Six major DHCP faults

Network problems are constantly emerging, which also causes headaches for administrators. Here we will introduce the content about DHCP troubleshooting. In a large LAN, the IP address of the workstation is usually obtained dynamically from the DHCP server, which can effectively save IP Address Resources; however, during the long-term contact with the DHCP server, we may encounter various network faults related to the DHCP service, which greatly affects the operating efficiency of the LAN network. In view of this, this article will explain several common network faults related to the DHCP service and provide corresponding countermeasures for various faults, I hope these contents will help you better manage your local network!

DHCP troubleshooting 1. Eliminate Authorization failure

Some servers in the network accidentally crashed, but after restarting the server system, it was found that the DHCP server could not automatically assign IP addresses to other workstations. when checking the specific parameters of the DHCP server, the system prompts that authorization is not performed. However, in the domain control server, the system prompts authorization. In the face of a failure in DHCP server authorization, how can we cope with this problem so that the DHCP server can be re-assigned with IP addresses automatically?

In fact, the reason why the domain control server shows that it has been authorized is that the domain control server automatically "remembers" the fault. To re-assign IP addresses to the DHCP server, you only need to cancel the original DHCP server authorization on the domain control server, and then authorize the DHCP server again, so that the DHCP server can take effect.

DHCP troubleshooting 2. IP address failure Elimination

A workstation in the LAN re-installs the operating system due to unexpected reasons. However, after the system is installed and the LAN parameters are set, the workstation cannot access the Internet. When a fixed IP address and other network parameters are set for the workstation, the workstation is immediately connected to the LAN. Obviously, when the workstation is set to automatically obtain the IP address, the DHCP server in the LAN cannot assign an IP address to the workstation.

To check whether there is a problem with the DHCP server, I went to another workstation in the LAN and executed the "ipconfig/renew" string command to find that the DHCP server can assign an IP address to the workstation, this phenomenon indicates that the DHCP server itself is normal. why can't the workstation of the reinstallation system obtain the IP address from the DHCP server? Is the available IP address of the DHCP server used up? For this reason, I came to the DHCP server and checked the server parameters. The result showed that the scope of the server was 192.168.1.10 ~ 192.168.1.42, 30 IP addresses in total. In addition, all IP addresses are used up in the lease cache settings of the DHCP server. However, I am puzzled that there are only 22 workstations in the LAN, so where are the other eight available IP addresses allocated? Later, in the lease cache settings of the DHCP server, I found that the other eight IP addresses were allocated to the laptop that was previously obtained from the Enterprise. Although these laptops only temporarily access the Internet, once they get an IP address, they will continue to occupy them until the "lease" is removed, why are these unused laptops occupying the IP addresses assigned to them all the time? In the past, there was a problem with the "lease" setting of the DHCP server. I found that the lease time of the server to the customer was set to "365" days. Obviously, this "lease" is too long, so that those laptops will forcibly occupy those IP addresses even if they are not connected to the network. To release the other eight IP addresses from the laptop, we only need to go to the settings page shown in 1, and set the "lease" period of the DHCP server to be slightly shorter, for example, in a week or so, you also need to delete the lease cache content from the DHCP server so that the other eight IP addresses occupied by the notebook can be released. Once the instance is successfully released, the system's workstation can automatically obtain the IP address from the DHCP server as soon as it is attached to the LAN.

Figure 1

DHCP Fault Handling 3. Eliminate DHCP unavailability

There is a computer installed with Windows 2003 Server in the LAN. It not only acts as a file Server for the LAN to back up all kinds of important file information, but also serves as a DHCP Server to assign IP addresses to the workstation. Workstation users often use FTP tools to transmit large multimedia files to the server for storage. One time, when a colleague transmitted data to the server in a workstation, he found that the workstation could not obtain the available IP address from the DHCP server, and the prompt that the DHCP server was unavailable appeared on the computer screen, in this way, the workstation will not be able to access the Internet, and the prompt will still appear after the workstation system is restarted.
After careful analysis of the fault, it is not difficult to find that the cause of the fault is definitely related to the DHCP service. When a DHCP server is working, the workstation usually sends an IP address request for broadcast information to the network. Once the DHCP server receives such broadcast information, it sends a Response Message to the workstation, this includes a valid IP address. If the DHCP server cannot respond to the wks request in a timely manner, the network connection initialization of the wks will fail, at this time, the wks system will automatically assign a random IP address for itself, and repeatedly send IP address request information to the network at intervals in the system background until the information of the DHCP server is received. If the workstation receives the IP address assigned to it by the DHCP server, it automatically uses the IP address to reconnect to the network. According to the working principle of the DHCP server, the reason why the workstation cannot obtain the IP address is that the DHCP server does not respond in time, or the workstation does not receive the response from the DHCP server due to network connection issues; according to this analysis, we first tested the network connection line with a professional wire measuring instrument. The test results show that all the connection lines are normal, in this way, we can be sure that the DHCP unavailable fault is caused by the failure of the DHCP server to respond to the wks request in time. Why does the DHCP server not respond to the wks request? After careful inspection, it was found that other colleagues in the LAN uploaded multimedia information to the server at the same time, resulting in the CPU resource usage of the server system reaching more than 80%, in this way, the server system will not be able to free up enough resources to respond to the wks IP address request. When all data transmission connections on the server are interrupted and all other invalid processes in the server system are recycled, the DHCP server in the workstation disappears immediately after it becomes unavailable. To sum up the above troubleshooting process, it is not difficult to see that if the server itself is heavily burdened, it will easily cause the DHCP server installed in it to fail to respond to the address application request from the workstation in a timely manner, in this way, the workstation is prone to failures that cannot access the Internet. In order to avoid such faults as much as possible, the hardware configuration of the server should be improved as much as possible, on the other hand, it is necessary to regularly recycle invalid application processes on the server to release the server resources that have been occupied for a long time.

DHCP troubleshooting 4. Eliminate Configuration Parameter loss

A lan server in the organization suffers from paralysis due to unexpected reasons. As a result, all DHCP configuration parameters installed on the server are lost. After the server system is reinstalled, the DHCP server can resume normal operation only after manual configuration. But if we cannot remember the previous configuration parameters, how can we make the DHCP server work properly according to the original configuration function? In fact, if we back up the configuration parameters when the DHCP server is running normally, then we can quickly "move" the previous configuration parameters to the newly installed DHCP server. Now I will take the Windows 2003 operating system as an example to introduce how to quickly and effectively back up and restore the configuration parameters of the DHCP server:

Before backing up the configuration parameters of the DHCP server, we should first check whether the server can normally provide IP Address Allocation services for the LAN workstation. If the DHCP server is working normally, then we can go to the desktop of the server operating system, and click "start"/"run" command in sequence. In the displayed system running dialog box, enter the string command "cmd ", click OK to switch the system interface to the MS-DOS working state;

At the doscommand prompt, enter the string command "netsh dhcp server export X: \ aaa.txt hosts" and copy the file to another safe location;

Figure 2

The netsh DHCP server import X: \ aaa.txt alltxt, in which the xforwarded‑aaa.txt file is located as the disk partition symbol. After you click the Enter key, the system will automatically resume the DHCP server configuration parameters;

When the system prompts that the recovery operation is successful, click "start", "set", "Control Panel", and "Management Tools" commands in sequence, double-click the "DHCP" icon on the interface to enter the DHCP Console window to reactivate the DHCP server. In this way, the DHCP server can work properly according to the original configuration function.

DHCP Troubleshooting 5. Eliminate the slow opening of the Configuration window

When we change or adjust the Internet parameters of LAN workstations, we often encounter such a fault, that is, the speed of opening the network parameter configuration interface is very slow, in severe cases, the system crashes. How can we solve this problem? In fact, this fault may occur because the workstation is set to dynamically obtain the IP address, workstations often communicate with DHCP servers on the LAN through broadcasting information. If the DHCP server does not respond in time, the workstation will send an IP address application to the DHCP server at intervals, in this case, the configuration window opens slowly. To avoid the slow opening of the Configuration window, we recommend that you set the IP address of the workstation to a static address, it is best to ensure a stable connection between the workstation and the DHCP server, and ensure that the DHCP server cannot be overly burdened.

DHCP troubleshooting 6. Eliminate the problem of address reservation being robbed

Generally, in order to prevent the IP address of the workstation from being "occupied" by others, we often use the "arp-s ip mac" command to bind the IP address to the physical address of the specified Nic, in this way, other people will not be able to connect to the Internet even if the IP address is used. However, if the IP address of the workstation uses the IP address reserved by the DHCP server, we cannot bind the IP address according to the previous method. So how can we avoid the DHCP server's reserved address being randomly "occupied" by others? In fact, with the built-in Netsh command in Windows, we can also bind the dynamic reserved IP address with the physical address of the specified Nic. Let's take a look at how to bind it together:

Click Start or run to open the system running dialog box, enter the string command cmd, and click OK, switches the system to MS-DOS mode;

Then, enter Netsh Dhcp Server xxx at the doscommand prompt. xxx. xxx. xxx Scope yyy. yyy. yyy. yyy Add reservedip zzz. zzz. zzz. zzz MAC "username" "Client" "Both string command, where xxx. xxx. xxx. xxx refers to the IP address of the DHCP server, yyy. yyy. yyy. yyy indicates the subnet network address of the DHCP server, zzz. zzz. zzz. zzz refers to the reserved IP address assigned to the DHCP server, MAC refers to the physical address of the workstation NIC that uses the reserved IP address, and username refers to the name of the specific account that accesses the DHCP server, the Client is used to bind the IP address from the workstation. By default, this parameter can be ignored and the Both parameter is used to obtain the dynamic reserved IP address from the DHCP server.

For example, if we want to bind the IP address of 192.168.10.18 to the NIC with the MAC address 66-48-53-88-99-00 on the DHCP server of 192.168.10.10, and wks access the DHCP server by the name "8888", then we can go to the MS-DOS command interface in the workstation with the MAC address 66-48-53-88-99-00, run Netsh Dhcp Server 192.168.10.10 Scope 192.168.10.0 Add reservedip 192.168.10.18 664853889900 "8888" both "in the command prompt line.