Application Layer: DNS Domain Name Resolution System

Application Layer: DNS Domain Name Resolution System

[Computer Network] Application Layer: DNS Domain Name Resolution System (Domain Name | Domain Name Server | Domain Name Resolution Process | Recursive Query | Iterative Query | Cache)

domain name:

① Domain name representation method: letters, numbers, "-" symbols, "." symbols;

② Label: A string separated by "." is called a label, and each label cannot exceed 63 characters ;

③ Label level: The label level in the domain name, from left to right, with priority from low to high;

④ Example of domain name: www.baidu.com, where com is the top-level domain name, baidu is the second-level domain name, and www is the third-level domain name;



Detailed analysis of each part of the domain name:



① Root: "www.baidu.com." is the full form of the domain name, and the root is the last dot ".", which can be omitted;

② TLD:

ccTLD: cn
gTLDs: com , org
Infrastructure domain name: reverse domain name arpa
③ Second-level domain name:

Category domains: com , org , gov
Administrative area domain name: province, autonomous region, municipality directly under the Central Government, bj , js
self-registered domain name
④ Third-level domain name: After obtaining the second-level domain name, the third-level domain name can be resolved;

⑤ Fourth-level domain name: www , mail

2. DNS Domain Name Resolution System.Domain Name Server

Domain Name Server:

① Root DNS: The root DNS knows the addresses of all TLD servers;

② Top-level domain name server: The top-level domain name server manages all the second-level domain names registered by the top-level domain name server; such as com top-level domain name server, org top-level domain name server;

③ Authority domain name server: The domain name server responsible for a zone; such as a.org domain name server, aaorg domain name server, these two domain name servers are equal;

④ Local domain name server: The host sends a DNS query request to the local domain name server;

3. DNS Domain Name Resolution System.Domain name resolution process

Domain name resolution process:

recursive query
iterative query

DNS Domain Name Resolution System.Fourth, recursive query

Recursive query: delegate to others to find;


DNS Domain Name Resolution System.Delegated search: first search in the local domain name server, if not, delegate the root domain name server to search, if not, the root domain name server delegates the search in the top-level domain name server, if not, the top-level domain name server delegates the authority domain name server to search;
Reverse return result: After finding the IP address corresponding to the domain name, according to the delegation order, authority domain name server -> top-level domain name server -> root domain name server -> local domain name server -> host, return the IP address corresponding to the searched domain name;


Five, iterative query

Iterative query: find by yourself; the core is the local name server;

DNS Domain Name Resolution System.The host queries the local domain name server for the domain name;
If the local domain name server is not found, search the root domain name server, and if the domain name server is not found, pass the top-level domain name server address to the local domain name server;
The local domain name server continues to query the domain name from the top-level domain name server. If it is not found, it will send back the authority domain name server address to the local domain name server;

The local domain name server continues to look up the authority domain name server, and if the address corresponding to the domain name is found, the result is returned;
The local domain name server transmits the final query result to the host;




6. Cache

Cache: Cache is set in the host, local domain name server, root domain name server, top-level domain name server, and authority domain name server to cache domain name information; when searching for domain names recursively or iteratively, the domain name is searched from the cache first, if not, and then perform related operations;

the top-level domain name server address is stored in the cache of the local domain name server, there is no need to visit the root domain name server, and the top-level domain name server can be accessed directly;