名称解析可能会失败，因为没有与主机名关联的IP，或者因为无法访问DNS服务器。不幸的是，在两种情况下，Python的socket.create_connection和socket.gethostbyname函数似乎会引发相同的错误：
$ 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
有什么方法可以区分这两种情况，这些情况不需要我对“已知有效”的主机名进行第二次查找？
该解决方案应在Linux下运行。
最佳答案
您可以使用dnslib库客户端自己进行DNS请求。客户端提供类似dig的功能，该功能可以指示与仅解析失败（不幸的是，阻止-请参见下面的修补程序）相比，解析失败的地址（NXDOMAIN）。
您可以这样使用它：
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.
与不存在的DNS服务器建立连接时出现问题。每次尝试此操作时，请求都将挂起。 （当我在命令行中使用netcat之类的命令发出相同的请求时，该请求也将挂起。我可能选择的IP很差，并且遭受了防火墙的攻击，这些防火墙只是丢弃了数据包）
无论如何，您可以更改源代码以添加超时。您可以在源here中查看相关方法（也已在github上进行镜像）。我更改的是：
--- 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。