add 2020-09-29 and 2020-10-{06,13} slidesHEADmaster

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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
diff --git a/2020-10-06.md b/2020-10-06.md
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+---
+title: WTF is Linux
+author: Rémi Nicole <remi.nicole@smile.fr>
+date: 2020-10-06
+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
+
+## Practical work practicalities
+
+- One or two group is going to have only 1 Raspberry
+- SD card readers are provided
+
+# Buildroot
+
+## Overview
+
+![Buildroot overview](./res/plantuml/buildroot-overview.png){ height=80% }
+
+## Single-project process
+
+![Buildroot single-project process](./res/plantuml/buildroot-project.png){ height=80% }
+
+## Overall process
+
+![Buildroot overall process](./res/plantuml/buildroot-process.png){ height=80% }
+
+## Configuring Buildroot
+
+- Same as the Linux kernel
+- Allows you to set:
+        - Target architecture
+        - Target packages
+        - Build options
+        - ...
+
+## Default configuration
+
+- To list available default configurations:
+        - `make list-defconfigs`
+- To set the configurations as one of the default:
+        - `make <configuration>_defconfig`
+
+::: notes
+
+We're going to use `raspberrypi2_defconfig`
+
+:::
+
+
+## Hierarchy of a Buildroot project
+
+- package/
+: All the available packages
+- toolchain/
+: Packages for building stuff
+- linux/
+: The Linux kernel package
+- system/skeleton/
+: The skeleton of the target Linux system
+- board/
+: All the board-specific files
+- docs/
+: The documentation of Buildroot
+- And other things
+
+::: notes
+
+By board-specific files, we can mean: kernel config & patches, scripts, etc.
+
+:::
+
+## Hierarchy of a Buildroot output
+
+Everything into `output/`
+
+- images/
+: The images to flash
+- target/
+: The target root filesystem
+- And other things
+
+## Demo time
+
+- _[Raspberry PI 2's pins](https://www.electronicwings.com/public/images/user_images/images/Raspberry%20Pi/RaspberryPi_UART/Raspberry%20pi%203%20UART%20pins.png)_
+- _[Serial cable's datasheet](https://docs.rs-online.com/12f1/0900766b811b9e83.pdf)_ page 7
+
+::: notes
+
+- Show Buildroot's download page
+- Go to extracted Buildroot directory
+- Show available defconfigs
+- Use `raspberrypi2_defconfig`
+- `make all`
+- Show output directory
+- Flash image on sdcard
+- Put sdcard on raspberry pi
+- Show
+- Boot raspberry
+
+:::
+
+# Installing GNU/Linux
+
+
+
+# References
diff --git a/2020-10-13.md b/2020-10-13.md
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+++ b/2020-10-13.md
@@ -0,0 +1,432 @@
+---
+title: WTF is Linux
+author: Rémi Nicole <remi.nicole@smile.fr>
+date: 2020-10-13
+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
+---
+
+# Makefile primer
+
+## Goal
+
+Make is a tool to create files
+
+
+::: notes
+
+A Makefile is a file that will direct Make on how to create these files
+
+:::
+
+## Concepts---Rules[^rule-intro]
+
+Target
+: Usually the name of a file/directory that this rule creates
+
+Prerequisites
+: The files/directories that are needed to produce the target
+
+Recipe
+: The commands to execute in order to create the target
+
+```make
+my_executable: source_1.c source_2.c
+        gcc source_1.c source_2.c -o my_executable
+```
+
+[^rule-intro]: <https://www.gnu.org/software/make/manual/make.html#Rules>
+
+::: notes
+
+- The target can also be an action, but it is rarer
+- If one of the prerequisite changes, Make figures out that the target needs to
+  be created again
+
+:::
+
+## Concepts---Variables[^variables]
+
+Very similar to shell variables, but use `$(VAR)` to expand it.
+
+```make
+MY_SOURCES = source_1.c source_2.c
+
+my_executable: $(MY_SOURCES)
+        gcc $(MY_SOURCES) -o my_executable
+```
+
+[^variables]: <https://www.gnu.org/software/make/manual/make.html#Using-Variables>
+
+## Concepts---Implicit Variables[^implicit-variables]
+
+Some variables are set implicitly by Make, for example:
+
+`$(CC)`{.no_minted}
+: The C compiler
+
+`$(CXX)`{.no_minted} / `$(CPP)`{.no_minted}
+: The C++ compiler
+
+`$(RM)`{.no_minted}
+: The `rm -f` command
+
+`$(MAKE)`{.no_minted}
+: The `make` command
+
+[^implicit-variables]: <https://www.gnu.org/software/make/manual/make.html#Implicit-Variables>
+
+::: notes
+
+Yes, you can call "make" inside of a Makefile
+
+:::
+
+## Concepts--Automatic Variables[^automatic-variables]
+
+Some variables are set automatically per-rule, for example:
+
+`$@`{.no_minted}
+: The target of the current rule
+
+`$<`{.no_minted}
+: The first prerequisite of the current rule
+
+`$^`{.no_minted}
+: All prerequisites of the current rule
+
+...
+
+[^automatic-variables]: <https://www.gnu.org/software/make/manual/make.html#Automatic-Variables>
+
+## Our example redone
+
+```make
+MY_SOURCES = source_1.c source_2.c
+
+my_executable: $(MY_SOURCES)
+        $(CC) $^ -o $@
+```
+
+## Relation with Buildroot
+
+Buildroot uses Makefiles to:
+
+- Figure out which packages to (re-)compile
+- How to compile them
+- Create the outputs (images, root filesystem, etc.) in general.
+
+But it is heavily abstracted
+
+# Setting up Buildroot for our modifications
+
+## For additional packages
+
+- We create the directory `package/<company>`
+- We put our additional packages in it
+- We create the `my_company.mk` and `Config.in` files[^adding-project-specific-packages]
+- And include the new `Config.in` in `package/Config.in`
+
+ [^adding-project-specific-packages]:
+        <https://buildroot.org/downloads/manual/manual.html#customize-packages>
+
+## Packages Makefile
+
+```makefile
+include $(sort $(wildcard package/my_company/*/*.mk))
+```
+
+## Packages configuration file
+
+In `package/my_company/Config.in`:
+
+```kconfig
+source "package/my_company/my_package1/Config.in"
+source "package/my_company/my_package2/Config.in"
+source "package/my_company/.../Config.in"
+# ...
+```
+
+## Packages configuration file inclusion
+
+In `package/Config.in`:
+
+```kconfig
+# ---8<--- Rest of the configuration
+
+# We add our own packages in our own submenu
+menu "My Company packages"
+        # We just created this file
+        source "package/my_company/Config.in"
+endmenu
+
+endmenu # <-- Was here before, it's the "Target packages" submenu
+```
+
+## For everything else
+
+- We create the directory `board/<company>/<board>`
+- We put our additional files, patches, configurations, scripts, etc. in it
+
+::: notes
+
+In our case, the board would be `raspberrypi2`.
+
+:::
+
+# Adding single files
+
+## Overlays
+
+A file hierarchy from the root directory that is going to get copied on top of
+the built system
+
+Can be used for:
+
+- Startup scripts
+- Program configuration files
+- Files containing data (like sound)
+
+::: notes
+
+However, if a startup script, configuration file, or data file is tied to
+a given package, it is usually better to install it with the package, instead
+of using an overlay.
+
+Overlays are more meant for files that are not tied to any specific package.
+
+:::
+
+## How to add an overlay
+
+- Create the directory `board/<company>/<board>/rootfs_overlay`
+- Add your file in that directory, as if it were the root directory
+- Configure Buildroot (`make menuconfig`) and set
+  `BR2_ROOTFS_OVERLAY`[^overlay-config] to
+  `board/<company>/<board>/rootfs-overlay`.
+
+[^overlay-config]:
+        Under "System configuration", "Root filesystem overlay directories"
+
+# Adding packages
+
+## Packages---Structure
+
+- Create the directory `package/<company>/<my-package>/`
+- Create the file `package/<company>/<my-package>/Config.in`[^config-files]
+- Create the file `package/<company>/<my-package>/<my-package>.mk`[^mk-files]
+
+[^config-files]: <https://buildroot.org/downloads/manual/manual.html#_config_files>
+
+[^mk-files]: <https://buildroot.org/downloads/manual/manual.html#_the_literal_mk_literal_file>
+
+<!-- _____ -->
+
+::: notes
+
+- The `Config.in` will allow Buildroot to add an option to install your
+  package. This will also specify your dependencies (what is needed to build or
+  run your package).
+- The `<my-package>.mk` is a Makefile which instructs how to compile and
+  install your package.
+
+:::
+
+## Packages---Configuration
+
+Example configuration:
+
+```kconfig
+config BR2_PACKAGE_DUMMY_PACKAGE
+        bool "dummy package"
+        default n
+        depends on BR2_PACKAGE_BUSYBOX || BR2_PACKAGE_BASH
+        help
+          A dummy package for demonstration purposes
+```
+
+## Packages---Makefile
+
+Example Makefile configuration:
+
+```makefile
+define DUMMY_PACKAGE_INSTALL_TARGET_CMDS
+        $(INSTALL) -m 0755 -D \
+                $(DUMMY_PACKAGE_PKGDIR)/dummy_script.sh \
+                $(TARGET_DIR)/usr/bin
+endef
+
+$(eval $(generic-package))
+```
+
+::: notes
+
+- This "package" just installs a script that is in the same directory of the
+`.mk` file
+- It is important that all variables 
+
+:::
+
+## Packages---More generic Makefile{.allowframebreaks}
+
+```makefile
+LIBFOO_VERSION = 1.0
+LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
+LIBFOO_SITE = http://www.foosoftware.org/download
+LIBFOO_LICENSE = GPL-3.0+
+LIBFOO_CONFIG_SCRIPTS = libfoo-config
+LIBFOO_DEPENDENCIES = host-libaaa libbbb
+
+# ---8<---
+```
+
+\framebreak
+
+If your package uses Makefiles to be built:
+
+```makefile
+# ---8<---
+
+define LIBFOO_BUILD_CMDS
+    $(MAKE) $(TARGET_CONFIGURE_OPTS) -C $(@D) all
+endef
+
+# ---8<---
+```
+
+`$(@D)`{.makefile} is the directory containing the downloaded sources.
+
+\framebreak
+
+```makefile
+define LIBFOO_INSTALL_TARGET_CMDS
+    $(MAKE) \
+        $(TARGET_CONFIGURE_OPTS) PREFIX=$(TARGET_DIR)/usr \
+        -C $(@D) \
+        install
+    # Or...
+    $(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(TARGET_DIR)/usr/lib
+    $(INSTALL) -d -m 0755 $(TARGET_DIR)/etc/foo.d
+endef
+
+$(eval $(generic-packages))
+```
+
+::: notes
+
+- In this instances, our package is called "libfoo"
+- `TARGET_CONFIGURE_OPTS` sets options like the compiler to use, the
+  compilation flags and other things
+
+:::
+
+## Packages---Notes
+
+- This was for a "generic package"
+- For package using common build frameworks (e.g. python, autotools, cmake,
+  etc.), some things are already done for you[^mk-files]
+- Don't forget to include the package's `Config.in` in
+  `package/my_company/Config.in`
+
+# Adding a service
+
+## Buildroot's Busybox init
+
+Every script or executable that is named like `/etc/init.d/S<something>` is
+going to be started on boot
+
+Two solutions then:
+
+- Add a script to the overlay (if not tied to one of your own package)
+- Install it with your package in the `<PACKAGE>_INSTALL_TARGET_CMDS`
+
+# Adding users
+
+## Using the board directory
+
+- Create the file `board/<company>/<board>/users.txt`[^adding-users]
+- Fill it with lines using the "makeusers" syntax[^makeusers-syntax]
+- Set `BR2_ROOTFS_USERS_TABLES`[^users-conf] to `board/<company>/<board>/users.tx`
+
+[^adding-users]: <https://buildroot.org/downloads/manual/manual.html#customize-users>
+
+[^makeusers-syntax]: <https://buildroot.org/downloads/manual/manual.html#makeuser-syntax>
+
+[^users-conf]: Under "System Configuration", "Path to the users tables"
+
+## Using the board directory---Example
+
+```kconfig
+#  ,---------------------- name
+#  |        ,------------- group      
+#  |        |      ,------ password
+#  |        |      |
+# vvv    vvvvvv    v
+  foo -1 libfoo -1 * - - - LibFoo daemon
+#     ^^        ^^         ^^^^^^^^^^^^^
+#      |         |               |
+#      |         |               '--- comment
+#      |         '------------------- GID
+#      '----------------------------- UID
+```
+
+::: notes
+
+- If UID or GID is "-1", buildroot will select an unused number
+- If password is "\*", then there is no password you cannot login as this user
+  (but you can still execute programs as this user)
+
+:::
+
+## In a package
+
+Inside the `.mk` file.
+
+```makefile
+define LIBFOO_USERS
+    foo -1 libfoo -1 * - - - LibFoo daemon
+endef
+```
+
+# Saving the configuration
+
+## Why it is important
+
+- Right now, every modification to the configuration (added packages, overlay,
+  etc.) is only in the build directory
+- We want to save it so that people cloning our modified Buildroot can have the
+  same configuration
+- Please often verify that your modifications are saved by deleting your
+  `output` directory, or downloading and compiling or project again.
+
+::: notes
+
+- This is necessary for me (the reviewer) since I want to build exactly what you
+  have.
+
+:::
+
+## How to save the Buildroot configuration
+
+Once you have done your configuration of Buildroot (`make menuconfig`), execute
+these commands:
+
+- Make sure the `BR2_DEFCONFIG` option[^defconfig-option] is set to
+  `configs/<my_company_board>_defconfig`
+- Execute `make savedefconfig` to save it
+
+And commit your changes
+
+[^defconfig-option]: Under "Build options", "Location to save buildroot config"
+
+# References
diff --git a/Makefile b/Makefile
index 17a8d13..71f1a88 100644
--- a/Makefile
+++ b/Makefile
@@ -4,7 +4,10 @@ MARKDOWN_FILES := $(wildcard *.md)
 TEX_FILES := $(patsubst %.md,build/%.tex,$(MARKDOWN_FILES))
 PDF_FILES := $(patsubst %.md,build/%.pdf,$(MARKDOWN_FILES))
 
-.SECONDARY: $(TEX_FILES)
+PLANTUML_FILES := $(wildcard res/plantuml/*.plantuml)
+PLANTUML_OUTPUTS := $(patsubst %.plantuml,build/%.png,$(PLANTUML_FILES))
+
+.SECONDARY: $(TEX_FILES) $(PLANTUML_OUTPUTS)
 
 all: $(PDF_FILES)
 
@@ -14,6 +17,9 @@ clean:
 build:
         mkdir -p build
 
+build/res/plantuml:
+        mkdir -p build/res/plantuml
+
 lua-filters/minted/minted.lua:
         git submodule update --init
 
@@ -26,7 +32,7 @@ build/%.tex: %.md build beamer-template.tex lua-filters/minted/minted.lua
                 --biblatex \
                 -so "$@"
 
-build/%.pdf: build/%.tex bibliography.bib
+build/%.pdf: build/%.tex bibliography.bib $(PLANTUML_OUTPUTS)
         cd build; latexmk \
                 -xelatex \
                 -interaction=nonstopmode \
@@ -36,3 +42,6 @@ build/%.pdf: build/%.tex bibliography.bib
                 -output-directory=./build \
                 "$*.tex"
         cp build/build/$*.pdf build
+
+build/res/plantuml/%.png: res/plantuml/%.plantuml build/res/plantuml
+        PLANTUML_LIMIT_SIZE=8192 plantuml -tpng "$<" -o "$(PWD)/build/res/plantuml"
diff --git a/bibliography.bib b/bibliography.bib
index df9700c..9b18999 100644
--- a/bibliography.bib
+++ b/bibliography.bib
@@ -84,3 +84,24 @@
   url = "https://commons.wikimedia.org/wiki/File:GUID_Partition_Table_Scheme.svg",
 }
 
+@misc{ corkami:rfc791,
+  title = "RFC791",
+  year = "2017",
+  author = "corkami",
+  url = "https://github.com/corkami/pics/blob/master/tracing/rfc791.pdf",
+}
+
+@misc{ wikimedia:network1,
+  title = "Network1",
+  year = "2017",
+  author = "Tsedenjav.Sh",
+  url = "https://commons.wikimedia.org/wiki/File:Network1.jpg",
+}
+
+@misc{ wikimedia:dhcp,
+  title = "DHCP session",
+  year = "2015",
+  author = "Gelmo96",
+  url = "https://commons.wikimedia.org/wiki/File:DHCP_session.svg",
+}
+
diff --git a/res/DHCP_session.png b/res/DHCP_session.png
new file mode 100644
index 0000000..60aada9
--- /dev/null
+++ b/res/DHCP_session.png
diff --git a/res/home-network.jpg b/res/home-network.jpg
new file mode 100644
index 0000000..d2cfe38
--- /dev/null
+++ b/res/home-network.jpg
diff --git a/res/plantuml/buildroot-overview.plantuml b/res/plantuml/buildroot-overview.plantuml
new file mode 100644
index 0000000..1b1277d
--- /dev/null
+++ b/res/plantuml/buildroot-overview.plantuml
@@ -0,0 +1,34 @@
+@startuml
+
+left to right direction
+
+skinparam backgroundcolor transparent
+skinparam component {
+        borderColor black
+        arrowColor #FA8100
+}
+skinparam package {
+        backgroundcolor white
+}
+
+[Buildroot] #B794F6
+
+package "External projects" {
+        [coreutils\n(ls, cp, mkdir, ...)] as coreutils #C6F68D
+        [util-linux\n(mount, mkfs, ...)] as utillinux #C6F68D
+        [Other projects...] as other_projects #C6F68D
+}
+
+package "Outputs" {
+        [Flashable image] as image #F186C0
+        [Other outputs...] as other_outputs #F186C0
+}
+
+coreutils --> Buildroot
+utillinux --> Buildroot
+other_projects --> Buildroot
+
+Buildroot --> image
+Buildroot --> other_outputs
+
+@enduml
diff --git a/res/plantuml/buildroot-process.plantuml b/res/plantuml/buildroot-process.plantuml
new file mode 100644
index 0000000..8fea3fe
--- /dev/null
+++ b/res/plantuml/buildroot-process.plantuml
@@ -0,0 +1,41 @@
+@startuml
+
+left to right direction
+
+skinparam backgroundcolor transparent
+skinparam component {
+        borderColor black
+        arrowColor #FA8100
+}
+skinparam package {
+        backgroundcolor white
+}
+
+package "External projects" #C6F68D {
+        [...] as project1
+        [...] as project2
+        [...] as project3
+}
+
+package "Buildroot" #B794F6 {
+        [build build tools\n(host)] as host_build
+        [build wanted projects\n(target)] as target_build
+        [post-build script] as post_build
+        [assemble the installation directory] as assemble
+}
+
+package "Outputs" {
+        [Flashable image] as image #F186C0
+        [Other outputs...] as other_outputs #F186C0
+}
+
+project1 --> host_build
+project3 --> target_build
+
+host_build -left-> target_build
+target_build --> post_build
+post_build -right-> assemble
+assemble --> image
+assemble --> other_outputs
+
+@enduml
diff --git a/res/plantuml/buildroot-project.plantuml b/res/plantuml/buildroot-project.plantuml
new file mode 100644
index 0000000..e2e48cf
--- /dev/null
+++ b/res/plantuml/buildroot-project.plantuml
@@ -0,0 +1,41 @@
+@startuml
+
+left to right direction
+
+skinparam backgroundcolor transparent
+skinparam component {
+        borderColor black
+        arrowColor #FA8100
+}
+skinparam package {
+        backgroundcolor white
+}
+
+package "External projects" #C6F68D {
+        [...] as project1
+        [...] as project2
+        [...] as project3
+}
+
+package "Buildroot" #B794F6 {
+        [download]
+        [check integrity] as checksum
+        [build]
+        [install]
+}
+
+package "Outputs" {
+        [Flashable image] as image #F186C0
+        [Other outputs...] as other_outputs #F186C0
+}
+
+project1 --> download
+
+download -left-> checksum
+checksum --> build
+build -right-> install
+
+install --> image
+install --> other_outputs
+
+@enduml
diff --git a/res/rfc791.pdf b/res/rfc791.pdf
new file mode 100644
index 0000000..da91b92
--- /dev/null
+++ b/res/rfc791.pdf
diff --git a/res/rfc791.png b/res/rfc791.png
new file mode 100644
index 0000000..77b836f
--- /dev/null
+++ b/res/rfc791.png