{
  "port": 3905,
  "protocol": "tcp",
  "service": "MUPDATE (Cyrus Murder)",
  "oneLiner": "MUPDATE — the distributed mailbox-database protocol that keeps a Cyrus Murder mail cluster's namespace consistent.",
  "whatItIs": "RFC 3656 defines the Mailbox Update protocol and IANA registered 3905 for it. In a Cyrus Murder, several IMAP/POP backends share one mailbox namespace: an MUPDATE master holds the authoritative mailbox database and makes operations on it atomic, while frontends and slaves connect to replicate it. The protocol is line-based and text-like, and a client authenticates over SASL after the server advertises its mechanisms. A listener on 3905 means a multi-server Cyrus mail cluster, not a single mail host.",
  "exposure": {
    "verdict": "never",
    "note": "The MUPDATE master is the mail cluster's source of truth for who owns which mailbox — a strictly internal, backend-to-backend service that should be confined to the cluster's own network."
  },
  "checkCommand": "nc -vz TARGET 3905",
  "links": [
    {
      "title": "RFC 3656 — The Mailbox Update (MUPDATE) Distributed Mailbox Database Protocol",
      "url": "https://www.rfc-editor.org/rfc/rfc3656.html",
      "kind": "spec"
    },
    {
      "title": "Cyrus IMAP — Murder MUPDATE details",
      "url": "https://www.cyrusimap.org/imap/reference/admin/murder/murder-mupdate-details.html",
      "kind": "official-docs"
    },
    {
      "title": "cyrusimap/cyrus-imapd on GitHub",
      "url": "https://github.com/cyrusimap/cyrus-imapd",
      "kind": "github"
    },
    {
      "title": "IANA port registry — 3905 (mupdate)",
      "url": "https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=3905",
      "kind": "iana"
    }
  ],
  "ianaName": "mupdate"
}