1 // Copyright 2012 Google, Inc. All rights reserved.
3 // Use of this source code is governed by a BSD-style license
4 // that can be found in the LICENSE file in the root of the source
7 // Package pcapgo provides some native PCAP support, not requiring
8 // C libpcap to be installed.
17 "github.com/google/gopacket"
18 "github.com/google/gopacket/layers"
21 // Writer wraps an underlying io.Writer to write packet data in PCAP
22 // format. See http://wiki.wireshark.org/Development/LibpcapFileFormat
23 // for information on the file format.
25 // For those that care, we currently write v2.4 files with nanosecond
26 // timestamp resolution and little-endian encoding.
31 const magicMicroseconds = 0xA1B2C3D4
32 const versionMajor = 2
33 const versionMinor = 4
35 // NewWriter returns a new writer object, for writing packet data out
36 // to the given writer. If this is a new empty writer (as opposed to
37 // an append), you must call WriteFileHeader before WritePacket.
39 // // Write a new file:
40 // f, _ := os.Create("/tmp/file.pcap")
41 // w := pcapgo.NewWriter(f)
42 // w.WriteFileHeader(65536, layers.LinkTypeEthernet) // new file, must do this.
43 // w.WritePacket(gopacket.CaptureInfo{...}, data1)
45 // // Append to existing file (must have same snaplen and linktype)
46 // f2, _ := os.OpenFile("/tmp/file.pcap", os.O_APPEND, 0700)
47 // w2 := pcapgo.NewWriter(f2)
48 // // no need for file header, it's already written.
49 // w2.WritePacket(gopacket.CaptureInfo{...}, data2)
51 func NewWriter(w io.Writer) *Writer {
55 // WriteFileHeader writes a file header out to the writer.
56 // This must be called exactly once per output.
57 func (w *Writer) WriteFileHeader(snaplen uint32, linktype layers.LinkType) error {
59 binary.LittleEndian.PutUint32(buf[0:4], magicMicroseconds)
60 binary.LittleEndian.PutUint16(buf[4:6], versionMajor)
61 binary.LittleEndian.PutUint16(buf[6:8], versionMinor)
62 // bytes 8:12 stay 0 (timezone = UTC)
63 // bytes 12:16 stay 0 (sigfigs is always set to zero, according to
64 // http://wiki.wireshark.org/Development/LibpcapFileFormat
65 binary.LittleEndian.PutUint32(buf[16:20], snaplen)
66 binary.LittleEndian.PutUint32(buf[20:24], uint32(linktype))
67 _, err := w.w.Write(buf[:])
71 const nanosPerMicro = 1000
73 func (w *Writer) writePacketHeader(ci gopacket.CaptureInfo) error {
81 usecs := t.Nanosecond() / nanosPerMicro
82 binary.LittleEndian.PutUint32(buf[0:4], uint32(secs))
83 binary.LittleEndian.PutUint32(buf[4:8], uint32(usecs))
84 binary.LittleEndian.PutUint32(buf[8:12], uint32(ci.CaptureLength))
85 binary.LittleEndian.PutUint32(buf[12:16], uint32(ci.Length))
86 _, err := w.w.Write(buf[:])
90 // WritePacket writes the given packet data out to the file.
91 func (w *Writer) WritePacket(ci gopacket.CaptureInfo, data []byte) error {
92 if ci.CaptureLength != len(data) {
93 return fmt.Errorf("capture length %d does not match data length %d", ci.CaptureLength, len(data))
95 if ci.CaptureLength > ci.Length {
96 return fmt.Errorf("invalid capture info %+v: capture length > length", ci)
98 if err := w.writePacketHeader(ci); err != nil {
99 return fmt.Errorf("error writing packet header: %v", err)
101 _, err := w.w.Write(data)