DNS basics

1, DNS is the domain name server, his task is to determine the resolution of the domain name, such as A record MX records and so on. 2. Any domain name has at least one DNS, usually two. Why do you want more than 2? Because DNS can be processed in turn, the first one can fail to find the second one. As long as there is a DNS resolution normal, it will not affect the normal use of the domain name. 3. How to determine the DNS of the domain name? It's very simple to go to http://www.internic.net/whois.html and enter the domain name you want to query. This is the International Domain Name Management Center. The only authority. As long as a domain name can be found here, it means that the domain name is valid. It says when you expire and when it expires. 4. The valid DNS indicates who the DNS server is currently working. For example, the query result is NS.XINNETDNS.COM, NS.XINNET.CN (new network letter sea), indicating that the current domain name is NS.XINNETDNS.COM, NS .XINNET.CN (new network letter sea) is responsible for analysis. Other DNS settings are invalid. 5. DNS can be modified. After the modification takes 24-72 hours, the world can be refreshed. Internic information is generally visible after 24 hours. In addition, the process of modification does not mean that the domain name will stop parsing, as long as you have resolved on both sides. If it works, the new DNS is working. If it does not take effect. That is, the old DNS is working. Either it will take effect or not. There are no two times when they don't work. Therefore, domain name resolution will not be interrupted. The premise is that both sides have been parsed. 6, DNS is cached. 1. The visitor's computer; 2. Your ISP access provider. A simple example: For example, if you visit www.tydncn.com, your computer first queries whether there is a cache of www.tydncn.com on this machine. If there is a direct call, no longer look up. That is to say, if you have just visited www.tydncn.com before, this time even the telecommunications DNS and NS.XINNETDNS.COM, NS.XINNET.CN (new network letter sea) can not be resolved. It is also able to resolve the domain name normally. Clearing the native DNS caching method is simple. Close IE and clear the history, or restart your computer. Then there is a cache of DNS for the isp access provider. Isp is the local network access provider. For example, Fujian Telecom here; Fuzhou Netcom, Nanping Railcom, etc. Every place is different. The DNS of isp is different from DNS such as NS.XINNETDNS.COM and NS.XINNET.CN (new network letter sea). NS.XINNETDNS.COM, NS.XINNET.CN (new network letter sea) is only responsible for the specific analysis, not responsible for caching. Isp's DNS is only responsible for querying and caching, and is not responsible for parsing. Briefly describe the situation just visited www.tydncn.com. If there is no record of www.tydncn.com on this machine. Your computer will go to the local ISP's DNS. Isp's DNS is only cached. That is to say he will check if there is a cache of www.tydncn.com. If so, he will send the record of www.tydncn.com directly to the user. Users can also access it. If there is no record of www.tydncn.com in the ISP's cache, then what is the DNS of tydncn.com? Then go to the corresponding DNS to get the data directly and return it to the user. When the first user visited www.tydncn.com, the dns of the isp began to cache the records of www.tydncn.com. In the future, he will not have to go to NS.XINNETDNS.COM, NS.XINNET.CN (new network letter sea) to find. He will check it unless there is a new domain name. For example, when visiting bbs.tydncn.com, he will check it again. 7, isp's DNS cache is time-limited. It is usually 1 hour. After two hours between the two times, he went to the DNS of the domain name to retrieve the data. Here is the comparison between the previous and the current. That is to say, if the time gap is large, then go to the DNS server of the domain name again. So refreshing is necessary, otherwise it will be cached once. The domain name record has been changed. The ISP will never find a new record. After knowing this principle, everyone will understand why the registration of the original record does not take effect. Modifications will take effect slowly. It is because of the cache. But if there is no cache, the access efficiency will be very low, because any time you enter www.tydncn.com you have to go to NS.XINNETDNS.COM, NS.XINNET.CN (new network letter sea) to query the record. Note: Many domain name providers' domain name resolution systems are not refreshed in real time. Usually set the time, such as 10 minutes. That is to say, after you set a new A record, the domain name server will be added for you within 10 minutes. The goal is to save server resources. Afraid of the customer's DNS constantly refreshing records. Refreshing the record definitely requires a certain amount of resources. And it can't be resolved during the refresh process. The refresh process is about 5 seconds. That is to say, the domain name provider's DNS is not available within 5 seconds.