Wraps your net.Conn and saves data in it to pcap

LICENSE

MIT License

Copyright (c) 2025 Ilya Lukyanov

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

pcap_netconn_logger.go

var (
	pcapWriter *pcapgo.Writer
	tcpPort    uint16 = 1023
)

func init() {
	f, err := os.Create("/tmp/test.pcap")
	if err != nil {
		panic(err)
	}
	pcapWriter = pcapgo.NewWriter(f)
	pcapWriter.WriteFileHeader(65536, layers.LinkTypeIPv4)
}

type pcapLoggingNetConn struct {
	net.Conn

	tcpPort uint16
	sentSrc uint32
	sentDst uint32
}

func newPcapLoggingNetConn(conn net.Conn) net.Conn {
	// that's not safe, huh
	tcpPort++

	p := &pcapLoggingNetConn{Conn: conn, tcpPort: tcpPort, sentSrc: 1, sentDst: 1}

	writePacket(
		p.getIP(false),
		p.getTCP(false, true, false, false, 0, 0),
	)
	writePacket(
		p.getIP(true),
		p.getTCP(true, true, true, false, 0, 1),
	)
	writePacket(
		p.getIP(false),
		p.getTCP(false, false, true, false, 1, 1),
	)
	return p
}

func (c *pcapLoggingNetConn) Read(b []byte) (int, error) {
	n, err := c.Conn.Read(b)
	if err != nil {
		return n, err
	}
	if n > 0 {
		c.writePacket(b[:n], false)
	}
	return n, nil
}

func (c *pcapLoggingNetConn) Write(b []byte) (int, error) {
	n, err := c.Conn.Write(b)
	if err != nil {
		return n, err
	}
	if n > 0 {
		c.writePacket(b[:n], true)
	}
	return n, nil
}

func (c *pcapLoggingNetConn) writePacket(b []byte, isWrite bool) {
	seq, ack := &c.sentSrc, &c.sentDst
	if isWrite {
		seq, ack = &c.sentDst, &c.sentSrc
	}
  glog.V(10).Infof("writing packet: (port=%d,len=%d,wr=%v,seq=%v,ack=%v) %v", c.tcpPort, len(b), isWrite, *seq, *ack, b)

	writePacket(
		c.getIP(isWrite),
		c.getTCP(isWrite, false, true, true, *seq, *ack),
		gopacket.Payload(b),
	)
	*seq += uint32(len(b))
	writePacket(
		c.getIP(!isWrite),
		c.getTCP(!isWrite, false, true, false, *ack, *seq),
	)
}

func writePacket(layers ...gopacket.SerializableLayer) {
	buf := gopacket.NewSerializeBuffer()
	opts := gopacket.SerializeOptions{FixLengths: true}
	err := gopacket.SerializeLayers(buf, opts, layers...)
	if err != nil {
		glog.V(10).Infof("error serializing packet: %v", err)
	}
	ci := gopacket.CaptureInfo{
		Timestamp:      time.Now(),
		CaptureLength:  len(buf.Bytes()),
		Length:         len(buf.Bytes()),
		InterfaceIndex: 0,
	}
	err = pcapWriter.WritePacket(ci, buf.Bytes())
	if err != nil {
		glog.V(10).Infof("error writing packet: %v", err)
	}
}

func (c *pcapLoggingNetConn) getIP(rev bool) gopacket.SerializableLayer {
	res := &layers.IPv4{
		Version:  4,
		TTL:      64,
		SrcIP:    net.IP{127, 0, 0, byte(c.tcpPort % 256)},
		DstIP:    net.IP{32, 0, 0, byte(c.tcpPort % 256)},
		Protocol: layers.IPProtocolTCP,
	}
	if rev {
		res.SrcIP, res.DstIP = res.DstIP, res.SrcIP
	}
	return res
}

func (c *pcapLoggingNetConn) getTCP(rev, SYN, ACK, PSH bool, seq, ack uint32) *layers.TCP {
	src, dst := layers.TCPPort(c.tcpPort), layers.TCPPort(80)
	if rev {
		src, dst = dst, src
	}
	return &layers.TCP{
		Window:  65535,
		SrcPort: src,
		DstPort: dst,
		SYN:     SYN,
		PSH:     PSH,
		ACK:     ACK,
		Seq:     seq,
		Ack:     ack,
	}
}