Port / UDP
67DHCP server
DHCP servers listen here for broadcast discovery and request messages, and hand out address leases and network configuration.
If it is internet-facing: Port 67 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Trusted network only
Transport
Cleartext
Risk if public
Elevated
Exposure verdict
Port 67 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
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
1 note- A rogue DHCP server can redirect a whole subnet's gateway and DNS, which is a full traffic interception.
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 67 proto udpiptables
sudo iptables -A INPUT -p udp -s 10.0.0.0/8 --dport 67 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 67 -j DROPIf you do need it open to everyone
sudo ufw allow 67/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 = :67'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:67macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 67 <host>A version probe confirms whether DHCP server is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 67The quick yes/no when you only need to know whether a path exists through the firewall.