Shulou(Shulou.com)06/01 Report--

Basic concepts of DNS

Root domain

Root domain is often said ".", such as Baidu URL www.baidu.com, its real domain name should be www.baidu.com. (last but not least), but for convenience, the latter point will be omitted when typing in the browser, which has now become a habit of users.

The root domain server has 13 IP addresses, but the number of machines is not 13. Because these IP addresses use anycast technology, these IP mirroring sites can be set up all over the world. The IP accessed by users is not the only host.

Domain division

Below the root domain is the top-level domain or first-level domain, and there are two ways to divide it. One is com.,net, which is divided according to the nature of the industry when the Internet was just emerging. And so on, one is classified by country, such as cn.,jp. Wait.

Each domain will have a domain name server, also known as an authoritative domain name server.

Baidu.com is a top-level domain name, while www.baidu.com is not a top-level domain name. It is a host called www in the domain of baidu.com.

After the first-level domain, there are second-level domain and third-level domain. If a user buys a top-level domain, sets up his own BIND server (or other software) and registers to the Internet, then he can add a few more domains in front of him at will (of course, the length is limited).

Domain name server

A domain name server is a server that can provide domain name resolution. The above record types can be A (address) records, NS (name server) records, MX (mail), CNAME and so on.

A record is to record an IP address and a host name. For example, a domain name server is located in test.baidu.com, which is a second-level domain name, and then there is an A record in the domain name server, which records the IP of a host.

Now if you want to query a.test.baidu.com through the domain name server baidu.com, then the top-level domain name server will find that the requested URL is in the domain test.baidu.com. The baidu.com domain name server records the IP of the domain name server test.baidu.com of this secondary domain. When querying, the IP of NS will be returned to the user, and the user will use this IP to check the address of host a.

The domain name servers in these domains are called authoritative servers and provide DNS query services directly. (these servers do not do recursion)

Analytical process

The following is a brief description of the common domain name resolution process:

Now that there is a computer connected to the Internet through ISP, ISP will assign it a DNS server. This DNS server is not an authoritative server, but a proxy DNS parsing server. It will help PC iterate the responses returned by the authoritative server, and then return the final IP to PC.

At this point, the computer initiates a request to this ISPDNS to query the domain name www.baidu.com (actually, it is not ISPDNS, but the DNS in the network settings of the user's computer, not necessarily ISPDNS, such as 8.8.8.8).

After ISPDNS gets the request, check whether there is this address in the local cache, and return it directly if so. The IP address obtained at this time will be marked as a reply from a non-authoritative server.

If not in the cache, ISPDNS will read the addresses of 13 root domain name servers from the configuration file (these addresses are unchanged, directly in the BIND configuration file)

And then make a request to one of them.

After the root server gets the request, it knows that it is com. Under this top-level domain, so the NS records in the com domain are returned, usually 13 hostnames and IP.

Then ISPDNS initiates a request to one of them again, and the server of com domain finds that the request belongs to baidu.com domain and finds out the NS of this domain, so it returns its IP (Baidu currently has 4 top-level domain name servers of baidu.com).

ISPDNS once again initiates a request to the authoritative server in the baidu.com domain. After baidu.com receives it, it looks for the www host and returns the host IP to ISPDNS.

After ISPDNS gets it, it returns it to the client and stores the IP in the cache.

At this point, the whole domain name resolution process is completed.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.