Engineering and technology notes

Run-levels in Ubuntu 16.04 LTS

Jovan Stosic March 31, 2019

Ubuntu 16.04 has moved from using init to systemd. Thus, the  concept of run-levels is replaced by the term targets.  The advantages over choosing systemd is discussed in the article The Story Behind ‘init’ and ‘systemd’: Why ‘init’ Needed to be Replaced with ‘systemd’ in Linux . The seven run-levels of init can be mapped with the targets as:

 

Run-levels Targets
0 poweroff.target
1 rescue.target
2,3,4 multi-user.target
5 graphical.target
6 reboot.target

To change the run-level non-GUI:

Source: Run-levels in Ubuntu 16.04 LTS – CodeChunk

User space

Jovan Stosic March 31, 2019

A modern computer operating system usually segregates virtual memory into kernel space and user space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.

Kernel space is strictly reserved for running a privileged operating system kernel, kernel extensions, and most device drivers. In contrast, user space is the memory area where application software and some drivers execute.

Source: User space – Wikipedia

Linux startup process – Wikipedia

Jovan Stosic March 30, 2019

Linux startup process is the multi-stage initialization process performed during booting a Linux installation. It is in many ways similar to the BSD and other Unix-style boot processes, from which it derives.

Booting a Linux installation involves multiple stages and software components, including firmware initialization, execution of a boot loader, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. For each of these stages and components there are different variations and approaches; for example, GRUB, LILO, SYSLINUX or Loadlin can be used as boot loaders, while the startup scripts can be either traditional init-style, or the system configuration can be performed through modern alternatives such as systemd or Upstart.

Source: Linux startup process – Wikipedia

systemd

Jovan Stosic March 30, 2019

The systemd software suite provides fundamental building blocks for a Linux operating system. It includes the systemd “System and Service Manager”, an init system used to bootstrap user space and manage user processes.

systemd aims to unify service configuration and behavior across Linux distributions.[8] It replaces the UNIX System V and BSD init systems. Since 2015, the majority of Linux distributions have adopted systemd, and it is considered a de facto standard.[1]

The name systemd adheres to the Unix convention of naming daemons by appending the letter d. It also plays on the term “System D“, which refers to a person’s ability to adapt quickly and improvise to solve problems.

SysV->upstart->systemd

Source: systemd – Wikipedia

UNIX System V

Jovan Stosic March 30, 2019

UNIX System V (pronounced: “System Five”) is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, and 4. System V Release 4, or SVR4, was commercially the most successful version, being the result of an effort, marketed as “Unix System Unification”, which solicited the collaboration of the major Unix vendors. It was the source of several common commercial Unix features. System V is sometimes abbreviated to SysV.

As of 2012, the Unix market is divided between five System V variants: IBM‘s AIX, Hewlett-Packard‘s HP-UX, Oracle‘s Solaris,[1] Xinuos‘s UnixWare and illumos distributions being the open-source OpenSolaris continuation.[2]

Source: UNIX System V – Wikipedia

Create and upload an Ubuntu Linux VHD in Azure

Jovan Stosic March 28, 2019

This article assumes that you have already installed an Ubuntu Linux operating system to a virtual hard disk. Multiple tools exist to create .vhd files, for example a virtualization solution such as Hyper-V. For instructions, see Install the Hyper-V Role and Configure a Virtual Machine.

Ubuntu installation notes

  • Please see also General Linux Installation Notes for more tips on preparing Linux for Azure.
  • The VHDX format is not supported in Azure, only fixed VHD. You can convert the disk to VHD format using Hyper-V Manager or the convert-vhd cmdlet.
  • When installing the Linux system it is recommended that you use standard partitions rather than LVM (often the default for many installations). This will avoid LVM name conflicts with cloned VMs, particularly if an OS disk ever needs to be attached to another VM for troubleshooting. LVM or RAID may be used on data disks if preferred.
  • Do not configure a swap partition on the OS disk. The Linux agent can be configured to create a swap file on the temporary resource disk. More information about this can be found in the steps below.
  • All VHDs on Azure must have a virtual size aligned to 1MB. When converting from a raw disk to VHD you must ensure that the raw disk size is a multiple of 1MB before conversion. See Linux Installation Notes for more information.

Source: Create and upload an Ubuntu Linux VHD in Azure | Microsoft Docs

Moodle – Reverse proxy frontend

Jovan Stosic March 25, 2019

A reverse proxy or surrogate is a proxy server that is installed in a server network. Typically, reverse proxies are used in front of Web servers. All connections coming from the Internet addressed to one of the Web servers are routed through the proxy server, which may either deal with the request itself or pass the request wholly or partially to the main web servers.

Contents

Requirements

  • You need ssl for authentication.
  • You run Apache 2.2 both in the backend and the frontend.
  • You run RHEL 5.2 Application stack 2 in the backend (php 5.2.6 + mysql 5 + apache 2.2.10).
  • You run Apache 2.2 as the frontend in any modern OS using trainer.moodle.org as url (My frontend apache runs on Windows 2003).
  • You installed moodle 1.9 to run from “/” the internal http server with SSL support on 10.1.1.24.
  • You have an external facing apache using SSL.

Configuration for the external server

http.conf

(snip)
ProxyPass / http://10.1.1.24/
ProxyPassReverse / http://10.1.1.24/
ProxyPreserveHost On
(snip)
Include conf/http-ssl.conf

http-ssl.conf

(snip)
ProxyPass / http://10.1.1.24/
ProxyPassReverse / http://10.1.1.24/
(snip)

config.php

<?php  /// Moodle Configuration File 

unset($CFG);

$CFG->dbtype    = 'mysql';
$CFG->dbhost    = 'localhost';
$CFG->dbname    = 'moodle';
$CFG->dbuser    = 'moodleuser';
$CFG->dbpass    = 'XXXXXXXX';
$CFG->dbpersist =  false;
$CFG->prefix    = 'mdl_';

$CFG->wwwroot   = 'http://trainer.moodle.org';
$CFG->dirroot   = '/var/www/moodle';
$CFG->dataroot  = '/opt/moodle_data';
$CFG->admin     = 'admin';

$CFG->directorypermissions = 00777;  // try 02777 on a server in Safe Mode

require_once("$CFG->dirroot/lib/setup.php");
// MAKE SURE WHEN YOU EDIT THIS FILE THAT THERE ARE NO SPACES, BLANK LINES,
// RETURNS, OR ANYTHING ELSE AFTER THE TWO CHARACTERS ON THE NEXT LINE.
?>

The tricky part

  • Go to Administration ► Security ► HTTP security and set Use HTTPS for loginsloginhttps to true.
  • You need to have the previous setup ready or you may lock yourself out of the server.

 

Source: Reverse proxy frontend – MoodleDocs