adds pihole exercise

modules/resources/exercise_pihole.md

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+# Setting up a pihole at home
+
+What you'll need to get started:
+
+* a raspberry pi
+* a USB power supply 
+* an SD card you can erase
+* a keyboard
+* an HDMI cable
+
+## Your current situation
+
+Your current situation looks probably *very* similar to the image below.
+You have some sort of router/modem/switch/wirelessAP combination box sitting somewhere in your house.
+To this device you have your PC's, printer, phone's connected either with a cable or over WiFi.
+
+![without pihole](./network_home_without_pihole.png)
+
+I advise you to take some notes on your network settings *before* starting to mess around.
+Your `nameserver` is probably the IP address of your router and the `DHCP` server for both your wired LAN and wireless WLAN is probably the same IP address.
+This means your *bbox* does a few different things:
+
+* it's a modem that gives you a public IP address (from your ISP)
+* it does NAT so that your LAN devices can access the internet
+* it has a builtin DHCP and DNS server
+* it creates a wireless access point so you can connect your phone
+* it has a builtin hardware switch so you can connect multiple PC's
+
+As you found out during our exercises, a Linux machine can do all of these things if you configure it properly.
+We'll try and replace two, or for the brave ones three, of those *features* with a raspberry pi.
+This will be done **step by step** but it's worth pointing out that a basic pihole installation is not invasive at all.
+You won't break the internet by just installing the pihole package on your raspberry pi.
+Throughout the full installation it is however possible you'll have some downtime.
+If there are people in your household depending on the internet to work it might be best to schedule your installation with them.
+
+## Hardware installation
+
+It's probably easiest, and recommended, to do the installation with the raspberry pi connected to your network over Ethernet.
+You can perform the installation over `ssh` or with a screen and keyboard connected, it's fully your choice.
+You don't need an extra LAN adaptor, just plugging in the raspberry pi into the switch suffices.
+Once plugged in your network layout will look similar to the image below.
+
+![with raspberry pi](./network_home_with_pihole.png)
+
+I recommend you start from a fresh Raspberry PI OS installation, either with or without desktop environment.
+The raspberry pi should receive an IP address from your *bbox* and you should be able to `ping` devices in your LAN and out on the internet.
+
+## Installing pihole
+
+I won't explain this as it's part of the exercise but there are a lot of tutorials around.
+First you should **only** use the DNS feature of the pihole.
+This is completely **non invasive** to your network, nor towards the other computers in your home network.
+Check the installation by logging into the web interface of your fresh pihole install!
+
+## Testing the DNS aspect of the pihole
+
+Your pihole is now a DNS server!
+You can try it out by changing the DNS settings of a PC that's connected to your network.
+Depending on your OS it will be done in a different way.
+On Linux you'll find the `nameserver` you're using in `/etc/resolv.conf`.
+If you did it right, you should see a client connected in the web interface of the pihole.
+Try to go to some websites you know have a lot of advertisements and see if the pihole blocks them properly.
+
+## Time to take over the DHCP role
+
+The pihole can also be in charge of the IP addresses on your LAN.
+This is the **invasive** part of the setup.
+First you should turn off the DHCP server that is currently running on you network.
+As mentioned before, this is *probably* your *bbox*.
+Log into it and figure our how to deactive it.
+Once this is done you can enable the DHCP server on the pihole.
+This is done via the web interface.
+Try disconnecting and reconnecting a PC from the network and see if it comes up in the leases list.
+If it does, bravo!
+
+## Undoing your installation
+
+If you want to revert back to your original network layout the main thing you have to do is turn the DHCP server in your *bbox* back on.
+Do keep in mind you can't have two DHCP servers on your network.
+You can either turn off the raspberry pi completely or just stop the pihole service via `systemctl`.
+Try to disconnect and reconnect one PC first to see if it get's back the normal settings from before.
+If it does you successfully removed the pihole from your network!
+
+## For the brave
+
+The raspberry pi has a builtin WiFi adapter you can use to create your own WLAN network.
+You can either have the clients join in the same range as your LAN network, or create a secondary subnet.
+If you opt for the latter you'll have to tweak you DHCP settings a bit.
+The package that turns your pi into a wireless access point is called [hostapd](https://en.wikipedia.org/wiki/Hostapd) and you can find heaps of [tutorials](https://www.raspberrypi.org/documentation/configuration/wireless/access-point-routed.md) online.
+
+## For the super brave
+
+We'll set up a VPN in an upcoming class but the pihole is often used together with [pivpn](https://pivpn.io/).
+If you decide to try this out keep in mind that you'll have to add a port forward from your router to the pi and that this **will expose** the VPN service to the internet!
+There is nothing inherently *wrong* with that but you'll see hacking attempts almost immediately.
+**Use good passwords or preferably encryption keys.**