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tailscale/net/packet/ip4.go

145 lines
4.1 KiB
Go

// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package packet
import (
"encoding/binary"
"fmt"
"inet.af/netaddr"
)
// IP4 is an IPv4 address.
type IP4 uint32
// IPFromNetaddr converts a netaddr.IP to an IP4. Panics if !ip.Is4.
func IP4FromNetaddr(ip netaddr.IP) IP4 {
ipbytes := ip.As4()
return IP4(binary.BigEndian.Uint32(ipbytes[:]))
}
// Netaddr converts ip to a netaddr.IP.
func (ip IP4) Netaddr() netaddr.IP {
return netaddr.IPv4(byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip))
}
func (ip IP4) String() string {
return fmt.Sprintf("%d.%d.%d.%d", byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip))
}
// IsMulticast returns whether ip is a multicast address.
func (ip IP4) IsMulticast() bool {
return byte(ip>>24)&0xf0 == 0xe0
}
// IsLinkLocalUnicast returns whether ip is a link-local unicast
// address.
func (ip IP4) IsLinkLocalUnicast() bool {
return byte(ip>>24) == 169 && byte(ip>>16) == 254
}
// IsMostLinkLocalUnicast returns whether ip is a link-local unicast
// address other than the magical "169.254.169.254" address used by
// GCP DNS.
func (ip IP4) IsMostLinkLocalUnicast() bool {
return ip.IsLinkLocalUnicast() && ip != 0xA9FEA9FE
}
// 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
IPID uint16
SrcIP IP4
DstIP IP4
}
// Len implements Header.
func (h IP4Header) Len() int {
return ip4HeaderLength
}
// Marshal implements Header.
func (h IP4Header) Marshal(buf []byte) error {
if len(buf) < h.Len() {
return errSmallBuffer
}
if len(buf) > maxPacketLength {
return errLargePacket
}
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)
binary.BigEndian.PutUint32(buf[12:16], uint32(h.SrcIP)) // Src
binary.BigEndian.PutUint32(buf[16:20], uint32(h.DstIP)) // Dst
binary.BigEndian.PutUint16(buf[10:12], ip4Checksum(buf[0:20])) // Checksum
return nil
}
// ToResponse implements Header.
func (h *IP4Header) ToResponse() {
h.SrcIP, h.DstIP = h.DstIP, h.SrcIP
// 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()
binary.BigEndian.PutUint32(buf[8:12], uint32(h.SrcIP))
binary.BigEndian.PutUint32(buf[12:16], uint32(h.DstIP))
buf[16] = 0x0
buf[17] = uint8(h.IPProto)
binary.BigEndian.PutUint16(buf[18:20], uint16(length))
return nil
}