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net/packet: documentation pass. 

Signed-off-by: David Anderson <danderson@tailscale.com>
This commit is contained in:
David Anderson 2020-11-10 22:26:00 -08:00
parent c2cc3acbaf
commit a38e28da07
6 changed files with 116 additions and 83 deletions
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30
net/packet/icmp4.go
View File

@ -6,6 +6,13 @@ package packet
import "encoding/binary"
// icmp4HeaderLength is the size of the ICMPv4 packet header, not
// including the outer IP layer or the variable "response data"
// trailer.
const icmp4HeaderLength = 4
// ICMP4Type is an ICMPv4 type, as specified in
// https://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml
type ICMP4Type uint8
const (
@ -30,31 +37,29 @@ func (t ICMP4Type) String() string {
}
}
// ICMP4Code is an ICMPv4 code, as specified in
// https://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml
type ICMP4Code uint8
const (
ICMP4NoCode ICMP4Code = 0
)
// ICMP4Header represents an ICMPv4 packet header.
// ICMP4Header is an IPv4+ICMPv4 header.
type ICMP4Header struct {
IP4Header
Type ICMP4Type
Code ICMP4Code
}
const (
icmp4HeaderLength = 4
// icmp4AllHeadersLength is the length of all headers in a ICMPv4 packet.
icmp4AllHeadersLength = ip4HeaderLength + icmp4HeaderLength
)
func (ICMP4Header) Len() int {
return icmp4AllHeadersLength
// Len implements Header.
func (h ICMP4Header) Len() int {
return h.IP4Header.Len() + icmp4HeaderLength
}
// Marshal implements Header.
func (h ICMP4Header) Marshal(buf []byte) error {
if len(buf) < icmp4AllHeadersLength {
if len(buf) < h.Len() {
return errSmallBuffer
}
if len(buf) > maxPacketLength {
@ -68,11 +73,14 @@ func (h ICMP4Header) Marshal(buf []byte) error {
h.IP4Header.Marshal(buf)
binary.BigEndian.PutUint16(buf[22:24], ipChecksum(buf))
binary.BigEndian.PutUint16(buf[22:24], ip4Checksum(buf))
return nil
}
// ToResponse implements Header. TODO: it doesn't implement it
// correctly, instead it statically generates an ICMP Echo Reply
// packet.
func (h *ICMP4Header) ToResponse() {
// TODO: this doesn't implement ToResponse correctly, as it
// assumes the ICMP request type.

11
net/packet/icmp6.go
View File

@ -4,6 +4,13 @@
package packet
// icmp6HeaderLength is the size of the ICMPv6 packet header, not
// including the outer IP layer or the variable "response data"
// trailer.
const icmp6HeaderLength = 4
// ICMP6Type is an ICMPv6 type, as specified in
// https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
type ICMP6Type uint8
const (
@ -28,10 +35,10 @@ func (t ICMP6Type) String() string {
}
}
// ICMP6Code is an ICMPv6 code, as specified in
// https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
type ICMP6Code uint8
const (
ICMP6NoCode ICMP6Code = 0
)
const icmp6HeaderLength = 4

110
net/packet/ip4.go
View File

@ -14,13 +14,13 @@ import (
// IP4 is an IPv4 address.
type IP4 uint32
// IPFromNetaddr converts a netaddr.IP to an IP. Panics if !ip.Is4.
// 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 an IP to a netaddr.IP.
// 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))
}
@ -29,15 +29,21 @@ 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
}
// IPHeader represents an IP packet header.
// 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
@ -45,59 +51,87 @@ type IP4Header struct {
DstIP IP4
}
const ip4HeaderLength = 20
func (IP4Header) Len() int {
// Len implements Header.
func (h IP4Header) Len() int {
return ip4HeaderLength
}
// Marshal implements Header.
func (h IP4Header) Marshal(buf []byte) error {
if len(buf) < ip4HeaderLength {
if len(buf) < h.Len() {
return errSmallBuffer
}
if len(buf) > maxPacketLength {
return errLargePacket
}
buf[0] = 0x40 | (ip4HeaderLength >> 2) // IPv4
buf[1] = 0x00 // DHCP, ECN
binary.BigEndian.PutUint16(buf[2:4], uint16(len(buf)))
binary.BigEndian.PutUint16(buf[4:6], h.IPID)
binary.BigEndian.PutUint16(buf[6:8], 0) // flags, offset
buf[8] = 64 // TTL
buf[9] = uint8(h.IPProto)
binary.BigEndian.PutUint16(buf[10:12], 0) // blank IP header checksum
binary.BigEndian.PutUint32(buf[12:16], uint32(h.SrcIP))
binary.BigEndian.PutUint32(buf[16:20], uint32(h.DstIP))
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], ipChecksum(buf[0:20]))
binary.BigEndian.PutUint16(buf[10:12], ip4Checksum(buf[0:20])) // Checksum
return nil
}
// MarshalPseudo serializes the header into buf in the "pseudo-header"
// form required when calculating UDP checksums. Overwrites the first
// h.Length() bytes of buf.
func (h IP4Header) MarshalPseudo(buf []byte) error {
if len(buf) < ip4HeaderLength {
return errSmallBuffer
}
if len(buf) > maxPacketLength {
return errLargePacket
}
length := len(buf) - ip4HeaderLength
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
}
// 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
}

4
net/packet/ip6.go
View File

@ -10,8 +10,10 @@ import (
"inet.af/netaddr"
)
// IP6 is an IPv6 address.
type IP6 [16]byte
// IP6FromNetaddr converts a netaddr.IP to an IP6. Panics if !ip.Is6.
func IP6FromNetaddr(ip netaddr.IP) IP6 {
if !ip.Is6() {
panic(fmt.Sprintf("IP6FromNetaddr called with non-v6 addr %q", ip))
@ -19,6 +21,7 @@ func IP6FromNetaddr(ip netaddr.IP) IP6 {
return IP6(ip.As16())
}
// Netaddr converts ip to a netaddr.IP.
func (ip IP6) Netaddr() netaddr.IP {
return netaddr.IPFrom16(ip)
}
@ -27,4 +30,5 @@ func (ip IP6) String() string {
return ip.Netaddr().String()
}
// ip6HeaderLength is the length of an IPv6 header with no IP options.
const ip6HeaderLength = 40

21
net/packet/packet.go
View File

@ -22,8 +22,6 @@ const (
)
// Parsed is a minimal decoding of a packet suitable for use in filters.
//
// In general, it only supports IPv4. The IPv6 parsing is very minimal.
type Parsed struct {
// b is the byte buffer that this decodes.
b []byte
@ -100,25 +98,6 @@ func writeIP6Port(sb *strbuilder.Builder, ip IP6, port uint16) {
sb.WriteUint(uint64(port))
}
// based on https://tools.ietf.org/html/rfc1071
func ipChecksum(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)
}
// Decode extracts data from the packet in b into q.
// It performs extremely simple packet decoding for basic IPv4 packet types.
// It extracts only the subprotocol id, IP addresses, and (if any) ports,

23
net/packet/udp4.go
View File

@ -6,23 +6,23 @@ package packet
import "encoding/binary"
// UDPHeader represents an UDP packet header.
// udpHeaderLength is the size of the UDP packet header, not including
// the outer IP header.
const udpHeaderLength = 8
// UDP4Header is an IPv4+UDP header.
type UDP4Header struct {
IP4Header
SrcPort uint16
DstPort uint16
}
const (
// udpHeaderLength is the size of the UDP packet header, not
// including the outer IP header.
udpHeaderLength = 8
)
func (UDP4Header) Len() int {
return ip4HeaderLength + udpHeaderLength
// Len implements Header.
func (h UDP4Header) Len() int {
return h.IP4Header.Len() + udpHeaderLength
}
// Marshal implements Header.
func (h UDP4Header) Marshal(buf []byte) error {
if len(buf) < h.Len() {
return errSmallBuffer
@ -40,14 +40,15 @@ func (h UDP4Header) Marshal(buf []byte) error {
binary.BigEndian.PutUint16(buf[26:28], 0) // blank checksum
// UDP checksum with IP pseudo header.
h.IP4Header.MarshalPseudo(buf)
binary.BigEndian.PutUint16(buf[26:28], ipChecksum(buf[8:]))
h.IP4Header.marshalPseudo(buf)
binary.BigEndian.PutUint16(buf[26:28], ip4Checksum(buf[ip4PseudoHeaderOffset:]))
h.IP4Header.Marshal(buf)
return nil
}
// ToResponse implements Header.
func (h *UDP4Header) ToResponse() {
h.SrcPort, h.DstPort = h.DstPort, h.SrcPort
h.IP4Header.ToResponse()