tailscale/net/packet/ip4.go

117 lines
3.1 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package packet
import (
"encoding/binary"
"errors"
"net/netip"
"tailscale.com/types/ipproto"
)
// ip4HeaderLength is the length of an IPv4 header with no IP options.
const ip4HeaderLength = 20
// IP4Header represents an IPv4 packet header.
type IP4Header struct {
IPProto ipproto.Proto
IPID uint16
Src netip.Addr
Dst netip.Addr
}
// Len implements Header.
func (h IP4Header) Len() int {
return ip4HeaderLength
}
var errWrongFamily = errors.New("wrong address family for src/dst IP")
// Marshal implements Header.
func (h IP4Header) Marshal(buf []byte) error {
if len(buf) < h.Len() {
return errSmallBuffer
}
if len(buf) > maxPacketLength {
return errLargePacket
}
if !h.Src.Is4() || !h.Dst.Is4() {
return errWrongFamily
}
buf[0] = 0x40 | (byte(h.Len() >> 2)) // IPv4 + IHL
buf[1] = 0x00 // DSCP + ECN
binary.BigEndian.PutUint16(buf[2:4], uint16(len(buf))) // Total length
binary.BigEndian.PutUint16(buf[4:6], h.IPID) // ID
binary.BigEndian.PutUint16(buf[6:8], 0) // Flags + fragment offset
buf[8] = 64 // TTL
buf[9] = uint8(h.IPProto) // Inner protocol
// Blank checksum. This is necessary even though we overwrite
// it later, because the checksum computation runs over these
// bytes and expects them to be zero.
binary.BigEndian.PutUint16(buf[10:12], 0)
src := h.Src.As4()
dst := h.Dst.As4()
copy(buf[12:16], src[:])
copy(buf[16:20], dst[:])
binary.BigEndian.PutUint16(buf[10:12], ip4Checksum(buf[0:20])) // Checksum
return nil
}
// ToResponse implements Header.
func (h *IP4Header) ToResponse() {
h.Src, h.Dst = h.Dst, h.Src
// Flip the bits in the IPID. If incoming IPIDs are distinct, so are these.
h.IPID = ^h.IPID
}
// ip4Checksum computes an IPv4 checksum, as specified in
// https://tools.ietf.org/html/rfc1071
func ip4Checksum(b []byte) uint16 {
var ac uint32
i := 0
n := len(b)
for n >= 2 {
ac += uint32(binary.BigEndian.Uint16(b[i : i+2]))
n -= 2
i += 2
}
if n == 1 {
ac += uint32(b[i]) << 8
}
for (ac >> 16) > 0 {
ac = (ac >> 16) + (ac & 0xffff)
}
return uint16(^ac)
}
// ip4PseudoHeaderOffset is the number of bytes by which the IPv4 UDP
// pseudo-header is smaller than the real IPv4 header.
const ip4PseudoHeaderOffset = 8
// marshalPseudo serializes h into buf in the "pseudo-header" form
// required when calculating UDP checksums. The pseudo-header starts
// at buf[ip4PseudoHeaderOffset] so as to abut the following UDP
// header, while leaving enough space in buf for a full IPv4 header.
func (h IP4Header) marshalPseudo(buf []byte) error {
if len(buf) < h.Len() {
return errSmallBuffer
}
if len(buf) > maxPacketLength {
return errLargePacket
}
length := len(buf) - h.Len()
src, dst := h.Src.As4(), h.Dst.As4()
copy(buf[8:12], src[:])
copy(buf[12:16], dst[:])
buf[16] = 0x0
buf[17] = uint8(h.IPProto)
binary.BigEndian.PutUint16(buf[18:20], uint16(length))
return nil
}