How To: DHCP Service Troubleshoot

tcpdump -i <interface> -n port 67 or port 68

Ports 67 and 68 are specifically for DHCP traffic – 67 for requests and 68 for responses.

Server Success

Running dhclient from the client computer should result in traffic like this on the dhcp server if successful: notice in the last line, the IP address for the dhcp server is shown responding back (BOOTP/DHCP, Reply) to the client with its new IP address.

eyrie:~# tcpdump -i eth1 -n port 67 or port 68
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
15:08:32.100367 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:50:04:b2:eb:a2 (oui Unknown), length 300
15:08:32.104525 IP 192.168.1.254.bootps > 192.168.1.100.bootpc: BOOTP/DHCP, Reply, length 300

Server Failure

If no traffic occurs, then the request isn’t going through the server, which means something may be wrong with the network in between. This can be tested with pings.

Alternatively, traffic like the following indicates that the server sees the request but is ignoring it. Make sure that /etc/dhcp3/dhcpd.conf is set up correctly to hand out leases and then the MAC address for this client is correct.

eyrie:~# tcpdump -i <interface> -n port 67 or port 68
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
15:04:03.105460 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:50:04:b2:eb:a2 (oui Unknown), length 300
15:04:07.105566 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:50:04:b2:eb:a2 (oui Unknown), length 300