{
  "port": 199,
  "protocol": "tcp",
  "service": "SMUX",
  "oneLiner": "SNMP multiplexing — how a subagent attaches its MIB subtree to the main SNMP agent.",
  "whatItIs": "SMUX, RFC 1227, lets a separate process register a MIB subtree with the local SNMP agent so queries for that subtree are relayed to it; the agent listens on TCP 199 for those peer connections. Routing daemons like gated and later quagga/zebra used it to expose routing MIBs, and net-snmp can be built with SMUX support. It is an experimental protocol that AgentX (RFC 2741) superseded, so new deployments use AgentX instead. An open 199 next to 161/udp marks an older SNMP stack.",
  "exposure": {
    "verdict": "never",
    "note": "SMUX peers authenticate with a plaintext community-style password and gain control of part of the agent's MIB; it is an internal agent-to-subagent channel, not an internet service."
  },
  "checkCommand": "sudo lsof -nP -iTCP:199 -sTCP:LISTEN",
  "links": [
    {
      "title": "RFC 1227 — SNMP MUX Protocol and MIB",
      "url": "https://www.rfc-editor.org/rfc/rfc1227.html",
      "kind": "spec"
    },
    {
      "title": "Net-SNMP source repository",
      "url": "https://github.com/net-snmp/net-snmp",
      "kind": "github"
    },
    {
      "title": "IANA Service Name and Transport Protocol Port Number Registry",
      "url": "https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml",
      "kind": "iana"
    }
  ]
}