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// Copyright 2012 Google, Inc. All rights reserved.
// Copyright 2009-2011 Andreas Krennmair. 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 layers
import (
"errors"
"fmt"
"github.com/google/gopacket"
)
// Checksum computation for TCP/UDP.
type tcpipchecksum struct {
pseudoheader tcpipPseudoHeader
}
type tcpipPseudoHeader interface {
pseudoheaderChecksum() (uint32, error)
}
func (ip *IPv4) pseudoheaderChecksum() (csum uint32, err error) {
if err := ip.AddressTo4(); err != nil {
return 0, err
}
csum += (uint32(ip.SrcIP[0]) + uint32(ip.SrcIP[2])) << 8
csum += uint32(ip.SrcIP[1]) + uint32(ip.SrcIP[3])
csum += (uint32(ip.DstIP[0]) + uint32(ip.DstIP[2])) << 8
csum += uint32(ip.DstIP[1]) + uint32(ip.DstIP[3])
return csum, nil
}
func (ip *IPv6) pseudoheaderChecksum() (csum uint32, err error) {
if err := ip.AddressTo16(); err != nil {
return 0, err
}
for i := 0; i < 16; i += 2 {
csum += uint32(ip.SrcIP[i]) << 8
csum += uint32(ip.SrcIP[i+1])
csum += uint32(ip.DstIP[i]) << 8
csum += uint32(ip.DstIP[i+1])
}
return csum, nil
}
// Calculate the TCP/IP checksum defined in rfc1071. The passed-in csum is any
// initial checksum data that's already been computed.
func tcpipChecksum(data []byte, csum uint32) uint16 {
// to handle odd lengths, we loop to length - 1, incrementing by 2, then
// handle the last byte specifically by checking against the original
// length.
length := len(data) - 1
for i := 0; i < length; i += 2 {
// For our test packet, doing this manually is about 25% faster
// (740 ns vs. 1000ns) than doing it by calling binary.BigEndian.Uint16.
csum += uint32(data[i]) << 8
csum += uint32(data[i+1])
}
if len(data)%2 == 1 {
csum += uint32(data[length]) << 8
}
for csum > 0xffff {
csum = (csum >> 16) + (csum & 0xffff)
}
return ^uint16(csum)
}
// computeChecksum computes a TCP or UDP checksum. headerAndPayload is the
// serialized TCP or UDP header plus its payload, with the checksum zero'd
// out. headerProtocol is the IP protocol number of the upper-layer header.
func (c *tcpipchecksum) computeChecksum(headerAndPayload []byte, headerProtocol IPProtocol) (uint16, error) {
if c.pseudoheader == nil {
return 0, errors.New("TCP/IP layer 4 checksum cannot be computed without network layer... call SetNetworkLayerForChecksum to set which layer to use")
}
length := uint32(len(headerAndPayload))
csum, err := c.pseudoheader.pseudoheaderChecksum()
if err != nil {
return 0, err
}
csum += uint32(headerProtocol)
csum += length & 0xffff
csum += length >> 16
return tcpipChecksum(headerAndPayload, csum), nil
}
// SetNetworkLayerForChecksum tells this layer which network layer is wrapping it.
// This is needed for computing the checksum when serializing, since TCP/IP transport
// layer checksums depends on fields in the IPv4 or IPv6 layer that contains it.
// The passed in layer must be an *IPv4 or *IPv6.
func (i *tcpipchecksum) SetNetworkLayerForChecksum(l gopacket.NetworkLayer) error {
switch v := l.(type) {
case *IPv4:
i.pseudoheader = v
case *IPv6:
i.pseudoheader = v
default:
return fmt.Errorf("cannot use layer type %v for tcp checksum network layer", l.LayerType())
}
return nil
}
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