Shallow into DNS

Domain Name System DNS mainly resolves the domain name and IP address conversion, it is designed as an online distributed database system, and the use of customer-server model. One of the advantages of DNS is that most domain names are parsed locally, and only a small amount of parsing is required to communicate over the Internet, so DNS is highly efficient and because DNS is a distributed system, a single node error does not affect the operation of the entire system. The domain name server can be divided into four categories, according to the tree hierarchy structure is as follows.

Root name server: It is the highest level of the domain name server, is also the most important domain name server, all the root domain server has saved all the top-level domain name server IP domain name table. Regardless of the local domain name server, if it is not resolved locally it will first request the root name server, so if the root name server is paralyzed, the entire DNS system will be paralyzed. In general, the root name server does not hold all the domain name of the IP address mapping table, it is not possible to store so many, root name server often is to tell the client next to request which top-level domain name server.

Top-level Nameservers: It is responsible for managing all two-level domain names registered by the top-level domain server, and when it receives a request from the client, the answer may be the final result or the IP address of the top-level domain server that should be requested next.

Domain Name server: It is responsible for a domain name server, a domain has one or more zones, it receives the client's request may give the final IP address or the next step should be to request the right domain name server.

Local domain Name server: This is also important for the domain Name System, possibly a university or a department with a local domain name server. When we configure the IP address, we fill in a DNS server address, which is our local name server, which is generally closer to the user. When a host sends a DNS query request, this query request message is sent to the local domain name server.

Now understand the 4 kinds of domain name server, the most important is to understand the domain name server layering idea. Next is the process of a complete request. First, the application process call resolver constructs the UDP data packet and becomes a DNS client, this message has the unresolved domain name. This packet is then sent to the local domain name server, if the local domain name server stored the corresponding domain name address mapping table will return the IP address of the domain name results to the client. If the local domain name server does not find the results, the local name server will now become a new customer. It will go to the root name server, the root server can either give the results or tell the local name server next to request which top-level nameservers, and then the local domain name server to request the next top-level name server, it will tell the local name server to request a domain name server, Next, the domain name server will return the IP address corresponding to this domain name. The last local domain name server then returns the result to the host.

In the local domain name server there will be a DNS cache, and the ARP cache is a concept that stores the domain name IP address mapping table. There is also a cache in the local host, which requests the local domain name server when no results are found in the native cache. Many hosts synchronize the DNS cache of the local domain name server at startup, and then dynamically update the cache database.

Shallow into DNS