Installing GNS3 on Ubuntu 10.04+

First, you have to install the dynagen and python-qt4 packages.

 sudo apt-get install dynagen python-qt4

After that, you have to download GNS3 source for Linux. You can either
 download it from their website or you can use the terminal to download it for you.

cd /opt
sudo wget http://downloads.sourceforge.net/gns-3/GNS3-0.7.4-src.tar.bz2?download
sudo tar -xjvf GNS3-0.7.4-src.tar.bz2?download
sudo mv GNS3-0.7.4-src /opt/GNS3

During my installation, the latest version is v0.7.4. Check this website first to see what is the latest version: http://www.gns3.net/download
Create the following directories inside the GNS3 directory: Dynamips, IOS, Project, Cache and tmp.

cd /opt/GNS3
sudo mkdir Dynamips
sudo mkdir IOS
sudo mkdir Project
sudo mkdir Cache
sudo mkdir tmp

Add Read and Write permissions for the other users to the following directories: Project, tmp and IOS.

sudo chmod o+rw -R ./Project
sudo chmod o+rw -R ./tmp
sudo chmod o+rw -R ./IOS

Download Dynamips and make it executable:

cd Dynamips
sudo wget http://www.ipflow.utc.fr/dynamips/dynamips-0.2.8-RC2-x86.bin
sudo chmod +x ./dynamips-0.2.8-RC2-x86.bin

The latest version available is 0.2.8. Check this website first to see what is the latest version: http://www.ipflow.utc.fr/dynamips/
Install the Qemu package.

sudo apt-get install qemu

That ends the installation of GNS3. The next steps will tell you how to configure GNS3.
Create a launcher for GNS3
In the Desktop, create a new launcher and use the following command:

python "/opt/GNS3/gns3"

Or you can use the Main Menu configuration to add one.
Launch GNS3 and go to Edit > Preferences > Terminal Settings > Preconfigurated Terminal Commands
Select Gnome Terminal (Linux)
You will see that the Terminal Command will be similar to this:

gnome-terminal -t %d -e 'telnet %h %p' >/dev/null 2>&1 &

In the General Settings tab, look for Paths:
Configure the Project directory:

/opt/GNS3/Project

Image directory:

/opt/GNS3/IOS

Go to the Dynamips tab and look for the Executable path:

/opt/GNS3/Dynamips/dynamips-0.2.8-RC2-x86.bin

Working Directory:

/opt/GNS3/tmp

Go to the Capture tab and look for the working directory for capture files:

/opt/GNS3/Project

 Go to the Qemu tab and set the Path to qemu:

/usr/bin/qemu

Path to qemu-img

/usr/bin/qemu-img

That’s it. You now have installed and configured GNS3 to run on Ubuntu. You just have to own an IOS to start setting up your routers.