382 lines
12 KiB
Go
382 lines
12 KiB
Go
// Package inet recalculates length fields and checksums of IPv4/IPv6 packets
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// and their transport headers (TCP, UDP, ICMP, ICMPv6, SCTP).
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package inet
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import (
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"encoding/binary"
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"errors"
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"fmt"
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"hash/crc32"
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)
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// IP protocol numbers.
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const (
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protoHopByHop = 0
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protoICMP = 1
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protoTCP = 6
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protoUDP = 17
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protoRouting = 43
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protoFragment = 44
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protoAH = 51
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protoICMPv6 = 58
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protoDstOpts = 60
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protoSCTP = 132
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)
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// Change records a field rewritten by Fix.
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type Change struct {
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Field string
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Old string
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New string
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}
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// Result describes what Fix did.
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type Result struct {
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Changes []Change // fields that were rewritten (including ones whose value did not change)
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Notes []string // informational messages, e.g. why a checksum was skipped
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}
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func (r *Result) change16(field string, b []byte, v uint16) {
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old := binary.BigEndian.Uint16(b)
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binary.BigEndian.PutUint16(b, v)
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r.Changes = append(r.Changes, Change{field, fmt.Sprintf("0x%04x (%d)", old, old), fmt.Sprintf("0x%04x (%d)", v, v)})
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}
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func (r *Result) notef(format string, args ...any) {
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r.Notes = append(r.Notes, fmt.Sprintf(format, args...))
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}
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// Fix rewrites, in place, the length fields and checksums of the IPv4 or IPv6
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// packet in pkt. pkt must start at the IP header and extend to the end of the
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// packet; everything after the IP header is taken as the IP payload.
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//
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// hdrLen is the number of leading bytes of pkt that are headers, i.e. where
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// the user's header input ends and the payload begins, or 0 if unknown. When
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// known it is used to set the TCP data offset and the length of the SCTP
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// chunk that the payload belongs to; when unknown those fields are left alone.
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// The IPv4 IHL and IPv6 extension header lengths are always taken as given.
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func Fix(pkt []byte, hdrLen int) (Result, error) {
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if len(pkt) == 0 {
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return Result{}, errors.New("no IP header data")
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}
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switch v := pkt[0] >> 4; v {
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case 4:
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return fixIPv4(pkt, hdrLen)
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case 6:
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return fixIPv6(pkt, hdrLen)
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default:
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return Result{}, fmt.Errorf("not an IPv4/IPv6 packet: IP version nibble is %d", v)
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}
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}
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func fixIPv4(pkt []byte, hdrLen int) (Result, error) {
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var r Result
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if len(pkt) < 20 {
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return r, fmt.Errorf("IPv4 header truncated: %d bytes, need at least 20", len(pkt))
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}
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ihl := int(pkt[0]&0x0f) * 4
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if ihl < 20 {
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return r, fmt.Errorf("invalid IPv4 header length (IHL) %d bytes", ihl)
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}
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if ihl > len(pkt) {
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return r, fmt.Errorf("IPv4 header length (IHL) %d exceeds packet length %d", ihl, len(pkt))
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}
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if len(pkt) > 0xffff {
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return r, fmt.Errorf("IPv4 packet length %d exceeds 65535", len(pkt))
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}
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if off := badIPv4Option(pkt[20:ihl]); off >= 0 {
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r.notef("IPv4 option at offset %d does not fit in the header length (IHL %d bytes); check IHL", 20+off, ihl)
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}
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r.change16("IPv4 total length", pkt[2:4], uint16(len(pkt)))
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binary.BigEndian.PutUint16(pkt[10:12], 0)
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r.change16("IPv4 header checksum", pkt[10:12], checksum(pkt[:ihl], 0))
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flagsFrag := binary.BigEndian.Uint16(pkt[6:8])
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if flagsFrag&0x2000 != 0 || flagsFrag&0x1fff != 0 {
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r.notef("IPv4 packet is a fragment; transport header not changed")
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return r, nil
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}
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proto := pkt[9]
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payload := pkt[ihl:]
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pseudo := sum(pkt[12:16], 0)
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pseudo = sum(pkt[16:20], pseudo)
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pseudo += uint32(proto) + uint32(len(payload))
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fixTransport(&r, proto, payload, transportHdrLen(&r, hdrLen, ihl), pseudo, false)
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return r, nil
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}
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// badIPv4Option walks IPv4 options and returns the offset of the first option
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// that runs past the end of opts, or -1 if they are well formed.
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func badIPv4Option(opts []byte) int {
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for i := 0; i < len(opts); {
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switch opts[i] {
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case 0: // end of option list, the rest is padding
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return -1
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case 1: // no-op
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i++
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continue
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}
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if i+1 >= len(opts) || opts[i+1] < 2 || i+int(opts[i+1]) > len(opts) {
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return i
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}
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i += int(opts[i+1])
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}
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return -1
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}
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func fixIPv6(pkt []byte, hdrLen int) (Result, error) {
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var r Result
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if len(pkt) < 40 {
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return r, fmt.Errorf("IPv6 header truncated: %d bytes, need 40", len(pkt))
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}
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if len(pkt)-40 > 0xffff {
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return r, fmt.Errorf("IPv6 payload length %d exceeds 65535 (jumbograms not supported)", len(pkt)-40)
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}
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r.change16("IPv6 payload length", pkt[4:6], uint16(len(pkt)-40))
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dst := pkt[24:40]
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next := pkt[6]
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off := 40
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for isIPv6ExtHeader(next) {
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if off+8 > len(pkt) {
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r.notef("IPv6 extension header truncated; transport header not changed")
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return r, nil
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}
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var hlen int
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switch next {
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case protoFragment:
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hlen = 8
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fragOffMore := binary.BigEndian.Uint16(pkt[off+2 : off+4])
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if fragOffMore&0xfff9 != 0 { // fragment offset != 0 or M flag set
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r.notef("IPv6 packet is a fragment; transport header not changed")
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return r, nil
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}
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case protoAH:
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hlen = (int(pkt[off+1]) + 2) * 4
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default:
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hlen = (int(pkt[off+1]) + 1) * 8
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}
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if off+hlen > len(pkt) {
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r.notef("IPv6 extension header truncated; transport header not changed")
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return r, nil
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}
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if next == protoRouting {
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if d := routingFinalDst(pkt[off : off+hlen]); d != nil {
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dst = d
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}
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}
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next = pkt[off]
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off += hlen
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}
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payload := pkt[off:]
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pseudo := sum(pkt[8:24], 0)
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pseudo = sum(dst, pseudo)
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pseudo += uint32(len(payload)) + uint32(next)
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fixTransport(&r, next, payload, transportHdrLen(&r, hdrLen, off), pseudo, true)
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return r, nil
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}
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// transportHdrLen converts the header length given to Fix into the number of
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// transport-layer header bytes, or -1 if unknown.
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func transportHdrLen(r *Result, hdrLen, ipHdrLen int) int {
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if hdrLen <= 0 {
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return -1
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}
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if hdrLen < ipHdrLen {
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r.notef("IP header (%d bytes including options/extension headers) extends past the layer 3 field (%d bytes)", ipHdrLen, hdrLen)
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return 0
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}
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return hdrLen - ipHdrLen
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}
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func isIPv6ExtHeader(proto byte) bool {
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switch proto {
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case protoHopByHop, protoRouting, protoFragment, protoAH, protoDstOpts:
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return true
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}
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return false
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}
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// routingFinalDst returns the final destination address carried in an IPv6
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// routing header, which the upper-layer pseudo header must use (RFC 8200 8.1).
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// It returns nil when the IPv6 destination address field should be used.
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func routingFinalDst(rh []byte) []byte {
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routingType, segmentsLeft := rh[2], rh[3]
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if segmentsLeft == 0 {
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return nil
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}
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switch routingType {
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case 0: // deprecated source route: last address in the list
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n := int(rh[1]) / 2
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if n > 0 && 8+n*16 <= len(rh) {
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return rh[8+(n-1)*16 : 8+n*16]
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}
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case 2, 4: // Mobile IPv6 home address / Segment Routing: first address in the list
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if len(rh) >= 24 {
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return rh[8:24]
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}
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}
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return nil
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}
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// fixTransport fixes the transport header at the start of payload. hdrLen is
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// the number of transport header bytes the user entered before the payload,
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// or -1 if unknown.
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func fixTransport(r *Result, proto byte, payload []byte, hdrLen int, pseudo uint32, ipv6 bool) {
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switch {
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case proto == protoTCP:
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if len(payload) < 20 {
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r.notef("TCP header truncated (%d bytes); TCP checksum not recalculated", len(payload))
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return
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}
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fixTCPDataOffset(r, payload, hdrLen)
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binary.BigEndian.PutUint16(payload[16:18], 0)
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r.change16("TCP checksum", payload[16:18], checksum(payload, pseudo))
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case proto == protoUDP:
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if len(payload) < 8 {
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r.notef("UDP header truncated (%d bytes); UDP length and checksum not recalculated", len(payload))
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return
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}
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r.change16("UDP length", payload[4:6], uint16(len(payload)))
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binary.BigEndian.PutUint16(payload[6:8], 0)
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cs := checksum(payload, pseudo)
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if cs == 0 {
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cs = 0xffff // zero means "no checksum" in UDP
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}
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r.change16("UDP checksum", payload[6:8], cs)
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case proto == protoICMP && !ipv6:
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if len(payload) < 4 {
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r.notef("ICMP header truncated (%d bytes); ICMP checksum not recalculated", len(payload))
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return
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}
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binary.BigEndian.PutUint16(payload[2:4], 0)
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r.change16("ICMP checksum", payload[2:4], checksum(payload, 0))
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case proto == protoICMPv6 && ipv6:
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if len(payload) < 4 {
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r.notef("ICMPv6 header truncated (%d bytes); ICMPv6 checksum not recalculated", len(payload))
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return
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}
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binary.BigEndian.PutUint16(payload[2:4], 0)
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r.change16("ICMPv6 checksum", payload[2:4], checksum(payload, pseudo))
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case proto == protoSCTP:
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if len(payload) < 12 {
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r.notef("SCTP common header truncated (%d bytes); SCTP checksum not recalculated", len(payload))
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return
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}
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fixSCTPChunkLength(r, payload, hdrLen)
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old := binary.BigEndian.Uint32(payload[8:12])
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binary.BigEndian.PutUint32(payload[8:12], 0)
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// CRC32c is transmitted least significant byte first (RFC 9260 appendix A).
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binary.LittleEndian.PutUint32(payload[8:12], crc32.Checksum(payload, castagnoli))
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v := binary.BigEndian.Uint32(payload[8:12])
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r.Changes = append(r.Changes, Change{"SCTP checksum (CRC32c)", fmt.Sprintf("0x%08x", old), fmt.Sprintf("0x%08x", v)})
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default:
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r.notef("IP protocol %d: no transport checksum to recalculate", proto)
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}
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}
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// fixTCPDataOffset sets the TCP data offset to the TCP header length (with
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// options) entered in the layer 3 field.
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func fixTCPDataOffset(r *Result, tcp []byte, hdrLen int) {
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switch {
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case hdrLen < 0:
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return
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case hdrLen == 0:
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r.notef("TCP header is in the payload field; TCP data offset not changed")
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return
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case hdrLen < 20 || hdrLen > 60 || hdrLen%4 != 0:
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r.notef("TCP header in the layer 3 field is %d bytes (must be 20-60 and a multiple of 4); TCP data offset not changed", hdrLen)
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return
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}
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old := int(tcp[12] >> 4)
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tcp[12] = byte(hdrLen/4)<<4 | tcp[12]&0x0f
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r.Changes = append(r.Changes, Change{"TCP data offset",
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fmt.Sprintf("%d (%d bytes)", old, old*4), fmt.Sprintf("%d (%d bytes)", hdrLen/4, hdrLen)})
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}
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// fixSCTPChunkLength sets the length of the SCTP chunk that the payload
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// belongs to: the last chunk whose header starts in the layer 3 field. Chunks
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// before it are skipped using their own length fields. The chunk is taken to
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// extend to the end of the packet, so the payload must not include padding
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// unless the chunk length is already correct.
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func fixSCTPChunkLength(r *Result, sctp []byte, hdrLen int) {
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switch {
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case len(sctp) == 12: // no chunks
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return
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case hdrLen < 0:
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r.notef("SCTP chunk lengths are only recalculated when the layer 3 field is used; not changed")
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return
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case hdrLen < 16:
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r.notef("SCTP chunk header is not in the layer 3 field; chunk length not changed")
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return
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}
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off := 12
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for {
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clen := int(binary.BigEndian.Uint16(sctp[off+2:]))
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next := off + (clen+3)&^3
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if clen >= 4 && next+4 <= hdrLen {
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off = next
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continue
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}
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rem := len(sctp) - off
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newLen := rem
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if clen >= 4 && clen <= rem && (clen+3)&^3 == rem {
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newLen = clen // already correct; the rest is padding
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}
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if newLen > 0xffff {
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r.notef("SCTP chunk would be %d bytes, more than 65535; chunk length not changed", newLen)
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return
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}
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r.change16(fmt.Sprintf("SCTP %s chunk length", sctpChunkName(sctp[off])), sctp[off+2:off+4], uint16(newLen))
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if rem%4 != 0 {
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r.notef("SCTP %s chunk is %d bytes, not padded to a multiple of 4", sctpChunkName(sctp[off]), rem)
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}
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return
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}
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}
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func sctpChunkName(t byte) string {
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names := map[byte]string{
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0: "DATA", 1: "INIT", 2: "INIT ACK", 3: "SACK", 4: "HEARTBEAT", 5: "HEARTBEAT ACK",
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6: "ABORT", 7: "SHUTDOWN", 8: "SHUTDOWN ACK", 9: "ERROR", 10: "COOKIE ECHO",
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11: "COOKIE ACK", 14: "SHUTDOWN COMPLETE", 64: "I-DATA",
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}
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if n, ok := names[t]; ok {
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return n
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}
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return fmt.Sprintf("type %d", t)
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}
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var castagnoli = crc32.MakeTable(crc32.Castagnoli)
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// sum adds data as big-endian 16-bit words to s (RFC 1071).
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func sum(data []byte, s uint32) uint32 {
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n := len(data) &^ 1
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for i := 0; i < n; i += 2 {
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s += uint32(data[i])<<8 | uint32(data[i+1])
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}
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if len(data)%2 == 1 {
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s += uint32(data[len(data)-1]) << 8
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}
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return s
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}
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// checksum returns the Internet checksum of data, starting from the partial sum s.
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func checksum(data []byte, s uint32) uint16 {
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s = sum(data, s)
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for s>>16 != 0 {
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s = s&0xffff + s>>16
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}
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return ^uint16(s)
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}
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