Python:如何区分不可用DNS服务器和不存在的地址

Question

名称解析可能会失败，因为没有与主机名关联的ip，或者因为无法到达DNS服务器。不幸的是，Python的socket.create_connection和socket.gethostbyname函数在这两种情况下似乎都会产生相同的错误：

$ python3 -c 'import socket; socket.create_connection(("www.google.com_bar", 80))'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.4/socket.py", line 491, in create_connection
    for res in getaddrinfo(host, port, 0, SOCK_STREAM):
  File "/usr/lib/python3.4/socket.py", line 530, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -2] Name or service not known

$ python3 -c 'import socket; socket.gethostbyname("www.google_bar.com")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
socket.gaierror: [Errno -5] No address associated with hostname

$ sudo vim /etc/resolv.conf # point to non-existing nameserver

$ python3 -c 'import socket; socket.create_connection(("www.google.com", 80))'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.4/socket.py", line 491, in create_connection
    for res in getaddrinfo(host, port, 0, SOCK_STREAM):
  File "/usr/lib/python3.4/socket.py", line 530, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -2] Name or service not known

$ python3 -c 'import socket; socket.gethostbyname("www.google.com")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
socket.gaierror: [Errno -5] No address associated with hostname

有没有办法区分这两种情况，而不需要对“已知-好”主机名执行第二次查找？

这个解决方案应该在Linux下工作。

Answer 1

您可以使用德斯利布库客户端自己提出DNS请求。客户端提供了类似于dig的功能，它可以指示地址是否失败解析(NXDOMAIN)，而不只是解析失败(不幸的是，这只是块-参见下面的修补程序)。

你就这样用它：

from dnslib import DNSRecord, RCODE

# I have dnsmasq running locally, so I can make requests to localhost.
# You need to find the address of the DNS server.
# The /etc/resolv.conf file is quite easily parsed, so you can just do that.
DNS_SERVER = "127.0.0.1"

query = DNSRecord.question("google.com")
response = DNSRecord.parse(query.send(DNS_SERVER, 53, False))
print RCODE[response.header.rcode]  # prints 'NOERROR'

query = DNSRecord.question("google.com_bar")
response = DNSRecord.parse(query.send(DNS_SERVER, 53, False))
print RCODE[response.header.rcode]  # prints 'NXDOMAIN'

# To avoid making the DNS request again when using the socket
# you can get the resolved IP address from the response.

当连接到不存在的DNS服务器时，问题就出现了。每次我试着这样做，请求就会挂起。(当我在命令行上发出相同的请求时，使用类似于netcat的东西，请求也会挂起。我可能选择的随机IP很差，并且遭受防火墙的痛苦，只是丢包)

无论如何，您可以修改源代码以添加超时。您可以在源这里中查看相关的方法(也可以在github上镜像)。我改变的是：

--- a/dns.py
+++ b/dns.py
@@ -357,6 +357,7 @@
             response = response[2:]
         else:
             sock = socket.socket(socket.AF_INET,socket.SOCK_DGRAM)
+            sock.settimeout(10)
             sock.sendto(self.pack(),(dest,port))
             response,server = sock.recvfrom(8192)
             sock.close()

完成此操作后，DNS请求超时。