updates the networking doc

modules/resources/exercise_networking.md

@@ -29,6 +29,23 @@ If you set both machines with addresses in the same range, you should be able to
 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`.
+
+```bash
+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.
+
+```bash
+sudo ifdown $INTERFACENAME
+sudo ifup $INTERFACENAME
+```
+
 ## Forwarding traffic
 
 One of our machines is supposed to be a router and the other a client.
@@ -86,9 +103,53 @@ On Debian this is done with [iptables](https://serverfault.com/questions/532569/
 
 ## DHCP
 
-We'll start from scratch again now.
+It gets real tiring real quick to fix the IP address for every new machine we add to the network.
-I would like you to create a new mini network of machines.
+A solution for this is to install a [DHCP](https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol) server onto our network.
-You can install a few new Debian machines, each with a different `hostname`
+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](https://wiki.debian.org/DHCP_Server)
+* [dnsmasq](https://wiki.debian.org/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](https://en.wikipedia.org/wiki/Domain_Name_System) 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](https://wiki.debian.org/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.
+
+```bash
+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:
+
+```bash
+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
 
 ![big network](./network_big.png)