Discussion on DHCP service troubleshooting

For the DHCP service, we have explained its definition, settings, and application content in some previous articles. So here we will discuss in detail the content of DHCP service troubleshooting. The following is a netizen's article for your reference. Recently, my friend's organization expanded the LAN and added more than 10 terminals to meet the Internet access requirements of more employees. I thought this expansion work was very simple, but who once thought that there was a strange fault after the newly purchased terminal was correctly set and connected to the LAN environment, some terminals can normally access resources in the LAN, but other terminals cannot access the Internet regardless of their settings. After repeated tests and practices, it was found that some terminals could not obtain valid Online parameters from the DHCP server due to improper DHCP settings. Now, this article will write down the troubleshooting process of this strange network fault for your reference and correction.

DHCP service troubleshooting: Fault symptom

The local area network of a friend's organization is not very large. There are about 80 common computers, all of which are set as members of a specific domain. To facilitate management, in the server system where the specified domain controller is located, a DHCP server is installed and all computers use dynamic addresses to dynamically obtain Internet access parameters from the DHCP server. In normal times, every common computer can log on to a specified domain to access Shared Resources in the LAN. However, a friend recently encountered a strange network fault, that is to say, the organization has bought 10 General computers. After the operating systems of these computers are installed and appropriate Internet parameters are set, my friend found that none of these new terminals could successfully log on to the specified domain on the LAN. Why?

DHCP service troubleshooting: Fault tracking

Considering that all new terminals cannot log on to the specified domain in the LAN, a friend subconsciously thinks that the switch connecting to the new computer has a problem. Therefore, he immediately rushed to the switch site connecting to the new computer, when observing the signal light status on the control panel of the switch, there was no exception, but my friend worried that the switch had a software error. In order to eliminate this problem, A friend specially restarted the vswitch background system and found that the newly added 10 computers still could not log on to the specified LAN domain. Later, he simply used his laptop, to test the working status of each switch port in sequence, and find that the switch connected to the new computer is working completely normal.

Since there is no problem with the switch connecting to the new computer, what causes 10 newly added General computers to be unable to access the lan? Considering that all new computers use dynamic IP addresses to access the Internet, will they not obtain valid IP addresses from the DHCP server of the LAN, as a result, they cannot log on to the specified domain controller on the LAN? When I think of this, my friend decides to test whether the newly purchased normal computer has obtained the correct Internet address. If I want to do so, I can log on to a new terminal at will, click Start or run. In the displayed system run dialog box, run the string command cmd to switch the system to the doscommand line status; at the command line prompt in this status, enter the string command "ipconfig/all" and click the Enter key. A friend is surprised to see that the address used by the terminal is 169.254.10.11, the subnet mask address has also changed to 255.255.0.0. What is the problem? Because the 169.254.10.11 address is the Internet address automatically assigned to the client by the Windows system, rather than the IP address allocated by the DHCP server in the LAN, no wonder the new terminal cannot log on to the specified domain controller of the LAN normally, the obtained Internet parameters are incorrect. Why cannot the new terminal obtain the correct Internet address from the DHCP server? Is it because the new terminal cannot access the DHCP server?

Because the first terminal in the LAN can use the DHCP server to normally obtain the Internet address and log on to the specified domain controller of the LAN, this indicates that the DHCP server is working normally, is there a problem with the status of a common computer, or is the network connection disconnected? After all, all new terminals cannot log on to the specified domain controller of the LAN at the same time, and the operating systems of these computers are almost just installed and configured, they work normally.

DHCP service troubleshooting: Find the slave

I decided to manually allocate a static address for these newly purchased terminals to see if they can access the LAN normally. In a new terminal, click Start, set, and network connection commands. In the displayed network connection List window, right-click the local connection icon and click the "properties" command from the shortcut menu to open the attribute Setting dialog box for the local connection. On the General tab page of the dialog box, select the TCP/IP protocol option and click "properties" to go To the TCP/IP protocol option dialog box, set the Internet address of the local terminal to the static address in the same working subnet as the domain controller host, set the gateway address and DNS server address, and restart the local terminal, the result shows that after the IP address is manually allocated, the new terminal can access the Internet normally and log on to the specified domain controller of the LAN smoothly. Obviously, the terminal is working normally, at the same time, there is no problem with the network connection. As we have determined, the possibility of problems with the DHCP server is relatively high.

In view of the ability of the DHCP server to provide dynamic Address Allocation services for other common computers in the LAN, a friend thinks that the DHCP server's working status should be normal, and the problem is probably due to its parameter configuration, in particular, its address pool parameters may be incorrectly set. Based on this inference, a friend immediately enters the domain controller host system of the LAN, and then click the "Start", "set", and "Control Panel" commands, from the pop-up system control panel window, double-click the DHCP icon to expand the DHCP Console window. Right-click the target host name in the list area on the left side of the page and choose "properties" from the shortcut menu, the properties Setting dialog box for the DHCP server is displayed. Here, my friend found that the address pool of the DHCP server ranges from 10.176.1.20 ~ 10.176.0.110, and almost all IP addresses in the address pool have been allocated. Generally, there are only about 80 common computers in the LAN, the total number of addresses that can be used in the address pool reaches 90. Theoretically, there should be the remaining IP addresses. When I checked the corresponding address Lease Term parameters, my friend found that there was no limit on the use time of all Internet addresses. Obviously, this time was too long, as a result, the valid IP address in the address pool cannot be released for a long time. So there are some idle IP addresses. which computers are occupying them? At this time, my friend accidentally remembered that the organization had concentrated on replacing Nic devices of some general computers, because there were no restrictions on the use time of these replaced Nic addresses, therefore, the idle Internet addresses must be occupied by the old network card devices. As a result, the DHCP server cannot correctly provide Dynamic Address Allocation services for the newly added General computers in the LAN.

DHCP service troubleshooting: Solves the fault

After finding the "sin" of the fault, it is easy to solve the fault. A friend immediately opens the DHCP server attribute Setting Dialog Box in the DHCP Console window, in which the IP address lease time is adjusted to the default 8 days, at the same time, all content in the address lease will be deleted, so that the DHCP server will re-allocate the Internet parameters for all common computers in the LAN in the future. After the above settings, my friend went online to test the newly purchased normal computer. The result showed that these general computers could successfully obtain the IP address from the DHCP server, you can also log on to the specified domain of the LAN.