Document ICMPv6 and TUN/TAP

src/yggdrasil/icmpv6.go

@@ -1,8 +1,13 @@
 package yggdrasil
 
-// The NDP functions are needed when you are running with a
-// TAP adapter - as the operating system expects neighbor solicitations
-// for on-link traffic, this goroutine provides them
+// The ICMPv6 module implements functions to easily create ICMPv6
+// packets. These functions, when mixed with the built-in Go IPv6
+// and ICMP libraries, can be used to send control messages back
+// to the host. Examples include:
+// - NDP messages, when running in TAP mode
+// - Packet Too Big messages, when packets exceed the session MTU
+// - Destination Unreachable messages, when a session prohibits
+//   incoming traffic
 
 import "net"
 import "golang.org/x/net/ipv6"
@@ -39,6 +44,9 @@ func ipv6Header_Marshal(h *ipv6.Header) ([]byte, error) {
 	return b, nil
 }
 
+// Initialises the ICMPv6 module by assigning our link-local IPv6 address and
+// our MAC address. ICMPv6 messages will always appear to originate from these
+// addresses.
 func (i *icmpv6) init(t *tunDevice) {
 	i.tun = t
 
@@ -50,6 +58,10 @@ func (i *icmpv6) init(t *tunDevice) {
 		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0xFE}
 }
 
+// Parses an incoming ICMPv6 packet. The packet provided may be either an
+// ethernet frame containing an IP packet, or the IP packet alone. This is
+// determined by whether the TUN/TAP adapter is running in TUN (layer 3) or
+// TAP (layer 2) mode.
 func (i *icmpv6) parse_packet(datain []byte) {
 	var response []byte
 	var err error
@@ -69,6 +81,10 @@ func (i *icmpv6) parse_packet(datain []byte) {
 	i.tun.iface.Write(response)
 }
 
+// Unwraps the ethernet headers of an incoming ICMPv6 packet and hands off
+// the IP packet to the parse_packet_tun function for further processing.
+// A response buffer is also created for the response message, also complete
+// with ethernet headers.
 func (i *icmpv6) parse_packet_tap(datain []byte) ([]byte, error) {
 	// Store the peer MAC address
 	copy(i.peermac[:6], datain[6:12])
@@ -97,6 +113,10 @@ func (i *icmpv6) parse_packet_tap(datain []byte) ([]byte, error) {
 	return dataout, nil
 }
 
+// Unwraps the IP headers of an incoming IPv6 packet and performs various
+// sanity checks on the packet - i.e. is the packet an ICMPv6 packet, does the
+// ICMPv6 message match a known expected type. The relevant handler function
+// is then called and a response packet may be returned.
 func (i *icmpv6) parse_packet_tun(datain []byte) ([]byte, error) {
 	// Parse the IPv6 packet headers
 	ipv6Header, err := ipv6.ParseHeader(datain[:ipv6.HeaderLen])
@@ -149,6 +169,9 @@ func (i *icmpv6) parse_packet_tun(datain []byte) ([]byte, error) {
 	return nil, errors.New("ICMPv6 type not matched")
 }
 
+// Creates an ICMPv6 packet based on the given icmp.MessageBody and other
+// parameters, complete with ethernet and IP headers, which can be written
+// directly to a TAP adapter.
 func (i *icmpv6) create_icmpv6_tap(dstmac macAddress, dst net.IP, src net.IP, mtype ipv6.ICMPType, mcode int, mbody icmp.MessageBody) ([]byte, error) {
 	// Pass through to create_icmpv6_tun
 	ipv6packet, err := i.create_icmpv6_tun(dst, src, mtype, mcode, mbody)
@@ -169,6 +192,10 @@ func (i *icmpv6) create_icmpv6_tap(dstmac macAddress, dst net.IP, src net.IP, mt
 	return dataout, nil
 }
 
+// Creates an ICMPv6 packet based on the given icmp.MessageBody and other
+// parameters, complete with IP headers only, which can be written directly to
+// a TUN adapter, or called directly by the create_icmpv6_tap function when
+// generating a message for TAP adapters.
 func (i *icmpv6) create_icmpv6_tun(dst net.IP, src net.IP, mtype ipv6.ICMPType, mcode int, mbody icmp.MessageBody) ([]byte, error) {
 	// Create the ICMPv6 message
 	icmpMessage := icmp.Message{
@@ -208,6 +235,11 @@ func (i *icmpv6) create_icmpv6_tun(dst net.IP, src net.IP, mtype ipv6.ICMPType,
 	return responsePacket, nil
 }
 
+// Generates a response to an NDP discovery packet. This is effectively called
+// when the host operating system generates an NDP request for any address in
+// the fd00::/8 range, so that the operating system knows to route that traffic
+// to the Yggdrasil TAP adapter.
+// TODO: Make this respect the value of address_prefix in address.go
 func (i *icmpv6) handle_ndp(in []byte) ([]byte, error) {
 	// Ignore NDP requests for anything outside of fd00::/8
 	if in[8] != 0xFD {

src/yggdrasil/tun.go

@@ -8,6 +8,7 @@ import "github.com/yggdrasil-network/water"
 const tun_IPv6_HEADER_LENGTH = 40
 const tun_ETHER_HEADER_LENGTH = 14
 
+// Represents a running TUN/TAP interface.
 type tunDevice struct {
 	core   *Core
 	icmpv6 icmpv6
@@ -17,6 +18,9 @@ type tunDevice struct {
 	iface  *water.Interface
 }
 
+// Defines which parameters are expected by default for a TUN/TAP adapter on a
+// specific platform. These values are populated in the relevant tun_*.go for
+// the platform being targeted. They must be set.
 type tunDefaultParameters struct {
 	maximumIfMTU     int
 	defaultIfMTU     int
@@ -24,6 +28,8 @@ type tunDefaultParameters struct {
 	defaultIfTAPMode bool
 }
 
+// Gets the maximum supported MTU for the platform based on the defaults in
+// getDefaults().
 func getSupportedMTU(mtu int) int {
 	if mtu > getDefaults().maximumIfMTU {
 		return getDefaults().maximumIfMTU
@@ -31,11 +37,14 @@ func getSupportedMTU(mtu int) int {
 	return mtu
 }
 
+// Initialises the TUN/TAP adapter.
 func (tun *tunDevice) init(core *Core) {
 	tun.core = core
 	tun.icmpv6.init(tun)
 }
 
+// Starts the setup process for the TUN/TAP adapter, and if successful, starts
+// the read/write goroutines to handle packets on that interface.
 func (tun *tunDevice) start(ifname string, iftapmode bool, addr string, mtu int) error {
 	if ifname == "none" {
 		return nil
@@ -48,6 +57,9 @@ func (tun *tunDevice) start(ifname string, iftapmode bool, addr string, mtu int)
 	return nil
 }
 
+// Writes a packet to the TUN/TAP adapter. If the adapter is running in TAP
+// mode then additional ethernet encapsulation is added for the benefit of the
+// host operating system.
 func (tun *tunDevice) write() error {
 	for {
 		data := <-tun.recv
@@ -75,6 +87,10 @@ func (tun *tunDevice) write() error {
 	}
 }
 
+// Reads any packets that are waiting on the TUN/TAP adapter. If the adapter
+// is running in TAP mode then the ethernet headers will automatically be
+// processed and stripped if necessary. If an ICMPv6 packet is found, then
+// the relevant helper functions in icmpv6.go are called.
 func (tun *tunDevice) read() error {
 	mtu := tun.mtu
 	if tun.iface.IsTAP() {
@@ -109,6 +125,9 @@ func (tun *tunDevice) read() error {
 	}
 }
 
+// Closes the TUN/TAP adapter. This is only usually called when the Yggdrasil
+// process stops. Typically this operation will happen quickly, but on macOS
+// it can block until a read operation is completed. 
 func (tun *tunDevice) close() error {
 	if tun.iface == nil {
 		return nil

src/yggdrasil/tun_bsd.go

@@ -70,6 +70,11 @@ type in6_ifreq_lifetime struct {
 	ifru_addrlifetime in6_addrlifetime
 }
 
+// Sets the IPv6 address of the utun adapter. On all BSD platforms (FreeBSD,
+// OpenBSD, NetBSD) an attempt is made to set the adapter properties by using
+// a system socket and making syscalls to the kernel. This is not refined though
+// and often doesn't work (if at all), therefore if a call fails, it resorts
+// to calling "ifconfig" instead. 
 func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int) error {
 	var config water.Config
 	if ifname[:4] == "auto" {

src/yggdrasil/tun_darwin.go

@@ -10,6 +10,8 @@ import "golang.org/x/sys/unix"
 
 import water "github.com/yggdrasil-network/water"
 
+// Sane defaults for the Darwin/macOS platform. The "default" options may be
+// may be replaced by the running configuration.
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     65535,
@@ -19,6 +21,7 @@ func getDefaults() tunDefaultParameters {
 	}
 }
 
+// Configures the "utun" adapter with the correct IPv6 address and MTU.
 func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int) error {
 	if iftapmode {
 		tun.core.log.Printf("TAP mode is not supported on this platform, defaulting to TUN")
@@ -65,6 +68,8 @@ type ifreq struct {
 	ifru_mtu uint32
 }
 
+// Sets the IPv6 address of the utun adapter. On Darwin/macOS this is done using
+// a system socket and making direct syscalls to the kernel. 
 func (tun *tunDevice) setupAddress(addr string) error {
 	var fd int
 	var err error

src/yggdrasil/tun_freebsd.go

@@ -1,5 +1,7 @@
 package yggdrasil
 
+// Sane defaults for the FreeBSD platform. The "default" options may be
+// may be replaced by the running configuration.
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     32767,

src/yggdrasil/tun_linux.go

@@ -1,7 +1,6 @@
 package yggdrasil
 
 // The linux platform specific tun parts
-// It depends on iproute2 being installed to set things on the tun device
 
 import "errors"
 import "fmt"
@@ -11,6 +10,8 @@ import water "github.com/yggdrasil-network/water"
 
 import "github.com/docker/libcontainer/netlink"
 
+// Sane defaults for the Linux platform. The "default" options may be
+// may be replaced by the running configuration.
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     65535,
@@ -20,6 +21,7 @@ func getDefaults() tunDefaultParameters {
 	}
 }
 
+// Configures the TAP adapter with the correct IPv6 address and MTU.
 func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int) error {
 	var config water.Config
 	if iftapmode {
@@ -39,6 +41,10 @@ func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int)
 	return tun.setupAddress(addr)
 }
 
+// Configures the TAP adapter with the correct IPv6 address and MTU. Netlink
+// is used to do this, so there is not a hard requirement on "ip" or "ifconfig"
+// to exist on the system, but this will fail if Netlink is not present in the
+// kernel (it nearly always is).
 func (tun *tunDevice) setupAddress(addr string) error {
 	// Set address
 	var netIF *net.Interface

src/yggdrasil/tun_netbsd.go

@@ -1,5 +1,7 @@
 package yggdrasil
 
+// Sane defaults for the NetBSD platform. The "default" options may be
+// may be replaced by the running configuration.
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     9000,

src/yggdrasil/tun_openbsd.go

@@ -1,5 +1,7 @@
 package yggdrasil
 
+// Sane defaults for the OpenBSD platform. The "default" options may be
+// may be replaced by the running configuration.
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     16384,

src/yggdrasil/tun_other.go

@@ -7,6 +7,8 @@ import water "github.com/yggdrasil-network/water"
 // This is to catch unsupported platforms
 // If your platform supports tun devices, you could try configuring it manually
 
+// These are sane defaults for any platform that has not been matched by one of
+// the other tun_*.go files.
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     65535,
@@ -16,6 +18,8 @@ func getDefaults() tunDefaultParameters {
 	}
 }
 
+// Creates the TUN/TAP adapter, if supported by the Water library. Note that
+// no guarantees are made at this point on an unsupported platform.
 func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int) error {
 	var config water.Config
 	if iftapmode {
@@ -32,6 +36,8 @@ func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int)
 	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 *tunDevice) setupAddress(addr string) error {
 	tun.core.log.Println("Platform not supported, you must set the address of", tun.iface.Name(), "to", addr)
 	return nil

src/yggdrasil/tun_windows.go

@@ -7,6 +7,8 @@ import "fmt"
 
 // This is to catch Windows platforms
 
+// Sane defaults for the Windows platform. The "default" options may be
+// may be replaced by the running configuration.
 func getDefaults() tunDefaultParameters {
 	return tunDefaultParameters{
 		maximumIfMTU:     65535,
@@ -16,6 +18,9 @@ func getDefaults() tunDefaultParameters {
 	}
 }
 
+// Configures the TAP adapter with the correct IPv6 address and MTU. On Windows
+// we don't make use of a direct operating system API to do this - we instead
+// delegate the hard work to "netsh".
 func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int) error {
 	if !iftapmode {
 		tun.core.log.Printf("TUN mode is not supported on this platform, defaulting to TAP")
@@ -63,6 +68,7 @@ func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int)
 	return tun.setupAddress(addr)
 }
 
+// Sets the MTU of the TAP adapter.
 func (tun *tunDevice) setupMTU(mtu int) error {
 	// Set MTU
 	cmd := exec.Command("netsh", "interface", "ipv6", "set", "subinterface",
@@ -79,6 +85,7 @@ func (tun *tunDevice) setupMTU(mtu int) error {
 	return nil
 }
 
+// Sets the IPv6 address of the TAP adapter.
 func (tun *tunDevice) setupAddress(addr string) error {
 	// Set address
 	cmd := exec.Command("netsh", "interface", "ipv6", "add", "address",