wgengine/packet: refactor and expose UDP header marshaling (#408)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/packet/icmp.go

@@ -0,0 +1,78 @@
+// 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
+
+type ICMPType uint8
+
+const (
+	ICMPEchoReply    ICMPType = 0x00
+	ICMPEchoRequest  ICMPType = 0x08
+	ICMPUnreachable  ICMPType = 0x03
+	ICMPTimeExceeded ICMPType = 0x0b
+)
+
+func (t ICMPType) String() string {
+	switch t {
+	case ICMPEchoReply:
+		return "EchoReply"
+	case ICMPEchoRequest:
+		return "EchoRequest"
+	case ICMPUnreachable:
+		return "Unreachable"
+	case ICMPTimeExceeded:
+		return "TimeExceeded"
+	default:
+		return "Unknown"
+	}
+}
+
+type ICMPCode uint8
+
+const (
+	ICMPNoCode ICMPCode = 0
+)
+
+// ICMPHeader represents an ICMP packet header.
+type ICMPHeader struct {
+	IPHeader
+	Type ICMPType
+	Code ICMPCode
+}
+
+const (
+	icmpHeaderLength = 4
+	// icmpTotalHeaderLength is the length of all headers in a ICMP packet.
+	icmpAllHeadersLength = ipHeaderLength + icmpHeaderLength
+)
+
+func (ICMPHeader) Len() int {
+	return icmpAllHeadersLength
+}
+
+func (h ICMPHeader) Marshal(buf []byte) error {
+	if len(buf) < icmpAllHeadersLength {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+	// The caller does not need to set this.
+	h.IPProto = ICMP
+
+	buf[20] = uint8(h.Type)
+	buf[21] = uint8(h.Code)
+
+	h.IPHeader.Marshal(buf)
+
+	put16(buf[22:24], ipChecksum(buf))
+
+	return nil
+}
+
+func (h *ICMPHeader) ToResponse() {
+	h.Type = ICMPEchoReply
+	h.Code = ICMPNoCode
+	h.IPHeader.ToResponse()
+}