Welcome to saghul’s notes¶

This is a collection of personal notes, tips and tricks or something like that so I don’t forget things.

Contents:

Debian¶

Some Debian tips and tricks.

Check for obsolete packages¶

Obsolete packages are those which cannot be installed from any of the configured repositories.

aptitude search ?obsolete

I have no idea if this can be accomplished with plain apt-get.

Override DNS resolvers received over DHCP¶

If you want to override the DNS resolvers received over DHCP, add the following to /etc/dhcp/dhclient.conf

interface "eth0" {
    supersede domain-name-servers 8.8.8.8, 8.8.4.4;
}

Adjust accordingly.

Install all build dependencies for a source package¶

Sometimes you can’t use apt-get build-dep (the first time you are building the package for example). We are going to use mk-build-deps from the devscripts package (equivs also needs to be installed.

mk-build-deps -ir -t "apt-get -qq -y --no-install-recommends"

Put a package on hold¶

Putting a package on hold:

sudo apt-mark hold package

Putting a package out of hold:

sudo apt-mark unhold package

Docker¶

The container technology everyone talks about. Website.

Using official Docker on Debian Sid¶

In order to use the latest (and greatest?) version it’s best to use the repositories provided by Docker. As root:

apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
echo "deb https://apt.dockerproject.org/repo debian-stretch main" > /etc/apt/sources.list.d/docker.list
apt-get update
apt-get install docker-engine

Using OverlayFS as the storage backend¶

The default storage backend on Debian seems to be devicemapper using a loop file. If you want to use OverlayFS, a systemd drop-in file is needed. Create a file in /etc/systemd/system/docker.service with the following content:

[Service]
ExecStart=
ExecStart=/usr/bin/docker daemon -H fd:// --graph="/storage/docker" --storage-driver=overlay

Remove all containers at once¶

Sometimes you gotta start clean.

docker rm -f $(docker ps -a -q)

Remove all images at once¶

Sometimes you gotta start clean.

docker rmi -f $(docker images -q)

Git¶

Using patches in mbox format¶

Git allows the creation of patch files in mbox format, which can be then sent over email or used in any other way.

First, we will use git format-patch to create the mbox file:

git format-patch --stdout HEAD~3 > tmp.patch

In the above example we created a tmp.patch file which contains the last 3 commits in the current branch. We can now use git am to apply them:

git am tmp.patch

And the 3 commits will be applied in the git tree,.

Aborting an incorrect amend¶

Isn’t it annoying when you do a git commit –amend when you didn’t intent to amend? If you are using wim to edit the commit message, you can abort the commit by exiting vim like this:

:cq

Caching HTTP(S) credentials¶

When cloning over HTTP(S) the following can be done so the credentials acre cached (by default for 15 minutes) so one doesn’t need to enter them all the time:

git config --global credential.helper cache

Reference.

Hardware¶

Things!

Arduino¶

Official Website

Signed kext for CH34x chipsets in clones¶

Some Arduino clones use a CH34X USB-serial bridge, which is not immediately recognized by OSX. A signed kext is available here.

BeagleBone Black¶

Official Wiki

Fix second boot on FreedomBox¶

As part of its first boot, FreedomBox uses the flash-kernel utility to update the kernel on the BBB. Unfortunately this doesn’t work. To prevent this from happening create the follopwing empty file before the first boot: /var/freedombox/dont-tweak-kernel

Raspberry Pi¶

The tiny computer everyone loves.

Extra current for the USB ports (B+/2 only)¶

Raspberry Pi models B+ and 2 are able to supply up to 1.2A over USB using a little trick. This allows us to connect portable hard drives without using powered USB hubs.

Edit /boot/config.txt and add the following:

# extra power for USB ports
max_usb_current=1

Warning

Do this at your own risk. A good power supply giving a stable current of 2A is required.

Linux¶

Or GNU/Linux.

Map Caps Lock to Control¶

But not the other way around. Put this in .profile or .bashrc:

setxkbmap -layout us -option ctrl:nocaps

LUKS¶

Simple use, for external devices, using a passphrase. For more options check this out.

Creating a LUKS volume¶

cryptsetup -v luksFormat /dev/sdaX
cryptsetup open /dev/sdaX LUKS001
mkfs -t ext4 /dev/mapper/LUKS001
cryptsetup close LUKS001

Mounting a LUKS volume¶

cryptsetup open /dev/sdaX LUKS001
mount -t ext4 /dev/mapper/LUKS001 /mnt/usb

Unmounting a LUKS volume¶

umount /mnt/usb
cryptsetup close LUKS001

Creating a ‘fake’ webcam device¶

For testing, one might need some ‘fake’ webcam, here is how to do it.

Install v4l2loopback - this is typically available as a package in your distro
Load the kernel module with the amount of virtual video devices you want:

sudo modprobe v4l2loopback devices=X

Get GStreamer 0.10 - looks like 1.0 doesn’t work well with the videotestsrc plugin see here
Launch GStreamer as follows:

gst-launch-0.10 -v videotestsrc pattern=snow ! "video/x-raw-yuv,width=640,height=480,framerate=15/1,format=(fourcc)YUY2" ! v4l2sink device=/dev/videoX

The pattern can be a number between 0 and 16 or the name of a pattern. More examples here.

SMART¶

Monitor the hard drives health with SMART. Full guide here.

Configuration¶

The following configuration will:

Monitor all drives
Except the ones in standby
Run a short self-test every day between 2-3am, and an extended self test weekly on Saturdays between 3-4am
Log temperature changes of 4 degrees or more, log when temp reaches 35 degrees, and log/email a warning when temp reaches 40
Alert using Pushover

/etc/smartd.conf

DEVICESCAN -a -n standby,q -s (S/../.././02|L/../../6/03) -W 4,35,40 -m root -M exec /usr/share/smartmontools/smartd-runner

/etc/smartmontools/run.d/10pushover

#!/bin/bash

# Send notification
echo "$SMARTD_MESSAGE" | pushover-notify -t "SMART failure: $SMARTD_FAILTYPE"

/usr/local/bin/pushover-notify

#!/usr/bin/env python

import argparse
import requests
import sys

APP_TOKEN = 'TODO'
USER_KEY  = 'TODO'
PUSHOVER_URL = 'https://api.pushover.net/1/messages.json'


parser = argparse.ArgumentParser()
parser.add_argument('-t', '--title', help='Title for the notification')
args = parser.parse_args()

title = args.title or ''
data = sys.stdin.read()

payload = {'token': APP_TOKEN, 'user': USER_KEY, 'title': title, 'message': data}
requests.post(PUSHOVER_URL, data=payload)

CUPS¶

chroot¶

chroot is a great way for repairing a system that won’t boot. In order to do so, run the following commands from a rescue shell (a Debian installer shell, for example):

# Mount the target in /mnt
cd /mnt
mount -t proc proc proc/
mount --rbind /sys sys/
mount --rbind /dev dev/
cd ..
chroot /mnt /bin/bash

OpenBSD¶

Some OpenBSD tips and tricks.

Start a network interface¶

sh /etc/netstart re0

Replace re0 with the interface name.

Use a serial console for installation¶

At the boot prompt:

> stty com0 115200
> set tty com0

Adjust the baudrate accordingly.

Reload pf rules¶

pfctl -f /etc/pf.conf -F all

The -F all part can be omitted if you don’t want to flush the current rules.

Programming¶

C¶

Compilation with musl¶

First install the musl-gcc wrapper (on Debian):

apt-get install musl-tools

Now set some environment variables:

export CC=musl-gcc
export LINK=musl-gcc
export CFLAGS=-static

That’s it, you can now compile your library and it will be linked with musl instead of glibc.

Feature test macros¶

Interesting article here.

CSS¶

Vertically center a div within another div¶

.parent-element {
    -webkit-transform-style: preserve-3d;
    -moz-transform-style: preserve-3d;
    transform-style: preserve-3d;
}

.element {
    position: relative;
    top: 50%;
    transform: translateY(-50%);
}

Source here.

Miscellaneous¶

/dev/random vs /dev/urandom¶

Miths about /dev/urandom High quality random bits in os.urandom (Python) Python random

Syscalls¶

SSH¶

Using a jump host¶

$ ssh -o ProxyCommand="ssh -W %h:%p firewall.example.com" server2.example.org

or

$ ssh -tt firewall.example.com ssh -tt server2.example.org