x/net/ipv6: add Header, ParseHeader

This CL adds Header type and ParseHeader function for convenience
in parsing ICMP error messages such as destination unreachable,
packet too big, time exceeded and parameter problem.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/177530043

ipv6/header.go

@@ -0,0 +1,55 @@
+// Copyright 2014 The Go 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 ipv6
+
+import (
+	"errors"
+	"fmt"
+	"net"
+)
+
+const (
+	Version   = 6  // protocol version
+	HeaderLen = 40 // header length
+)
+
+// A Header represents an IPv6 base header.
+type Header struct {
+	Version      int    // protocol version
+	TrafficClass int    // traffic class
+	FlowLabel    int    // flow label
+	PayloadLen   int    // payload length
+	NextHeader   int    // next header
+	HopLimit     int    // hop limit
+	Src          net.IP // source address
+	Dst          net.IP // destination address
+}
+
+func (h *Header) String() string {
+	if h == nil {
+		return "<nil>"
+	}
+	return fmt.Sprintf("ver: %v, tclass: %#x, flowlbl: %#x, payloadlen: %v, nxthdr: %v, hoplim: %v, src: %v, dst: %v", h.Version, h.TrafficClass, h.FlowLabel, h.PayloadLen, h.NextHeader, h.HopLimit, h.Src, h.Dst)
+}
+
+// ParseHeader parses b as an IPv6 base header.
+func ParseHeader(b []byte) (*Header, error) {
+	if len(b) < HeaderLen {
+		return nil, errors.New("header too short")
+	}
+	h := &Header{
+		Version:      int(b[0]) >> 4,
+		TrafficClass: int(b[0]&0x0f)<<4 | int(b[1])>>4,
+		FlowLabel:    int(b[1]&0x0f)<<16 | int(b[2])<<8 | int(b[3]),
+		PayloadLen:   int(b[4])<<8 | int(b[5]),
+		NextHeader:   int(b[6]),
+		HopLimit:     int(b[7]),
+	}
+	h.Src = make(net.IP, net.IPv6len)
+	copy(h.Src, b[8:24])
+	h.Dst = make(net.IP, net.IPv6len)
+	copy(h.Dst, b[24:40])
+	return h, nil
+}

ipv6/header_test.go

@@ -0,0 +1,50 @@
+// Copyright 2014 The Go 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 ipv6_test
+
+import (
+	"net"
+	"reflect"
+	"testing"
+
+	"golang.org/x/net/internal/iana"
+	"golang.org/x/net/ipv6"
+)
+
+var (
+	wireHeaderFromKernel = [ipv6.HeaderLen]byte{
+		0x69, 0x8b, 0xee, 0xf1,
+		0xca, 0xfe, 0x2c, 0x01,
+		0x20, 0x01, 0x0d, 0xb8,
+		0x00, 0x01, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x01,
+		0x20, 0x01, 0x0d, 0xb8,
+		0x00, 0x02, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x01,
+	}
+
+	testHeader = &ipv6.Header{
+		Version:      ipv6.Version,
+		TrafficClass: iana.DiffServAF43,
+		FlowLabel:    0xbeef1,
+		PayloadLen:   0xcafe,
+		NextHeader:   iana.ProtocolIPv6Frag,
+		HopLimit:     1,
+		Src:          net.ParseIP("2001:db8:1::1"),
+		Dst:          net.ParseIP("2001:db8:2::1"),
+	}
+)
+
+func TestParseHeader(t *testing.T) {
+	h, err := ipv6.ParseHeader(wireHeaderFromKernel[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(h, testHeader) {
+		t.Fatalf("got %#v; want %#v", h, testHeader)
+	}
+}