Installing Skype

Follow the steps.........
1.echo "deb http://download.skype.com/linux/repos/debian/ stable non-free #Skype" | sudo tee -a /etc/apt/sources.list > /dev/null

2. sudo apt-key adv --keyserver pgp.mit.edu --recv-keys 0xd66b746e

3. sudo aptitude update && sudo aptitude install skype

Stop brute force attacks with these iptables examples

First let's define with the help of Wikipedia what a dictionary attack is:

In cryptanalysis and computer security, a dictionary attack is a technique for defeating a cipher or authentication mechanism by trying to determine its decryption key or passphrase by searching likely possibilities.

A dictionary attack uses a brute-force technique of successively trying all the words in an exhaustive list called a dictionary (from a pre-arranged list of values). In contrast with a normal brute force attack, where a large proportion key space is searched systematically, a dictionary attack tries only those possibilities which are most likely to succeed, typically derived from a list of words for example a dictionary (hence the phase dictionary attack) or a bible etc. Generally, dictionary attacks succeed because many people have a tendency to choose passwords which are short (7 characters or fewer), single words found in dictionaries or simple, easily-predicted variations on words, such as appending a digit.

So as you can see, we have two "types of brute force attacks" those which use dictionary and those that does not. With this technique we will be protected from both of them.

This technique, uses iptables to block a particular IP, that has passed the threshold of a certain number of connections in a given period of time.

I will show here, some basic IPtables rules to protect a web server from brute force attacks, but this example can be adapted to other scenarios.

   
Basic rules, only open port 80 (http) and 22 (ssh)

This is written as a script that may be run each time your server start, or can configured to run iptables as daemon, as I will show you later.

iptables -F
iptables -A INPUT -i lo -p all -j ACCEPT
iptables -A OUTPUT -o lo -p all -j ACCEPT                   
iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp ! --tcp-option 2 -j REJECT --reject-with tcp-reset
iptables -A INPUT -p tcp --dport ssh -j ACCEPT
iptables -A INPUT -p tcp --dport www -j ACCEPT
iptables -P INPUT DROP

This IPtables script example, will close all port but ssh and www ports, but our server is still open to brute force attacks, so let's close this by adding two more rules that will only permit a certain number of connections to our server from a given IP.
Stop brute force attacks

Here is the example that will stop the brute force attacks.

iptables -F
iptables -A INPUT -i lo -p all -j ACCEPT
iptables -A OUTPUT -o lo -p all -j ACCEPT                   
iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp ! --tcp-option 2 -j REJECT --reject-with tcp-reset
iptables -A INPUT -p tcp --dport ssh -j ACCEPT
iptables -A INPUT -p tcp --dport www -j ACCEPT
iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --update --seconds 600 \
--hitcount 2 -j DROP
iptables -P INPUT DROP

If we now run

sudo iptables -L

This is the output

Chain INPUT (policy DROP)
target     prot opt source               destination
DROP       tcp  --  anywhere             anywhere            tcp dpt:ssh state NEW recent: UPDATE seconds: 600 hit_count: 2 name: DEFAULT side: source
           tcp  --  anywhere             anywhere            tcp dpt:ssh state NEW recent: SET name: DEFAULT side: source
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
REJECT     tcp  --  anywhere             anywhere            tcp option=!2 reject-with tcp-reset
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:www

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere

The last two lines do the trick. Here is a simple explanation of what they do:

This line:

iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --set

Starts a table with each IP that starts a connection to ssh port.

And this one:

iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --update --seconds 600 \
--hitcount 2 -j DROP

Counts the number of connections that IP makes to our server in time frame of 600 seconds, if the number of connectios passed 2 (hitcount). The server will not accept any more connections from that IP for 600 seconds.

You can adjust those values to better fit your needs.

Make it automatic

If you are running Debian or Ubuntu you may run:

sudo /etc/init.d/iptables save

If you are running Arch Linux run:

sudo /etc/rc.d/iptables save

And add iptable to the daemons part in the /etc/rc.conf file.
Logging the connections

If you want to keep a log of the failed connections write something like this:

iptables -F
iptables -A INPUT -i lo -p all -j ACCEPT
iptables -A OUTPUT -o lo -p all -j ACCEPT                   
iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp ! --tcp-option 2 -j REJECT --reject-with tcp-reset
iptables -A INPUT -p tcp --dport ssh -j ACCEPT
iptables -A INPUT -p tcp --dport www -j ACCEPT
iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --update --seconds 600 \
--hitcount 2 -j LOG
iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --update --seconds 600 \
--hitcount 3 -j DROP
iptables -P INPUT DROP

Look that the LOG line has a hitcount number minor that the DROP line, this will make iptables to write a line like this:

Apr 26 20:44:44 arch kernel: IN=eth0 OUT= MAC=00:19:d1:ea:e6:3f:00:11:2f:8f:f8:f8:08:00 SRC=97.107.x.x DST=200.87.x.x LEN=60 TOS=0x00 PREC=0x00 TTL=52 ID=37839 DF PROTO=TCP SPT=50094 DPT=22 WINDOW=5840 RES=0x00 SYN URGP=0

Networking Tools

http://www.bwmonitor.com/

http://www.cacti.net

http://www.topology.org/comms/netmon.html

10 Tools to add beauty in SHELL SCRIPT..............

There are some misconceptions that shell scripts are only for a CLI environment. You can easily use various tools to write GUI and/or network (socket) scripts under KDE or Gnome desktops. Shell scripts can make use of some of the GUI widget (menus, warning boxs, progress bars etc). You can always control the final output, cursor position on screen, various output effects, and so on. With the following tools you can build powerful, interactive, user friendly UNIX / Linux bash shell scripts.

Creating GUI application is not just expensive task but task that takes time and patience. Luckily, both UNIX and Linux ships with plenty of tools to write beautiful GUI scripts. The following tools are tested on FreeBSD and Linux operating systems but should work under other UNIX like operating systems.

#1: notify-send Command

The notify-send command allows you to send desktop notifications to the user via a notification daemon from the command line. This is useful to inform the desktop user about an event or display some form of information without getting in the user's way. You need to install the following package:
$ sudo apt-get install libnotify-bin
In this example, send simple desktop notification from the command line, enter:

 
notify-send "rsnapshot done :)"
 

Sample outputs:

Fig:01: notify-send in action

Here is another code with additional options:

 
....
alert=18000
live=$(lynx --dump http://money.rediff.com/ | grep 'BSE LIVE' | awk '{ print $5}' | sed 's/,//g;s/\.[0-9]*//g')
[ $notify_counter -eq 0 ] && [ $live -ge $alert ] && { notify-send -t 5000 -u low -i   "BSE Sensex touched 18k";  notify_counter=1; }
...

Sample outputs:

Fig.02: notify-send with timeouts and other options

Where,

• -t 5000: Specifies the timeout in milliseconds ( 5000 milliseconds = 5 seconds)
• -u low : Set the urgency level (i.e. low, normal, or critical).
• -i gtk-dialog-info : Set an icon filename or stock icon to display (you can set path as -i /path/to/your-icon.png).

For more information on use of the notify-send utility, please refer to the notify-send man page, viewable by typing man notify-send from the command line:
man notify-send

#2: tput Command

The tput command is used to set terminal features. With tput you can set:

• Move the cursor around the screen.
• Get information about terminal.
• Set colors (background and foreground).
• Set bold mode.
• Set reverse mode and much more.

Here is a sample code:

#!/bin/bash
 
# clear the screen
tput clear
 
# Move cursor to screen location X,Y (top left is 0,0)
tput cup 3 15
 
# Set a foreground colour using ANSI escape
tput setaf 3
echo "XYX Corp LTD."
tput sgr0
 
tput cup 5 17
# Set reverse video mode
tput rev
echo "M A I N - M E N U"
tput sgr0
 
tput cup 7 15
echo "1. User Management"
 
tput cup 8 15
echo "2. Service Management"
 
tput cup 9 15
echo "3. Process Management"
 
tput cup 10 15
echo "4. Backup"
 
# Set bold mode
tput bold
tput cup 12 15
read -p "Enter your choice [1-4] " choice
 
tput clear
tput sgr0
tput rc

Sample outputs:

Fig.03: tput in action

For more detail concerning the tput command, see the following man page:
man 5 terminfo
man tput

#3: setleds Command

The setleds command allows you to set the keyboard leds. In this example, set NumLock on:

setleds -D +num

To turn it off NumLock, enter:

setleds -D -num

• -caps : Clear CapsLock.
• +caps : Set CapsLock.
• -scroll : Clear ScrollLock.
• +scroll : Set ScrollLock.

See setleds command man page for more information and options:
man setleds

#4: zenity Command

The zenity commadn will display GTK+ dialogs box, and return the users input. This allows you to present information, and ask for information from the user, from all manner of shell scripts. Here is a sample GUI client for the whois directory service for given domain name:

#!/bin/bash
# Get domain name
_zenity="/usr/bin/zenity"
_out="/tmp/whois.output.$$"
domain=$(${_zenity} --title  "Enter domain" \
             --entry --text "Enter the domain you would like to see whois info" )
 
if [ $? -eq 0 ]
then
  # Display a progress dialog while searching whois database
  whois $domain  | tee >(${_zenity} --width=200 --height=100 \
          --title="whois" --progress \
               --pulsate --text="Searching domain info..." \
                                    --auto-kill --auto-close \
                                    --percentage=10) >${_out}
 
  # Display back output
  ${_zenity} --width=800 --height=600  \
      --title "Whois info for $domain" \
      --text-info --filename="${_out}"
else
  ${_zenity} --error \
      --text="No input provided"
fi

Sample outputs:

Fig.04: zenity in Action

See the zenity man page for more information and all other supports GTK+ widgets:
zenity --help
man zenity

#5: kdialog Command

kdialog is just like zenity but it is designed for KDE desktop / qt apps. You can display dialogs using kdialog. The following will display message on screen:

 
kdialog --dontagain myscript:nofilemsg --msgbox "File: '~/.backup/config' not found."
 

Sample outputs:

Fig.05: Suppressing the display of a dialog

See shell scripting with KDE Dialogs tutorial for more information.

#6: Dialog

Dialog is an application used in shell scripts which displays text user interface widgets. It uses the curses or ncurses library. Here is a sample code:

>#!/bin/bash
dialog --title "Delete file" \
--backtitle "Linux Shell Script Tutorial Example" \
--yesno "Are you sure you want to permanently delete \"/tmp/foo.txt\"?" 7 60
 
# Get exit status
# 0 means user hit [yes] button.
# 1 means user hit [no] button.
# 255 means user hit [Esc] key.
response=$?
case $response in
   0) echo "File deleted.";;
   1) echo "File not deleted.";;
   255) echo "[ESC] key pressed.";;
esac

See the dialog man page for details:
man dialog

A Note About Other User Interface Widgets Tools

UNIX and Linux comes with lots of other tools to display and control apps from the command line, and shell scripts can make use of some of the KDE / Gnome / X widget set:

• gmessage - a GTK-based xmessage clone.
• xmessage - display a message or query in a window (X-based /bin/echo)
• whiptail - display dialog boxes from shell scripts
• python-dialog - Python module for making simple Text/Console-mode user interfaces

#7: logger command

The logger command writes entries in the system log file such as /var/log/messages. It provides a shell command interface to the syslog system log module:

 
logger "MySQL database backup failed."
tail -f /var/log/messages
logger -t mysqld -p daemon.error "Database Server failed"
tail -f /var/log/syslog
 

Sample outputs:

Apr 20 00:11:45 vivek-desktop kernel: [38600.515354] CPU0: Temperature/speed normal
Apr 20 00:12:20 vivek-desktop mysqld: Database Server failed

See howto write message to a syslog / log file for more information. Alternatively, you can see the logger man page for details:
man logger

#8: setterm Command

The setterm command can set various terminal attributes. In this example, force screen to turn black in 15 minutes. Monitor standby will occur at 60 minutes:

 
setterm -blank 15 -powersave powerdown -powerdown 60
 

In this example show underlined text for xterm window:

 
setterm -underline on;
echo "Add Your Important Message Here"
setterm -underline off
 

Another useful option is to turn on or off cursor:

 
setterm -cursor off
 

Turn it on:

 
setterm -cursor on
 

See the setterm command man page for details:
man setterm

#9: smbclient: Sending Messages To MS-Windows Workstations

The smbclient command can talk to an SMB/CIFS server. It can send a message to selected users or all users on MS-Windows systems:

smbclient -M WinXPPro <1
Message 2
...
..
EOF

OR

 
echo "${Message}" | smbclient -M salesguy2
 

See smbclient man page or read our previous post about "sending a message to Windows Workstation" with smbclient command:
man smbclient

#10: Bash Socket Programming

Under bash you can open a socket to pass some data through it. You don't have to use curl or lynx commands to just grab data from remote server. Bash comes with two special device files which can be used to open network sockets. From the bash man page:

1. /dev/tcp/host/port - If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open a TCP connection to the corresponding socket.
2. /dev/udp/host/port - If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open a UDP connection to the corresponding socket.

You can use this technquie to dermine if port is open or closed on local or remote server without using nmap or other port scanner:

# find out if TCP port 25 open or not
(echo >/dev/tcp/localhost/25) &>/dev/null && echo "TCP port 25 open" || echo "TCP port 25 close"

You can use bash loop and find out open ports with the snippets:

 
echo "Scanning TCP ports..."
for p in {1..1023}
do
  (echo >/dev/tcp/localhost/$p) >/dev/null 2>&1 && echo "$p open"
done

Sample outputs:

Scanning TCP ports...
22 open
53 open
80 open
139 open
445 open
631 open

In this example, you bash script act as an HTTP client:

#!/bin/bash
exec 3<> /dev/tcp/${1:-www.cyberciti.biz}/80
 
printf "GET / HTTP/1.0\r\n" >&3
printf "Accept: text/html, text/plain\r\n" >&3
printf "Accept-Language: en\r\n" >&3
printf "User-Agent: nixCraft_BashScript v.%s\r\n" "${BASH_VERSION}"   >&3
printf "\r\n" >&3
 
while read LINE <&3
do
   # do something on $LINE
   # or send $LINE to grep or awk for grabbing data
   # or simply display back data with echo command
   echo $LINE
done

See the bash man page for more information:
man bash

A Note About GUI Tools and Cronjob

You need to request local display/input service using export DISPLAY=[user's machine]:0 command if you are using cronjob to call your scripts. For example, call /home/vivek/scripts/monitor.stock.sh as follows which uses zenity tool:
@hourly DISPLAY=:0.0 /home/vivek/scripts/monitor.stock.sh

Have a favorite UNIX tool to spice up shell script? Share it in the comments below.