SNMP multiplexing — how a subagent attaches its MIB subtree to the main SNMP agent.
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 It is an experimental protocol that AgentX (RFC 2741) superseded, so new deployments use AgentX instead.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.
Do not expose
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.
$ sudo lsof -nP -iTCP:199 -sTCP:LISTENSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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