30 June 2008 25 Comments

　　Binding GNS3/dynamips Routers Ethernet port to the Hosts physical interface.

　　While browsing the http://7200emu.hacki.at forums, I noticed a few posts were asking for help on how to bridge a Routers ethernet port in Dynamips to the Linux hosts ethernet card. The only responses that seemed to show up stated that it was difficult as the tap interface needs to be created wen you start the lab. Well this article is here to address the issue and make things easier.

　　All the commands mentioned on this page should be executed with root privleges. To do this either use the su command to switch user, Or if you use ubuntu type sudo before each of the commands I have listed. Configuring your system in this manner allows you to run gns3/dynamips as a standard user and still have network access!

　　Firstly the terminology

　　Eth0,Wlan0 - If your machine has a wired ethernet card, It should show up as eth0 or eth1. If its an 802.11 card its possible to see something like wlan0.

　　Tap0,Tap1 - The Tap interface is a virtual interface created in linux, This interface is the one that the routers port is bound to.

　　Br0 - This is a Bridge for linux networking, This device is the one that ties eth0 and tap0 together so traffic can be passed.

　　Part 1 - Information Gathering

　　We are going to start with gathering some information about your network settings. If you use Dhcp you can skip straight to Part 2

　　bash:#ip a

　　1: lo:  mtu 16436 qdisc noqueue

　　link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

　　inet 127.0.0.1/8 scope host lo

　　inet6 ::1/128 scope host

　　valid_lft forever preferred_lft forever

　　2: eth1:  mtu 1500 qdisc noop qlen 1000

　　link/ether 00:1e:0b:33:6e:4c brd ff:ff:ff:ff:ff:ff

　　3: eth0:  mtu 1500 qdisc pfifo_fast qlen 1000

　　link/ether 00:15:77:9b:ec:07 brd ff:ff:ff:ff:ff:ff

　　inet 192.168.1.11/24 brd 192.168.1.255 scope global eth0

　　inet6 fe80::215:77ff:fe9b:ec07/64 scope link

　　valid_lft forever preferred_lft forever

　　Notice card number 3: eth0. Has the ip address 192.168.1.11 with a /24 mask. To find the default gateway enter this command

　　bash:#ip r

　　192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.11

　　169.254.0.0/16 dev eth0 scope link

　　127.0.0.0/8 dev lo scope link

　　default via 192.168.1.1 dev eth0

　　My default gateway is shown by the line default via 192.168.1.1 dev eth0. So the gateway ip address is 192.168.1.1. Does everyone remember the 169.254.0.0/16 network? The APIPA range?. Well most modern linux distro’s include a route to that network just in case.

　　Part 2 - Setting up the Network

　　Welcome back to the Dhcp users. To kick this off we need to create a bridge interface on your machine. The command is brctl and is usually part of a package called “Bridge-utils”

　　To create the bridge we can do the following

　　brctl addbr br0

　　Execute ip a to verify that it was created and you should see a new entry like this

　　3: eth0:  mtu 1500 qdisc pfifo_fast qlen 1000

　　link/ether 00:15:77:9b:ec:07 brd ff:ff:ff:ff:ff:ff

　　inet 192.168.1.11/24 brd 192.168.1.255 scope global eth0

　　inet6 fe80::215:77ff:fe9b:ec07/64 scope link

　　valid_lft forever preferred_lft forever

　　4: br0:  mtu 1500 qdisc noqueue

　　link/ether 00:15:77:9b:ec:07 brd ff:ff:ff:ff:ff:ff

　　Notice how on the bridge line there is no “UP,LOWER_UP” statement after multicast?, This indicates that the bridge interface is shutdown.

　　To activate the bridge we need to perform the following

　　ip l s dev br0 up

　　Now execute ip a again to verify interface br0 now shows as up.

　　Now to create the tap interface we need the following command “tunctl”. Normally in a package called “uml-utilities”

　　To create the tap interface execute the following where username! is your linux login

　　tunctl -t tap0 -u username!

　　Again to verify the creation of the interface execute ip a

　　3: eth0:  mtu 1500 qdisc pfifo_fast qlen 1000

　　link/ether 00:15:77:9b:ec:07 brd ff:ff:ff:ff:ff:ff

　　inet 192.168.1.11/24 brd 192.168.1.255 scope global eth0

　　inet6 fe80::215:77ff:fe9b:ec07/64 scope link

　　valid_lft forever preferred_lft forever

　　4: br0:  mtu 1500 qdisc noqueue

　　link/ether 00:15:77:9b:ec:07 brd ff:ff:ff:ff:ff:ff

　　5: tap0:  mtu 1500 qdisc pfifo_fast qlen 500

　　link/ether 00:ff:da:10:31:61 brd ff:ff:ff:ff:ff:ff

　　Again, the Tap0 interface is shutdown by default. to Activate it execute

　　ip l s dev tap0 up

　　So now we have all the interfaces we need created. Time to start adding interfaces to the bridge.

　　To associate both the Eth0 and tap0 interface with the bridge, enter the following to commands. This *links the interfaces together

　　brctl addif br0 tap0

　　brctl addif br0 eth0

　　To verify that this worked, try

　　brctl show br0

　　You output should be similar to this

　　bridge name bridge id STP enabled interfacesbr0 8000.00123f43a4b3 no tap0 eth0

　　Now we need to change the eth0 card to promiscuous mode and remove the ip address from it

　　ifconfig eth0 0.0.0.0 promisc

　　So you can still access your network, We need to configure the br0 interface with your network settings, If you use a static ip address execute the following commands

　　ip a a 192.168.1.11/24 dev br0

　　ip r a default via 192.168.1.1

　　If you use dhcp, We can start the dhcp client on br0 with the following

　　dhclient br0

　　Thats it for the network!. Try to ping your default gateway or another device on your network. Easy hey!

　　Next part is dynamips! And its easy to.

　　Inside the .net file of your chosen topology.

　　autostart = True

　　[localhost:7210]

　　workingdir = /data/GNS3/ts_working

　　[[3640]]

　　image = /data/GNS3/cisco/ios/C3640-IK.BIN

　　chassis = 3640

　　[[ROUTER R0]]

　　model = 3640

　　console = 2000

　　cnfg = /data/GNS3/ts_configs/R0.cfg

　　slot0 = NM-1FE-TX

　　f0/0 = nio_tap:tap0

　　slot1 = NM-1FE-TX>

　　[[Cloud C0]]

　　connections = R0:f0/0:nio_tap:tap0

　　Note the device called Cloud C0, It’s connection line is the key to making this work. It states that router R0 has the interface Fastethernet0/0 which links to nio_tap:tap0. Where tap0 is the tap interface we created earlier

　　Router R0 has the matching line which completes the configuration.

　　If you’re a GNS3 user, Then its just as simple, Add the Cloud object to your network and create an ethernet link from the Cloud to your chosen router. Then right click the cloud and select configure.

　　Select the “NIO TAP” tab and enter your tap name.