Port 161 SNMP · UDP

What runs on port 161

SNMP: how monitoring systems read counters from switches, routers, printers and servers (get/walk), and how some write to them (set). Versions 1 and 2c authenticate with a plaintext "community string", usually still "public"; only SNMPv3 has real authentication and encryption. Reachable from the internet it hands out interface tables, routing tables and device models to anyone.

Software that uses it: Net-SNMP (snmpd, snmpwalk) · LibreNMS · Zabbix · Prometheus snmp_exporter · PRTG

Should port 161 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• snmpwalk -v2c -c public host system is the first test; if that works from outside your network, it is a finding.
• Prefer v3 with authPriv; at minimum change the community and restrict by source.

Check port 161

# on the host: is anything listening, and which process?
ss -lunp 'sport = :161'
sudo lsof -i :161

# from another machine: is it reachable?
nc -zuv host 161
sudo nmap -sU -p 161 host

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 161

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 161 proto udp
sudo ufw deny 161/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 161 accept
nft add rule inet filter input udp dport 161 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 161 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 161 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=161 protocol=udp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 161" with no source is just "open".

Related ports

• 162 SNMP trap
• 514 syslog — Remote logging (tcp 514 is rsh/rlogin historically)
• 9100 JetDirect / node_exporter — Raw printing, Prometheus node exporter

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions