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forwarder.go
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forwarder.go
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package router
import (
"code.google.com/p/gopacket"
"code.google.com/p/gopacket/layers"
"log"
"net"
"syscall"
"time"
)
func (conn *LocalConnection) ensureForwarders() error {
if conn.forwardChan != nil || conn.forwardChanDF != nil {
return nil
}
udpSender := NewSimpleUDPSender(conn)
udpSenderDF, err := NewRawUDPSender(conn) // only thing that can error, so do it early
if err != nil {
return err
}
usingPassword := conn.SessionKey != nil
var encryptor, encryptorDF Encryptor
if usingPassword {
encryptor = NewNaClEncryptor(conn.local.NameByte, conn, false)
encryptorDF = NewNaClEncryptor(conn.local.NameByte, conn, true)
} else {
encryptor = NewNonEncryptor(conn.local.NameByte)
encryptorDF = NewNonEncryptor(conn.local.NameByte)
}
var (
forwardChan = make(chan *ForwardedFrame, ChannelSize)
forwardChanDF = make(chan *ForwardedFrame, ChannelSize)
stopForward = make(chan interface{}, 0)
stopForwardDF = make(chan interface{}, 0)
verifyPMTU = make(chan int, ChannelSize)
)
//NB: only forwarderDF can ever encounter EMSGSIZE errors, and
//thus perform PMTU verification
forwarder := NewForwarder(conn, forwardChan, stopForward, nil, encryptor, udpSender, DefaultPMTU)
forwarderDF := NewForwarder(conn, forwardChanDF, stopForwardDF, verifyPMTU, encryptorDF, udpSenderDF, DefaultPMTU)
// Various fields in the conn struct are read by other processes,
// so we have to use locks.
conn.Lock()
conn.forwardChan = forwardChan
conn.forwardChanDF = forwardChanDF
conn.stopForward = stopForward
conn.stopForwardDF = stopForwardDF
conn.verifyPMTU = verifyPMTU
conn.effectivePMTU = forwarder.unverifiedPMTU
conn.Unlock()
go forwarder.Run()
go forwarderDF.Run()
return nil
}
func (conn *LocalConnection) stopForwarders() {
conn.Lock()
conn.forwardChan = nil
conn.forwardChanDF = nil
conn.Unlock()
// Now signal the forwarder loops to exit. They will drain the
// forwarder chans in order to unblock any router processes
// blocked on sending.
if conn.stopForward != nil {
conn.stopForward <- nil
conn.stopForwardDF <- nil
}
}
// Called from peer.Relay[Broadcast] which is itself invoked from
// router (both UDP listener process and sniffer process). Also called
// from connection's heartbeat process, and from the connection's TCP
// receiver process.
func (conn *LocalConnection) Forward(df bool, frame *ForwardedFrame, dec *EthernetDecoder) error {
conn.RLock()
var (
forwardChan = conn.forwardChan
forwardChanDF = conn.forwardChanDF
effectivePMTU = conn.effectivePMTU
stackFrag = conn.stackFrag
)
conn.RUnlock()
if forwardChan == nil || forwardChanDF == nil {
conn.log("Cannot forward frame yet - awaiting contact")
return nil
}
// We could use non-blocking channel sends here, i.e. drop frames
// on the floor when the forwarder is busy. This would allow our
// caller - the capturing loop in the router - to read frames more
// quickly when under load, i.e. we'd drop fewer frames on the
// floor during capture. And we could maximise CPU utilisation
// since we aren't stalling a thread. However, a lot of work has
// already been done by the time we get here. Since any packet we
// drop will likely get re-transmitted we end up paying that cost
// multiple times. So it's better to drop things at the beginning
// of our pipeline.
if df {
if !frameTooBig(frame, effectivePMTU) {
forwardChanDF <- frame
return nil
}
return FrameTooBigError{EPMTU: effectivePMTU}
} else {
if stackFrag || dec == nil || len(dec.decoded) < 2 {
forwardChan <- frame
return nil
}
// Don't have trustworthy stack, so we're going to have to
// send it DF in any case.
if !frameTooBig(frame, effectivePMTU) {
forwardChanDF <- frame
return nil
}
conn.Router.LogFrame("Fragmenting", frame.frame, &dec.eth)
// We can't trust the stack to fragment, we have IP, and we
// have a frame that's too big for the MTU, so we have to
// fragment it ourself.
return fragment(dec.eth, dec.ip, effectivePMTU, frame, func(segFrame *ForwardedFrame) {
forwardChanDF <- segFrame
})
}
}
func frameTooBig(frame *ForwardedFrame, effectivePMTU int) bool {
// We capture/forward complete ethernet frames. Therefore the
// frame length includes the ethernet header. However, MTUs
// operate at the IP layer and thus do not include the ethernet
// header. To put it another way, when a sender that was told an
// MTU of M sends an IP packet of exactly that length, we will
// capture/forward M + EthernetOverhead bytes of data.
return len(frame.frame) > effectivePMTU+EthernetOverhead
}
func fragment(eth layers.Ethernet, ip layers.IPv4, pmtu int, frame *ForwardedFrame, forward func(*ForwardedFrame)) error {
// We are not doing any sort of NAT, so we don't need to worry
// about checksums of IP payload (eg UDP checksum).
headerSize := int(ip.IHL) * 4
// &^ is bit clear (AND NOT). So here we're clearing the lowest 3
// bits.
maxSegmentSize := (pmtu - headerSize) &^ 7
opts := gopacket.SerializeOptions{
FixLengths: false,
ComputeChecksums: true}
payloadSize := int(ip.Length) - headerSize
payload := ip.BaseLayer.Payload[:payloadSize]
offsetBase := int(ip.FragOffset) << 3
origFlags := ip.Flags
ip.Flags = ip.Flags | layers.IPv4MoreFragments
ip.Length = uint16(headerSize + maxSegmentSize)
if eth.EthernetType == layers.EthernetTypeLLC {
// using LLC, so must set eth length correctly. eth length
// is just the length of the payload
eth.Length = ip.Length
} else {
eth.Length = 0
}
for offset := 0; offset < payloadSize; offset += maxSegmentSize {
var segmentPayload []byte
if len(payload) <= maxSegmentSize {
// last one
segmentPayload = payload
ip.Length = uint16(len(payload) + headerSize)
ip.Flags = origFlags
if eth.EthernetType == layers.EthernetTypeLLC {
eth.Length = ip.Length
} else {
eth.Length = 0
}
} else {
segmentPayload = payload[:maxSegmentSize]
payload = payload[maxSegmentSize:]
}
ip.FragOffset = uint16((offset + offsetBase) >> 3)
buf := gopacket.NewSerializeBuffer()
segPayload := gopacket.Payload(segmentPayload)
err := gopacket.SerializeLayers(buf, opts, ð, &ip, &segPayload)
if err != nil {
return err
}
// make copies of the frame we received
var segFrame ForwardedFrame = *frame
segFrame.frame = buf.Bytes()
forward(&segFrame)
}
return nil
}
// Forwarder
func NewForwarder(conn *LocalConnection, ch <-chan *ForwardedFrame, stop <-chan interface{}, verifyPMTU <-chan int, enc Encryptor, udpSender UDPSender, pmtu int) *Forwarder {
fwd := &Forwarder{
conn: conn,
ch: ch,
stop: stop,
verifyPMTU: verifyPMTU,
enc: enc,
udpSender: udpSender}
fwd.unverifiedPMTU = pmtu - fwd.effectiveOverhead()
fwd.maxPayload = pmtu - UDPOverhead
return fwd
}
func (fwd *Forwarder) Run() {
defer fwd.udpSender.Shutdown()
var flushed, ok bool
var frame *ForwardedFrame
for {
flushed = false
select {
case <-fwd.stop:
fwd.drain()
return
case <-fwd.verifyPMTUTick:
// We only do this case here when we know the buffers are
// all empty so that we don't risk appending verify-frames
// to other data.
fwd.verifyPMTUTick = nil
if fwd.pmtuVerified {
continue
}
if fwd.pmtuVerifyCount > 0 {
fwd.pmtuVerifyCount--
fwd.attemptVerifyEffectivePMTU()
} else {
// we've exceeded the verification attempts of the
// unverifiedPMTU
fwd.lowestBadPMTU = fwd.unverifiedPMTU
fwd.verifyEffectivePMTU((fwd.highestGoodPMTU + fwd.lowestBadPMTU) / 2)
}
case epmtu := <-fwd.verifyPMTU:
if fwd.pmtuVerified || epmtu != fwd.unverifiedPMTU {
continue
}
if epmtu+1 < fwd.lowestBadPMTU {
fwd.highestGoodPMTU = fwd.unverifiedPMTU // = epmtu
fwd.verifyEffectivePMTU((fwd.highestGoodPMTU + fwd.lowestBadPMTU) / 2)
} else {
fwd.pmtuVerified = true
fwd.maxPayload = epmtu + fwd.effectiveOverhead() - UDPOverhead
fwd.conn.setEffectivePMTU(epmtu)
fwd.conn.log("Effective PMTU verified at", epmtu)
}
case frame = <-fwd.ch:
if !fwd.appendFrame(frame) {
fwd.logDrop(frame)
continue
}
for !flushed {
select {
case frame, ok = <-fwd.ch:
if !ok {
return
}
if !fwd.appendFrame(frame) {
fwd.flush()
if !fwd.appendFrame(frame) {
fwd.logDrop(frame)
flushed = true
}
}
default:
fwd.flush()
flushed = true
}
}
}
}
}
func (fwd *Forwarder) effectiveOverhead() int {
return UDPOverhead + fwd.enc.PacketOverhead() + fwd.enc.FrameOverhead() + EthernetOverhead
}
func (fwd *Forwarder) verifyEffectivePMTU(newUnverifiedPMTU int) {
fwd.unverifiedPMTU = newUnverifiedPMTU
fwd.pmtuVerifyCount = PMTUVerifyAttempts
fwd.attemptVerifyEffectivePMTU()
}
func (fwd *Forwarder) attemptVerifyEffectivePMTU() {
pmtuVerifyFrame := &ForwardedFrame{
srcPeer: fwd.conn.local,
dstPeer: fwd.conn.remote,
frame: make([]byte, fwd.unverifiedPMTU+EthernetOverhead)}
fwd.enc.AppendFrame(pmtuVerifyFrame)
fwd.flush()
if fwd.verifyPMTUTick == nil {
fwd.verifyPMTUTick = time.After(PMTUVerifyTimeout << (PMTUVerifyAttempts - fwd.pmtuVerifyCount))
}
}
func (fwd *Forwarder) appendFrame(frame *ForwardedFrame) bool {
frameLen := len(frame.frame)
if fwd.enc.TotalLen()+fwd.enc.FrameOverhead()+frameLen > fwd.maxPayload {
return false
}
fwd.enc.AppendFrame(frame)
return true
}
func (fwd *Forwarder) flush() {
err := fwd.udpSender.Send(fwd.enc.Bytes())
if err != nil {
if mtbe, ok := err.(MsgTooBigError); ok {
newUnverifiedPMTU := mtbe.PMTU - fwd.effectiveOverhead()
if newUnverifiedPMTU >= fwd.unverifiedPMTU {
return
}
fwd.pmtuVerified = false
fwd.maxPayload = mtbe.PMTU - UDPOverhead
fwd.highestGoodPMTU = 8
fwd.lowestBadPMTU = newUnverifiedPMTU + 1
fwd.conn.setEffectivePMTU(newUnverifiedPMTU)
fwd.verifyEffectivePMTU(newUnverifiedPMTU)
} else if PosixError(err) == syscall.ENOBUFS {
// TODO handle this better
} else {
fwd.conn.CheckFatal(err)
}
}
}
func (fwd *Forwarder) drain() {
// We want to drain before exiting otherwise we could get the
// packet sniffer or udp listener blocked on sending to a full
// chan
for {
select {
case <-fwd.ch:
default:
return
}
}
}
func (fwd *Forwarder) logDrop(frame *ForwardedFrame) {
fwd.conn.log("Dropping too big frame during forwarding: frame len:", len(frame.frame), "; effective PMTU:", fwd.maxPayload+UDPOverhead-fwd.effectiveOverhead())
}
// UDP Senders
func NewSimpleUDPSender(conn *LocalConnection) *SimpleUDPSender {
return &SimpleUDPSender{udpConn: conn.Router.UDPListener, conn: conn}
}
func (sender *SimpleUDPSender) Send(msg []byte) error {
_, err := sender.udpConn.WriteToUDP(msg, sender.conn.RemoteUDPAddr())
return err
}
func (sender *SimpleUDPSender) Shutdown() error {
return nil
}
func NewRawUDPSender(conn *LocalConnection) (*RawUDPSender, error) {
ipSocket, err := dialIP(conn)
if err != nil {
return nil, err
}
udpHeader := &layers.UDP{SrcPort: layers.UDPPort(Port)}
ipBuf := gopacket.NewSerializeBuffer()
opts := gopacket.SerializeOptions{
FixLengths: true,
// UDP header is calculated with a phantom IP
// header. Yes, it's totally nuts. Thankfully, for UDP
// over IPv4, the checksum is optional. It's not
// optional for IPv6, but we'll ignore that for
// now. TODO
ComputeChecksums: false}
return &RawUDPSender{
ipBuf: ipBuf,
opts: opts,
udpHeader: udpHeader,
socket: ipSocket,
conn: conn}, nil
}
func (sender *RawUDPSender) Send(msg []byte) error {
payload := gopacket.Payload(msg)
sender.udpHeader.DstPort = layers.UDPPort(sender.conn.RemoteUDPAddr().Port)
err := gopacket.SerializeLayers(sender.ipBuf, sender.opts, sender.udpHeader, &payload)
if err != nil {
return err
}
packet := sender.ipBuf.Bytes()
_, err = sender.socket.Write(packet)
if err == nil || PosixError(err) != syscall.EMSGSIZE {
return err
}
f, err := sender.socket.File()
if err != nil {
return err
}
defer f.Close()
fd := int(f.Fd())
log.Println("EMSGSIZE on send, expecting PMTU update (IP packet was",
len(packet), "bytes, payload was", len(msg), "bytes)")
pmtu, err := syscall.GetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_MTU)
if err != nil {
return err
}
return MsgTooBigError{PMTU: pmtu}
}
func (sender *RawUDPSender) Shutdown() error {
defer func() { sender.socket = nil }()
return sender.socket.Close()
}
func dialIP(conn *LocalConnection) (*net.IPConn, error) {
ipLocalAddr, err := ipAddr(conn.TCPConn.LocalAddr())
if err != nil {
return nil, err
}
ipRemoteAddr, err := ipAddr(conn.TCPConn.RemoteAddr())
if err != nil {
return nil, err
}
ipSocket, err := net.DialIP("ip4:UDP", ipLocalAddr, ipRemoteAddr)
if err != nil {
return nil, err
}
f, err := ipSocket.File()
if err != nil {
return nil, err
}
defer f.Close()
fd := int(f.Fd())
// This Makes sure all packets we send out have DF set on them.
err = syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
if err != nil {
return nil, err
}
return ipSocket, nil
}
func ipAddr(addr net.Addr) (*net.IPAddr, error) {
host, _, err := net.SplitHostPort(addr.String())
if err != nil {
return nil, err
}
return &net.IPAddr{
IP: net.ParseIP(host),
Zone: ""}, nil
}