{
  "port": 6699,
  "protocol": "tcp",
  "service": "reserved (peer-to-peer file sharing, historically)",
  "oneLiner": "Reserved at IANA on TCP; remembered as the Napster and WinMX file-sharing port.",
  "whatItIs": "IANA marks 6699/tcp as Reserved with no assignee, while 6699/udp was registered in 2020 for the Babel routing protocol over DTLS (RFC 8968). The TCP side is famous for something else entirely: nmap's service table still names it napster, from the era when Napster-style and WinMX clients defaulted to this port for peer connections. Those networks are gone, so an open 6699/tcp today is either a long-abandoned client or unrelated software on a free port.",
  "exposure": {
    "verdict": "careful",
    "note": "A live listener here is worth investigating — either an unmaintained file-sharing client or something you have not identified. It is not a service you should be deliberately publishing."
  },
  "checkCommand": "sudo lsof -nP -iTCP:6699 -sTCP:LISTEN",
  "links": [
    {
      "title": "IANA Service Name and Transport Protocol Port Number Registry — 6699",
      "url": "https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=6699",
      "kind": "iana"
    },
    {
      "title": "RFC 8968 — Babel Routing Protocol over Datagram Transport Layer Security",
      "url": "https://www.rfc-editor.org/rfc/rfc8968.html",
      "kind": "spec"
    },
    {
      "title": "nmap-services (Nmap's port/service frequency database)",
      "url": "https://github.com/nmap/nmap/blob/master/nmap-services",
      "kind": "github"
    }
  ]
}