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References:

• Ubuntu Community Wiki - PinningHowto
• Debian Wiki - AptPreferences

Overview

Priority values

From man page: man apt_preferences

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sysrq magic keys

This is useful for trying to recover from an unresponsive system. However, by default, Ubuntu disables the key to gain raw keyboard access.\

Sometimes part of the issue is that X/Wayland display servers have crashed due to bad graphics drivers and keyboard input isn't processed anymore, whereby changing to another console, e.g. ctrl + alt + F1 doesn't work. Since the default disables keyboard control, alt + sysrq + r, can't help salvage the situation. Hence the suggestion to at least enable system request raw keyboard control when trying to recover a system.

Ubuntu doesn't enable control of keyboard by default

Ubuntu 16.04 LTS default value:

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Overview

Some notes about:

• Explaining why current day Linux memory swap thrashing still happens (as of 2016).
• Mitigating "stop the world" type thrashing issues on a Linux workstation when it's under high memory pressure and where responsiveness is more important than process completion.
• Prioritizing and limiting memory use.
• Older ulimit versus newer CGroup options.

These notes assume some basic background knowledge about memory management, ulimits and cgroups.

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Enable persistent storage for the systemd journal log

Overview

The assumed default setting in /etc/systemd/journald.conf is Storage=auto which implies that systemd journaling will only persist the journal if the expected storage location is available. Otherwise, the journal data is stored in memory and lost between reboots. On Ubuntu 16.04, /var/log/journal does not exist by default. Create it to keep and query events from previous boots.

Considerations:

• Syslog still provides the persistant log records for Ubuntu 16.04, so enabling persistant systemd journal logging does cause a level of duplicaiton.
• There are sane defaults:

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The newer versions of bash (>= 3.0) include a regex operator =~

Simple example

$ re='t(es)t'
$ [[ "test" =~ $re ]]
$ echo $?
0
$ echo ${BASH_REMATCH[1]}
es

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Logging bash history to syslog

Overview

Bash history was a convenience feature to help a user recall previous commands and not intended to meet any security requirements.

The Linux audit system (or alternate kernel level audit OS facility) is a more robust way to ensure user and process log events are recorded.

Security issues with bash history files and $BASH_COMMAND

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In this example

• Multiline input can be caputured by spreading openening and closing quote accross lines
• Newlines can be replaced using built-in bash text substitution
• The main trick is knowing that $'\n' is the way to specify the newling within the subsitution
• Note, echo output of a var without quotation replaces newlines with spaces

$ t='1
> 2

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Why?

Useful for:

• updating current terminal's shell env proxy settings, or
• wrapping exec in .desktop files to inject env proxy settings

GNOME proxy settings should normally get propergated into the shell (bash) environment via gnome-terminal, e.g. http_proxy and no_proxy. However:

• I noticed that applications exectuted via .desktop entries sometimes don't work. This happens when an app ignores org.gnome.proxy settings but can often use proxy env vars from the shell.
• If you're proxy settings change, existing shell processes will still have the old proxy enviroment variables, so this can help refresh them whithout having to create a new shell.

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A quick set of notes looking into Ubuntu mirror locations for South Africa from my home fiber (Vumatel). The examples should apply in general.

Debian Package Info and Downloads

There are two phases

1. Getting lists of packages
2. The actual package download

When testing performance of package list info downloads via upt-get update, clear previous package info downloaded. If this isn't done, cached package info could skew results.

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Delete all containers

$ docker ps -q -a | xargs docker rm

• -q prints only the container IDs
• -a prints all containers

Notice that it uses xargs to issue a remove container command for each container ID