Port 68 — DHCP client

Q: How do I check if port 68 is open?

Locally, sudo lsof -i :68 on macOS or Linux, or netstat -ano | findstr :68 on Windows. From outside, nc -zv host 68 tells you whether anything answers.

UDP well-known

What runs on port 68

The client side of DHCP address assignment.

The client half of DHCP. Nothing listens here on a server; it is where a workstation waits for its address offer. If a machine ends up with a 169.254.x.x address, the DHCP exchange failed and the operating system fell back to link-local addressing. That is a symptom of the network, not the machine — cable, VLAN, or an exhausted DHCP pool.

Security considerations

Local network only; nothing to expose.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :68
sudo ss -lntp | grep :68

# Windows
netstat -ano | findstr :68
Get-NetTCPConnection -LocalPort 68

# is it reachable from outside?
nc -zv example.com 68
curl -v telnet://example.com:68

Freeing the port

# find the process, then stop it
sudo lsof -ti :68 | xargs kill        # Linux / macOS
netstat -ano | findstr :68            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Should this port be open to the internet?

Usually not. Expose it only if a specific external client needs it, and restrict by source address where you can.

Opening or blocking port 68

Finding the port is usually the first half of the job. Start narrow — the second rule is there for services that genuinely serve the public, and most do not:

# ufw — Debian, Ubuntu
sudo ufw allow from 10.0.0.0/8 to any port 68 proto udp
sudo ufw allow 68/udp                   # only if it genuinely serves the public

# firewalld — RHEL, Fedora, CentOS
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=68 protocol=udp accept'
sudo firewall-cmd --reload

# iptables, if you are still there
sudo iptables -A INPUT -p udp --dport 68 -s 10.0.0.0/8 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 68 -j DROP

# Windows
New-NetFirewallRule -DisplayName "DHCP client 68" -Direction Inbound -Protocol UDP -LocalPort 68 -RemoteAddress 10.0.0.0/8 -Action Allow

10.0.0.0/8 stands in for a private range — substitute your own. Note that a firewall rule does not undo a service bound to 0.0.0.0; it only stops packets reaching it, and a misconfigured or disabled firewall then exposes it immediately.

Quick reference

Port	68
Protocol	UDP
Service	DHCP client
Range	Well-known (0–1023) — binding requires root on Unix

Frequently asked questions

What is port 68 used for?

The client side of DHCP address assignment.

Is it safe to open port 68?

Local network only; nothing to expose.

How do I check if port 68 is open?

Locally, sudo lsof -i :68 on macOS or Linux, or netstat -ano | findstr :68 on Windows. From outside, nc -zv host 68 tells you whether anything answers.

Why do I get "address already in use" on port 68?

Another process is bound to it — often a previous run of your own program that did not exit cleanly. Find it with lsof -ti :68 and stop it, or configure your application to use a different port.

Can I change the port this service uses?

Almost always yes, in the service's configuration. Moving off a default port reduces automated scan noise, but it is obfuscation rather than security — a real attacker scans all 65,535.

Other common ports

All port numbers