Script to block annoying IPs

izanami.py

import socket
import subprocess
import random
import string

BAD_THRESHOLD = 5    # If there are more than these failed attempt then IP will be blocked
iptables_str = "/sbin/iptables -I INPUT -s IP-HERE -j DROP\n"    
identifier = ''.join(random.choice(string.lowercase) for i in range(6))
script = open("/tmp/iptables.sh-" + identifier,"w")

def is_valid_ip(ip):
    try:
        socket.inet_aton(ip)
        return True
    except socket.error:  
        return False

ip_dict={}
with open("/var/log/secure") as logf:
    for line in logf:
        if "sshd" in line and "preauth" in line:
            ip = line.split()[-2]
            if is_valid_ip(ip):
                if ip in ip_dict:
                    ip_dict[ip] += 1
                else:
                    ip_dict[ip] = 1

for ip in ip_dict:
    if ip_dict[ip] > BAD_THRESHOLD:
        script.write(iptables_str.replace("IP-HERE", ip))

script.close()
print "Please examine and execute /tmp/iptables.sh-" + identifier

Use tempfile.mkstemp instead of random.choice. Also use collections.defaultdict to simplify things a bit.