.Println("Added CKR destination subnet", cidr)
+		return nil
+	}
+
+	return errors.New("Unspecified error")
+}
+
+// Looks up the most specific route for the given address (with the address
+// length specified in bytes) from the crypto-key routing table. An error is
+// returned if the address is not suitable or no route was found.
+func (c *cryptokey) getPublicKeyForAddress(addr address, addrlen int) (boxPubKey, error) {
+	// Check if the address is a valid Yggdrasil address - if so it
+	// is exempt from all CKR checking
+	if addr.isValid() {
+		return boxPubKey{}, errors.New("Cannot look up CKR for Yggdrasil addresses")
+	}
+
+	// Build our references to the routing table and cache
+	var routingtable *[]cryptokey_route
+	var routingcache *map[address]cryptokey_route
+
+	// Check if the prefix is IPv4 or IPv6
+	if addrlen == net.IPv6len {
+		routingtable = &c.ipv6routes
+		routingcache = &c.ipv6cache
+	} else if addrlen == net.IPv4len {
+		routingtable = &c.ipv4routes
+		routingcache = &c.ipv4cache
+	} else {
+		return boxPubKey{}, errors.New("Unexpected prefix size")
+	}
+
+	// Check if there's a cache entry for this addr
+	if route, ok := (*routingcache)[addr]; ok {
+		var box boxPubKey
+		copy(box[:boxPubKeyLen], route.destination)
+		return box, nil
+	}
+
+	// No cache was found - start by converting the address into a net.IP
+	ip := make(net.IP, addrlen)
+	copy(ip[:addrlen], addr[:])
+
+	// Check if we have a route. At this point c.ipv6routes should be
+	// pre-sorted so that the most specific routes are first
+	for _, route := range *routingtable {
+		// Does this subnet match the given IP?
+		if route.subnet.Contains(ip) {
+			// Cache the entry for future packets to get a faster lookup
+			(*routingcache)[addr] = route
+
+			// Return the boxPubKey
+			var box boxPubKey
+			copy(box[:boxPubKeyLen], route.destination)
+			return box, nil
+		}
+	}
+
+	// No route was found if we got to this point
+	return boxPubKey{}, errors.New(fmt.Sprintf("No route to %s", ip.String()))
+}

src/yggdrasil/config/config.go

@@ -4,8 +4,8 @@ package config
 type NodeConfig struct {
 	Listen                      string              `comment:"Listen address for peer connections. Default is to listen for all\nTCP connections over IPv4 and IPv6 with a random port."`
 	AdminListen                 string              `comment:"Listen address for admin connections Default is to listen for local\nconnections either on TCP/9001 or a UNIX socket depending on your\nplatform. Use this value for yggdrasilctl -endpoint=X."`
-	Peers                       []string            `comment:"List of connection strings for static peers in URI format, i.e.\ntcp://a.b.c.d:e or socks://a.b.c.d:e/f.g.h.i:j."`
+	Peers                       []string            `comment:"List of connection strings for static peers in URI format, e.g.\ntcp://a.b.c.d:e or socks://a.b.c.d:e/f.g.h.i:j."`
-	InterfacePeers              map[string][]string `comment:"List of connection strings for static peers in URI format, arranged\nby source interface, i.e. { \"eth0\": [ tcp://a.b.c.d:e ] }. Note that\nSOCKS peerings will NOT be affected by this option and should go in\nthe \"Peers\" section instead."`
+	InterfacePeers              map[string][]string `comment:"List of connection strings for static peers in URI format, arranged\nby source interface, e.g. { \"eth0\": [ tcp://a.b.c.d:e ] }. Note that\nSOCKS peerings will NOT be affected by this option and should go in\nthe \"Peers\" section instead."`
 	ReadTimeout                 int32               `comment:"Read timeout for connections, specified in milliseconds. If less\nthan 6000 and not negative, 6000 (the default) is used. If negative,\nreads won't time out."`
 	AllowedEncryptionPublicKeys []string            `comment:"List of peer encryption public keys to allow or incoming TCP\nconnections from. If left empty/undefined then all connections\nwill be allowed by default."`
 	EncryptionPublicKey         string              `comment:"Your public encryption key. Your peers may ask you for this to put\ninto their AllowedEncryptionPublicKeys configuration."`
@@ -17,6 +17,7 @@ type NodeConfig struct {
 	IfTAPMode                   bool                `comment:"Set local network interface to TAP mode rather than TUN mode if\nsupported by your platform - option will be ignored if not."`
 	IfMTU                       int                 `comment:"Maximux Transmission Unit (MTU) size for your local TUN/TAP interface.\nDefault is the largest supported size for your platform. The lowest\npossible value is 1280."`
 	SessionFirewall             SessionFirewall     `comment:"The session firewall controls who can send/receive network traffic\nto/from. This is useful if you want to protect this node without\nresorting to using a real firewall. This does not affect traffic\nbeing routed via this node to somewhere else. Rules are prioritised as\nfollows: blacklist, whitelist, always allow outgoing, direct, remote."`
+	TunnelRouting               TunnelRouting       `comment:"Allow tunneling non-Yggdrasil traffic over Yggdrasil. This effectively\nallows you to use Yggdrasil to route to, or to bridge other networks,\nsimilar to a VPN tunnel. Tunnelling works between any two nodes and\ndoes not require them to be directly peered."`
 	//Net                         NetConfig `comment:"Extended options for connecting to peers over other networks."`
 }
 
@@ -26,6 +27,7 @@ type NetConfig struct {
 	I2P I2PConfig `comment:"Experimental options for configuring peerings over I2P."`
 }
 
+// SessionFirewall controls the session firewall configuration
 type SessionFirewall struct {
 	Enable                        bool     `comment:"Enable or disable the session firewall. If disabled, network traffic\nfrom any node will be allowed. If enabled, the below rules apply."`
 	AllowFromDirect               bool     `comment:"Allow network traffic from directly connected peers."`
@@ -34,3 +36,12 @@ type SessionFirewall struct {
 	WhitelistEncryptionPublicKeys []string `comment:"List of public keys from which network traffic is always accepted,\nregardless of AllowFromDirect or AllowFromRemote."`
 	BlacklistEncryptionPublicKeys []string `comment:"List of public keys from which network traffic is always rejected,\nregardless of the whitelist, AllowFromDirect or AllowFromRemote."`
 }
+
+// TunnelRouting contains the crypto-key routing tables for tunneling
+type TunnelRouting struct {
+	Enable           bool              `comment:"Enable or disable tunnel routing."`
+	IPv6Destinations map[string]string `comment:"IPv6 CIDR subnets, mapped to the EncryptionPublicKey to which they\nshould be routed, e.g. { \"aaaa:bbbb:cccc::/e\": \"boxpubkey\", ... }"`
+	IPv6Sources      []string          `comment:"Optional IPv6 source subnets which are allowed to be tunnelled in\naddition to this node's Yggdrasil address/subnet. If not\nspecified, only traffic originating from this node's Yggdrasil\naddress or subnet will be tunnelled."`
+	IPv4Destinations map[string]string `comment:"IPv4 CIDR subnets, mapped to the EncryptionPublicKey to which they\nshould be routed, e.g. { \"a.b.c.d/e\": \"boxpubkey\", ... }"`
+	IPv4Sources      []string          `comment:"IPv4 source subnets which are allowed to be tunnelled. Unlike for\nIPv6, this option is required for bridging IPv4 traffic. Only\ntraffic with a source matching these subnets will be tunnelled."`
+}

src/yggdrasil/core.go

@@ -121,6 +121,31 @@ func (c *Core) Start(nc *config.NodeConfig, log *log.Logger) error {
 		return err
 	}
 
+	c.router.cryptokey.setEnabled(nc.TunnelRouting.Enable)
+	if c.router.cryptokey.isEnabled() {
+		c.log.Println("Crypto-key routing enabled")
+		for ipv6, pubkey := range nc.TunnelRouting.IPv6Destinations {
+			if err := c.router.cryptokey.addRoute(ipv6, pubkey); err != nil {
+				panic(err)
+			}
+		}
+		for _, source := range nc.TunnelRouting.IPv6Sources {
+			if c.router.cryptokey.addSourceSubnet(source); err != nil {
+				panic(err)
+			}
+		}
+		for ipv4, pubkey := range nc.TunnelRouting.IPv4Destinations {
+			if err := c.router.cryptokey.addRoute(ipv4, pubkey); err != nil {
+				panic(err)
+			}
+		}
+		for _, source := range nc.TunnelRouting.IPv4Sources {
+			if c.router.cryptokey.addSourceSubnet(source); err != nil {
+				panic(err)
+			}
+		}
+	}
+
 	if err := c.admin.start(); err != nil {
 		c.log.Println("Failed to start admin socket")
 		return err

src/yggdrasil/router.go

@@ -23,6 +23,7 @@ package yggdrasil
 //  The router then runs some sanity checks before passing it to the tun
 
 import (
+	"bytes"
 	"time"
 
 	"golang.org/x/net/icmp"
@@ -34,18 +35,21 @@ import (
 type router struct {
 	core      *Core
 	addr      address
+	subnet    subnet
 	in        <-chan []byte // packets we received from the network, link to peer's "out"
 	out       func([]byte)  // packets we're sending to the network, link to peer's "in"
 	recv      chan<- []byte // place where the tun pulls received packets from
 	send      <-chan []byte // place where the tun puts outgoing packets
 	reset     chan struct{} // signal that coords changed (re-init sessions/dht)
 	admin     chan func()   // pass a lambda for the admin socket to query stuff
+	cryptokey cryptokey
 }
 
 // Initializes the router struct, which includes setting up channels to/from the tun/tap.
 func (r *router) init(core *Core) {
 	r.core = core
 	r.addr = *address_addrForNodeID(&r.core.dht.nodeID)
+	r.subnet = *address_subnetForNodeID(&r.core.dht.nodeID)
 	in := make(chan []byte, 32) // TODO something better than this...
 	p := r.core.peers.newPeer(&r.core.boxPub, &r.core.sigPub, &boxSharedKey{}, "(self)")
 	p.out = func(packet []byte) {
@@ -67,6 +71,7 @@ func (r *router) init(core *Core) {
 	r.core.tun.send = send
 	r.reset = make(chan struct{}, 1)
 	r.admin = make(chan func())
+	r.cryptokey.init(r.core)
 	// go r.mainLoop()
 }
 
@@ -117,30 +122,82 @@ func (r *router) mainLoop() {
 // If the session hasn't responded recently, it triggers a ping or search to keep things alive or deal with broken coords *relatively* quickly.
 // It also deals with oversized packets if there are MTU issues by calling into icmpv6.go to spoof PacketTooBig traffic, or DestinationUnreachable if the other side has their tun/tap disabled.
 func (r *router) sendPacket(bs []byte) {
-	if len(bs) < 40 {
-		panic("Tried to send a packet shorter than a header...")
-	}
 	var sourceAddr address
-	var sourceSubnet subnet
+	var destAddr address
-	copy(sourceAddr[:], bs[8:])
+	var destSnet subnet
-	copy(sourceSubnet[:], bs[8:])
+	var destPubKey *boxPubKey
-	if !sourceAddr.isValid() && !sourceSubnet.isValid() {
+	var destNodeID *NodeID
+	var addrlen int
+	if bs[0]&0xf0 == 0x60 {
+		// Check if we have a fully-sized header
+		if len(bs) < 40 {
+			panic("Tried to send a packet shorter than an IPv6 header...")
+		}
+		// IPv6 address
+		addrlen = 16
+		copy(sourceAddr[:addrlen], bs[8:])
+		copy(destAddr[:addrlen], bs[24:])
+		copy(destSnet[:addrlen/2], bs[24:])
+	} else if bs[0]&0xf0 == 0x40 {
+		// Check if we have a fully-sized header
+		if len(bs) < 20 {
+			panic("Tried to send a packet shorter than an IPv4 header...")
+		}
+		// IPv4 address
+		addrlen = 4
+		copy(sourceAddr[:addrlen], bs[12:])
+		copy(destAddr[:addrlen], bs[16:])
+	} else {
+		// Unknown address length
 		return
 	}
-	var dest address
+	if !r.cryptokey.isValidSource(sourceAddr, addrlen) {
-	copy(dest[:], bs[24:])
+		// The packet had a source address that doesn't belong to us or our
-	var snet subnet
+		// configured crypto-key routing source subnets
-	copy(snet[:], bs[24:])
-	if !dest.isValid() && !snet.isValid() {
 		return
 	}
+	if !destAddr.isValid() && !destSnet.isValid() {
+		// The addresses didn't match valid Yggdrasil node addresses so let's see
+		// whether it matches a crypto-key routing range instead
+		if key, err := r.cryptokey.getPublicKeyForAddress(destAddr, addrlen); err == nil {
+			// A public key was found, get the node ID for the search
+			destPubKey = &key
+			destNodeID = getNodeID(destPubKey)
+			// Do a quick check to ensure that the node ID refers to a vaild Yggdrasil
+			// address or subnet - this might be superfluous
+			addr := *address_addrForNodeID(destNodeID)
+			copy(destAddr[:], addr[:])
+			copy(destSnet[:], addr[:])
+			if !destAddr.isValid() && !destSnet.isValid() {
+				return
+			}
+		} else {
+			// No public key was found in the CKR table so we've exhausted our options
+			return
+		}
+	}
 	doSearch := func(packet []byte) {
 		var nodeID, mask *NodeID
-		if dest.isValid() {
+		switch {
-			nodeID, mask = dest.getNodeIDandMask()
+		case destNodeID != nil:
+			// We already know the full node ID, probably because it's from a CKR
+			// route in which the public key is known ahead of time
+			nodeID = destNodeID
+			var m NodeID
+			for i := range m {
+				m[i] = 0xFF
 			}
-		if snet.isValid() {
+			mask = &m
-			nodeID, mask = snet.getNodeIDandMask()
+		case destAddr.isValid():
+			// We don't know the full node ID - try and use the address to generate
+			// a truncated node ID
+			nodeID, mask = destAddr.getNodeIDandMask()
+		case destSnet.isValid():
+			// We don't know the full node ID - try and use the subnet to generate
+			// a truncated node ID
+			nodeID, mask = destSnet.getNodeIDandMask()
+		default:
+			return
 		}
 		sinfo, isIn := r.core.searches.searches[*nodeID]
 		if !isIn {
@@ -153,11 +210,11 @@ func (r *router) sendPacket(bs []byte) {
 	}
 	var sinfo *sessionInfo
 	var isIn bool
-	if dest.isValid() {
+	if destAddr.isValid() {
-		sinfo, isIn = r.core.sessions.getByTheirAddr(&dest)
+		sinfo, isIn = r.core.sessions.getByTheirAddr(&destAddr)
 	}
-	if snet.isValid() {
+	if destSnet.isValid() {
-		sinfo, isIn = r.core.sessions.getByTheirSubnet(&snet)
+		sinfo, isIn = r.core.sessions.getByTheirSubnet(&destSnet)
 	}
 	switch {
 	case !isIn || !sinfo.init:
@@ -186,6 +243,14 @@ func (r *router) sendPacket(bs []byte) {
 		}
 		fallthrough // Also send the packet
 	default:
+		// If we know the public key ahead of time (i.e. a CKR route) then check
+		// if the session perm pub key matches before we send the packet to it
+		if destPubKey != nil {
+			if !bytes.Equal((*destPubKey)[:], sinfo.theirPermPub[:]) {
+				return
+			}
+		}
+
 		// Drop packets if the session MTU is 0 - this means that one or other
 		// side probably has their TUN adapter disabled
 		if sinfo.getMTU() == 0 {
@@ -236,29 +301,55 @@ func (r *router) sendPacket(bs []byte) {
 			// Don't continue - drop the packet
 			return
 		}
+
 		sinfo.send <- bs
 	}
 }
 
 // Called for incoming traffic by the session worker for that connection.
 // Checks that the IP address is correct (matches the session) and passes the packet to the tun/tap.
-func (r *router) recvPacket(bs []byte, theirAddr *address, theirSubnet *subnet) {
+func (r *router) recvPacket(bs []byte, sinfo *sessionInfo) {
 	// Note: called directly by the session worker, not the router goroutine
 	if len(bs) < 24 {
 		util_putBytes(bs)
 		return
 	}
-	var source address
+	var sourceAddr address
-	copy(source[:], bs[8:])
+	var dest address
 	var snet subnet
-	copy(snet[:], bs[8:])
+	var addrlen int
-	switch {
+	if bs[0]&0xf0 == 0x60 {
-	case source.isValid() && source == *theirAddr:
+		// IPv6 address
-	case snet.isValid() && snet == *theirSubnet:
+		addrlen = 16
-	default:
+		copy(sourceAddr[:addrlen], bs[8:])
+		copy(dest[:addrlen], bs[24:])
+		copy(snet[:addrlen/2], bs[24:])
+	} else if bs[0]&0xf0 == 0x40 {
+		// IPv4 address
+		addrlen = 4
+		copy(sourceAddr[:addrlen], bs[12:])
+		copy(dest[:addrlen], bs[16:])
+	} else {
+		// Unknown address length
+		return
+	}
+	// Check that the packet is destined for either our Yggdrasil address or
+	// subnet, or that it matches one of the crypto-key routing source routes
+	if !r.cryptokey.isValidSource(dest, addrlen) {
 		util_putBytes(bs)
 		return
 	}
+	// See whether the packet they sent should have originated from this session
+	switch {
+	case sourceAddr.isValid() && sourceAddr == sinfo.theirAddr:
+	case snet.isValid() && snet == sinfo.theirSubnet:
+	default:
+		key, err := r.cryptokey.getPublicKeyForAddress(sourceAddr, addrlen)
+		if err != nil || key != sinfo.theirPermPub {
+			util_putBytes(bs)
+			return
+		}
+	}
 	//go func() { r.recv<-bs }()
 	r.recv <- bs
 }

src/yggdrasil/session.go

@@ -589,5 +589,5 @@ func (sinfo *sessionInfo) doRecv(p *wire_trafficPacket) {
 	sinfo.updateNonce(&p.Nonce)
 	sinfo.time = time.Now()
 	sinfo.bytesRecvd += uint64(len(bs))
-	sinfo.core.router.recvPacket(bs, &sinfo.theirAddr, &sinfo.theirSubnet)
+	sinfo.core.router.recvPacket(bs, sinfo)
 }

src/yggdrasil/tun.go

@@ -101,10 +101,10 @@ func (tun *tunDevice) read() error {
 		if tun.iface.IsTAP() {
 			o = tun_ETHER_HEADER_LENGTH
 		}
-		if buf[o]&0xf0 != 0x60 ||
+		switch {
-			n != 256*int(buf[o+4])+int(buf[o+5])+tun_IPv6_HEADER_LENGTH+o {
+		case buf[o]&0xf0 == 0x60 && n == 256*int(buf[o+4])+int(buf[o+5])+tun_IPv6_HEADER_LENGTH+o:
-			// Either not an IPv6 packet or not the complete packet for some reason
+		case buf[o]&0xf0 == 0x40 && n == 256*int(buf[o+2])+int(buf[o+3])+o:
-			//panic("Should not happen in testing")
+		default:
 			continue
 		}
 		if buf[o+6] == 58 {