Port / UDP
161SNMP
SNMP polls and sets management data on switches, routers, printers, UPS units, and server agents.
If it is internet-facing: Finding 161 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Trusted network only
Transport
Encryption optional
Risk if public
High
Exposure verdict
Port 161 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Encryption is available but negotiated or configured rather than guaranteed. A client that does not insist on it will silently fall back to plaintext.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- SNMPv1 and v2c authenticate with a community string sent in cleartext, and 'public' is still a live default.
- A readable MIB tree describes interfaces, routes, ARP tables, and running processes.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 161 proto udpiptables
sudo iptables -A INPUT -p udp -s 10.0.0.0/8 --dport 161 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 161 -j DROPIf you do need it open to everyone
sudo ufw allow 161/udpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -ulpn 'sport = :161'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iUDP:161macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 161 <host>A version probe confirms whether SNMP is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 161The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
no port changeChange the configuration, not the port
SNMPv3 on the same port adds real authentication and privacy — configure it rather than moving ports.
The fix here lives in the service configuration rather than in a different port assignment.