Port 161 (UDP) — SNMP

Simple Network Management Protocol polling.

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Default "public" community strings leak device info. Use SNMPv3 and restrict by IP.

Check if port 161 is open

On the server, see what is listening:

sudo ss -tulpn | grep ':161 '

From another machine, test reachability:

nc -zv your-server.example.com 161

With UFW (Ubuntu/Debian):

sudo ufw deny 161

With firewalld (RHEL/AlmaLinux/Rocky):

sudo firewall-cmd --permanent --remove-port=161/udp && sudo firewall-cmd --reload