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7eaf5e509f
Updates #5162 Change-Id: Id7bdec303b25471f69d542f8ce43805328d56c12 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
118 lines
3.2 KiB
Go
118 lines
3.2 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package packet
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import (
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"encoding/binary"
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"errors"
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"tailscale.com/net/netaddr"
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"tailscale.com/types/ipproto"
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)
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// ip4HeaderLength is the length of an IPv4 header with no IP options.
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const ip4HeaderLength = 20
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// IP4Header represents an IPv4 packet header.
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type IP4Header struct {
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IPProto ipproto.Proto
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IPID uint16
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Src netaddr.IP
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Dst netaddr.IP
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}
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// Len implements Header.
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func (h IP4Header) Len() int {
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return ip4HeaderLength
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}
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var errWrongFamily = errors.New("wrong address family for src/dst IP")
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// Marshal implements Header.
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func (h IP4Header) Marshal(buf []byte) error {
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if len(buf) < h.Len() {
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return errSmallBuffer
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}
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if len(buf) > maxPacketLength {
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return errLargePacket
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}
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if !h.Src.Is4() || !h.Dst.Is4() {
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return errWrongFamily
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}
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buf[0] = 0x40 | (byte(h.Len() >> 2)) // IPv4 + IHL
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buf[1] = 0x00 // DSCP + ECN
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binary.BigEndian.PutUint16(buf[2:4], uint16(len(buf))) // Total length
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binary.BigEndian.PutUint16(buf[4:6], h.IPID) // ID
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binary.BigEndian.PutUint16(buf[6:8], 0) // Flags + fragment offset
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buf[8] = 64 // TTL
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buf[9] = uint8(h.IPProto) // Inner protocol
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// Blank checksum. This is necessary even though we overwrite
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// it later, because the checksum computation runs over these
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// bytes and expects them to be zero.
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binary.BigEndian.PutUint16(buf[10:12], 0)
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src := h.Src.As4()
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dst := h.Dst.As4()
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copy(buf[12:16], src[:])
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copy(buf[16:20], dst[:])
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binary.BigEndian.PutUint16(buf[10:12], ip4Checksum(buf[0:20])) // Checksum
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return nil
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}
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// ToResponse implements Header.
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func (h *IP4Header) ToResponse() {
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h.Src, h.Dst = h.Dst, h.Src
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// Flip the bits in the IPID. If incoming IPIDs are distinct, so are these.
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h.IPID = ^h.IPID
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}
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// ip4Checksum computes an IPv4 checksum, as specified in
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// https://tools.ietf.org/html/rfc1071
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func ip4Checksum(b []byte) uint16 {
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var ac uint32
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i := 0
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n := len(b)
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for n >= 2 {
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ac += uint32(binary.BigEndian.Uint16(b[i : i+2]))
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n -= 2
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i += 2
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}
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if n == 1 {
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ac += uint32(b[i]) << 8
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}
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for (ac >> 16) > 0 {
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ac = (ac >> 16) + (ac & 0xffff)
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}
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return uint16(^ac)
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}
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// ip4PseudoHeaderOffset is the number of bytes by which the IPv4 UDP
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// pseudo-header is smaller than the real IPv4 header.
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const ip4PseudoHeaderOffset = 8
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// marshalPseudo serializes h into buf in the "pseudo-header" form
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// required when calculating UDP checksums. The pseudo-header starts
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// at buf[ip4PseudoHeaderOffset] so as to abut the following UDP
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// header, while leaving enough space in buf for a full IPv4 header.
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func (h IP4Header) marshalPseudo(buf []byte) error {
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if len(buf) < h.Len() {
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return errSmallBuffer
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}
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if len(buf) > maxPacketLength {
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return errLargePacket
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}
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length := len(buf) - h.Len()
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src, dst := h.Src.As4(), h.Dst.As4()
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copy(buf[8:12], src[:])
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copy(buf[12:16], dst[:])
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buf[16] = 0x0
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buf[17] = uint8(h.IPProto)
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binary.BigEndian.PutUint16(buf[18:20], uint16(length))
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return nil
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}
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