DNS server Introduction

DNS, domainnamesystem, or domainnameservice (Domain Name System or multiple-Name Service ). The domain name system assigns domain name addresses and IP addresses to hosts on the Internet. When a user uses a domain name address, the system automatically converts the domain name address to an IP address. Domain Name Service is an internet tool that runs the domain name system. The server that executes the Domain Name Service is called the DNS server, and the DNS server is used to respond to the query of the Domain Name Service.
1. DNS is the Domain Name Server, and its task is to determine domain name resolution, such as a record MX records.
2. Any domain name must have at least one DNS, usually two. But why more than two? Because DNS can be processed in a loop, you can find the second one if the first resolution fails. In this way, as long as a DNS resolution is normal, the normal use of the domain name will not be affected.
3. How to Determine the DNS of a domain name
It's easy to enter the domain name you want to query at www.internic.net/whois.html. This is the international domain name management center. The only authority. If a domain name can be found here, it indicates that the domain name takes effect. It indicates when you expire, that is, when you expire.
4. valid DNS indicates who is currently working on the DNS server, for example, the query result is ns. xinnetdns. com, NS. xinnet. CN indicates that the current domain name is composed of ns. xinnetdns. com, NS. xinnet. CN (xinnet Xinhai) is responsible for parsing. Other DNS settings are invalid.
5. DNS can be modified. After modification, it takes 24-72 hours for the whole world to refresh. The InterNIC information is generally displayed 24 hours later. In addition, the modification process does not mean that the domain name will stop parsing, as long as you have completed resolution on both sides. If it takes effect, the new DNS will take effect. If not. That is, the old DNS is working. It either takes effect or does not take effect. There are no two periods that do not work. DNS has a cache.
1) Visitor's computer; 2) your ISP access provider.
For example, if you access www.askbaidu.com, your computer first queries whether www.askbaidu.com records are cached on the local computer. If yes, you can directly call it to stop searching. That is to say, if you have just visited www.askbaidu.com, the DNS and NS. xinnetdns. com and NS. xinnet. CN of China Telecom cannot be resolved. The domain name can also be resolved normally.
It is easy to clear the local DNS cache. Disable IE, Clear history records, or restart your computer. Then there is the DNS cache of the ISP access provider. ISP is the local network access provider. For example, Fujian Telecom, Fuzhou Netcom, and Nanping tietong. Every place is different. The DNS of the ISP is different from that of NS. xinnetdns. com and NS. xinnet. CN (xinnet xinxin Hai. NS. xinnetdns. com and NS. xinnet. CN (xinnet xinxin Hai) are only responsible for specific resolution and are not responsible for caching. Isp dns is only responsible for query and cache, not for resolution.
Briefly describe the access to www.askbaidu.com. If the www.askbaidu.com record does not exist on the local machine. Your computer will query the DNS of your local ISP. Isp dns only has cache. That is to say, he will check whether there is any cache for www.askbaidu.com. If yes, he will directly send the records of www.askbaidu.com to the user. The user can access it. If the ISP cache does not contain any records from www.askbaidu.com, then what is the DNS of askbaidu.com? Then, the data is directly obtained on the corresponding DNS and returned to the user. When the first user accesses www.askbaidu.com, the records of www.askbaidu.com are cached on the DNS of the ISP. In the future, he will not have to go to NS. xinnetdns. com or NS. xinnet. CN (xinnet xinxin Hai) again. It will not be checked unless there is a new domain name. For example, when you access bbs.askbaidu.com, you need to re-query it.
7. there is a time limit on the DNS cache of the ISP. It generally takes 1 hour. Two times before and after the interval of one hour, he will go to the DNS of the domain name to obtain data again. This is the first comparison with the current one. That is to say, if the time difference is large, you can try again on the DNS server of the domain name. Therefore, it is necessary to refresh the data. Otherwise, the data is cached once. After the Domain Name Record is changed. ISP will never find a new record. After understanding this principle, you will understand why records that have not been registered and become effective soon. The modification takes effect very slowly. This is because of the cache. However, if no cache is available, the access efficiency will be very low, because any time you enter www.askbaidu.com, you have to run to NS. xinnetdns. com, NS. xinnet. CN (xinxinxinxinxinxin Hai) to query records.
Note: domain name resolution systems of many domain name vendors are not refreshed in real time. Generally, the time is set, for example, 10 minutes. That is to say, after you set a new a record, the domain name server will add it to you within 10 minutes. The purpose is to save server resources. The customer's DNS keeps refreshing records. Refresh records must consume a certain amount of resources. It cannot be parsed during the refresh process. In addition, the refresh process takes about 5 seconds. That is to say, the DNS of the domain name provider cannot be used within five seconds.