Category: Ubuntu/Debian

Stuff about Ubuntu and Debian GNU/Linux

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How to: Quick and Dirty Web Server Load Balancing with IPTables in Linux

Load balancing is a method to distribute workload across multiple computer over a network. The purpose of load balance in web server is to avoid one web server from being overwhelmed by requests which eventually leads the machine to come down to a crawling halt.

Assuming that you have 3 web server to assign the load to each with this IP Address:
10.20.20.1
10.20.20.2
10.20.20.3

You can drive the traffic to each of this on every third packet with this iptables rules:

iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every 3 --packet 0 -j DNAT --to-destination 10.20.20.1:80

iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every 3 --packet 1 -j DNAT --to-destination 10.20.20.2:80

iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every 3 --packet 2 -j DNAT --to-destination 10.20.20.3:80

This will ensure that every 3rd packet of the request will be properly distributed among the three servers to balance the load. Note that this is only useful in simple website which serves static content or for a download servers that serve large files over the internet (CD or DVD iso downloading)

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Crack zip file password with FCrackzip

Fcrackzip is a tool that can be used to crack zip files encrypted with ZipCrypto algorithm through dictionary-based and brute-force attack.

The brute force attack can be configured to use the combination of lower,upper, numerical characters or with other symbols or punctuation marks.

Example usage:

  • fcrackzip -u -v -l 1-6 -c a example.zip
  • fcrackzip -u -v -l 1-6 -c aA1 example.zip
  • fcrackzip -u -v -D -p wordlist-dict.txt example.zip (dictionary attack)

Switch Explanation:

  • -v : verbose output, display the progress of current crack, may slow the progress a little bit
  • -l : length of password to brute-force in this case (1 to 6 characters)
  • -c : character set to try (a – lower-alphabet, A-uppercase alphabet, 1-numeric, ! – include [!:$%&/()=?[]+*~#])
  • -u : verify the zip password in case of multiple possible matches

p/s: It is strongly suggested to use dictionary attack first before going down with brute-force as passwords longer than 6 characters may take (a long) time to crack. A collection of wordlist can be found at PacketStormSecurity website

Debian and Ubuntu users can get fcrackzip from the default apt-get repository.
Windows may download fcrackzip win32 binaries from Schmorp.de website

Recommended Reading

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How to solve Apache – Could not reliably determine the server’s fully qualified name – error in Ubuntu

Apache2 web server will almost always display this information message :
"Could not reliably determine the server's fully qualified name"
when it is first started in Ubuntu and Debian server.

The reason behind this message is because the web server fails to find the suitable domain name in the system.

How to remove the message
First, you need to edit “/etc/hosts” file and put your server name of choice in the file. For example:

127.0.0.1 server.mylocal

Then you need to add “ServerName” directive in the “/etc/apache2/apache2.conf” file.

ServerName server.mylocal

Finally, restart the web server for the changes to take effect. You will notice that the information message is gone now.

$ sudo service apache2 restart

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Enable Hardware Notification on Ubuntu Desktop when plugging devices – udev-notify

udev-notify enables hardware notification on desktop whenever devices are plugged to the computer. It provides a convenient way for users to get visual feedback on whether the devices has been successfully recognized by the computer or otherwise.

udev-notify is known to be compatible with Debian and Ubuntu (to some extend Fedora 15) under GNOME and XFCE. udev-notify display the type,name and model of the device connected and recognized by the computer on the desktop.

Udev
Udev
Udev
Udev

Installing udev-notify

echo "deb http://download.learnfree.eu/repository/skss / #SKSS" | sudo tee -a /etc/apt/sources.list
wget http://download.learnfree.eu/repository/skss/repo.pub.asc -q -O- | sudo apt-key add -
sudo apt-get update
sudo apt-get install udev-notify

And you may try plugging in devices to your computer to see if it works!