AutoSSH Module for Prey


Prey is software that allows you to track computers and other devices in case they are stolen. After it’s installed on a machine, it periodically checks to see if you have reported it as stolen, and if you have, it will contact you with details about the machine’s current status. This information includes its IP address, GPS position, a screenshot of the current desktop and a picture (or video) of the perpetrator using the webcam.

This makes it much easier to recover your property. However, I also wanted to be able to have full control of the machine in order to access my data and to be able to directly monitor the thief. The easiest option is to ssh directly into the machine; however, this is only possible if the router they are connected to doesn’t block outside connections. One solution is to set up Prey to open a remote port on another machine that forwards back to itself; this way, your laptop initiates an outbound connection and evades the block.


To accomplish this, you will need root access on a server that’s running OpenSSH so that you can add a user that’s only used for Prey. You could conceivably use your own account instead of creating a new one, but if your laptop is stolen, the thief would be given full access to your account. Your laptop also needs to be running Linux (the same procedures should work for other *nix systems, but modifications will have to be made to the directory structure of the download).

Account preparation

To allow Prey to access your server, we will be setting up SSH keys that allow automatic login onto a server. Because of this, it is very important that you create a new user on the server and disable their shell, so that if your laptop is stolen, the thief won’t be given access to an account.

On the server, run the following as root to add a new user:
root@server:~# adduser preyuser
Set a password and whatever other options it asks for.

On your laptop, run all of the following commands as root:
root@laptop:~# ssh-keygen -t rsa
Do not enter a passphrase.
Now use ssh to create a ~/.ssh directory by running:
root@laptop:~# ssh preyuser@server mkdir -p .ssh
Enter the password for the user you just created.
Now append the laptop’s public key to the authorized keys on the server:
root@laptop:~# cat ~/.ssh/ | ssh preyuser@server 'cat >> .ssh/authorized_keys'
You should now be able to log into the server as preyuser without entering a password.

Finally, back on the server, disable preyuser‘s ability to login by disabling their login shell:
root@server:~# which nologin
root@server:~# chsh -s /usr/sbin/nologin preyuser

Trying to log into the server should now give give you the message that the account is currently not available.

Configuring your laptop

The only thing to do now is to configure Prey. Download the AutoSSH module onto your laptop and decompress it:
root@laptop:~# wget
root@laptop:~# tar -xzvf prey-autossh.tgz
Edit the autossh/config file and fill in your server, username, and remote port. When you’re done, move the folder into the Prey modules directory:
root@laptop:~# mv autossh /usr/share/prey/modules/
That’s it!

Prey should now open a forwarded tunnel from your server to your laptop when activated. However, if the connection is interrupted, the tunnel will die. You may want to consider installing autossh which will start and actively monitor an SSH connection and restart it as necessary. This module will use autossh if it is available and fall back to plain ssh otherwise.

Logging into your stolen laptop

Now, if your laptop is stolen, Prey should activate and send you a report. To access your laptop, login to your server and run:
user@server:~$ ssh user@localhost -p port

Enter the laptop’s user‘s password,and you should be in. Now go catch the bastard!

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No sound for Flash in OpenSuse 11.2

I installed OpenSuse 11.2 on my laptop and everything was working fine. except I wasn’t getting any sound out of Flash (e.g. Youtube). Sound was working fine in other programs, so I knew my sound card was working. The problem was, alsamixer had my Pulse Code Modulation muted. To fix it, just run this in a terminal:
and move over to PCM with the arrows and hold the up arrow until it’s maxed out. Hopefully that will fix your problem!

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A Subversion Pre-Commit Hook

Subversion allows you to run commands, called hook scripts, whenever actions take place in your repository. The most useful of these that I’ve found is the pre-commit hook, which allows you check a transaction for problems before it is committed.

Word Aligned has an excellent introduction on setting up pre-commit hooks and includes a small example. I use a more complex script on my own server, and thought it might be of use to somebody else. It verifies the following:

  • The commit message is at least 10 characters long
  • The commit message is different from the previous message (handy when hit the up arrow and accidentally try to commit again)
  • Tabs are used for indentation instead of spaces (sorry, I’m a tabs guy – you can change it if you need to)
  • PHP files don’t have syntax errors
  • C and C++ files don’t have any digraphs or trigraphs

import sys

# These should point to the respective commands
SVNLOOK = '/usr/bin/svnlook'
PHP = '/usr/bin/php'

# Gets a command's output
def commandOutput(command):
	import subprocess
	process = subprocess.Popen(command.split(), stdout = subprocess.PIPE)
	return process.communicate()[0] # 0 is stdout

# Returns an array of the changed files' names
def getChangedFiles(svnPath, transaction):
	# Run svnlook to find the files that were changed
	output = commandOutput('%s changed %s --transaction %s' % (SVNLOOK, svnPath, transaction))

	# The output from svnlook looks like the following:
	# U   folder/file1.cpp
	# A   folder/file2.cpp
	# where U means updated and A means added
	def changed(fileName):
		return len(line) > 0 and line[0] in ('A', 'U')
	changedFiles = [line[4:] for line in output.split('\n') if changed(line)]

	# svnlook inserts an empty line, so output.split() will have an extra
	# line with nothing in it - ignore the last lines if they're empty
	while 0 == len(changedFiles[-1]):
		changedFiles = changedFiles[:-1]

	return changedFiles

# Checks that the message is a minimum length
def checkMessageLen(svnPath, transaction, minLength):
	# Run svnlook to get the message log
	output = commandOutput('%s log %s --transaction %s' % (SVNLOOK, svnPath, transaction))

	if len(output) < minLength:
		sys.stderr.write('Message must be at least %d characters\n' % minLength)
		return 1
		return 0

# Checks that the message is different from the last message
def checkRepeatMessage(svnPath, transaction):
	# Run svnlook to get the message log
	output = commandOutput('%s log %s --transaction %s' % (SVNLOOK, svnPath, transaction))

	currentMessage = output

	lastRevision = int(transaction.split('-')[0]) - 1

	# Empty repositories have no messages
	if lastRevision  0:
			sys.stderr.write('Regex \'%s\' matched \'%s\' on line(s): %s\n' % (regex, fileName, badLines))

	return matches

# Checks a PHP file for syntax errors
def checkPHPSyntax(svnPath, transaction, fileName):
	# Run svnlook to get the file contents
	output = commandOutput('%s cat %s %s --transaction %s' % (SVNLOOK, svnPath, fileName, transaction))

	# Run PHP syntax checking on the file contents
	command = (PHP, '-l')
	process = subprocess.Popen(command, stdin = subprocess.PIPE, stdout = subprocess.PIPE, stderr = subprocess.PIPE)
	output = process.communicate(fileContents)[0]

	if 0 != process.returncode:
		sys.stderr.write('Found syntax errors in \'%s\':' % fileName)
		# Print the offending lines that were returned from php -l
		for line in output:
		return 1 # Error found

	return 0 # No errors

svnPath = sys.argv[1]
transaction = sys.argv[2]
files = getChangedFiles(svnPath, transaction)

errorCount = checkMessageLen(svnPath, transaction, MIN_MESSAGE_LENGTH)
errorCount += checkRepeatMessage(svnPath, transaction)

for file in files:
	# Prefer tabs for indentation
	# Skip .txt files
	if not file.endswith(".txt") and not file.endswith(".sql"):
		errorCount = errorCount + stringsInFile(svnPath, transaction, [' {4}'], file)

	# Check PHP syntax
	if file.endswith('.php'):
		errorCount += checkPHPSyntax(svnPath, transaction, file)

	# C and C++ files should not use digraphs or trigraphs
	digraph = '%s|%s|%s|%s|%s' % ('<:', '', ':>', '%:')
	trigraph = '\?\?[=/\'()!-]'
	if file.endswith(('.c', '.cpp', '.cxx', '.cc', '.C', '.h', '.hpp')):
		errorCount += stringsInFile(svnPath, transaction, [digraph, trigraph], file)

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Compression before encryption

If you’re going to be compressing and encrypting some data, you should do the compression first. Why? There are several reasons:

  • Compressing it last won’t reduce the file size much. Good encryption should make any input data (especially redundant data) appear random. But compression works by removing redundancy, and doesn’t work well on random data. You can see a good example of this here, where encrypting a file and then compressing it actually made it larger than the original!
  • Compressing it should decrease the effectiveness of some attacks. Compression works by reducing the redundancy in the data. A common cryptanalysis method is frequency analysis, which relies on finding repeated data. Compressing it should reduce its effectiveness!
  • Brute force attacks will take longer. Brute force attacks work by trying various keys and decrypting the data and checking if the output data makes any sense. By compressing it first, an attacker must decrypt the data and then decompress it before seeing if the output data makes any sense. This takes much longer, and if an attacker doesn’t know you’re compressing the data at all, they might never break the encryption.

I wanted to see how effective the third point was, so I wrote a Python script that encrypted a short message and used a brute force attack to break it. Then I repeated the experiment, but compressed it using gzip before encrypting it. Here’s how long it took on average, in seconds, to guess a single password:

Password length: Zipped: Not zipped:
2 letters 0.021 0.002
3 letters 0.546 0.061
4 letters 13.612 1.551

As you can see, compressing it before encrypting it took about 9 times as long to break.

Details: A short message was chosen, specifically, “a message“, to encrypt. Because gzip is a block compression algorithm, an attacker only needs to decompress the first bytes rather than the whole file, so I wanted to keep the message short to simulate this. I used 128-bit AES, using a password with only lower case letters. In each iteration, a random password was chosen and both the zipped and unzipped versions were tested. The test was run 1000 times for each password length.

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Test MySQL password strength

Newer versions of MySQL stores passwords using a double SHA-1 hash. Because of this, administrators can’t tell what someone’s password is. What if you want to make sure no users are using weak passwords?

This is a simple Python script that will connect to your local MySQL database, pull out the password hashes for all users, and either run a dictionary search or a brute force search on it. Either give it an argument for the dictionary file, or give it a number for the length of passwords you want to test. If you want to use a different character set (e.g. include special characters), change the characterSet variable near the top of the script.

On my system, I was able to test around 50,000 words per second. Note that this cannot be used to recover the root MySQL password, as it requires the root password to connect to the database in the first place.

import MySQLdb
import sha
import sys
import os
import getpass
alpha = "abcdefghijklmnopqrstuvwxyz"
numbers = "0123456789"
# Set this to a string with the character set you want to check
# Examples:
characterSet = alpha
#characterSet = alpha + alpha.upper()
#characterSet = numbers
#characterSet = alpha + alpha.upper() + numbers
def recurse(width, logins):
  for i in range(0, width + 1):
    print "Checking width %d" % i
    recurse2(i, logins, 0, "")
    print ""
def recurse2(width, logins, position, string):
  for char in characterSet:
    # Show progress
    if position == 0:
      print char,
    if (position < width - 1):
      recurse2(width, logins, position + 1, string + char)
      hash = + char).digest())\
      for login in logins:
        if hash == login[2]:
          print "\n'%s'@'%s': '%s'\n" %\
              (login[0], login[1], string + char)
if len(sys.argv) > 1:
  password = getpass.getpass('Enter the root password: ')
  conn = MySQLdb.connect(host = "localhost",
    user = "root",
    passwd = password,
    db = "mysql")
  cursor = conn.cursor()
  cursor.execute("SELECT User, Host, Password from user;")
  logins = cursor.fetchall()
  # Preprocess the logins
  processed = []
  for login in logins:
    ignore = False
    # Ignore empty passwords
    if 0 == len(login[2]):
      print "'%s'@'%s' has empty password, ignoring" %\
          (login[0], login[1])
      ignore = True
    # Ignore duplicates
    for p in processed:
      if p[2] == login[2][1:].lower():
        print "'%s'@'%s' has same password as '%s'@'%s', ignoring" %\
            (login[0], login[1], p[0], p[1])
        ignore = True
    if not ignore:
      processed.append((login[0], login[1], login[2][1:].lower()))
  if os.path.isfile(sys.argv[1]):
    f = open(sys.argv[1])
    lines = f.readlines()
    for line in lines:
      hash =[:-1]).digest()).hexdigest()
      for login in processed:
        if hash == login[2]:
          print "'%s'@'%s': '%s'" % (login[0], login[1], line[:-1])
      depth = int(sys.argv[1])
      print "Usage: %s [dictionary file | brute force depth]" % sys.argv[0]
    recurse(depth, processed)
  print "Usage: %s [dictionary file | brute force depth]" % sys.argv[0]
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Convert mkv files to xvid for Xbox 360

Ushare is an excellent media server that allows you to watch videos, listen to music and view pictures on your Xbox 360 that are stored on another computer. Unfortunately, the Xbox can be somewhat finicky about which formats it supports – and it doesn’t support mkv files. To convert it to a file that the Xbox does support, you can use mencoder as follows:

mencoder inputFile.mkv -ffourcc XVID -ovc lavc -lavcopts vcodec=mpeg4:threads=8:vbitrate=1000:cmp=2:subcmp=2:trell=yes:v4mv=yes:mbd=2 -oac lavc -lavcopts acodec=ac3:abitrate=128 -channels 2 -o outputFile.avi

If the original file is high quality, you’ll probably want to increase the vbitrate parameter to prevent the quality from degrading. If you have surround sound, you should set the number of channels to 6 and set the audio bitrate higher (384 is probably a good choice) as well.

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Using Javadoc with Eclipse in Ubuntu

When you’re using Eclipse to program in Java and you hover over a keyword (like a function name, a package, or a class), some Javadoc will be displayed, showing what the keyword does and how it is used. The default installation of Eclipse did not include Javadoc, so hovering over keywords only gave me a message that Javadoc wasn’t installed. Here’s how to get Javadoc installed and working.

First, enter
sudo apt-get install sun-java6-doc
The installer will prompt you to download a zip file containing the documentation from Sun. Bring up Firefox and head to Sun’s website. Click the Download link next to Java SE 6 Documentation, agree to the terms and click Continue. Click the zip file to download the documentation.

When the download finishes, in another terminal, enter
sudo mv /path/to/zip/file/ /tmp/
You need to rename the file to for the installer to work. Enter
sudo chown root:root /tmp/
to change the file permissions.

Back in the first terminal, hit enter, and the installer should continue. When it finishes, in Eclipse, click Window – Preferences – Java – Installed JREs, and click the JRE (mine was called java-sun- Click Edit, and use Shift+Click to select all the JRE system libraries. Click Javadoc Location, and enter file:/usr/lib/jvm/java-6-sun/docs/api/ in the box. Click Validate to test it (you can open it in your browser to double check), and click OK out of all the dialogs. Javadoc should now be working!

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