Troubleshooting a DHCP server

If you are using a DHCP server that automatically configure the TCP / IP configuration of workstations in your organization, a non-working DHCP may lead to disruption of the entire organization. In addition, if the workstation is unable to obtain an IP address, it will not be able to access resources on your network or the Internet. Here we are going to discuss some methods that can be used to resolve issues related to the DHCP server.

Assigning an incorrect address

The most common problem associated with DHCP, which is to assign the wrong IP address. For example, assume that your DHCP server that has been configured to use the IP address range from 192.168.0.1 to 192.168.50. You should expect that the network computer been assigned an IP address from this range. Now assume that a workstation on your network began to experience problems accessing other network servers. You need to use the command IPCONFIG / ALL to see the network configuration and IP address. Instead of the address of the expected range of addresses, we see that the workstation that has been assigned an address starts with 169.254. If a computer on your network unexpectedly assigned to an address that starts with 169.254, then you can be absolutely sure that this address was not given by your DHCP server. What happened is that your workstation is unable to connect to server DHCP server. If this occurs, the workstation it assigns an IP address itself by using the Windows' Automatic Private IP Addressing (APIPA or automatic addressing).

Microsoft has built an automatic addressing of the Windows operating system as an aid to those who use very little network. For example, if you set up a small network of Windows, you do not need to configure the IP address manually, even if there is no DHCP server on the network. APIPA can help you automatically assign a unique address to the class of every machine on the network. It is excellent for home networks, but is not applicable for large networks. If the workstation used the services of APIPA, it means that its request for an IP address does not come to an answer. The causes of this problem can be several. If you know that all the other computers on your network normally request an IP address from your DHCP server, then you can conclude that the reason behind the problem is not the DHCP server.

More than likely a problem with the network hardware that is installed on the workstation. For example, networks interface using the wrong driver. Another possible reason could be that the power cable is driven into the network card is not connected to the other side of the switch.

Of course, only that one computer has failed to obtain an IP address that does not mean that our server is the source of the problem. If other workstations successfully acquired IP, then be sure that the server is working properly. However, there may be possibilities that the server has reached the limit of IP addresses that it can assign to clients. You can easily detect such a problem, comparing the number of addresses included in the slot allocated to the DHCP server, the number of devices have requested an IP address from the DHCP server, Common problems of DHCP servers.

If multiple workstations are experiencing problems with obtaining IP addresses, the most likely problem is with the DHCP server. If you suspect that the problem is the DHCP server, then you can check this by using a few simple tests to verify the connection (ping test) and the availability of the DHCP server on the network.

If the DHCP server can communicate with other computers on the network, then I suggest that you verify the DHCP server assigned IP address, and that address is compatible with the same interval of addresses for which the server is configured for, assigning addresses to workstations. For example, if the range of addresses, the DHCP server assigns workstations range from 192.168.0.1 to 192.168.0.50, then the server will not be able to assign addresses to workstations until it itself will not be assigned a static IP address in the same segment of the subnet, for example, 192.168.0.0 or 192.168.0.51.

If this still does not resolve the problem, then I recommend checking out the basics. As an example, you have to make sure that the DHCP server is still logging in Active Directory to distribute IP addresses. You should also check that this interval is active, and that all required services are active on the DHCP server.

Conflicts of IP addresses

Another problem that I have observed is the conflict of IP addresses, which dynamically allocated. When you create an interval DHCP scope, the DHCP server is responsible for the fact that the addresses within the interval were unique to each machine. If this is true, then where there is a conflict dynamically assigned addresses?

There were two situations that I have encountered in the event of a problem. The first time I encountered this problem, I was able to determine which computers were assigned the same address. When I checked the TCP / IP configuration of these machines, I found that on one of these addresses have been configured manually. This is a rather long story, but briefly, the work of one of the application on the computer it was necessary that the car had a static IP address. User tired of constantly adjusting this app to my address, so he just took the address that was assigned to it dynamically and made it static.

The likelihood of such a situation today is quite small. When this situation occurs commonly used operating system is Windows 98. In Windows 98 lacks many tools for security that we have today. Properly configured security on a workstation running Windows XP or Windows Vista will disable any configuration changes to the user. But despite this, I still wanted to mention this situation, because sometimes it will help you solve the problem.

It is much more likely a problem with the address conflict occurs when using multiple DHCP servers and DHCP servers, these are overlapping sets of addresses. If you have only one DHCP server on your network, you do not make mistakes, and do not rule out the possibility of such a situation on your network. There is a possibility that your network has a pirate (rogue) DHCP server that conflicts with your main server DHCP.

Windows 2000 Server and Windows Server 2003 are designed in such a way as to avoid problems with piracy (rogue) DHCP servers. They DHCP server to assign IP addresses only after it has been authenticated Active Directory. But the problem is that it is applicable only for servers DHCP, which work on Windows. Server DHCP, running on other operating systems can assign IP addresses to clients without the need to be authenticated Active Directory.

So is there any difficulty installing pirated server DHCP, which runs on Linux? Probably not. The explanation that makes more sense is that the problem is a wireless access point or router. Such devices almost always have a built-in DHCP server. These devices generally use a 192.168.0.x address range or 192.168.1.x. If it so happens that the same IP address range used on your primary DHCP server that then you run into the situation where both the DHCP server assign addresses from the same interval, which leads to conflict.

Conclusion

In the above article, I explained that there are a number of potential cases in which failures may occur in the work of DHCP. In most cases, failures related to problems with the connection between the DHCP server and workstations that are trying to address.