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+---
+title: WTF is Linux
+author: Rémi Nicole <remi.nicole@smile.fr>
+date: 2020-09-29
+slide-level: 2
+aspectratio: 169
+
+theme: metropolis
+colortheme: owl
+beameroption: "show notes on second screen=right"
+
+toc: true
+highlightstyle: breezedark
+lang: en-US
+
+bibliography: ../bibliography.bib
+---
+
+# Before we get started
+
+## Reminder
+
+- Please fill the [form](https://mensuel.framapad.org/p/zufgimy2yy-9j3n)
+- Please have a GNU/Linux system with an SD card reader
+
+
+## Where were we
+
+::: notes
+
+- We managed to boot the system!
+- What we needed was:
+        - A file system to store files, and to be used as the root directory
+        - An init program in this file system, and started by the kernel
+
+:::
+
+
+# It works but...
+
+
+## The init problem
+
+- init is a shell
+- we want to automate some tasks:
+        - re-mounting the `/` directory in read-write mode
+        - networking?
+        - anything our system should do
+
+
+## The networking problem
+
+- We want to access the network:
+        - get updates
+        - execute our primary function
+
+
+## The user problem
+
+users, anyone?
+
+::: notes
+
+- At no point in our creation of a system we had to create a user, or add
+  a password
+
+:::
+
+
+# Automating tasks
+
+## Goal
+
+We want to execute programs or code when the system is starting
+
+
+## Attempt number 1
+
+Let's make a shell script
+
+::: notes
+
+- Instead of just launching a command-line as first program, let's execute some
+  commands automatically before, and *then* launch the command-line
+
+:::
+
+
+## Attempt number 2
+
+Let's use SysV-style init
+
+
+## SysV-style init
+
+- The init process reads `/etc/inittab`
+- Starts processes as specified by this configuration
+
+
+## inittab
+
+This file is a series of files in this format:
+
+```ini
+<id>:<runlevels>:<action>:<process>
+```
+
+::: notes
+
+Busybox does not support each of these features
+
+:::
+
+## inittab (Busybox version)
+
+```ini
+#  ,-- out of scope
+#  |
+# vvv
+ <tty>::<action>:<process>
+```
+
+`action` can be one of:
+
+- sysinit
+: normal process started at boot
+- shutdown
+: process started before shutting down
+- reboot
+: process started before rebooting
+- ...
+
+## inittab example
+
+```ini
+# Mount /proc and /sys, and remount / in read-write mode
+::sysinit:/etc/init.d/mount-special-filesystems
+
+# Setup the network
+::sysinit:/etc/init.d/setup-network start
+
+# Start additional services
+::sysinit:/etc/init.d/miniserve start
+
+# ...
+```
+
+::: notes
+
+- Note the start argument in the two last services
+
+:::
+
+## Changing to SysV-style init
+
+We just have to change the `init=` kernel parameter to Busybox's `init`.
+
+## The long-running processes problem
+
+- We need to launch long-running processes in the background
+- We also need a way to stop them
+
+::: notes
+
+- We might want to stop a process if something goes wrong, if we want to modify
+  the configuration, etc.
+
+:::
+
+
+## The `start-stop-daemon` command
+
+Can do multiple things:
+
+- Start a process in a background
+- Start a process as a different user
+- Write the PID of the launched process into a file (called a "PID file")
+- Kill a process with a PID file
+
+:::notes
+
+- We don't have our users figured out yet
+- Reminder, this is still the old/light way of doing things, so you might not
+  have this command on your Linux
+
+:::
+
+
+## Example service script
+
+```sh
+#!/bin/sh
+
+PIDFILE="/var/run/myprogram.pid"
+
+case "$1" in
+    start)
+       start-stop-daemon -S -p "$PIDFILE" -m -b \
+            -x myprogram -- args...
+       ;;
+    stop)
+       start-stop-daemon -K -p "$PIDFILE"
+       ;;
+esac
+```
+
+# Networking
+
+## What's in an IP packet
+
+![Anatomy of an IP packet[@corkami:rfc791]](../res/rfc791.png){ height=80% }
+
+::: notes
+
+- From this, we can conclude that we need an IP address for every packet sent
+  over IP
+
+- Also, we see that we cannot contact a server through it's domain name (like
+  google.com), we have to use IP addresses
+
+:::
+
+## A home network
+
+![Home network example[@wikimedia:network1]](../res/home-network.jpg){ height=80% }
+
+## Some definitions
+
+- IP address
+: Identifier that allows you to get messages/packets
+- Subnet
+: A defined subset of all IP addresses. For example 192.168.1.0/24 means all IP
+  addresses from 192.168.1.0 to 192.168.1.255
+- Router
+: Machine that connects one subnet to another. Usually, it is the subnet of
+  your home, to the global internet
+- Gateway
+: A machine where you send your internet packets, so that they are transferred
+  to the "real" recipient. It is usually your router.
+
+::: notes
+
+- The `/24` in the example subnet is what we call a "mask"
+
+It is nice to have a postal mail analogy. In this case:
+
+- IP address is your home address
+- Subnet is your town name, street name
+- Router and gateway would be your postman company (La Poste, UPS, Fedex, or
+  whatever)
+
+:::
+
+## Network interfaces
+
+- Represent a way to connect to a network
+- Stores each:
+  - One or more IP address
+  - A subnet
+
+::: notes
+
+- If you have two Ethernet ports, you could connect to two different networks
+- In this example you would have one interface for each of your Ethernet port
+- And maybe one more interface for your WiFi connection, if you have the
+  hardware for it
+
+:::
+
+## What you need to connect to a network
+
+- An IP address
+- The subnet of your network
+- A gateway, if you want to connect your network to other networks, like the
+  world wide web
+- A DNS server's IP address, if you want to resolve host names, like
+  <http://example.com>
+
+## Configuring a network interface
+
+- The static way
+- The dynamic way
+
+::: notes
+
+- The static way is simpler to configure, but you have to know in advance the
+  device that are going to connect to your network, and have a static IP
+  address for all of them
+  - You also have to know in advance the subnet and IP address of your router
+
+- The dynamic way is the one that you use on your personal computers
+  - This is the mode were you try to ask for all these parameters
+
+:::
+
+## The static way
+
+```sh
+# Set the IP address and subnet
+ip addr add $ip/$mask dev $interface
+# Activate this interface
+ip link set $interface up
+# Set the default gateway
+ip route add default via $gateway dev $interface
+# Add the default DNS server
+echo "nameserver $nameserver_ip" >> /etc/resolv.conf
+```
+
+::: notes
+
+For QEMU's defaults:
+
+- `busybox ip addr add 10.0.2.42/24 dev eth0`
+- `busybox ip link set eth0 up`
+- `busybox ip route add default via 10.0.2.2 dev eth0`
+- `echo "nameserver 10.0.2.3" >> /etc/resolv.conf`
+
+:::
+
+## The dynamic way
+
+- DHCP for the win!
+- meaning Dynamic Host Configuration Protocol
+
+. . .
+
+- we need a DHCP client on our machine and a DHCP server on the network
+
+::: notes
+
+DHCP is a protocol that allows us to get all of this information:
+
+- A new, unused IP address
+- The subnet of the network
+- The address of the gateway
+- The address of the DNS server
+
+:::
+
+## DHCP overview
+
+![DHCP overview[@wikimedia:dhcp]](../res/DHCP_session.png){ height=80% }
+
+## Implementation
+
+- Here we can use the `udhcpc` command of Busybox
+
+# Users
+
+## Current status
+
+Right now we have none
+
+::: notes
+
+Do a `ls -l` on the machine
+
+:::
+
+
+## The passwd file
+
+Each line of `/etc/passwd` contains:
+
+- Login name (what we were missing)
+- Field unused today (was the password, but it got moved to another file)
+- Numerical ID of the user
+- Numerical ID of the main group for user
+- User name or comment
+- Home directory
+- Optional default shell
+
+## Example
+
+```ini
+#    ,--- tells that the password is in another file
+#    |
+#    v
+root:x:0:0:System administrator:/root:/bin/sh
+```
+
+## Login "screen"
+
+- Now we can add one to the `inittab`
+- This is the `getty` command of `busybox`
+
+# References