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.