Troubleshoot your DHCP setup

A common problem you see is when your DHCP service fails and a client can't get an IP address. When you do an IPCONFIG or IFCONFIG command your network interface returns the address of the loopback adapter. After establishing that there are no hardware problems and that your connectivity is good you will then need to find out what has gone wrong with your DHCP server. DHCP works using a Discover/Offer/Request/and Acknowledge system. A client sends a DHCP broadcast, it receives an offer, broadcasts a request and then gets an acknowledgement that the offer has been accepted. Therefore you will need to test all four parts of this system to determine what the problem is.

When a client doesn't issue a DHCP Discover request you need to check the TCP/IP configuration to check if it is set to accept DHCP address or is set for a static address. Make sure that you haven't swapped out a NIC with an excluded MAC address. If you find that your client's DHCP is set correctly the next step is to determine the state of your server. Check if your DHCP server is accessible on the local network and if it is listening on port 67. You should also check the status of the DHCP service on your server. Other problems you might encounter are that the DHCP server is overloaded with requests or can't service your request because it is busy with other processes.

For a DHCP server on another subnet you need to check if the router is the problem. If the Discover frame can't be seen on the

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DHCP side of the router then you should suspect the router configuration, and you should also check the relay agent. If the Discover frame appears remotely then you need to check the DHCP server's network addressing and configuration. Check your DHCP error log, if one is kept.

Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.

This was first published in September 2003

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