Python for loop slows and evenutally hangs

Question

I'm totally new to Python (as of half an hour ago) and trying to write a simple script to enumerate users on an SMTP server.

The users file is a simple list (one per line) of usernames.

The script runs fine but with each iteration of the loop it slows until, around loop 14, it seems to hang completely. No error - I have to ^c.

Can anyone shed some light on the problem please?

TIA, Tom

#!/usr/bin/python

import socket
import sys

if len(sys.argv) != 2:
        print "Usage: vrfy.py <username file>"
        sys.exit(0)

#open user file
file=open(sys.argv[1], 'r')
users=[x.strip() for x in file.readlines()]
file.close

#Just for debugging
print users

# Create a Socket
s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Connect to the Server
connect=s.connect(('192.168.13.222',25))

for x in users:
        # VRFY a user
        s.send('VRFY ' + x + '\r\n')
        result=s.recv(1024)
        print result

# Close the socket
s.close()

Answer 1

Most likely your SMTP server is tarpitting your client connection. This is a defense against runaway clients, or clients which submit large volumes of "junk" commands. From the manpage for Postfix smtpd:

   smtpd_junk_command_limit (normal: 100, stress: 1)
          The number of junk commands (NOOP, VRFY, ETRN or  RSET)  that  a
          remote  SMTP  client  can  send  before  the Postfix SMTP server
          starts to increment the error counter with each junk command.

The smtpd daemon will insert a 1-second delay before replying after a certain amount of junk is seen. If you have root access to the smtp server in question, try an strace to see if nanosleep syscalls are being issued by the server.

Here is a trace from running your script against my local server. After 100 VRFY commands it starts sleeping between commands. Your server may have a lower limit of ~15 junk commands:

nanosleep({1, 0}, 0x7fffda9a67a0)       = 0
poll([{fd=9, events=POLLOUT}], 1, 300000) = 1 ([{fd=9, revents=POLLOUT}])
write(9, "252 2.0.0 pat\r\n", 15)       = 15
poll([{fd=9, events=POLLIN}], 1, 300000) = 1 ([{fd=9, revents=POLLIN}])
read(9, "VRFY pat\r\n", 4096)           = 10

Answer 2

s.recv blocks so if you have no more data on the socket then it will block forever.
You have to keep track of how much data you are receiving. You need to know this ahead of time so the client and the server can agree on the size.

Answer 3

Solving the exact same problem I also ran into the issue. I'm almost sure @samplebias is right. I found I could work around the "tarpitting" by abusing the poor system even more, tearing down and rebuilding every connection:

#[ ...Snip... ]
import smtplib
#[ ...Snip... ]
for USER in open(opts.USERS,'r'):
    smtpserver = smtplib.SMTP(HOST,PORT)
    smtpserver.ehlo()
    verifyuser = smtpserver.verify(USER)
    print("%s %s:  %s") % (HOST.rstrip(), USER.rstrip(), verifyuser)
    smtpserver.quit()

I'm curious whether this particular type of hammering would work in a live environment, but too certain it would make some people very unhappy.

PS, python: batteries included.