Check your HOSTS File!

#!/usr/bin/python

# Copyright: ME!
# License: The latest version of GPL as posted on the EFF website at the time
#          you copy this file.  As of 2008, that is GPLv3.

import re, sys
from collections import defaultdict

SE = sys.stderr

"DOES NOT STRICTLY CHECK FOR SIMILARITIES LIKE 127.1 vs 127.0.0.1"

comment_re = re.compile( r'^[ \t]*#.*$' )
valid_re   = re.compile( r'^(\d+\S+)[ \t]+(\w.*)+$' )
alias_re   = re.compile( r'^\w+(\.\w+)*$' )

alias_mapping = defaultdict( list )

#for noLine, line in enumerate( sys.stdin ):
for noLine, line in enumerate( open('/tmp/hosts', 'rb') ):
    noLine += 1   # Humans think from 1, apparently.

    # To catch variances in frickin' whitespace.  Fix your damn text editors,
    # people, and show your EOL whitespace!
    line = line.rstrip('\n')

    if not line:
        continue

    if comment_re.match( line ):
        continue

    match = valid_re.match( line )
    if match:
        ip4, aliases = match.groups()

        ip4 = ip4.split('.')
        if len( ip4 ) > 4:
            msg = "Line {}: Bad IPv4 address '{}'\n"
            SE.write( msg.format( noLine, ip4 ))
        else:
            for i in ip4:
                try:
                    i = int( i );
                    if not ( 0 <= i and i < 255 ):
                        msg = "Line {}: Bad IPv4 address '{}'\n"
                        SE.write( msg.format( noLine, ip4 ))
                        break
                except:
                    msg = "Line {}: Bad IPv4 address '{}'\n"
                    SE.write( msg.format( noLine, ip4 ))
                    break

        aliases = aliases.split()
        for alias in aliases:
            if not alias_re.match( alias ):
                msg = "Line {}: Invalid host alias name '{}'\n"
                SE.write( msg.format( noLine, alias ))

            elif alias.lower() != alias:
                msg = ('Line {}: Please use company standard lower case domain '
                  "name in alias '{}'\n")
                SE.write( msg.format( noLine, alias ))

            alias_mapping[ alias ].append( '.'.join(ip4) )

        continue

    SE.write( "Line {}: Invalid syntax\n".format( noLine ))


for alias, ip_addresses in sorted( alias_mapping.iteritems() ):
    if len( ip_addresses ) > 1:
        if len( set( ip_addresses )) == 1:
            msg = "Multiple definitions of alias '{}' to IP '{}'\n"
            SE.write( msg.format( alias, ip_addresses[0] ))
        else:
            msg = "Multiple IPs assigned to alias '{}': {}\n"
            SE.write( msg.format( alias, ', '.join( ip_addresses )))