DNS resolution failure

Source: Internet
Author: User
Tags domain name server nslookup nslookup command website ip
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The customer called at last night and said that the website could not be accessed. He immediately ran up, entered the domain name, and asked my friend to enter it.
Change access from IP address OK, DNS problem. I came to the company early this morning to find out what this is ---

 

The following information is from Baidu.

Resolution fault

In actual application, DNS resolution errors may occur, that is, when we access a domain name, it cannot be resolved to an IP address, however, you can directly enter the website IP address for normal access, which is caused by a DNS resolution failure. This phenomenon has a high probability of occurrence, so this article will teach you some basic troubleshooting methods from scratch.

  

Generally, the address we access is called a domain name, and it is well known that any host on the network is identified by an IP address. That is to say, only the IP address of the website can be successfully accessed.

 

However, because the IP address information is not easy to remember, the domain name is displayed on the network. During access, we need to enter this domain name that is easy to remember, there will be a server in the network that automatically resolves the corresponding domain name to an IP address. This is the DNS server. The machine that can implement DNS resolution can be either its own computer or a computer in the network. However, when DNS resolution fails, for example, resolving a domain name into an incorrect IP address, or, if we do not know the IP address corresponding to a domain name, we cannot access the site through the domain name. This is a DNS resolution failure.

The biggest symptom of a DNS resolution failure is that the IP address corresponding to the website is correct, but an error occurs when accessing the domain name.

When a DNS resolution fault occurs on our computer, don't worry. The solution is also simple.

  (1) Use NSLookup to determine whether a DNS resolution fault is true:

To fully determine whether a DNS resolution fault exists, you must use the NSLookup provided by the system.

Step 1: confirm that your system is Windows 2000 or above, and then press Start> RUN> Enter cmd to enter the command line mode.

Step 2: Enter the NSLookup command and press enter to go To the DNS resolution query page.

Step 3: the command line window displays the DNS server address used by the current system. For example, the IP address of the author's DNS server is 202.106.0.20.

Step 4: Enter the domain name of the site you cannot access. If it cannot be accessed, DNS resolution may fail. We will receive a prompt message for DNS request timed out and timeout was 2 seconds. This shows that our computer does have a DNS resolution fault.

Tip: If DNS resolution is normal, the correct IP address will be returned.

  (2) check whether the DNS server works properly:

In this case, we need to check the DNS address used by our computer and its running status.

Step 1: confirm that your system is Windows 2000 or above, and then press Start> RUN> Enter cmd to enter the command line mode.

Step 2: Enter the ipconfig/all command to query network parameters.

Step 3: In the ipconfig/all display information, we can see a place where DNS servers is written. This is the address of our DNS server. For example, the author is 202.106.0.20 and 202.106.46.151. It can be seen from this address that it is an Internet address. If an Internet DNS resolution error occurs, we can change the address of another DNS server to solve the problem.

Step 4: If the DNS server displays the internal network address of your company, it means that your company's DNS resolution work is handed over to the internal DNS server, in this case, we need to check the DNS server and perform the NSLookup operation on the DNS server to check whether the resolution can be normal. The DNS service fault on the DNS server can be solved in general.

  (3) Clear DNS Cache Information:

When a computer accesses a domain name, it does not need to seek help from the DNS server every time it accesses the domain name. Generally, after the resolution is completed, the resolution entry is saved in the DNS Cache list of the computer, if the DNS resolution changes at this time, the DNS Cache list information does not change, and the computer will not connect to the DNS server to obtain the latest resolution information when accessing the domain name, it will be resolved based on the cache correspondence saved on your computer, so that DNS resolution will fail. In this case, we should clear the DNS Cache command to solve the fault.

Step 1: Choose Start> RUN> Enter cmd to enter the command line mode.

Step 2: In command line mode, we can see in ipconfig /? There is a parameter named/flushdns, which is the command to clear DNS Cache Information.

Step 3: run the ipconfig/flushdns command. When the prompt "successfully flushed the DNS resolver cache" appears, the cache information of the current computer has been cleared successfully.

Step 4: When we access the domain name again, we will get the latest resolution address on the DNS server, and there will no longer be a resolution error caused by the previous cache.

  (4) modify the hosts file method:

The modification of the hosts method is to modify the DNS resolution correspondence in the hosts file to achieve correct resolution. When accessing a domain name on the local computer, the hosts file in the Local System will be checked first. The priority of the Resolution relationship in the hosts file is higher than that on the DNS server.

In this way, if you want to bind a domain name to an IP address, you can add a resolution entry to the hosts file.

Step 1: Use "start-> Search" to search for a file named hosts.

Step 2: Of course, readers who already know the path can directly find the hosts file in the C:/Windows/system32/Drivers/etc directory. If your system is Windows 2000, search for it in the C:/winnt/system32/Drivers/etc directory. (Access denied during local modification)

Step 3: double-click the hosts file and select to open it with the "Notepad" program.

Step 4: We will see all the content in the hosts file. By default, there is only one line of content "127.0.0.1 localhost ". (Other rows with # In front of them are not real content, but help information)

Step 5: add the entries you want to perform DNS resolution to the hosts file. The specific format is to first write the corresponding IP address of the domain name, and then use a space to connect the domain name information.

Step 6: After the settings are complete, the website will be automatically parsed Based on the Intranet or Internet.

DNS query

DNS query can be interpreted in two ways: one is to query the resource records (such as a record) on the specified DNS server, and the other is to query the resolution process of the fqdn name.

I. query resource records on the DNS server

On Windows, you can use the command line tool to enter NSLookup. The returned results include the IP address (a record) and alias (cname record) corresponding to the domain name. In addition to the above methods, you can also query the DNS information of domain names through some DNS query sites, such as those in foreign countries.

Ii. query the resolution process of FQDN names

If you want to track the resolution process of an FQDN name, input dig www + trace in Linux Shell. The returned results include the recursive or iterative process starting with the domain and the authoritative Domain Name Server.

 

Check again and find that modifying the host file can also block the examples of websites you do not want to visit.

If we write the following content in hosts:

127.0.0.1 # website a to be blocked

0.0.0.0 # website B to be blocked

In this way, when the computer resolves domain names a and B, it will resolve them to the IP address of the local machine or the wrong IP address, to shield websites A and B.

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