If you are on Gnome, you may want to use the Gnome terminal.

The proper command to set is :

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

However, it requires you to use a real network card with a fixed IP for doing that. This is not really handy if, llike me, you use GNS3 on a laptop whose connectivity is frequently switching between cable and wireless, and on different subnets.

I started to look for something more convenient like a virtual interface.

I first thought about declaring a virtual IP on eth0, but no way : briding is layer 2 (based on MAC address), you can’t add eth0:0 to a bridge.

Then I found the very handy dummy interface.

Load the module :

% sudo modprobe dummy
% lsmod | grep dummy
dummy                   3192  0
% ifconfig dummy0
dummy0    Link encap:Ethernet  HWaddr AE:89:91:BD:61:87
BROADCAST NOARP  MTU:1500  Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 lg file transmission:0
RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

To have the module loaded at boot time, do :

In debian :

% echo dummy >> /etc/modules

In openSUSE, edit this line in /etc/sysconfig/kernel :

MODULES_LOADED_ON_BOOT="dummy"

Then, you could go on with the tutorial linked above, replacing eth0 with dummy0, or use the script I made :

#!/bin/sh

/bin/tunctl -t tap0
/sbin/ifconfig tap0 0.0.0.0 promisc up
/sbin/ifconfig dummy0 0.0.0.0 promisc up
/sbin/brctl addbr br0
/sbin/brctl addif br0 tap0
/sbin/brctl addif br0 dummy0
/sbin/ifconfig br0 10.10.10.99/24 up
/path/to/GNS3-0.6-src/gns3

You may also insert this line if you want to interconnect your lab network with your other networks (it activates kernel’s IP forwarding) :

echo 1 > /proc/sys/net/ipv4/ip_forward

I saved it /usr/local/bin/gns and created a pretty shortcut for the application browser of Gnome :

% cat /usr/share/applications/gns3.desktop
[Desktop Entry]
X-SuSE-translate=true
Encoding=UTF-8
Name=gns3
GenericName=Cisco routers emulator
Exec=/usr/local/bin/gns-start
Terminal=false
Type=Application
X-KDE-SubstituteUID=true
Icon=gnome-window-manager
Categories=GNOME;Network;

This shortcut will prompt you for the root password (unfortunately, when dealing with tap interface, GNS3 requires root access – security could be tighten with SELinux or AppArmor) and launch GNS3 with most of the network stuff prepared.

Just in case you want to clear that all quickly, there is the gns-stop script :

#!/bin/sh
ifconfig br0 down
brctl delif br0 tap0
brctl delif br0 eth0
brctl delbr br0
tunctl -d tap0

I sometimes use Boson Netsim, which is not only non-free but not so reliable.

However, as I just started to use GNS3 and Dynamips, I don’t know it so well yet and won’t compare any further the two solutions.

On this page, I am just summarizing the few steps to set it up on your Linux system.

First, set up the prerequisite :

$ aptitude install python-qt4

Now, go to gns3.net and download the source code for Linux (direct link).

You may extract the archive in your local application folder :

$ wget http://pfe.epitech.net/frs/download.php/819/GNS3-0.5-src.tar.gz
$ tar -xzvf GNS3-0.5-src.tar.gz -C /opt

Then, you need the dynamips binary from the dynamips blog (direct links for x86 or amd64 platforms).
The file must be executable.

$ wget http://www.ipflow.utc.fr/dynamips/dynamips-0.2.8-RC2-amd64.bin
$ chmod u+x dynamips*.bin
$ mv dynamips-0.2.8-RC2-amd64.bin /opt

Now, start GNS3 :

$ /opt/GNS3-0.5-src/gns3

In the edit menu, select preferences and go the dynamips section.

Just browse to the dynamips binary you dowloaded, to fill the value of the executable path field.

Still from the edit menu, select IOS images and hypervisors. There, you have to add all the IOS images you want to use, one after another. Normally, the default settings for each file loaded are suitable.

Back to the main window, you can drag and drop routers and link them, creating the topology you wish.

For now, just add one router. Right click on it and select start to start it up. Right click again and select console.

Enjoy ! Of course, I strongly recommand that you start reading further from this page.

Resources :

http://www.ipflow.utc.fr/blog/
http://www.gns3.net/
http://www.blindhog.net/tutorials/gns3-linux-install/gns3-linux-install.html