Check hosts file for dodgy entries

hosts-check.py

#!/usr/bin/env python

import argparse, os, platform, sys

def is_safe(line):
    if (is_empty_line(line) or
        is_comment(line) or
        is_safe_redir(line)):
        return True
    return False

def is_empty_line(line):
    return len(line.strip()) == 0

def is_comment(line):
    return line.strip()[:1] == '#'

def is_safe_redir(line):
    safe_addrs = [
        '0.0.0.0',
        '127.0.0.1',
        '127.0.1.1',
        '127.0.0.53',
        '255.255.255.255',
        '::1',
        'fe80::1%lo0',
        'ff00::0',
        'ff02::1',
        'ff02::2',
        'ff02::3',
    ]
    for addr in safe_addrs:
        addr_w_space = addr + ' '
        if (len(line) >= len(addr_w_space) and
            line[:len(addr_w_space)] == addr_w_space):
            return True
        # Continue looping if not at last iteration
        elif not addr == safe_addrs[-1]:
            continue
    return False

def print_nonewline(line):
    sys.stdout.write(line)

def check_hosts(hosts_path):
    unsafe_lines = []
    with open(hosts_path, 'r') as f:
        for line in f:
            if not is_safe(line):
                unsafe_lines.append(line)
    
    num_results = len(unsafe_lines)
    print('Found ' + str(num_results) + ' lines to manually check.')
    linenum = 0
    for line in unsafe_lines:
        fline = '- line ' + str(linenum) + ':\t' + line
        print_nonewline(fline)
        linenum += 1
    print('')

def get_system_default_hosts_path():
    if platform.system().lower().startswith('win'):
        return os.fsdecode(os.path.join(os.environ['SystemRoot'],
            'System32' + os.sep + 'drivers' + os.sep + 'etc' + os.sep + 'hosts'))
    else:
        return '/etc/hosts'

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    
    help_str = 'hosts file location to perform checks on; if undefined, will use ' +\
        get_system_default_hosts_path()
    
    parser.add_argument('-f', '--file', help=help_str)
    args = parser.parse_args()
    if args.file:
        check_hosts(args.file)
    else:
        check_hosts(get_system_default_hosts_path())