Virtual networking labs – NAT and host-only networking with VirtualBox

When you work with virtualized environments, you will sooner or later realize that a large part of the complexity of such environments originates in the networking part. Networking itself is a non-trivial endeavor, and in the context of cloud and virtualization technology, you often stack different virtualization layers on top of each other. To provide the basics to understand all this, this series aims at introducing some of the more commonly used techniques using hands-on exercises.

Setup

To follow this series, I highly recommend to run the examples yourself. For that purpose, you will need Vagrant and VirtualBox installed on your machine, which we use for most of the examples. We will also use Docker at times, so this should be installed as well.

The setup of most examples is automated, using tools like Vagrant or Ansible that you will know when you have followed some earlier posts on this blog. The labs are stored in a Github reposítory that you should clone by running

git clone https://github.com/christianb93/networking-samples

on your machine.

Lab 1: NAT networking with VirtualBox

When you take a look at networking options of Linux based virtual machines like KVM, Xen or VirtualBox, you will find that certain networking modes tend to be common to all these virtualization solutions. First, there is typically a networking mode based on network address translation (NAT) to allow access to the internet from within the virtual machine. Then, there are networking modes which allow you to connect one or more virtual machine using software-emulated ethernet bridges. This can be combined with VLANs, the usage of routing tables or iptables firewall rules to realize advanced networking topologies. And finally, all these methods can be combined in a variety of different setups. Networking for VirtualBox is comparatively easy to understand, but still displays some of these ideas nicely. This is why I have chosen VirtualBox as an example hypervisor. The first networking mode that we will look at is called NAT networking and is actually the VirtualBox default.

To see this in action, switch to the lab1 directory and run Vagrant to bring up the example machine, then use Vagrant to SSH into the machine.

cd lab1
vagrant up
vagrant ssh

When you run this for the first time, Vagrant might have to download the used Ubuntu disk image, which might take a few minutes. Once you are logged into the machine, run ifconfig -a to get a list of all network devices.

You will find that there are two networking devices. First, there is of course the standard loopback device lo which is present on every Linux system. Then, there is an interface enp0s3 which looks like an ordinary Ethernet device (but is of course a virtual device). This device has a MAC address and an Ethernet address assigned to it, usually 10.0.2.15.

When you run route -n to list the content of the kernel routing tables, you will find that this is the default interface for outgoing traffic, with gateway IP address being 10.0.2.2. We can try this out – run

ping leftasexercise.com

to verify that you can actually reach servers on the Internet via this device.

How does this work? When an application within the virtual machine sends a TCP/IP packet to the virtual device, VirtualBox picks up the packet and performs a network address translation on it. It then forwards the resulting packet to the network on the host system. When the answer comes back, the reverse process is applied and to the application, it looks like the reply came from a real network device. In this way, we can reach any host which is also reachable from the host – including the host itself and any other virtual networks reachable from the host.

Let us try this out. On the host, start an NGINX container and determine its IP address.

docker run -d --rm --name=nginx  nginx:latest
docker inspect nginx | jq -r ".[0].NetworkSettings.IPAddress"

Let us suppose that the result is 172.17.0.2. Now switch back into the virtual machine and run

curl 172.17.0.2

and you should see the NGINX welcome page. To see the NAT’ing in action run

sudo netstat -t  -c -p

on the host and then run

telnet 172.17.0.2 80

inside the virtual machine to establish a long running connection to the NGINX server. When you stop the output of netstat and browse through it, you should find a connection established by the VBoxHeadless process that connects to port 80 on 172.17.0.2. What happens is that when we run the telnet command inside the virtual machine, VirtualBox will open a socket on the host machine and use that to connect to the target, similar to a NAT’ing device which proxies outgoing connections. So if you wanted to represent the setup diagramatically, the result would be something like this.

NATNetworking

By the way, if you are asking yourself how the configuration of the network within the virtual machine has worked, take a look at the file /etc/netplan/50-cloud-init.yaml inside the virtual machine – here we see that the configuration is done by cloud-init and that the IP address is obtained using a DHCP server, which again is emulated by VirtualBox.

But wait, there is still a problem. If we are conceptually behind a gateway, this implies that the virtual machine cannot be reached from the host network. But how can we then SSH into it? The answer is that VirtualBox (respectively Vagrant) has created a port mapping for us, similar like you would configure an incoming forwarding rule in a classical gateway. Let us try to print out this route using the VirtualBox machine manager. First, we retrieve the name of the machine that Vagrant has created for us, place it in an environment variable and then invoke the VMM again to list some details which we search for forwarding rules.

vm=$(vboxmanage list vms | grep "boxA" | awk '{print $1}' | sed s/\"//g)
vboxmanage showvminfo --machinereadable $vm  | grep "Forwarding"

In fact, we see that there is a forwarding rule that directs incoming traffic on port 2222 from the host to port 22 (SSH) in the virtual machine where the SSH daemon is listening. This makes it possible to reach the machine via SSH.

Lab2: host-only networking

Next, we try a slightly different combination. We will bring up a virtual machine with two network devices, one using NAT as before, and one using host-only networking, or, in Vagrant terminology, private networking.

To run this example, first shut down your existing lab, then switch over to lab2 and restart Vagrant from there.

vagrant destroy
cd ../lab2/
vagrant up

The first thing that you will realize by running ifconfig -a on the host is that VirtualBox has actually created a new networking device vboxnet0 with IP address 192.168.50.1 on the host. When you run ethtool -i on this device, you will see that this device is managed by a custom driver which comes with VirtualBox (see source code here). On the host, VirtualBox has also added a new route, sending all traffic for the network destination 192.168.50.0 to this device.

When you log into the machine and run ifconfig -a, you will see that inside the machine, a new interface enp0s8 with IP address 192.168.50.4 is visible. This is the newly created host-only virtual networking device. Internally, VirtualBox captures traffic sent to this device and re-routes it to the vboxnet0 device and vice versa. Graphically, this looks as follows.

HostOnlyNetworking

Let us briefly discuss how packets travel across this interface. First, inside the virtual machine, a new route has been added, sending traffic for the network
192.168.50.0 to this device. To test this route, let us first get rid of the NAT interface to have a clearer picture. To do this, we again use the VirtualBox machine manager.

vm=$(vboxmanage list vms | grep "boxA" | awk '{print $1}' | sed s/\"//g)
vboxmanage controlvm $vm setlinkstate1 off

If you have used vagrant ssh to SSH into the machine, this will of course kill your connection, as the connection uses the port forward rule associated to the NAT device. But we can easily get it back and, in doing so, also verify our first route, by using the IP address 192.168.50.4 to SSH into the machine from our host. This should work, as, on the host, we have a route to this destination via vboxnet0. However, we first need the location of the private SSH key file that Vagrant has created as part of the provisioning process using vagrant ssh-config, which will show you that the private key file is stored at .vagrant/machines/boxA/virtualbox/private_key. So we can run

ssh -o StrictHostKeyChecking=no -i .vagrant/machines/boxA/virtualbox/private_key vagrant@192.168.50.4

and should be back in our machine. Thus we can actually reach the machine from the host using vboxnet0. To verify that the reverse process also works, let us again bring up our Docker container for NGINX, but this time, we use port forwarding to bind it to a port on the host.

docker run -d --rm --name=nginx  -p 80:80 nginx:latest

This will of course only work if you do not already have a webserver running on port 80, but if there is one, what comes next should also work. If you now switch back to the virtual machine and run

curl 192.168.50.1

you will again see the NGINX default page.

It is also instructive to look at the ARP caches on both machines. First, on the host, when running arp -n, we see an entry for the MAC address of the enp0s8 interface registered with the outgoing interface vboxnet0. So on layer 2, the traffic seems to flow transparently between enp0s8 on the virtual machine and the vboxnet0 device on the host. When you run arp on the virtual machine, the picture is reversed, and we see an entry showing us that the MAC address of vboxnet0 is reachable via enp0s8.

How does all this work? First, let us see what happens when we try to reach 192.168.50.4 from the host, and let us start our investigation by looking at the source code of the VirtualBox network driver.

As every network driver, the Virtualbox network driver has a function hard_start_xmit which is responsible for the actual transmission of a frame. When you look at the source code of this driver, you will see that this function does nothing except updating the statistics. Logically, this means that the device points “into nowhere”. But how can the packet then reach the virtual machine?

This is where for me, things start to become a bit blurry, but I believe that the answer is hidden in the concept of a local route (ip_fib_local_table in the kernel). The local routing table is maintained by the Linux kernel, and when a network device comes up, an entry is added to it automatically. To inspect the table in our case, enter

ip route show table local

on the host. This should yield, among others, an entry for the destination 192.168.50.1 of type local. The presence of this entry means that when delivering IP packets to this destination, the hard_start_xmit function of the device is never actually invoked, but (see for instance chapter 35 of “Understanding Linux network internals” by C. Benvenuti) will be injected back into the kernel’s IP stack, as if the packet came in via vboxnet0. Thus, effectively, the device acts as a loopback device.

When the packet is picked up again on the IP layer, one of the first things that happens is that the netfilter mechanism is invoked. VirtualBox comes with an additional kernel module VBoxNetFlt that attaches itself to the virtual device vboxnet0 (look at the output of dmesg) and seems to divert traffic to and from the virtual network device so that they are processed by VirtualBox. Understanding the details of this mechanism is beyond my own expertise, but conceptually, this seems to be what is happening.

Combining host-only networking with LAN access

Before we close this post, let us try one more thing. We have seen that the virtual device vboxnet0 allows us to connect to the host network. As a Linux host can serve as a router, it should therefore also be possible to connect to the outside world. So let us pick some server on your LAN, for instance the router that you use to connect to the Internet. In my home network, the router is at 192.168.178.1, reachable from the host via the device eno1. The first thing that we have to do is to add a new default route inside the VM, as we have disconnected the NAT device to which the old default route was pointing. So in the VM, enter

sudo route add default gw 192.168.50.1 enp0s8

Next, we have to prepare the host to enable forwarding. First, we enable forwarding globally in the kernel. Then, we set up a set of forwarding rules. As my router is connected to the device eno1, we first allow all new connections from the virtual device to this device, using the conntrack matching extension.

sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
sudo iptables -A FORWARD -o eno1 -i vboxnet0 -s 192.168.50.0/24 -m conntrack --ctstate NEW -j ACCEPT

Next, we need to make sure that the reply is allowed back into the system, so we set up a rule that will enable forwarding for all established connections.

sudo iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

We also need to enable IP masquerading so that the reply is directed towards the host. The following two commands will first flush the POSTROUTING table (which might not be needed, you might want to try without this command first as it might interfere with existing rules) and then add a rule that will enable masquerading (i.e. replacing of the IP source address by the address of the outgoing interface) for all traffic going out via eno1.

sudo iptables -t nat -F POSTROUTING
sudo iptables -t nat -A POSTROUTING -o eno1 -j MASQUERADE

This is already sufficient to reach hosts in the LAN and globally using IP addresses. However, DNS lookups will be broken in the virtual machine. To fix this, edit the file /etc/systemd/resolv.conf in the virtual machine and change the line

#DNS

into

DNS=192.168.178.1

or whatever your preferred IP address is. Then pick up the configuration by running

sudo systemctl restart systemd-resolved

and DNS resolution should work again.

In this post, we have covered the basics of host-only networking and played a bit with only one virtual machine involved. However, with host-only networking, we can do more – we can also connect more than one virtual machine to the same virtual network. We will look into this in detail in the next post.

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