1.DNS查询过程:
以查询 www.baidu.com为例
(1)电脑向本地域名服务器发送解析www.baidu.com的请求
(2)本地域名服务器收到请求后,先查询本地的缓存,如果找到直接返回查询结果,如果没有该记录,本地域名服务器把www.baidu.com的请求发送给根域名服务器
(3)根域名服务器收到请求后,把.com域的服务器IP地址返回给本地域名服务器
(4)本地域名服务器连接.com服务器,向其请求解析域名www.baidu.com, .com服务器把baidu.com服务器的IP地址返回给本地DNS服务器
(5)本地DNS服务器向baidu.com服务器发送解析域名请求,baidu.com服务器将www.baidu.com的IP地址返回给本地DNS服务器
(6)本地DNS服务器把www.baidu.com的IP地址返回给电脑。
2.域名和IP地址的对应关系:
一个域名可以对应多个IP地址,但在同一时刻,一个域名只能有一个IP地址,一个IP地址可以对应多个域名。
3.查询DNS
Python可以实现DNS的正向和反向查询。下面是正向查询的代码:
#!/usr/bin/env python
import sys,socket
result=socket.getaddrinfo(sys,argv[1],None)
print result[0][4]
因为一个域名可以有多个IP地址,所以上述程序两次运行的结果可能会不同。
运行程序 :
结果为 :
反向查询:
#!/usr/bin/env python
import sys,socket
try:
result=socket.gethostbyaddr(sys.argv[1])
print "hostname is "+result[0]
except socket.herror,e:
print "can't look up"
运行程序
结果为:

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