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// 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()
}
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