To setup a virtual network lab, you get a lot of alternatives. You can for example use GNS3, Netkit, Marionnet or VNUML. Some of those tools, like GNS3, allow you to add some closed source equipments like a Cisco 7200 or a Juniper router.

All those tools are a great way to setup your network lab. Look at them! If you want to setup a virtual network lab for educational purpose, one of those should fit your purpose. However, none of those solution were a perfect match for me. I did not want to maintain some root filesystem. I wanted my lab to start in a few seconds. I wanted to keep all configuration files (including the ones for the virtual hosts) into one subdirectory of my home and be able to modify them while the lab was running. I also wanted to be able to plug some Cisco router using Dynamips/Dynagen.

None of the listed solution above matched all those criteria. Therefore, I setup my own lab script with User Mode Linux. This is not a complete solution, but is more like a home-made solution to match one particular need. You cannot use the final result without tweaking it. Again, look at the other solutions first.

We will go step by step to understand how the lab was built. If you are in a hurry, you can look at the result which is published on GitHub.

User Mode Linux§

User Mode Linux (or UML) is a Linux kernel running as a process instead of running on top of some processor. With Debian, you can install it using apt-get install user-mode-linux. You get a linux command that will run your Linux kernel as a simple process. No need to be root.

$ linux
Core dump limits :
    soft - 0
    hard - NONE
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Checking advanced syscall emulation patch for ptrace...OK
Checking for tmpfs mount on /dev/shm...nothing mounted on /dev/shm
Checking PROT_EXEC mmap in /tmp/user/500/...OK
Checking for the skas3 patch in the host:
  - /proc/mm...not found: No such file or directory
  - PTRACE_FAULTINFO...not found
  - PTRACE_LDT...not found
UML running in SKAS0 mode
Adding 5632000 bytes to physical memory to account for exec-shield gap
Initializing cgroup subsys cpuset
Linux version 2.6.32 (2.6.32) (root@tito) (gcc version 4.4.5 (Debian 4.4.5-10) ) #2 Thu Jan 27 12:49:46 UTC 2011
[...]
console [mc-1] enabled
Couldn't stat "root_fs" : err = 2
Failed to initialize ubd device 0 :Couldn't determine size of device's file
registered taskstats version 1
VFS: Cannot open root device "98:0" or unknown-block(98,0)
Please append a correct "root=" boot option; here are the available partitions:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(98,0)

You get a Linux kernel that will just panic because it does not find a disk to mount to run init from it. Usually, you build some root file system and give it as an argument to your UML kernel.

Basic setup§

However, you can also use your host filesystem as a root filesystem. We also specify /bin/sh as init instead of /sbin/init. This will be the first process to be started by our UML kernel.

$ linux init=/bin/sh rootfstype=hostfs
[...]
Linux version 2.6.32 (2.6.32) (root@tito) (gcc version 4.4.5 (Debian 4.4.5-10) ) #2 Thu Jan 27 12:49:46 UTC 2011
[...]
VFS: Mounted root (hostfs filesystem) readonly on device 0:12.
IRQ 3/console-write: IRQF_DISABLED is not guaranteed on shared IRQs
IRQ 2/console: IRQF_DISABLED is not guaranteed on shared IRQs
IRQ 10/winch: IRQF_DISABLED is not guaranteed on shared IRQs
/bin/sh: can't access tty; job control turned off
# uname -a
Linux (none) 2.6.32 #2 Thu Jan 27 12:49:46 UTC 2011 x86_64 GNU/Linux
# echo $$
1

You may want to setup a bit your environment:

# hostname -b R1
# export TERM=xterm
# export PATH=/usr/local/bin:/usr/bin:/bin:/sbin:/usr/local/sbin:/usr/sbin
# mount -t proc proc /proc
# mount -t sysfs sysfs /sys
# mount -t tmpfs tmpfs /var/run -o rw,nosuid,nodev
# mount -t tmpfs tmpfs /var/log -o rw,nosuid,nodev
# mount -o bind /usr/lib/uml/modules /lib/modules
# mount -t hostfs hostfs /home/bernat/mylab -o /home/bernat/mylab

We setup /proc and /sys correctly. We also turn /var/run and /var/log as tmpfs filesystems. This will allow most daemons to run correctly: they will be able to write runtime data in /var/run and to write logs in /var/log. You can even launch your favorite syslog daemon.

The line about /lib/modules is to ensure that our UML kernel is able to access its kernel modules. For example, we can now use modprobe tun. The line about /home allows us to mount the home directory of your host inside UML. This was already the case, but it was mounted as read-only.

One important thing to notice is that while you seem to be root inside UML, you did not run linux as root. Therefore, when the UML kernel tries to write a file it will still be bound by the permissions granted by your host kernel. See this example:

# mount -o remount,rw /
# touch /tmp/test1
# touch /etc/test1
touch: cannot touch `/etc/test1': Permission denied

This is a great way to ensure that your lab will not destroy your host. Do not run linux as root.

What is interesting with such a setup is that you share the filesystem with your host. You can modify some configuration file on your host and your UML will see the modification right away. You can also modify a file inside UML and see the modification on your host too.

Console & job control§

There is a slight problem with this environment. You may be used to it if you already started your own GNU/Linux system with init=/bin/sh. The problem is that you don’t have job control. Job control allows you to put tasks in background and interrupt them while they are running. It is job control that allows you to stop a program with ^C. Here, you don’t have job control. See:

# cat
^C^C^C^C

You are stuck. The only way to go out is to kill the linux process on your host.

Job control is a great mystery for me. I don’t know how to enable it by hand. However, getty is able to enable job control for you. Just type this command:

# exec getty -n -l /bin/sh 38400 /dev/tty0

You get job control. We keep /bin/sh, but you may prefer /bin/bash since on most systems, /bin/sh is rather basic.

Writable root filesystem§

Let’s get back to our root filesystem. Most daemons can run just fine with our setup. For example, we can start Nginx, a popular web server:

# mkdir /var/log/nginx
# /etc/init.d/nginx start
Starting nginx: nginx.

However, it will use the configuration file in /etc/nginx. You don’t want to modify this directory for all your labs. The best way to handle this is to start Nginx with a custom configuration file:

# /etc/init.d/nginx stop
# nginx -c /home/bernat/mylab/nginx.conf

While keeping the configuration files related to your lab in your home is an important objective (all your lab related files are in the same place and you can modify them at will), it may be more convenient to still use /etc/init.d/nginx script to control Nginx. Moreover, some daemons do not allow you to specify a configuration file. You will then need to modify your root filesystem.

This is a pretty common problem with Live CD and it is solved by using an union mount. This is a special kind of mount that will merge two filesystems into one. One of the two filesystems can be read-only and all the changes will be reported to the second one. We will use AUFS. Ensure that you have aufs-tools package installed. Let’s start from scratch for this example.

$ linux init=/bin/sh rootfstype=hostfs
[...]
# mount -n -t proc proc /proc
# mount -n -t sysfs sysfs /sys
# mount -o bind /usr/lib/uml/modules /lib/modules
# mount -n -t tmpfs tmpfs /tmp -o rw,nosuid,nodev
# mkdir /tmp/ro /tmp/rw /tmp/aufs
# mount -n -t hostfs hostfs /tmp/ro -o /,ro
# mount -n -t aufs aufs /tmp/aufs -o noatime,dirs=/tmp/rw:/tmp/ro=ro
# exec chroot /tmp/aufs /bin/bash

It seems that AUFS with hostfs is rather fragile. We could have merged our root filesystem with some directory in our lab using hostfs like this:

# mount -n -t hostfs hostfs /tmp/rw -o /home/bernat/mylab/root,rw
# mount -n -t hostfs hostfs /tmp/ro -o /,ro
# mount -n -t aufs aufs /tmp/aufs -o noatime,dirs=/tmp/rw:/tmp/ro=ro
# exec chroot /tmp/aufs /bin/bash

However, the kernel panics on most operations. Therefore, /tmp/rw stays in a tmpfs filesystem. This means that any change to the root filesystem will be lost. I also was unable to issue a correct pivot_root before chroot. This is not really important here.

UPDATED: It seems that there are other drawbacks to this setup. For some reason, if you have a separate /usr partition on your host system, you won’t be able to write to it until you specify the noxino option to AUFS. Another issue is that AUFS does not notice most changes on the hostfs filesystem. You can fix this with option udba=inotify. However, this is incompatible with the noxino option. Therefore, you need to choose what is best for your lab. You can also mount some parts of your home with hostfs (for example, the source directory of the piece of software that you would like to test).

Let’s consider that Nginx configuration is in /home/bernat/lab/nginx. We simply uses a symbolic link:

# rm -rf /etc/nginx
# ln -s /home/bernat/mylab/nginx /etc/nginx

Since we lose the modifications to the root filesystem, the symbolic link should be done each time we start the lab. However, we can modify directly the configuration files in the host or in the UML.

Networking§

We know how to start our UML. Let’s see how we can add some wires to it. UML supports several network adapters. The two most useful are TAP and VDE backend.

TAP§

A TAP interface is a virtual network kernel device which can be used by userland applications to inject layer 2 frames in it. If the kernel sends frames in a TAP device, they will be received by the application listening to it. This also works the other way around. A TAP interface is a regular ethernet device for the kernel. This means that you can bridge it or use tcpdump on it.

The drawback of such interfaces is that you need to be root to create them. The application using them does not need to be run as root.

I usually use this nifty shell function to setup TAP interfaces:

__add_to_bridge() {
    # Optionally, add it to given bridge
    [ -z "$2" ] || {
        [ -f /sys/class/net/$2/brforward ] || {
            sudo brctl addbr $2
            sudo brctl stp $2 off
            sudo ip link set $2 up
        }
        [ -f /sys/class/net/$2/brif/$1 ] || {
            # We need to check if it is in another bridge
            bridge=$(echo /sys/class/net/*/brif/$1 2> /dev/null | \
                sed 's+/sys/class/net/\([^/]*\)/.*+\1+') 2> /dev/null
            [ -n "$bridge" ] && \
                sudo brctl delif $bridge $1
            sudo brctl addif $2 $1
        }
    }
}
tap() {
    sudo tunctl -b -u $(whoami) -t $1 > /dev/null
    sudo ip link set up dev $1
    __add_to_bridge $1 $2
}

It will create the interface and put it into a bridge. For example, you can create two interfaces linked together through a bridge to allow two UML to talk each other:

$ tap tap-R1 br-R1R2
$ tap tap-R2 br-R1R2

We can setup two UML which will use those interfaces. The first one is setup like this:

$ linux init=/bin/sh rootfstype=hostfs eth0=tuntap,tap-R1
[...]
# ip link set up dev eth0
# ip addr add 192.168.0.1/24 dev eth0

The second one:

$ linux init=/bin/sh rootfstype=hostfs eth0=tuntap,tap-R2
[...]
# ip link set up dev eth0
# ip addr add 192.168.0.2/24 dev eth0
# ping 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
Warning: time of day goes back (-13832us), taking countermeasures.
64 bytes from 192.168.0.1: icmp_req=20 ttl=64 time=0.633 ms
64 bytes from 192.168.0.1: icmp_req=21 ttl=64 time=0.157 ms

If you are running a firewall on your host, ensure that you let your host forward those packets. By default, bridging code forwards the frames through Netfilter.

VDE§

A VDE switch is a software emulation of a regular network switch. It is a userland component and does not need to be run as root. You need to install vde2 package to make use of it. You can run a switch using vde_switch command. You will get a console by pressing Enter. From this console, you can configure the switch: add ports, configure VLAN, etc.

Like for TAP interfaces, let’s setup two UML connected to this switch. The first one is like this:

$ linux init=/bin/sh rootfstype=hostfs eth0=vde
[...]
# ip link set up dev eth0
# ip addr add 192.168.0.1/24 dev eth0

And the second one:

$ linux init=/bin/sh rootfstype=hostfs eth0=vde
[...]
# ip link set up dev eth0
# ip addr add 192.168.0.2/24 dev eth0
# ping 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
64 bytes from 192.168.0.1: icmp_req=1 ttl=64 time=2.93 ms
Warning: time of day goes back (-8101us), taking countermeasures.
64 bytes from 192.168.0.1: icmp_req=2 ttl=64 time=0.503 ms

The lab§

So far, we had a look at what can be used to setup a lab. We need to put everything together into some script to setup the lab in a quick way. For each lab that I want to setup, I copy my latest script to a new location and adapt to the topology I want. I have put everything described below in GitHub. Feel free to browse and fork.

Our lab will be about setting up a redundant VPN between two remote sites of the same company. This company is using OSPF as an IGP on each site and we will use BGP to let each site know the routes advertised in the other site. We will only use VDE switches for the layer 2 part.

Redundant VPN lab

Let’s implement this lab. You should grab the sources first:

$ git clone https://github.com/vincentbernat/network-lab.git
$ cd network-lab/lab-redundant-vpn

You have a setup script which will setup the whole lab for you. You also have one directory for each UML. Those directories only contain configuration for the various daemons used inside UML. To make this lab work, ensure that you have quagga (Quagga, an Internet routing daemon supporting both OSPF and BGP) and racoon packages.

UML configuration§

First look at the end of script. There is something like this:

case $$ in
    1)
        # Inside UML. Three states:
        [...]
        ;;
    *)
        TMP=$(mktemp -d)
        trap "rm -rf $TMP" EXIT
        check_dependencies
        setup_screen
        # Setup switches
        setup_switch site1
        setup_switch site101
        setup_switch internet
        # Start VM
        start_vm R1 eth0=vde,$TMP/switch-site1.sock
        start_vm R2 eth0=vde,$TMP/switch-site101.sock
        start_vm V1 eth0=vde,$TMP/switch-site1.sock eth1=vde,$TMP/switch-internet.sock
        start_vm V2 eth0=vde,$TMP/switch-site1.sock eth1=vde,$TMP/switch-internet.sock
        start_vm V3 eth0=vde,$TMP/switch-site101.sock eth1=vde,$TMP/switch-internet.sock
        start_vm V4 eth0=vde,$TMP/switch-site101.sock eth1=vde,$TMP/switch-internet.sock
        start_vm I1 eth0=vde,$TMP/switch-internet.sock
        display_help
        cleanup
        screen -X quit
        ;;
esac

The script will check its PID ($$). If it is 1, this means it has been invoked as init. Otherwise, it means it has been launched by the user. We will see the init part later. When called, we first check some dependencies, then we setup screen. Everything is run inside a screen session. No multiple windows. The next step is to setup the various switches needed for our lab. We take one switch for each site and one switch for “Internet”. The function that setups the switch just invokes vde_switch with the help of start-stop-daemon to record the PID (to shutdown the lab properly).

Then, we start our various UML. For each of them, we need to specify which network adapters we need. For R1 and R2, we don’t use a network adapter to connect to the internal network. We will use a dummy0 interface. To emulate Internet, we use another UML, I1.

For each UML, our script will be used as init. In this case, its PID will be 1. As we have seen earlier, the script checks if it is called as init. If it is the case, it will setup the UML. During this setup, host specific configuration will be applied:

echo "[+] Setup UML"
sysctl -w net.ipv4.ip_forward=1
case ${uts} in
    R1)
        modprobe dummy
        ip link set up dev dummy0
        ip addr add 192.168.15.1/24 dev dummy0
        ip addr add 192.168.1.10/24 dev eth0
        setup_quagga
        ;;
    R2)
        modprobe dummy
        ip link set up dev dummy0
        ip addr add 192.168.115.1/24 dev dummy0
        ip addr add 192.168.101.10/24 dev eth0
        setup_quagga
        ;;
    V1)
        ip addr add 192.168.1.11/24 dev eth0
        ip addr add 1.1.2.1/24 dev eth1
        ip route add default via 1.1.2.10
        setup_quagga
        setup_racoon 1.1.2.1 1.1.1.1 192.168.0.0/19-192.168.100.0/19
        ;;
    V2)
        ip addr add 192.168.1.12/24 dev eth0
        ip addr add 1.1.2.2/24 dev eth1
        ip route add default via 1.1.2.10
        setup_quagga
        setup_racoon 1.1.2.2 1.1.1.2 192.168.0.0/19-192.168.100.0/19
        ;;
    V3)
        ip addr add 192.168.101.13/24 dev eth0
        ip addr add 1.1.1.1/24 dev eth1
        ip route add default via 1.1.1.10
        setup_quagga
        setup_racoon 1.1.1.1 1.1.2.1 192.168.100.0/19-192.168.0.0/19
        ;;
    V4)
        ip addr add 192.168.101.14/24 dev eth0
        ip addr add 1.1.1.2/24 dev eth1
        ip route add default via 1.1.1.10
        setup_quagga
        setup_racoon 1.1.1.2 1.1.2.2 192.168.100.0/19-192.168.0.0/19
        ;;
    I1)
        ip addr add 1.1.1.10/24 dev eth0
        ip addr add 1.1.2.10/24 dev eth0
        ;;
esac

After all this, we drop to a shell to allow you to issue additional commands interactively.

Testing§

We need to wait one minute after the lab has been started. We can then check the routing table of various equipments. For example, here is R1 routing table (you need to type vtysh to get Quagga console):

R1# show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF,
       I - ISIS, B - BGP, > - selected route, * - FIB route

C>* 127.0.0.0/8 is directly connected, lo
O   192.168.1.0/24 [110/10] is directly connected, eth0, 00:25:42
C>* 192.168.1.0/24 is directly connected, eth0
O   192.168.15.0/24 [110/10] is directly connected, dummy0, 00:25:42
C>* 192.168.15.0/24 is directly connected, dummy0
O>* 192.168.115.0/24 [110/20] via 192.168.1.11, eth0, 00:02:24
  *                           via 192.168.1.12, eth0, 00:02:24

We can see that if this router wants to contact R2, it has learned a multipath route with OSPF and can then sends its packets through V1 or V2, as expected. Here is the routing table of V3:

V3# show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF,
       I - ISIS, B - BGP, > - selected route, * - FIB route

K>* 0.0.0.0/0 via 1.1.1.10, eth1
C>* 1.1.1.0/24 is directly connected, eth1
C>* 127.0.0.0/8 is directly connected, lo
O   192.168.15.0/24 [110/20] via 192.168.101.14, eth0, 00:04:26
B>* 192.168.15.0/24 [20/20] via 192.168.1.11 (recursive via 1.1.1.10), 00:07:16
O   192.168.101.0/24 [110/10] is directly connected, eth0, 00:07:30
C>* 192.168.101.0/24 is directly connected, eth0
O>* 192.168.115.0/24 [110/20] via 192.168.101.10, eth0, 00:07:18

We see that to contact R1, V3 knows two routes. One with BGP through the VPN. This is the one that will be used. The other one is through V4 but will not be used unless the BGP route is lost.

We can ping R2 from R1:

R1# ping -I 192.168.15.1 192.168.115.1
PING 192.168.115.1 (192.168.115.1) from 192.168.15.1 : 56(84) bytes of data.
Warning: time of day goes back (-459129us), taking countermeasures.
64 bytes from 192.168.115.1: icmp_req=1 ttl=62 time=1.10 ms

Let’s suppose we break the link between V1 and the “Internet” (using ip link set down eth1 on V1). The previous multipath route on R1 is now a single route through V2. Moreover, V3 has lost its BGP route to V1 and is now using its OSPF route through V4:

V3# show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF,
       I - ISIS, B - BGP, > - selected route, * - FIB route

K>* 0.0.0.0/0 via 1.1.1.10, eth1
C>* 1.1.1.0/24 is directly connected, eth1
C>* 127.0.0.0/8 is directly connected, lo
O>* 192.168.15.0/24 [110/20] via 192.168.101.14, eth0, 00:10:13
O   192.168.101.0/24 [110/10] is directly connected, eth0, 00:13:17
C>* 192.168.101.0/24 is directly connected, eth0
O>* 192.168.115.0/24 [110/20] via 192.168.101.10, eth0, 00:13:05

We could enhance the resiliance of such a setup by adding a dedicated link between V1 and V2 and between V3 and V4 and running OSPF on this link (or iBGP). This would allow us to support several outages on our network.

Conclusion§

The script used to setup this lab can be adapted to setup other labs with little effort. Please, note that it depends on the configuration of your environment. However, the whole lab is contained in a single directory and is very small. You can email it to your friends or publish it on GitHub.

It can also be extended to support Dynagen to include some Cisco equipments in your lab (for example, to experiment with VRF or MPLS).