Application layer protocol - DNS

[Foreword] After the user enters a URL in the browser, if the protocol name in the URL is followed by a domain name instead of an IP, the domain name resolution will be performed first, that is, the DNS protocol. The DNS protocol is to convert the domain name into an IP address.

DNS protocol is based on UDP transmission

1. Hierarchy of domain names

Attention Attention Attention!The root domain name is . by default. The common last one, such as com, is a top-level domain name and not a root domain name.

(1) The hierarchy of the domain name belongs to the relationship of management

There is only one root domain name, which is not identified, and is directly represented by .

The root domain knows the IPs of all top-level domains, and so on

(2) The following domain name:

Host.example.Microsoft.com.

In this domain name, Host is the fourth-level domain name, example is the third-level domain name, Microsoft is the second-level domain name, com is the top-level domain name, and the last . is the root domain name

Host is a fourth-level domain name, which is the most basic information. It is generally the host name of a computer. The common www represents a web server

2. Some TLDs

com commercial organization, such as www.baidu.com

cn China

net network support organization

gov government organizations

edu educational institutions

3. Domain name resolution process

3.1 Recursive query

Generally, the query request made by the DNS client is a recursive query method

The so-called recursive query means that after the DNS client sends a query request, if there is no required data in the DNS server, the DNS server will replace the client to query other DNS servers until the result is returned, that is, thisThe DNS server must reply to the DNS client

3.2 Iterative query

Generally, the query method between the DNS server and the DNS server is iterative query

The so-called iterative query refers to:

After the first DNS server makes a query request to the second DNS server, if the second DNS server does not find it, it will provide the IP address of the third DNS server to the first, so that the firststation directly to the third DNS server to query, and so on

If the last DNS server is not found, notify the first DNS server that the query failed

3.3 Example

For example, if the host whose domain name is me.abc.com needs to know the IP address of the target domain name a.xyz.com, the query process is as follows:

First, the host performs a recursive query to the local domain name server dns.abc.com

Then the local domain name server dns.abc.com uses an iterative query to query a root domain name server

The root domain name server tells the local domain name server the IP of the next top-level domain name server dns.com to query

The local domain name server goes to the top-level domain name server dns.com for query

The top-level domain name server then tells the local domain name server the IP of the next authoritative domain name server dns.xyz.com to query

Query the local domain name server to the authority domain name server dns.xyz.com

The authority domain name server found the IP of the target domain name a.xyz.com and told it to the local domain name server

Finally, the local domain name server tells the local host me.abc.com the final result of the query

4.Other

4.1 Cache mechanism

Each nameserver maintains a cache of recently used names and a record of where name mapping information was obtained

Therefore, when the same domain name is requested to be resolved again, the address in the cache can be used directly

Through the caching mechanism, not only the traffic of DNS query data network is reduced, but also the workload of upper-layer servers is reduced

4.2 related commands (under Windows system)

• Look up the default DNS server configured for the host - nslookup

• Display all DNS cache entries - ipconfig/displaydns