# WHATPORT(199)

## NAME

port 199 — SMUX — SNMP multiplexing — how a subagent attaches its MIB subtree to the main SNMP agent.

Protocol: TCP  
IANA registry

## DESCRIPTION

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 It is an experimental protocol that AgentX (RFC 2741) superseded, so new deployments use AgentX instead.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

**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.

## CHECK

```sh
sudo lsof -nP -iTCP:199 -sTCP:LISTEN
```

## SEE ALSO

- spec — [RFC 1227 — SNMP MUX Protocol and MIB](https://www.rfc-editor.org/rfc/rfc1227.html) (rfc-editor.org)
- source — [Net-SNMP source repository](https://github.com/net-snmp/net-snmp) (github.com)
- iana — [IANA Service Name and Transport Protocol Port Number Registry](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml) (iana.org)

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HTML: <https://whatport.net/199/> · JSON: <https://whatport.net/199.json> · full dataset: <https://whatport.net/ports.json>