Port 68 DHCP client · UDP

What runs on port 68

DHCP client side: the port a host listens on for OFFER and ACK replies while it has no address yet. Firewalls on laptops and servers must allow UDP 68 inbound from the server or the machine never gets a lease; most host firewalls special-case it.

Software that uses it: dhclient · systemd-networkd · NetworkManager · dhcpcd · Windows DHCP client

Should port 68 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.

• If a client never gets an address while others do, capture on UDP 67/68 (tcpdump port 67 or port 68) and watch for the missing step.

Check port 68

# on the host: is anything listening, and which process?
ss -lunp 'sport = :68'
sudo lsof -i :68

# from another machine: is it reachable?
nc -zuv host 68
sudo nmap -sU -p 68 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 68

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 68 proto udp
sudo ufw deny 68/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 68 accept
nft add rule inet filter input udp dport 68 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 68 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 68 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=68 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 68" with no source is just "open".

Related ports

• 67 DHCP server — Hands out IP addresses

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