Add some linux notes

src/articles/computer_networking.md

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+This is a work in progress article I'm using to build up my understanding of computer networking.
+
+
+# Standards #
+
+## [OSI model](https://en.wikipedia.org/wiki/OSI_model) ##
+
+The Open Systems Interconnection model is a basis for standards development in systems interconnection.
+
+### Physical Layer ###
+
+Converts digital bits into electrical, radio or optical signals
+
+* NIC: Network Interface Controller 
+* Ethernet hub
+* Network switch
+* Physical transmission media
+
+Standards at this layer include Bluetooth, Ethernet, USB
+
+### Data link layer ###
+
+* Medium Access Control Address (MAC) https://en.wikipedia.org/wiki/MAC_address
+* Logical link control (LLC)
+
+### Others ###
+
+* network
+* transport
+* session
+* presentation
+* application
+
+
+## [IEEE 802](https://en.wikipedia.org/wiki/IEEE_802) ##
+
+Family of standards for local area networks (LANs) and other types.
+
+# Terms #
+
+
+
+# Internet Protocol Suite #
+
+* RFC 1122
+* RFC 1123
+
+
+## Link Layer ##
+
+* Address Resolution Procotol (ARP) https://en.wikipedia.org/wiki/Address_Resolution_Protocol
+
+
+##  Transport Layer ##
+
+* TCP
+* UDP
+* QUIC
+

src/articles/linux_internals.md

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+This article is my attempt to explain Linux internals to myself and act as a structured way of keeping notes.
+
+Linux is an operating system kernel - which is distinct from the user-space where user applications will run. The kernel directly interfaces with hardware via its firmware and associated drivers. It also manages resources for sharing by user applications. Other open-source kernels that may be of interest are those based on BSD and the GNU Hurd microkernel.
+
+The [Linux kernel](https://www.kernel.org/doc/html/v4.13/index.html) can be interacted with or controlled from user space by means of a File based API or a C API which aim to be POSIX compatible. 
+
+The types of hardware interfaced in the kernel include:
+
+* graphics
+* audio
+* memory
+* cpu
+* gpu
+* network
+* keyboard and mouse
+* storage and i/o peripherals
+
+with the kernel handling control messages including power messages for associated devices.
+
+As part of its resource management the kernel includes:
+
+* filesystems and virtual file management
+* memory management functionality
+* process scheduling and interprocess communication
+* networking
+* security
+* media support
+
+In the user-space of operating systems that use the Linux kernel (will controversially refer to as Linux distros for brevity) some foundational elements and applications are:
+
+* an init system - often `systemd` and previously collections of 'sysvinit scripts' or niche alternatives like `shepherd`.
+* system daemons for administrative services
+* a graphics system - oten `wayland` now replacing `x org`
+* an audo or multimedia system - `pipewire` is becoming a common replacement for `pulseaudio` and others
+* a terminal emulator and shell
+* network interface management, such as ethernet or wifi
+* a package manager
+* a bootloader
+
+
+# APIs #
+
+https://en.wikipedia.org/wiki/Linux_kernel_interfaces#Linux_API
+
+The kernel tries to follow the [Portable Operating System Interface (POSIX)](https://en.wikipedia.org/wiki/POSIX) and [Single Unix Specification](https://en.wikipedia.org/wiki/Single_UNIX_Specification) where applicable. 
+
+## File Based ##
+
+Device drivers are interacted with in directories:
+
+* `/dev` https://en.wikipedia.org/wiki/Device_file#DEVFS
+* `/sys`
+
+Processes are interacted with in:
+
+* `/proc`
+* `/proc/sys`
+
+## System calls and similar ##
+
+https://en.wikipedia.org/wiki/System_call
+
+`ioctl` (input/output control) is a system call for device specific io operations.
+
+* `sysctl` (system control)
+* `ioctl` (io control)
+* `fcntl` (file control)
+
+Other communication mechanisms include [netlink](https://en.wikipedia.org/wiki/Netlink) sockets which allow IPC between both kernel and userspace programs. It is designed to be a more flexible successor to `ioctl`.
+
+# User Space #
+
+## Init Systems ##
+
+### Systemd ###
+
+Important utilities:
+
+* `systemctl`
+* `journalctl`
+* `notify`
+* `loginctl`
+* `systemd-boot`
+
+Important daemons:
+
+* `systemd`
+* `journald`
+* `resolved`
+* `networkd`
+* `logind`
+* `user-session`
+* `udevd`
+
+Important libraries:
+
+* `libnotify`
+* `libudev`
+
+
+
+
+
+## Devices ##
+
+Devices are managed in user space with the `udev` utility - which has the following parts:
+
+* libudev which can be used as a library for device info
+* udevd daemon for managing the `/dev` virtual file hierarchy
+* the `udevadm` command line utility for admin and diagnostics
+
+
+## Window management and device input events ##
+
+Wayland and X are the two most common display servers on Linux systems. They both follow a client-server approach, where the latter in Wayland can be part of the compositor.
+
+Display servers can react to device input events via the `libinput` library, which in turn uses `libevdev` to handle evdev `ioctls` from the kernel.
+
+## Network ##
+
+`iptables` allows configuration of IP packet filter ruls in the Linux kernel firewal, which are implemented as netfiler modules.
+
+[avahi](https://en.wikipedia.org/wiki/Avahi_(software)) is a zero-configuration networking implementation (allows network service use by freshly networked computers or peripherals) including multicast DNS and DNS service discovery. Apples' Bonjour and Systemd's `systemd-resolved` are other implementations.
+
+The Desktop-Bus (DBus) is a user space middleware allowing communication between multiple processes (e.g. IPC).
+
+## Other ##
+
+* avahi
+* dbus
+* udisks
+* cgroups
+* autofs
+* kdbus
+* Plokit (Policy Kit) https://en.wikipedia.org/wiki/Polkit
+* Pluggable Authentication Module (PAM) https://en.wikipedia.org/wiki/Pluggable_Authentication_Module
+* Name Service Switch (NSS) https://en.wikipedia.org/wiki/Name_Service_Switch
+* procfs https://en.wikipedia.org/wiki/Procfs
+* sysfs https://en.wikipedia.org/wiki/Sysfs
+
+# Kernel Space #
+
+
+## Input events ##
+
+* evdev
+* `/dev/input`
+
+## Filesystems ##
+
+* ext4
+* btrfs
+* xfs
+* jfs
+* fat32
+
+* FUSE
+
+## Storage ##
+
+* SCSI
+* libATA
+
+## Virtualization ##
+
+* KVM
+* Xen
+
+## Process Management ##
+
+* `clone(2)` and `clone3(2)`
+* `futex(7)` and `futex(2)`
+* Completely Fair Scheduler
+* https://en.wikipedia.org/wiki/Earliest_eligible_virtual_deadline_first_scheduling
+* Native Posix Thread Library (NPTL) is kernel side of pthreads
+
+## Security ##
+
+* Linux Security Modules
+* SELinux
+* AppArmor
+* POSIX ACLs
+
+## Memory ##
+
+* DMA buffers
+
+## Audio ##
+
+* Advanced Linux Sound Architecture (ALSA)
+
+
+## Graphics ##
+
+* Direct Rendering Manager (DRM)
+* Kernel Mode Setting (KMS)
+
+
+## Network ##
+
+* New API
+* mac80211
+* [Netfiler](https://en.wikipedia.org/wiki/Netfilter)
+
+
+## Power and Control ##
+
+* ACPI https://en.wikipedia.org/wiki/ACPI
+
+