使用Python的socket.gethostbyaddr()时遇到困难

Question

我正在用Python里的socket.gethostbyaddr()这个方法来反向查找一堆IP地址的主机名，但有些IP返回的是“未知主机”。而用dig这个工具查同样的IP却能得到主机名。此外，dig的速度似乎比用Python模块快很多，这有什么特别的原因吗？

import socket

# This returns 'Unknown Host' 
name, alias, addresslist = socket.gethostbyaddr('114.143.51.197')

Answer 1

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而使用命令 dig -x 114.143.51.197+short 可以让我得到这个IP的主机名。

抱歉，你搞错了。114.143.51.197 这个IP没有PTR记录……所以 socket.gethostbyaddr() 应该会报错……为了正确处理这种情况，你需要加一个 try / except 语句来捕获socket.herror错误。

>>> def dns_ptr_lookup(addr):
...     try:
...         return socket.gethostbyaddr(addr)
...     except socket.herror:
...         return None, None, None
...
>>> # At this time, 4.2.2.2 has a valid PTR
>>> name,alias,addresslist = dns_ptr_lookup('4.2.2.2')
>>> print(name)
vnsc-bak.sys.gtei.net
>>>
>>> # At this time, 114.143.51.197 does NOT have a valid PTR
>>> name,alias,addresslist = dns_ptr_lookup('114.143.51.197')
>>> print(name)
None
>>>

对114.143.51.197进行DNS反向查找……注意它没有有效的 PTR 记录。

[mpenning@Bucksnort ~]$ dig @8.8.8.8 -x 114.143.51.197

; <<>> DiG 9.6-ESV-R4 <<>> @8.8.8.8 -x 114.143.51.197
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 4735
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;197.51.143.114.in-addr.arpa.   IN      PTR

;; AUTHORITY SECTION:
114.in-addr.arpa.       1800    IN      SOA     ns1.apnic.net. read-txt-record-of-zone-first-dns-admin.apnic.net. 17812 7200 1800 604800 172800

;; Query time: 182 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Tue Nov 22 05:11:36 2011
;; MSG SIZE  rcvd: 134

[mpenning@Bucksnort ~]$ python
Python 2.5.2 (r252:60911, Jan 24 2010, 14:53:14)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> socket.gethostbyaddr('114.143.51.197')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
socket.herror: (1, 'Unknown host')
>>>

这是一个有效的 PTR 记录应该是什么样子的……

[mpenning@Bucksnort ~]$ dig -x 4.2.2.2

; <<>> DiG 9.6-ESV-R4 <<>> -x 4.2.2.2
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 61856
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 1

;; QUESTION SECTION:
;2.2.2.4.in-addr.arpa.          IN      PTR

;; ANSWER SECTION:
2.2.2.4.in-addr.arpa.   86400   IN      PTR     vnsc-bak.sys.gtei.net.

;; AUTHORITY SECTION:
2.4.in-addr.arpa.       86400   IN      NS      dnsauth2.sys.gtei.net.
2.4.in-addr.arpa.       86400   IN      NS      dnsauth1.sys.gtei.net.
2.4.in-addr.arpa.       86400   IN      NS      dnsauth3.sys.gtei.net.

;; ADDITIONAL SECTION:
dnsauth1.sys.gtei.net.  1800    IN      A       4.2.49.2

;; Query time: 308 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Nov 22 05:10:16 2011
;; MSG SIZE  rcvd: 158

[mpenning@Bucksnort ~]$ python
Python 2.5.2 (r252:60911, Jan 24 2010, 14:53:14)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> socket.gethostbyaddr('4.2.2.2')
('vnsc-bak.sys.gtei.net', [], ['4.2.2.2'])
>>>