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// Copyright 2012 Google, Inc. All rights reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the LICENSE file in the root of the source
// tree.
// This binary provides an example of connecting up bidirectional streams from
// the unidirectional streams provided by gopacket/tcpassembly.
package main
import (
"flag"
"fmt"
"github.com/google/gopacket"
"github.com/google/gopacket/examples/util"
"github.com/google/gopacket/layers"
"github.com/google/gopacket/pcap"
"github.com/google/gopacket/tcpassembly"
"log"
"time"
)
var iface = flag.String("i", "eth0", "Interface to get packets from")
var snaplen = flag.Int("s", 16<<10, "SnapLen for pcap packet capture")
var filter = flag.String("f", "tcp", "BPF filter for pcap")
var logAllPackets = flag.Bool("v", false, "Logs every packet in great detail")
// key is used to map bidirectional streams to each other.
type key struct {
net, transport gopacket.Flow
}
// String prints out the key in a human-readable fashion.
func (k key) String() string {
return fmt.Sprintf("%v:%v", k.net, k.transport)
}
// timeout is the length of time to wait befor flushing connections and
// bidirectional stream pairs.
const timeout time.Duration = time.Minute * 5
// myStream implements tcpassembly.Stream
type myStream struct {
bytes int64 // total bytes seen on this stream.
bidi *bidi // maps to my bidirectional twin.
done bool // if true, we've seen the last packet we're going to for this stream.
}
// bidi stores each unidirectional side of a bidirectional stream.
//
// When a new stream comes in, if we don't have an opposite stream, a bidi is
// created with 'a' set to the new stream. If we DO have an opposite stream,
// 'b' is set to the new stream.
type bidi struct {
key key // Key of the first stream, mostly for logging.
a, b *myStream // the two bidirectional streams.
lastPacketSeen time.Time // last time we saw a packet from either stream.
}
// myFactory implements tcpassmebly.StreamFactory
type myFactory struct {
// bidiMap maps keys to bidirectional stream pairs.
bidiMap map[key]*bidi
}
// New handles creating a new tcpassembly.Stream.
func (f *myFactory) New(netFlow, tcpFlow gopacket.Flow) tcpassembly.Stream {
// Create a new stream.
s := &myStream{}
// Find the bidi bidirectional struct for this stream, creating a new one if
// one doesn't already exist in the map.
k := key{netFlow, tcpFlow}
bd := f.bidiMap[k]
if bd == nil {
bd = &bidi{a: s, key: k}
log.Printf("[%v] created first side of bidirectional stream", bd.key)
// Register bidirectional with the reverse key, so the matching stream going
// the other direction will find it.
f.bidiMap[key{netFlow.Reverse(), tcpFlow.Reverse()}] = bd
} else {
log.Printf("[%v] found second side of bidirectional stream", bd.key)
bd.b = s
// Clear out the bidi we're using from the map, just in case.
delete(f.bidiMap, k)
}
s.bidi = bd
return s
}
// emptyStream is used to finish bidi that only have one stream, in
// collectOldStreams.
var emptyStream = &myStream{done: true}
// collectOldStreams finds any streams that haven't received a packet within
// 'timeout', and sets/finishes the 'b' stream inside them. The 'a' stream may
// still receive packets after this.
func (f *myFactory) collectOldStreams() {
cutoff := time.Now().Add(-timeout)
for k, bd := range f.bidiMap {
if bd.lastPacketSeen.Before(cutoff) {
log.Printf("[%v] timing out old stream", bd.key)
bd.b = emptyStream // stub out b with an empty stream.
delete(f.bidiMap, k) // remove it from our map.
bd.maybeFinish() // if b was the last stream we were waiting for, finish up.
}
}
}
// Reassembled handles reassembled TCP stream data.
func (s *myStream) Reassembled(rs []tcpassembly.Reassembly) {
for _, r := range rs {
// For now, we'll simply count the bytes on each side of the TCP stream.
s.bytes += int64(len(r.Bytes))
if r.Skip > 0 {
s.bytes += int64(r.Skip)
}
// Mark that we've received new packet data.
// We could just use time.Now, but by using r.Seen we handle the case
// where packets are being read from a file and could be very old.
if s.bidi.lastPacketSeen.After(r.Seen) {
s.bidi.lastPacketSeen = r.Seen
}
}
}
// ReassemblyComplete marks this stream as finished.
func (s *myStream) ReassemblyComplete() {
s.done = true
s.bidi.maybeFinish()
}
// maybeFinish will wait until both directions are complete, then print out
// stats.
func (bd *bidi) maybeFinish() {
switch {
case bd.a == nil:
log.Fatalf("[%v] a should always be non-nil, since it's set when bidis are created", bd.key)
case !bd.a.done:
log.Printf("[%v] still waiting on first stream", bd.key)
case bd.b == nil:
log.Printf("[%v] no second stream yet", bd.key)
case !bd.b.done:
log.Printf("[%v] still waiting on second stream", bd.key)
default:
log.Printf("[%v] FINISHED, bytes: %d tx, %d rx", bd.key, bd.a.bytes, bd.b.bytes)
}
}
func main() {
defer util.Run()()
log.Printf("starting capture on interface %q", *iface)
// Set up pcap packet capture
handle, err := pcap.OpenLive(*iface, int32(*snaplen), true, pcap.BlockForever)
if err != nil {
panic(err)
}
if err := handle.SetBPFFilter(*filter); err != nil {
panic(err)
}
// Set up assembly
streamFactory := &myFactory{bidiMap: make(map[key]*bidi)}
streamPool := tcpassembly.NewStreamPool(streamFactory)
assembler := tcpassembly.NewAssembler(streamPool)
log.Println("reading in packets")
// Read in packets, pass to assembler.
packetSource := gopacket.NewPacketSource(handle, handle.LinkType())
packets := packetSource.Packets()
ticker := time.Tick(timeout / 4)
for {
select {
case packet := <-packets:
if *logAllPackets {
log.Println(packet)
}
if packet.NetworkLayer() == nil || packet.TransportLayer() == nil || packet.TransportLayer().LayerType() != layers.LayerTypeTCP {
log.Println("Unusable packet")
continue
}
tcp := packet.TransportLayer().(*layers.TCP)
assembler.AssembleWithTimestamp(packet.NetworkLayer().NetworkFlow(), tcp, packet.Metadata().Timestamp)
case <-ticker:
// Every minute, flush connections that haven't seen activity in the past minute.
log.Println("---- FLUSHING ----")
assembler.FlushOlderThan(time.Now().Add(-timeout))
streamFactory.collectOldStreams()
}
}
}
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