Compdigitec Labs

Simple recursive DNS server with Unbound DNS

By admin | July 30, 2020

This is a simple configuration for running a recursive DNS server (passes DNS requests to another server and caches responses) with the Unbound DNS server.

Installation (Ubuntu):

sudo apt-get install -y unbound

Open the config

sudo vim /etc/unbound/unbound.conf


Replace below with the desired upstream DNS server.

# The following line includes additional configuration files from the
# /etc/unbound/unbound.conf.d directory.
#include: "/etc/unbound/unbound.conf.d/*.conf"
# NOTE: needed to comment out the above line avoid a "status: SERVFAIL" response

    # Enable verbose debugging messages
    verbosity: 1000

    # Run on all interfaces

    # Hide the server name and version
    hide-identity: yes
    hide-version: yes

    # Who should be able to query the server
    access-control: allow

    do-ip4: yes
    do-ip6: no

    do-udp: yes
    # Enable this to support TCP DNS which is required in some applications
    do-tcp: yes

    # Allow forwarding to another DNS server (e.g. another local dnsmasq or systemd-resolve)
    do-not-query-localhost: no

    name: "."
    # Replace with your desired upstream DNS server
    # You can have multiple forward-addr lines

Starting the server

sudo systemctl restart unbound

Debugging / Troubleshooting

Query the server

dig @your_server_here

Read the DNS server log

sudo systemctl status unbound -n 50


Topics: Linux | No Comments »

Generating POT files for WordPress plugins

By admin | July 6, 2020

The easiest non-GUI way appears to be using WP CLI.

Run the following in the plugin folder:

wp i18n make-pot . output.pot

Topics: HTTP (Internet) | No Comments »

Make fails silently

By admin | February 28, 2017

Run make <...> -d &> log.txt and examine the log. Likely there is some prerequisite which is missing, in the form of “File `foo’ does not exist.”

Topics: Linux | No Comments »

LaTeX macros ending in numbers

By admin | February 13, 2016

Only letters are directly permitted in macro names, but by using csname to create definitions along with a variable command, we can simulate them.

First, define the macros with numbers using \csname:

\expandafter\def\csname xR1\endcsname{-3}
\expandafter\def\csname xR2\endcsname{9}

Now, create a command which takes in one variable argument, and simply expands it to call the appropriate macro (“xR” + variable) defined above.

\newcommand{\xR}[1]{\expandafter\csname xR#1\endcsname}

And to use the macros:

The first is \xR1 and the second is \xR2.

Topics: Linux | No Comments »

Phabricator LDAP (and other) account integration table

By admin | January 17, 2016


See also

For LDAP, the following structure is used:

Topics: PHP | No Comments »

Possible crash in method_exists() with PHP 5.3

By admin | August 9, 2014

Apparently in some versions of PHP 5.3, calling method_exists() on a class which doesn’t exist seems to cause a crash. The solution, seen here, apparently involves checking class_exists() to prevent a potentially crashing call to method_exists().

This particular bug was causing the administration page of the MathJax-LaTeX WordPress plugin to be blank in the admin interface.

The trick is to replace the two lines in mathjax-latex-admin.php lines 91-92:

$wp_latex_disabled = method_exists('WP_LaTeX','init') ? "disabled='true'" : "";
$wp_latex_disabled_warning = method_exists('WP_LaTeX','init') ? "Disable wp-latex to use this syntax." : "";


$wp_latex_disabled = (class_exists('WP_LaTeX', false) && method_exists('WP_LaTeX','init')) ? "disabled='true'" : "";
$wp_latex_disabled_warning = (class_exists('WP_LaTeX', false) && method_exists('WP_LaTeX','init')) ? "Disable wp-latex to use this syntax." : "";

Topics: PHP | No Comments »

Disable dragging of an element in JSXGraph

By admin | July 20, 2014

Set the attribute ‘fixed‘ to true in JXG.GeometryElement:

var line = board.create('line', [ [0,0], [1,1] ], {dash:0, strokeColor:"#FF0000", strokeWidth:3, fixed: true});


var line = board.create('line', [ [0,0], [1,1] ], {dash:0, strokeColor:"#FF0000", strokeWidth:3});
line.setAttribute({ fixed: true });

Topics: (X)HTML, Windows | No Comments »

Reverse engineering the Fischertechnik blinker

By admin | July 13, 2014

The Fischertechnik blinker comes in a enclosed case with no other sensory inputs with two wires, a yellow wire and a blue wire.

The Fischertechnik blinker

The Fischertechnik blinker

As such, I reverse engineered the Fischertechnik blinker to understand how it works through desoldering, a camera, and a lot of drawing on the GIMP.

It turns out that the blinker is intended to be used like a switch in a series circuit before a load, with the yellow lead being the positive end and the blue lead being the negative end.

Series circuit with FT blinker on mySTEM board

Series circuit with FT blinker on mySTEM board

The main idea here is that the board uses a 555 timer (555C EZ606 with STMicroelectronics logo, surface-mounted) with fixed frequency (fixed R1, R2 and C) in astable mode to control a BSP 75N MOSFET transistor.

The power supply for the ICs is controlled in parallel with a diode and a 470Ω resistor to protect the ICs, while the MOSFET controls the connection to the other lead.

It features an unknown-model fuse (VW UG4 GP613) on the emitter of the transistor, presumably to protect the circuit from excess.

A diode and 100 µF capacitor cap are present, possibly to smooth out the power input for the ICs during temporary dislocations. Considering that it is a Fischertechnik component, its purpose is probably to ensure continued reliability when it is handled roughly, shaken, and dropped by students.

After complete disassembly:

Fischertechnik blinker, disassembled

Fischertechnik blinker, disassembled

With traces and PCB features:

Front view, with traces and PCB features

Front view, with traces and PCB features

Back view, with traces and PCB features

Back view, with traces and PCB features


Fischertechnik blinker schematic

Fischertechnik blinker schematic

Since C is a generic-looking ceramic capacitor with no markings, its capacitance was estimated by using the period.

The light was measured to blink 100 times in 41.21 seconds, giving:
T = 0.4121 \text{s} \\
f = 2.426 \text{s}^{-1}

Given R1 = 1 MΩ and R2 = 1.2 MΩ from above and f = 2.426, we can calculate C using the following equation:
f = \dfrac{1}{\ln 2 * C * (R_1 + 2*R_2)}

C then works out to about 0.175 µF.

Topics: Other | No Comments »

Reverse proxy with HTML rewriting using Apache

By admin | May 25, 2014

<VirtualHost *:80>

 ... other directives ...

 ProxyHTMLEnable On (comment in some cases)
 SetOutputFilter proxy-html
 SetOutputFilter inflate;proxy-html;deflate  (for compression issues - e.g. The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression)

 ProxyHTMLURLMap /
 ProxyHTMLURLMap /myfolders /

Topics: Linux | No Comments »

How to get the classic Firefox look back

By admin | November 20, 2013

Firefox, restored to a classic look

Firefox, restored to a classic look

While all the latest dynamic changes (such as the new Australis UX) may be interesting, you may be looking to get that classic Firefox look back for productivity reasons. Here is a list of recommended addons to restore your Firefox back to something to do work with:

Topics: HTTP (Internet) | No Comments »

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