linux_course_doc/modules/resources/exercise_networking.md

Essential Networking on Debian

VirtualBox has multiple options when it comes to it's networking settings. We've already used two different ones, NAT and bridged, but we'll now try to create our own internal network. For this we'll need a few minimal Debian installations. I invite you to install at the least two basic installations.

• One will become our router
• The other one our first client

Static routing

The most basic way of setting your network settings in Debian can be found in the /etc/network/interfaces file. When you open it you'll notice it mentions it sources a folder called interfaces.d. The choice is up to you whether you set your networking settings in this file or create a new file in the folder but I advise you to go for the second way. Don't just copy/paste the code below, check whether the interface names and ranges make sense!

auto eth0
iface eth0 inet static
	address 10.0.0.1
	netmask 255.255.255.0
	gateway 10.0.0.1

If you set both machines with addresses in the same range, you should be able to ping each other. Have a go at this until you can make it work. Which service do you have to restart of reload to apply your changes?

Restarting your network interfaces can be done in multiple ways. The most complete restart of all the interfaces can be done by restarting the networking.service run by systemd.

sudo systemctl restart networking.service

If this does not bring back your network settings you probably forgot to add the auto $INTERFACENAME line in the /etc/network/interfaces file.

A less brutal and more advised way of bringing an interface down and up is done with the following commands. It has the added advantage of giving a verbose output to STDOUT with what is happening which can be very handy for debugging purposes.

sudo ifdown $INTERFACENAME
sudo ifup $INTERFACENAME

Forwarding traffic

One of our machines is supposed to be a router and the other a client. Right now we can just ping between both machines but the outside world is completely invisible to us. How can we tackle this? Do we need more equipment?

An overview of what we would like to accomplish can be seen below.

In VirtualBox we can add more than one network adapter. On the router machine I would like you to add a second network interface and set it to bridged mode. When you reboot you should notice you have two network cards. Can you ping outside of your network now?

You could try and add a dhcp configuration to your /etc/network/interfaces file for this second interface. Once this is done, how do you ask for an IP address from the dhcp server? Have a look at the dhclient program to see how it works.

Now, if everything went OK your router should have two IP addresses, one in the 10.0.0.0/24 range and one in the 192.168.0.0/24 range. Who gave you this second address? Can the client ping both IP addresses? Can the client now ping outside of the network?

The easiest way to achieve routing between your internal network and the outside world is to enable NAT on your router. In order to do this, you need to do 2 things. First enable the kernel to actually forward packages, secondly iptables needs to do masquerading. You can do both these things with just to simple commands on you router.

1. IP forwarding needs to be setup on the router
2. NAT needs to be enabled

From here on out all your clients should have internet access but you won't be able to ping your clients from outside your network. Your clients can ping each other and the internet at large but for clients outside of their mini network the router will masquerade the IP address. This means that from outside your network, you'll never be able to ping a specific client, just the router itself.

sysctl net.ipv4.ip_forward=1
iptables -t nat -A POSTROUTING -j MASQUERADE

These settings won't save themselves so next time you reboot they'll be missing. For the kernel option you should have a look at /etc/sysctl.conf. In this file you can enable, disable or set kernel values.

To save iptables rules have a look online but this and this.

Extra Challenge

Your client machines are now behind a NAT. Can you think of a way to ssh into them? As you can only ping the router from outside of the network you'll have to setup port forwarding. On Debian this is done with iptables.

DHCP

It gets real tiring real quick to fix the IP address for every new machine we add to the network. A solution for this is to install a DHCP server onto our network. It can be installed onto any machine or ever a new machine but I advise you to install the DHCP server onto the router.

As with most thing Linux there are multiple servers to choose from. The two most popular ones are:

• isc-dhcp-server
• dnsmasq

We'll start with isc-dhcp-server because it's an industry standard for large scale networks. The other one, dnsmasq, is lighter and easier to use but consequently it has less features. It does however has the added benefit of being a DNS server as well! If you want to use isc-dhcp-server and add a DNS server to your network as well you'll have to install a secondary service. Large scale networks often combine it with bind9 which is a very powerful, but pretty complicated to configure, server. For our long term purposes dnsmasq is a better option but we'll start with isc-dhcp-server non the less.

isc-dhcp-server

Installing isc-dhcp-server is pretty straight forward. An sudo apt install isc-dhcp-server should sort you out but you'll get a bunch of errors. Don't panic, this is pretty normal because we haven't configured the server yet. A handy new command you'll learn here is journalctl. This is the main interface towards all logging done by all services systemd manages. The -x option will 'Augment log lines with explanation texts from the message catalog.' so will be more verbosen and the -e will jump to the end of the logs.

sudo journalctl -xe

We need to edit two files to successfully start the DHCP server. First we need to specify which interface the server should listen on because by default it listens on no interface. This first file can be found at /etc/default/isc-dhcp-server. Have a read of this configuration file and you'll quickly understand where the second file we need to edit is located.

In this second file we need to add a subnet on which the server will distribute IP addresses. A simple declaration is as follows:

subnet 10.0.1.0 netmask 255.255.255.0 {
	range 10.0.1.10 10.0.1.100;
}

This suffices to get the server up and running without any errors.

dnsmasq

Solo labo

Try to go as far as you can with following the network layout below. You'll have to create quite a few virtual machines machines so grouping them and having a consistent naming scheme is advised.

I would break it down as such:

• 1 VM to be the bridge between all your clients and the LAN of the class
  • 1 network interface in bridged mode (connects to my LAN)
  • 2 network interfaces in internal network mode (they should be named differently LSN/RSN)
• 1 VM on the left side with:
  • 4 network interfaces (LSN/LSN1/LSN2/LSN3)
  • can run the DHCP for all these subnets'
• 4 VM as clients per subnet (so 3 * 4 = 12)

The setup is mirrored on the right side so rinse and repeat.