Learn from me – DNS troubleshooting tips

Day green, wild boundless, website a paralysis, only tears two lines!! Customer jump, the boss called, analysis fault, heart startled and meat jump!!

For enterprise website, very afraid of the site can not open the situation, once happened, will find the company’s technology department to present a scene of mourning. In order to make the operation and maintenance of the brothers and sisters to do a delicate small white-collar, xiaobian special summed up for you a set of “domain name resolution troubleshooting skills practical operation of the network most complete manual”, and is divided into the “initial version” “advanced version”, with me to protect you in troubleshooting troubleshooting faults, cerebral circum god is clear, minutes can locate the problem. In order to help you reduce customer unavailable time, and win the trust of the boss, today let’s listen to xiaobian with everyone that domain name resolution thing.

DNS is the gateway for Internet traffic. If an enterprise website is found to be unavailable, o&M personnel need to immediately troubleshoot domain name resolution faults. Secondly, we need to know that domain name resolution failure and website access are two concepts, because the website access and domain name resolution, website server, website record, website procedures, network environment and many other factors are related, and this article focuses on the description of domain name resolution fault troubleshooting methods and ideas.

Most users report domain name resolution failure, in fact, is not necessarily a real domain name resolution problem, xiaobian here for you to summarize the four most common reasons, can help you quickly locate the cause: first, because the domain name expired, locked will lead to the normal access to the domain name. ① First of all, we need to know why domain name abnormalities directly lead to domain name resolution abnormalities? Domain names are upstream of domain name resolution. Therefore, if the domain name is abnormal during recursive resolution, the authoritative DNS server cannot receive DNS query requests and complete the response task. Secondly, we need to know the domain name in what scenarios will lead to domain name resolution abnormalities?

Finally, we further understand how to locate the cause and solve it. Aliyun DNS provides domain name detection tool for users, which can query domain name validity period and domain name status with one click.

First of all, we need to know that there is no direct relationship between domain name resolution and the record. Even if the website is abnormal, it does not affect the setting and effect of domain name resolution. However, abnormal filing will directly affect the access and use of the website. You can use the domain name detection tool to query the filing status with one click. If there is an anomaly in the filing check, you can contact the website server supplier to locate the cause of the anomaly and solve it.

Check whether the domain name and DNS server name have been changed within 48 hours

After a DNS server is modified, the effective time of resolution depends on the TTL of the DNS server name cached in the local DNS, which generally takes 24 to 48 hours. Therefore, within 48 hours after the DNS server is changed, the resolution data of the new DNS server and the old DNS server must be consistent. If you delete resolution records from the original DNS server, do not add resolution records to the new DNS server, or add resolution records to the new DNS server and the old DNS server are inconsistent, Can result in parsing not being available, or a site that is sometimes healthy and sometimes abnormal. Conclusion: If the fault is caused by this condition, you can perform operations on the new and old DNS service providers to ensure that the resolved data is consistent and fast recovery can be achieved.

① Check whether the resolution record is set correctly

You are advised to use the domain name detection tool to check the resolution result of the DNS service provider. If “DNS resolution record of this domain name is not detected” is displayed, you are advised to check whether the resolution record is set correctly. For example, is this domain name not set, or is deleted by “misoperation” or “accidentally”? Or was there a typo in modifying the parse record? In a word, the cause of the error is possible, we first exclude the abnormal error caused by configuration reasons.

② Check whether the resolution record has been modified recently. If you modify the resolution record, the resolution takes effect only after the local carrier’s Localdns cache expires. Generally, the validity time of the modified resolution depends on the TTL setting of the domain name. You can use the domain name detection tool to check the TTL validity time. Wait until the TTL validity time and check again.

(3) check whether the intelligent analytical lines is not set the default line cloud the analytical DNS smart line, parsing is in accordance with the “line” priority sorting application of the line, “default” line, if your visitor source does not belong to “beat” line, and the administrator has not set the “default” scene, can cause this part of the user cannot access the site.

If the above preliminary investigation, the results of the investigation are normal, then you can follow the xiaobian into the advanced version of learning.

Before entering the advanced version, to deeply understand the abnormal situation of domain name resolution, we must first understand the recursive process of domain name resolution, many students should not know the 110 routines of resolution, simply put, in the recursive DNS no cache scenario, 1 domain name resolution needs to experience 10 query interaction.

From the above analysis process, we can see that the process of domain name resolution is to experience a lot of query interaction, any problems in the link may lead to resolution does not take effect, so the next small series will teach how to check and resolve abnormal problems in each link. Generally, the resolution failure is defined as the failure to find the server IP address to which the domain name refers in a DNS query request or the queried IP address is different from the expected one. The authoritative DNS resolution result is inconsistent with the recursive DNS resolution result, local DNS resolution result, or public DNS resolution result, or no data is displayed in the query result.

The authoritative DNS is the DNS server that manages the mapping relationship between domain names and IP addresses. The cloud DNS provided by Ali Cloud is the authoritative DNS. Therefore, the first step, xiao Xiao teaches you to check the authoritative DNS resolution data first. Use the domain name detection tool to view the resolution result of the DNS service provider, as shown in the following figure.

(1) Check whether the resolution result is consistent with the resolution record set on the DNS server console. If yes, the authoritative DNS resolution takes effect. (2) If the resolution result of the DNS service provider indicates that the DNS resolution record is not set, check whether you forgot to set the resolution record or deleted the resolution record by mistake. ③ The check result is inconsistent with the resolution record set on the DNS server console. In this case, contact your DNS server to refresh the resolution data.

The local DNS server is a DNS server that responds to recursive requests from clients and finally traces the DNS server to obtain the resolution result. For example, DNS automatically assigned by the user or ISP can be called recursive DNS. Use the domain name detection tool to download the client query tool from the local DNS check module.

The result is as follows:

If the authoritative DNS resolution is correct and the local DNS resolution result is inconsistent with the authoritative DNS resolution result, domain name resolution does not take effect on the local user. So why does local parsing not work? The possible causes are as follows: (1) The cache information on the local DNS server does not expire. -> In this scenario, you only need to wait for the TTL displayed at the local DNS resolution office to expire and test again. (2) If the local DNS resolves the TTL after it expires and the result is still inconsistent with the authoritative DNS resolution result, try to check whether the public DNS resolution result is consistent. 3 If the public DNS resolution result is the same as the authoritative DNS resolution result, both the authoritative DNS and public DNS are valid. The local DNS does not take effect. In this case, refresh the local DNS server and test it, or change the local DNS server to a public DNS server.

Rest assured, in order to help you deepen your understanding and memory, xiaobian has carefully drawn a simplified version of the Check List for you, a table in hand, let us help you to “type down” the troubleshooting and parsing of abnormal faults.

Third, rule out whether the domain name is blocked to cause the domain name resolution exception

① Through the domain name detection tool, such as recursive resolution tracking error “domain name recursive resolution process is contaminated”.

Generally, the domain name is blocked by the superior network facilities during the normal resolution process, which is beyond the authoritative DNS service capability. In this case, xiaobian also regrets that it cannot provide a better solution, and generally recommends that users change their business domain names.

② Determine which part of the recursive parsing process is “polluted”

You can also run the dig+trace command. When the local DNS queries the root DNS server, the IP address is directly returned. Therefore, it can be judged that the DNS query is blocked at the root DNS server.

This time to share the first here, finally, I hope that xiaobian for you to summarize the “domain name resolution troubleshooting skills practical operation of the most complete network manual”, can effectively help you quickly locate the cause of the resolution exception and reduce the service unavailable time.

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