使用参考：

https://www.computerhope.com/unix/unslooku.htm

https://www.thegeekstuff.com/2012/02/dig-command-examples/

引申出问题：

What Is a Domain Name?    https://www.lifewire.com/what-is-a-domain-name-2483189

十分系统（www.a.shifen.com）是干什么的？和百度有什么关系？

引申出问题：

What is the difference between a domain.com and www.domain.com?

What is the difference between "google.com" and "google.co.in"?

引申出2个Google工具

Google webmaster   https://www.google.com/intl/en/webmasters/#?modal_active=none

Google Search Console  https://www.google.com/webmasters/tools/home?hl=zh-CN

---

正文开始

About nslookup

nslookup命令用于交互式查询 Internet name servers 以获取信息。

Overview

nslookup, which stands for "name server lookup", is a useful tool for finding out information about a named domain.

By default, nslookup will translate a domain name to an IP address (or vice versa). For instance, to find out what the IP address of microsoft.com is, you could run the command:

[root@51cto ~]# nslookup microsoft.com
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
Name:    microsoft.com
Address: 134.170.185.46
Name:    microsoft.com
Address: 134.170.188.221

Here, 8.8.8.8 is the address of our system's DNS Server. This is the server our system is configured to use to translate domain names into IP addresses. "#53" indicates that we are communicating with it on port 53, which is the standard port number DNS servers use to accept queries.

8.8.8.8在配置文件/etc/resolv.conf中指定。

Below this, we have our lookup information for microsoft.com. Our name server returned two entries, 134.170.185.46 and 134.170.188.221. This indicates that microsoft.com uses a round robin setup to distribute server load. When you access micrsoft.com, you may be directed to either of these servers and your packets will be routed to the correct destination.

You can see that we have received a "Non-authoritative answer" to our query. An answer is "authoritative" only if our DNS has the complete zone file information for the domain in question. More often, our DNS will have a cache of information representing the last authoritative answer it received when it made a similar query; this information is passed on to you, but the server qualifies it as "non-authoritative": the information was recently received from an authoritative source, but the DNS server is not itself that authority.