go.net/ipv4: add Flags field to Header

Fixes golang/go#6453.

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/13832043

ipv4/header.go

@@ -36,10 +36,8 @@ const (
 	maxHeaderLen = 60 // sensible default, revisit if later RFCs define new usage of version and header length fields
 )
 
-type headerField int
-
 const (
-	posTOS      headerField = 1  // type-of-service
+	posTOS      = 1  // type-of-service
 	posTotalLen = 2  // packet total length
 	posID       = 4  // identification
 	posFragOff  = 6  // fragment offset
@@ -50,6 +48,13 @@ const (
 	posDst      = 16 // destination address
 )
 
+type HeaderFlags int
+
+const (
+	MoreFragments HeaderFlags = 1 << iota // more fragments flag
+	DontFragment                          // don't fragment flag
+)
+
 // A Header represents an IPv4 header.
 type Header struct {
 	Version  int         // protocol version
@@ -57,6 +62,7 @@ type Header struct {
 	TOS      int         // type-of-service
 	TotalLen int         // packet total length
 	ID       int         // identification
+	Flags    HeaderFlags // flags
 	FragOff  int         // fragment offset
 	TTL      int         // time-to-live
 	Protocol int         // next protocol
@@ -70,7 +76,7 @@ func (h *Header) String() string {
 	if h == nil {
 		return "<nil>"
 	}
-	return fmt.Sprintf("ver: %v, hdrlen: %v, tos: %#x, totallen: %v, id: %#x, fragoff: %#x, ttl: %v, proto: %v, cksum: %#x, src: %v, dst: %v", h.Version, h.Len, h.TOS, h.TotalLen, h.ID, h.FragOff, h.TTL, h.Protocol, h.Checksum, h.Src, h.Dst)
+	return fmt.Sprintf("ver: %v, hdrlen: %v, tos: %#x, totallen: %v, id: %#x, flags: %#x, fragoff: %#x, ttl: %v, proto: %v, cksum: %#x, src: %v, dst: %v", h.Version, h.Len, h.TOS, h.TotalLen, h.ID, h.Flags, h.FragOff, h.TTL, h.Protocol, h.Checksum, h.Src, h.Dst)
 }
 
 // Please refer to the online manual; IP(4) on Darwin, FreeBSD and
@@ -89,12 +95,13 @@ func (h *Header) Marshal() ([]byte, error) {
 	b := make([]byte, hdrlen)
 	b[0] = byte(Version<<4 | (hdrlen >> 2 & 0x0f))
 	b[posTOS] = byte(h.TOS)
+	flagsAndFragOff := (h.FragOff & 0x1fff) | int(h.Flags<<13)
 	if supportsNewIPInput {
 		b[posTotalLen], b[posTotalLen+1] = byte(h.TotalLen>>8), byte(h.TotalLen)
-		b[posFragOff], b[posFragOff+1] = byte(h.FragOff>>8), byte(h.FragOff)
+		b[posFragOff], b[posFragOff+1] = byte(flagsAndFragOff>>8), byte(flagsAndFragOff)
 	} else {
 		*(*uint16)(unsafe.Pointer(&b[posTotalLen : posTotalLen+1][0])) = uint16(h.TotalLen)
-		*(*uint16)(unsafe.Pointer(&b[posFragOff : posFragOff+1][0])) = uint16(h.FragOff)
+		*(*uint16)(unsafe.Pointer(&b[posFragOff : posFragOff+1][0])) = uint16(flagsAndFragOff)
 	}
 	b[posID], b[posID+1] = byte(h.ID>>8), byte(h.ID)
 	b[posTTL] = byte(h.TTL)
@@ -135,6 +142,8 @@ func ParseHeader(b []byte) (*Header, error) {
 		h.TotalLen += hdrlen
 		h.FragOff = int(*(*uint16)(unsafe.Pointer(&b[posFragOff : posFragOff+1][0])))
 	}
+	h.Flags = HeaderFlags(h.FragOff&0xe000) >> 13
+	h.FragOff = h.FragOff & 0x1fff
 	h.ID = int(b[posID])<<8 | int(b[posID+1])
 	h.TTL = int(b[posTTL])
 	h.Protocol = int(b[posProtocol])

ipv4/header_test.go

@@ -16,28 +16,28 @@ import (
 var (
 	wireHeaderFromKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xbe, 0xef,
-		0xca, 0xfe, 0x05, 0xdc,
+		0xca, 0xfe, 0x45, 0xdc,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
 	}
 	wireHeaderToKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xbe, 0xef,
-		0xca, 0xfe, 0x05, 0xdc,
+		0xca, 0xfe, 0x45, 0xdc,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
 	}
 	wireHeaderFromTradBSDKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xdb, 0xbe,
-		0xca, 0xfe, 0xdc, 0x05,
+		0xca, 0xfe, 0xdc, 0x45,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
 	}
 	wireHeaderToTradBSDKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xef, 0xbe,
-		0xca, 0xfe, 0xdc, 0x05,
+		0xca, 0xfe, 0xdc, 0x45,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
@@ -51,6 +51,7 @@ var (
 		TOS:      1,
 		TotalLen: 0xbeef,
 		ID:       0xcafe,
+		Flags:    ipv4.DontFragment,
 		FragOff:  1500,
 		TTL:      255,
 		Protocol: 1,