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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.
// Package pcapgo provides some native PCAP support, not requiring
// C libpcap to be installed.
package pcapgo
import (
"code.google.com/p/gopacket"
"code.google.com/p/gopacket/layers"
"encoding/binary"
"fmt"
"io"
)
// Writer wraps an underlying io.Writer to write packet data in PCAP
// format. See http://wiki.wireshark.org/Development/LibpcapFileFormat
// for information on the file format.
//
// For those that care, we currently write v2.4 files with nanosecond
// timestamp resolution and little-endian encoding.
type Writer struct {
w io.Writer
}
const magicNanoseconds = 0xA1B2C3D4
const versionMajor = 2
const versionMinor = 4
// NewWriter returns a new writer object, for writing packet data out
// to the given writer. If this is a new empty writer (as opposed to
// an append), you must call WriteFileHeader before WritePacket.
//
// // Write a new file:
// f, _ := os.Create("/tmp/file.pcap")
// w := pcapgo.NewWriter(f)
// w.WriteFileHeader(65536, layers.LinkTypeEthernet) // new file, must do this.
// w.WritePacket(gopacket.CaptureInfo{...}, data1)
// f.Close()
// // Append to existing file (must have same snaplen and linktype)
// f2, _ := os.OpenFile("/tmp/file.pcap", os.O_APPEND, 0700)
// w2 := pcapgo.NewWriter(f2)
// // no need for file header, it's already written.
// w2.WritePacket(gopacket.CaptureInfo{...}, data2)
// f2.Close()
func NewWriter(w io.Writer) *Writer {
return &Writer{w: w}
}
// WriteFileHeader writes a file header out to the writer.
// This must be called exactly once per output.
func (w *Writer) WriteFileHeader(snaplen uint32, linktype layers.LinkType) error {
var buf [24]byte
binary.LittleEndian.PutUint32(buf[0:4], magicNanoseconds)
binary.LittleEndian.PutUint16(buf[4:6], versionMajor)
binary.LittleEndian.PutUint16(buf[6:8], versionMinor)
// bytes 8:12 stay 0 (timezone = UTC)
// bytes 12:16 stay 0 (sigfigs is always set to zero, according to
// http://wiki.wireshark.org/Development/LibpcapFileFormat
binary.LittleEndian.PutUint32(buf[16:20], snaplen)
binary.LittleEndian.PutUint32(buf[20:24], uint32(linktype))
_, err := w.w.Write(buf[:])
return err
}
const nanosPerMicro = 1000
const microsPerSecond = 1000000
func (w *Writer) writePacketHeader(ci gopacket.CaptureInfo) error {
var buf [16]byte
micros := ci.Timestamp.UnixNano() / nanosPerMicro
secs, usecs := uint32(micros/microsPerSecond), uint32(micros%microsPerSecond)
binary.LittleEndian.PutUint32(buf[0:4], secs)
binary.LittleEndian.PutUint32(buf[4:8], usecs)
binary.LittleEndian.PutUint32(buf[8:12], uint32(ci.CaptureLength))
binary.LittleEndian.PutUint32(buf[12:16], uint32(ci.Length))
_, err := w.w.Write(buf[:])
return err
}
// WritePacket writes the given packet data out to the file.
func (w *Writer) WritePacket(ci gopacket.CaptureInfo, data []byte) error {
if ci.CaptureLength != len(data) {
return fmt.Errorf("capture length %d does not match data length %d", ci.CaptureLength, len(data))
}
if ci.CaptureLength > ci.Length {
return fmt.Errorf("invalid capture info %+v: capture length > length", ci)
}
if err := w.writePacketHeader(ci); err != nil {
return fmt.Errorf("error writing packet header: %v", err)
}
_, err := w.w.Write(data)
return err
}
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