path: root/2020-09-29.md

---
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