Internet-Security.com TCP port 161 details

TCP port 161 is used by the Simple Network Management Protocol (SNMP), in addition to the more common UDP/161 (per RFC 3417).
Net-SNMP’s snmpd (widely shipped on Linux/BSD) can bind to tcp:161; real-world deployments include pfSense, OpenWrt, Synology DSM, and F5 BIG-IP, all of which use Net-SNMP and support TCP transport when configured.
Commercial/embedded agents such as SNMP Research EMANATE/OneAgent also implement SNMP over TCP/161 and are used in various network and server products.
The Net-SNMP client tools (snmpget/snmpwalk/snmpset) can query agents via tcp:host:161, and are used by many admins and automation scripts.
Organizations sometimes enable SNMP-over-TCP/161 to traverse strict firewalls or to handle large responses more reliably than UDP.
Security note: SNMP on 161 (including TCP) is frequently probed for default community strings and misconfigurations, leading to information disclosure or device control on poorly secured equipment.

TCP 161