Add Loopbacks in Ubuntu for GNS3

Running GNS3 on Linux and trying to reach the outside world, you will not be able to use your host PC to telnet to the virtual routers unless you create loopbacks. see related post in GNS3 forum
Below I post the steps for creating loopback on Ubuntu. After that you will be able to use your PC to telnet or ping to the virtual routers. (more details here)



First we need to install the utility programs and bridge-utils to be able to create network bridges

 sudo apt-get install uml-utilities   
 sudo apt-get install bridge-utils

Load module for tunneling

 modprobe tun  

Backup the current interfaces file with a copy (that has the current date)

 sudo cp /etc/network/interfaces /etc/network/interfaces.`date +%F-%T`   

Create a TUN/TAP device
The setup needs to be done as root, but once that's done, there is no need for root assistance. Setting up the device is done as follows

 sudo tunctl -t tap0  

Remove ip addressing and set eth0 and tap0 to promiscuous mode 

 sudo ifconfig eth0 0.0.0.0 promisc up   
 sudo ifconfig tap0 0.0.0.0 promisc up  

Create a new bridge interface

 sudo brctl addbr br0  

Check the created bridge

 brctl show  
  bridge name  bridge id    STP enabled  interfaces  
 br0    8000.000000000000  no      

Add tap0 and eth0 to the bridge group

 sudo brctl addif br0 tap0   
 sudo brctl addif br0 eth0   

To see the new interfaces under the bridge

 brctl show  
 bridge name  bridge id    STP enabled  interfaces  
 br0    8000.001fc675588b  no    eth0  
                                 tap0  

Enable the bridge interface and give it an ip address

 sudo ifconfig br0 up  
 sudo ifconfig br0 192.168.0.77/24  

Or force interface to get IP address from DHCP

 sudo dhclient br0  

Configure the default route

 sudo route add default gw 192.168.0.1   

For easy copy paste

 sudo tunctl  
 sudo ifconfig eth0 0.0.0.0 promisc up   
 sudo ifconfig tap0 0.0.0.0 promisc up  
 sudo brctl addbr br0  
 sudo brctl addif br0 tap0  
 sudo brctl addif br0 eth0  
 sudo ifconfig br0 up  
 sudo ifconfig br0 192.168.0.77/24  
 sudo route add default gw 192.168.0.1  

My example in GNS3

Create the cloud, then right on it and go to configuration
Then goto NIO TAP - and type tap0

Example of Cloud configuration

 Router#show arp  
 Protocol Address     Age (min) Hardware Addr  Type  Interface  
 Internet 192.168.0.1       6  000d.29aa.b1b1 ARPA  FastEthernet1/0  
 Internet 192.168.0.88      -  ca00.0c26.001c ARPA  FastEthernet1/0  
 Internet 192.168.0.104      6  3a16.33c4.9982 ARPA  FastEthernet1/0  
 Internet 192.168.0.105      0  Incomplete   ARPA    

On R3

 interface FastEthernet1/0  
  ip address 192.168.0.88 255.255.255.0  

On PC

 ip addr show   
 2: eth0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000  
 5: tap0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 500  
 6: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN   
   inet 192.168.0.104/24 brd 192.168.0.255 scope global br0  

From R3 to PC

 R3#ping 192.168.0.104  
 Type escape sequence to abort.  
 Sending 5, 100-byte ICMP Echos to 192.168.0.104, timeout is 2 seconds:  
 .!!!!  
 Success rate is 80 percent (4/5), round-trip min/avg/max = 4/8/12 ms  
 Router#ping 192.168.0.1   
 Type escape sequence to abort.  
 Sending 5, 100-byte ICMP Echos to 192.168.0.1, timeout is 2 seconds:  
 !!!!!  
 Success rate is 100 percent (5/5), round-trip min/avg/max = 4/11/24 ms  

From PC to R3

 ping 192.168.0.88  
 PING 192.168.0.88 (192.168.0.88) 56(84) bytes of data.  
 64 bytes from 192.168.0.88: icmp_req=1 ttl=255 time=4.80 ms  
 64 bytes from 192.168.0.88: icmp_req=2 ttl=255 time=2.22 ms