Port 631 IPP / CUPS · TCP and UDP

What runs on port 631

IPP: the Internet Printing Protocol, carried over HTTP(S). CUPS listens on 631 for both printing and its web administration page; modern printers support IPP Everywhere, which is why they work without drivers. UDP 631 is CUPS browsing (printer announcements), largely replaced by mDNS.

Software that uses it: CUPS · IPP Everywhere / AirPrint printers · Windows IPP client

Should port 631 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.

• http://localhost:631 is the CUPS admin page; remote access needs "Listen" and "Allow" lines in cupsd.conf.
• A 2024 CUPS vulnerability chain started with UDP 631 exposed to the internet; keep it internal.

Check port 631

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :631'
ss -lunp 'sport = :631'
sudo lsof -i :631

# from another machine: is it reachable?
nc -zv host 631
nmap -p 631 host
nc -zuv host 631
sudo nmap -sU -p 631 host
curl -v telnet://host:631      # raw TCP connect test, Ctrl-C to leave

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 631

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 631 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 631 proto udp
sudo ufw deny 631/tcp
sudo ufw deny 631/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 631 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 631 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 631 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 631 -j DROP
iptables -A INPUT -p udp --dport 631 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 631 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=631 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 631" with no source is just "open".

Related ports

• 515 LPD — Line printer daemon
• 9100 JetDirect / node_exporter — Raw printing, Prometheus node exporter
• 5353 mDNS — Bonjour / Avahi discovery

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