[Tutor] Socket Module

[Martin A. Brown]

Hello Nym,

> Here is the updated code: https://bpaste.net/show/358583e1a0bd

It's short.  I have included inline here:

   import socket

   ListOfIPAddresses = []

   with open('top500ips.csv', 'r') as f:
       for line in f:
           line = line.strip()
           ListOfIPAddresses.append(line)

   for address in ListOfIPAddresses:
       try:
           ResolvedAddresses = socket.gethostbyaddr(address)
       except:
           print('No Resolution Available')

   print(ResolvedAddresses)

> The issue that I am running into now is that for some reason, the 
> script is not resolving known-public IP addresses that I am 
> passing through. For example, some of the IPs, that I have used 
> are for sites like facebook (173.252.120.6) github 
> (207.97.227.239), however the script is not able to resolve them.
>
> But its interesting that I am able to resolve them using nslookup 
> on windows command prompt. Which means my laptop's DNS setting is 
> fine.

The apparent (?) DNS lookup failure
-----------------------------------
At time X, you run your Python program and something (perhaps in the 
DNS resolution process) fails and you see "No Resolution Available", 
but you do not know what has failed, nor for which address lookup.

At time Y, you run 'nslookup' at the shell prompt, receive an answer 
and conclude that your script is operating properly.  While this is 
may appear logical, it is an incorrect conclusion.

One coding error (a dangerous habit to perpetuate)
--------------------------------------------------
When performing the DNS lookup, you are using something called 
a 'bare except'.  This will catch any and all errors, even if it's 
something unrelated like, for example, a signal.  This is a bad and 
dangerous habit.  In general, you should catch only the exceptions 
that you can do something about.

In addition, this will offer you more information about the problem. 
Here's a simple example, where I'm only changing two lines:

   for address in ListOfIPAddresses:
       try:
           ResolvedAddresses = socket.gethostbyaddr(address)
       except socket.herror as e:
           print("No resolution available for %s: %s" % (address, e))

This will give you a little more information about what, 
specifically, the failure is in your call to socket.gethostbyaddr()

Comment on NXdomain responses
-----------------------------
I picked at random an address that had no PTR record and tried to 
call socket.gethostbyaddr('8.97.227.2').  What do you think I got 
for my trouble?  When running through the code block above, I saw 
the following output to my terminal:

   No resolution available for 8.97.227.2: [Errno 0] Resolver Error 0 (no error)

In short, there is no guarantee that anybody has properly set up 
reverse DNS entries (DNS PTR records) for the addresses you are 
looking up.  Although the vast majority of lookups occur 
successfully and smoothly, there are many things that can go wrong 
in the network and on an end host which can cause transient errors 
during DNS lookups, and it is possible that you have already 
encountered some of these problems (even though I would not expect 
to hit very many errors looking up PTR records for only 500 IPs).

May I wish you good luck resolving not just your addresses, but also 
your problem!

-Martin

-- 
Martin A. Brown
http://linux-ip.net/