Shulou(Shulou.com)06/03 Report--

DNS (Domain Name System) is the abbreviation of "domain name system". It is a computer and network service naming system organized into a domain hierarchy. It is used in TCP/IP networks. The services it provides are used to convert host names and domain names into IP addresses. DNS is such a "translator", and its basic working principle can be shown in the following figure.

How the DNS domain name space is organized:

Basic process:

1. Clients access the website through a browser: www.baidu.com

In this process, you need to map the domain name to an IP address. First, call the resolution function to put the domain name to be translated in the DNS request and send it to the local domain name server as a UDP message.

two。 After the local domain name server finds the domain name, it returns the corresponding IP address in the reply message.

3. The colleague domain name server must also have information to connect to other servers to support forwarding when it cannot be resolved.

4. If the domain name server cannot answer the request, the domain name server temporarily becomes another client in DNS and sends a request for resolution to the root domain name server. The root domain name server must be able to find all the domain name servers of the second-level domain name below, and so on, until the requested domain name is queried.

1. Enter the www.baidu.com domain name in the browser, and the operating system will first check whether the local hosts file has this URL mapping relationship. If so, call the IP address mapping first to complete the domain name resolution.

2. If there is no mapping of this domain name in hosts, look for the local DNS parser cache to see if there is this URL mapping relationship. If so, return directly to complete the domain name resolution.

3. If there is no corresponding URL mapping relationship between hosts and the local DNS parser cache, we will first find the preferred DNS server set in the TCP/ip parameter, here we call it the local DNS server. When this server receives the query, if the domain name to be queried is included in the local configuration zone resources, the resolution result will be returned to the client to complete the domain name resolution.

4. If the domain name to be queried is not resolved by the local DNS server region, but the server has cached the URL mapping, the IP address mapping is called to complete the domain name resolution. This resolution is not authoritative.

5. If the local DNS server local zone file and cache resolution are invalid, the query will be made according to the settings of the local DNS server (whether to set the forwarder). If the forwarding mode is not used, the local DNS will send the request to 13 root DNS. After receiving the request, the root DNS server will determine who is responsible for the authorization and management of the domain name (.com), and will return an IP responsible for the top-level domain name server. When the local DNS server receives the IP information, it will contact the server responsible for the .com domain. After the server in charge of the .com domain receives the request, if it cannot resolve it, it will find a DNS server address (qq.com) to manage the next level of the .com domain to the local DNS server. When the local DNS server receives this address, it will find the qq.com domain server, repeat the above action, and query until the www.qq.com host is found.

6. If the forwarding mode is used, the DNS server will forward the request to the DNS server at a higher level, which will be parsed by the server at the next level. If the server at the next level cannot resolve the request, it can either find the root DNS or transfer the request to the superior in a loop. Whether it is used by the local DNS server or forwarded or prompted by the root, the result is finally returned to the local DNS server, from which the DNS server returns to the client.

From the client to the local DNS server is a recursive query, and the interactive query between DNS servers is iterative query.

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