wgengine, net/packet, cmd/tailscale: add ICMP echo

Updates tailscale/corp#754

Signed-off-by: James Tucker <james@tailscale.com>

net/packet/icmp.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2022 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
+
+import (
+	crand "crypto/rand"
+
+	"encoding/binary"
+)
+
+// ICMPEchoPayload generates a new random ID/Sequence pair, and returns a uint32
+// derived from them, along with the id, sequence and given payload in a buffer.
+// It returns an error if the random source could not be read.
+func ICMPEchoPayload(payload []byte) (idSeq uint32, buf []byte) {
+	buf = make([]byte, len(payload)+4)
+
+	// make a completely random id/sequence combo, which is very unlikely to
+	// collide with a running ping sequence on the host system. Errors are
+	// ignored, that would result in collisions, but errors reading from the
+	// random device are rare, and will cause this process universe to soon end.
+	crand.Read(buf[:4])
+
+	idSeq = binary.LittleEndian.Uint32(buf)
+	copy(buf[4:], payload)
+
+	return
+}

net/packet/packet.go

@@ -434,6 +434,29 @@ func (q *Parsed) IsEchoResponse() bool {
 	}
 }
 
+// EchoIDSeq extracts the identifier/sequence bytes from an ICMP Echo response,
+// and returns them as a uint32, used to lookup internally routed ICMP echo
+// responses. This function is intentionally lightweight as it is called on
+// every incoming ICMP packet.
+func (q *Parsed) EchoIDSeq() uint32 {
+	switch q.IPProto {
+	case ipproto.ICMPv4:
+		offset := ip4HeaderLength + icmp4HeaderLength
+		if len(q.b) < offset+4 {
+			return 0
+		}
+		return binary.LittleEndian.Uint32(q.b[offset:])
+	case ipproto.ICMPv6:
+		offset := ip6HeaderLength + icmp6HeaderLength
+		if len(q.b) < offset+4 {
+			return 0
+		}
+		return binary.LittleEndian.Uint32(q.b[offset:])
+	default:
+		return 0
+	}
+}
+
 func Hexdump(b []byte) string {
 	out := new(strings.Builder)
 	for i := 0; i < len(b); i += 16 {