Port 161 (SNMP)

Simple Network Management Protocol polling of routers, switches, printers, UPSes and servers.

Well-known (system) port, 0-1023 UDP Monitoring and logging Do not expose to the internet

Port 161 is normally used over UDP; the checker tests TCP only, so a "filtered" result does not mean the UDP service is down.

What port 161 is used for

Simple Network Management Protocol polling of routers, switches, printers, UPSes and servers.

Security: should port 161 be open?

Do not expose to the internet. Never expose. SNMPv1/v2c use a community string sent in clear text, and many devices ship with 'public'/'private'; with write access an attacker can reconfigure the device. Open SNMP is also used for amplification. Use SNMPv3 with auth and privacy and allow only your monitoring host.

IANA assignment

Service nameProtoDescriptionAssigneeReference

snmp TCP SNMP - -

snmp UDP SNMP - -

Check what is listening on port 161

Linux: UDP 161

sudo ss -ulnp 'sport = :161'

Linux / macOS: which process owns the port

sudo lsof -nP -i :161

Windows (Command Prompt)

netstat -ano | findstr :161

Windows (PowerShell)

Get-NetUDPEndpoint -LocalPort 161

Test it from outside

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 161

Block port 161 in the firewall

ufw

sudo ufw deny 161/udp

iptables

sudo iptables -A INPUT -p udp --dport 161 -j DROP

nftables

sudo nft add rule inet filter input udp dport 161 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 161' -Direction Inbound -Protocol UDP -LocalPort 161 -Action Block

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.