disco: move disco pcap helper to disco package

Updates tailscale/corp#13464

Signed-off-by: David Anderson <danderson@tailscale.com>

disco/pcap.go

@@ -0,0 +1,40 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package disco
+
+import (
+	"bytes"
+	"encoding/binary"
+	"net/netip"
+
+	"tailscale.com/tailcfg"
+	"tailscale.com/types/key"
+)
+
+// ToPCAPFrame marshals the bytes for a pcap record that describe a disco frame.
+//
+// Warning: Alloc garbage. Acceptable while capturing.
+func ToPCAPFrame(src netip.AddrPort, derpNodeSrc key.NodePublic, payload []byte) []byte {
+	var (
+		b    bytes.Buffer
+		flag uint8
+	)
+	b.Grow(128) // Most disco frames will probably be smaller than this.
+
+	if src.Addr() == tailcfg.DerpMagicIPAddr {
+		flag |= 0x01
+	}
+	b.WriteByte(flag) // 1b: flag
+
+	derpSrc := derpNodeSrc.Raw32()
+	b.Write(derpSrc[:])                                       // 32b: derp public key
+	binary.Write(&b, binary.LittleEndian, uint16(src.Port())) // 2b: port
+	addr, _ := src.Addr().MarshalBinary()
+	binary.Write(&b, binary.LittleEndian, uint16(len(addr)))    // 2b: len(addr)
+	b.Write(addr)                                               // Xb: addr
+	binary.Write(&b, binary.LittleEndian, uint16(len(payload))) // 2b: len(payload)
+	b.Write(payload)                                            // Xb: payload
+
+	return b.Bytes()
+}