Commands for DHCP Troubleshooting

What should happen

DHCP clients broadcast Discover; a server or relay answers Offer; the client Requests; the server Acknowledges with address, mask, gateway, DNS, and lease time. When any leg vanishes, the client keeps a link-local or stale address—symptoms look like “Wi-Fi connected, no internet.”

Capture and commands

Perspective	What to run	Looks for
Client Linux	`journalctl -u NetworkManager -f` or `dhcpcd -d`	NAK, timeout, rogue server
Wire sniff	`tcpdump -ni eth0 port 67 or port 68`	Discover without Offer, duplicate MAC
Server	Lease file / SQL / Kea logs	Pool exhaustion, class mismatch
Relay	Verify `giaddr` and circuit-id	Wrong VLAN mapped to scope

Windows quick checks

ipconfig /all shows DHCP enabled state and server ID; ipconfig /renew forces a new cycle—pair with a simultaneous capture on the default gateway port if possible.

Frequently Asked Questions

Q.Why does the client get an address from the wrong subnet?

Multiple DHCP servers on the same VLAN, a rogue device, or a relay giaddr pointing at the wrong shared-network. Use tcpdump to read server identifiers and option 54.

Q.What does DHCPNAK mean in practice?

The server refused the client’s Request—often because the offered lease expired, the MAC moved VLANs, or reservations conflict. Follow with a fresh Discover cycle capture.

Q.Why would renew work but reboot fail?

Boot-time timing or firmware DHCP options differ from renew unicasts. Check switchport STP edge settings and whether the client needs DHCP option 121 for classless static routes.

Q.How do I prove the relay is stripping Option 82?

Compare captures on the client VLAN versus the server-facing interface. If circuit-id disappears, the relay config—not the DHCP scope—is broken.

Q.The pool is empty but only 20 devices exist—why?

Abandoned leases, short lease times with sleeping IoT, duplicate subnets, or stale reservations consuming addresses. Audit lease DB and shorten cleanup timers cautiously.

Q.Does static IP on the gateway break DHCP for everyone?

If the gateway IP collides with the pool or the server cannot route to relays, yes. Validate that the router helper-address matches the DHCP service IP and L3 path.

Q.Why does IPv6 SLAAC work but DHCPv4 fails?

Separate stacks—RA may advertise DNS while IPv4 DHCP is blocked by ACL or missing helper. Troubleshoot each family independently.

Q.Is it safe to run dhclient -r in production SSH sessions?

Risky on remote servers—you can drop the address serving your session. Prefer maintenance windows or out-of-band console when renewing on headless boxes.