User access to Web page process, DNS resolution process

First, the user access process

Second, the DNS resolution process

DNS, the domain Name System, is the acronym for a domain-organized hierarchy of computers and network service naming systems that are used in TCP/IP networks that provide services that are used to convert host names and domain names to IP addresses. As the saying goes, DNS is the conversion of URLs to external IP addresses.

　　

The first step: the browser will check if there is no resolved IP address for this domain name in the cache, and if so, the parsing process will end. Browser cache domain names are also limited, including the time and size of the cache, which can be set by the TTL attribute.

Second Step: If the user's browser is not in the cache, the operating system will first check whether the local Hosts file has this URL mapping relationship, if so, first call this IP address mapping, complete the domain name resolution.

Third Step: if the hosts do not have the mapping of this domain name, then find the local DNS parser cache, whether there is this URL mapping relationship, if there is, direct return, complete the domain name resolution.

Fourth Step: if the hosts and the local DNS resolver cache do not have a corresponding URL mapping relationship, first will find the TCP/IP parameters set in the preferred DNS server, in this we call it a local DNS server, this server receives the query, if you want to query the domain name, Included in the local Configuration zone resource, the parse result is returned to the client to complete the domain name resolution, which is authoritative.

Fifth Step: if the domain name to be queried is not resolved by the local DNS server zone, but the server has cached this URL mapping relationship, call this IP address mapping to complete the domain name resolution, this resolution is not authoritative.

Sixth step: if the local DNS server local zone file and cache resolution are invalidated, the query is based on the local DNS server's settings (whether to set forwarders), and if the forwarding mode is not used, local DNS sends the request to 13 root DNS, the root DNS When the server receives the request, it will determine who the domain name (. com) is authorized to administer and will return an IP that is responsible for the top-level domain name server.

After the local DNS server receives the IP information, it will contact the server responsible for the. com domain. After the server that is responsible for the. com domain receives the request, if it cannot resolve itself, it will find an administrative. com domain with the next-level DNS server address to the local DNS server. When the local DNS server receives this address, it will find the domain name server, repeat the above action, query, until the domain name corresponding to find the host.

Seventh Step: If the use of the forwarding mode, this DNS server will forward the request to the first level of DNS server, by the top level of the server to resolve, the previous level of the server if it can not resolve, or find root DNS or transfer requests to the upper ancestor, in this cycle. Whether the local DNS server uses forwarding or root hints, the result is returned to the local DNS server and returned to the client by the DNS server.

User access to Web page process, DNS resolution process