Installing SoftEther VPN server on Ubuntu

This tutorial is to show you how to install a SoftEther VPN on Ubuntu 12.04 & Ubuntu 14.04.
After installing a fresh copy of Ubuntu or your machine, run following commands to update the packages.


After updating add SoftEther PPA:


Update package list again & install softether package.

After installing run following commands to check for any errors.

Select option 1.  And press Enter for the rest. Then run command check.


2015-12-25 19_51_20-Clipboard


If you get output same as above image, then you have successfully installed SoftEther VPN server on your machine.


Introduction to Erlang part 1

Erlang is a functional programming language. If you have ever worked with imperative languages, statements such as i++ may be normal to you; in functional programming they are not allowed. In fact, changing the value of any variable is strictly forbidden.

We will start with installing Erlang.


In Erlang, you can test most of your stuff in an emulator; it will run your scripts
and  it will also let you edit stuff live. To start,  open a terminal and then type $ erl. If you’ve set up everything fine, you should see text like this:


Shell commands:

If you type in i then c, Erlang should stop the currently running code and bring you back to a responsive shell. J will give you a list of processes running (a star after a number indicates this is the job you are currently running), which you can then interrupt with i followed by the number. If you use k , you will kill the shell as it is instead of just interrupting it. Press s to start a new one.

Now we will go through some basics

In the Erlang shell, expressions have to be terminated with a period followed by whitespace(line break, a space etc.), otherwise they won’t be executed. You can separate expressions with commas, but only the result of the last one will be shown (the others are still executed). This is certainly unusual syntax for most people and it comes from the days Erlang was implemented directly in Prolog, a logic programming language.

Here is example,


Erlang doesn’t care if you enter floating point numbers or integers: both types are supported when dealing with arithmetic.Integers and floating values are pretty much the only types of data Erlang’s mathematical operators will handle transparently for you.

Note that we can use several operators in a single expression, and mathematical operations obey the normal precedence rules.

If you want to express integers in other bases than base 10, just enter the number as Base#Value (given Base is in the range 2..36):


Invariable Variables

Doing arithmetic is alright, but you won’t go far without being able to store results somewhere. For that, we’ll use variables. If you have read the intro to this book, you’ll know that variables can’t be variable in functional programming. The basic behavior of variables can be demonstrated with these 7 expressions (note that variables begin with an uppercase letter):

The first thing these commands tell us is that you can assign a value to a variable exactly once; then you can ‘pretend’ to assign a value to a variable if it’s the same value it already has. If it’s different, Erlang will complain. It’s a correct observation, but the explanation is a bit more complex and depends on the = operator. The = operator (not the variables) has the role of comparing values and complaining if they’re different. If they’re the same, it returns the value:


Data Types:

1) Terms: A piece of data of any data type is called a term.

2) Number: There are two types of numeric literals, integers and floats. Besides the conventional notation, there are two Erlang-specific notations:

$char ASCII value or unicode code-point of the character char.

base#value Integer with the base base, that must be an integer in the range 2..36. In Erlang 5.2/OTP R9B and earlier versions, the allowed range is 2..16.


3) Atom


4) Bit Strings and Binaries

A bit string is used to store an area of untyped memory. Bit strings are expressed using the bit syntax. Bit strings that consist of a number of bits that are evenly divisible by eight, are called binaries.


5) Reference

A reference is a term that is unique in an Erlang runtime system, created by calling make_ref/0.

6) Fun

A fun is a functional object. Funs make it possible to create an anonymous function and pass the function itself — not its name — as argument to other functions.


7) Port Identifier

A port identifier identifies an Erlang port. open_port/2, which is used to create ports, returns a value of this data type.

8) Pid

A process identifier, pid, identifies a process. The following BIFs, which are used to create processes, return values of this data type:

spawn/1,2,3,4 spawn_link/1,2,3,4 spawn_opt/4


9) Tuple

A tuple is a compound data type with a fixed number of terms: Each term Term in the tuple is called an element. The number of elements is said to be the size of the tuple. There exists a number of BIFs to manipulate tuples.


10) Map

A map is a compound data type with a variable number of key-value associations: ex: {Key1=>Value1,…,KeyN=>ValueN}

Each key-value association in the map is called an association pair. The key and value parts of the pair are called elements. The number of association pairs is said to be the size of the map.


11) List

A list is a compound data type with a variable number of terms. [Term1,…,TermN] Each term Term in the list is called an element. The number of elements is said to be the length of the list.

Formally, a list is either the empty list [] or consists of a head (first element) and a tail (remainder of the list). The tail is also a list. The latter can be expressed as [H|T]. The notation [Term1,…,TermN] above is equivalent with the list [Term1|[…|[TermN|[]]]].

A list where the tail is a list is sometimes called a proper list. It is allowed to have a list where the tail is not a list, for example, [a|b]. However, this type of list is of little practical use.


13) Record A record is a data structure for storing a fixed number of elements. It has named fields and is similar to a struct in C. However, a record is not a true data type. Instead, record expressions are translated to tuple expressions during compilation. Therefore, record expressions are not understood by the shell unless special actions are taken.

14) Boolean

There is no Boolean data type in Erlang. Instead the atoms true and false are used to denote Boolean values.


15) Type converstions



How to integrate Celery into Django project

What is Celery?

Celery is a distributed task queue that allows us to execute jobs in background. This article explains how to set up Celery with Django to perform a background task.


  • Large or small, Celery makes scheduling such periodic tasks easy.
  • You never want end users to have to wait unnecessarily for pages to load or actions to complete. If a long process is part of your application’s workflow, you can use Celery to execute that process in the background, as resources become available, so that your application can continue to respond to client requests.

Celery uses brokers to pass messages between a Django Project and the Celery workers. We will use Redis as the message broker.


Before diving into Celery, follow the below setup points

Create a new virtualenv ‘venv’ using following command:

To activate the environment use command:

Install django and create a django project ‘myproject’. Make sure to activate a virtualenv, create a requirements.txt file and run the migrations. Then fire up the server and navigate to http://localhost:8000/ in your browser. You should see the familiar “Congratulations on your first Django-powered page” text. When done, kill the sever.

Let’s install Celery:

Now we will integrate Celery into our Django project with the following steps:

Step 1:

Inside the myproject directory i.e beside your create a new file called and add the following code in that:

Let’s break down what happens in the first module, first we import absolute imports from the future, so that our module will not clash with the library:

Then we set the default DJANGO_SETTINGS_MODULE for the celery command-line program:

Specifying the settings here means the celery command line program will know where your Django project is. This statement must always appear before the app instance is created, which is what we do next:

This is your instance of the library, you can have many instances but there’s probably no reason for that when using Django.

We also add the Django settings module as a configuration source for Celery. This means that you don’t have to use multiple configuration files, and instead configure Celery directly from the Django settings.

You can pass the object directly here, but using a string is better since then the worker doesn’t have to serialize the object when using Windows or execv:

Next, a common practice for reusable apps is to define all tasks in a separate module, and Celery does have a way to autodiscover these modules:

Step 2:

To ensure that the Celery app is loaded when Django starts, add the following code into the file that sits next to your file:

Project layout should look like:

Step 3:

Celery uses “brokers” to pass messages between a Django Project and the Celery workers. In this article, we will use Redis as the message broker.

First, install Redis from the official download page and then turn to your terminal, in a new terminal window, fire up the server:

You can test that Redis is working properly by typing this into your terminal:

Redis should reply with PONG – try it!

Once Redis is up, add the following code to your file:

You also need to add Redis as a dependency in the Django Project:

Test that the Celery worker is ready to receive tasks:

Kill the process with CTRL-C. Now, test that the Celery task scheduler is ready for action:

That’s it! You can now use Celery with Django. For more information on setting up Celery with Django, please check out the official Celery documentation.

Sending emails asynchronously using Twisted – Part 2

In Part 1 of article, we saw how to send blocking emails using ‘smtplib’ module & non-blocking emails using Twisted framework. In this part, we will see how to send asynchronous emails to multiple recipients using Twisted

  • Sending multiple emails

    Refer following script.This script sends emails to given recipients asynchronously. Here we have used twisted.internet.defer.DeferredList API. This API is very useful in some scenarios. Suppose you have to finish multiple task asynchronously and then you have to finish one final task. For examples, your program is connected to 4 different clients & and before shutting it down, you have to make sure that all connections are closed properly. In such cases, DeferredList API is used. Create deferrands of each task & make their list. Pass this list to ‘DeferredList‘ API which will return you another deferrand. This final deferrand will be fired when all deferrands in list will be fired.

  • Sending multiple emails using coiterator

    Though above script runs fine, there is one problem. Here, recipients number is very small. But suppose you have to send emails to millions recipients then will this code work ?. Refer function ‘send_multiple_emails’.

    Here we have used ‘for’ loop which is blocking. So until this ‘for’ loop is iterated, program will not move to next line of code. For 3 recipients iteration will not take much time however for millions of recipients, it will not work.
    So lets modify our code to work like generators.

    Here, we have used twisted.internet.task.coiterate API. This API iterates over iterator by dividing reactor runtime between all iterators. Thus we can send millions of emails asynchronously.

Sending emails asynchronously using Twisted – Part 1

  • Using ‘smtplib‘ module

It is very easy to send emails using ‘smtplib‘ module of python. Check following recipe.

But ‘smtplib’ module sends emails synchronously. So code execution is blocked until email is sent. To overcome this, lets try to send email asynchornously.

  • Using Twisted

For this tutorial we are going to use Twisted framework. Twisted is event-driven networking engine. It uses reactor-pattern. Twisted uses deferred objects to address waiting IOs. Deferred is more like subset of promises. Check following recipe to send asynchronously MIME message using Twisted.



Building hello world using Python Tkinter

Continuing our series of desktop GUIs, in this post we are going to go over the topic of building a small hello world program using Python Tkinter.

What is Tkinter ?

Tkinter is python wrapper/binding to Tk library. Tk was developed as GUI library for Tcl language by John Ousterhout. For many other high level language authors coming to programming scene in 1990s this seemed like easy tool to capitalize on in bringing GUI library to their language. That’s why you can find many different language bindings to Tk. Tk brought easy GUI building to programming masses.

Often times many people write off Tk or Tkinter as ugly old GUI library. Their claims are not completely unfounded. But, Tk in recent times made strides to bring modern look by implementing Tile, themeing engine. Tile is also called as ttk. With ttk in place look and feel issues of Tk are addressed.

ttk xp blue theme demo
Tile xp blue theme demo

But, Tkinter still lags behind in terms available collection of default widgets compared to other libraries like PySide or wxPython . However, Tkinter is still a good candidate if you quickly want to dish out a GUI without having programming styles imposed on you. (You will see these style restrictions in our next posts about wxPython and PySide)

Hello world

Now, we will go over building a simple hello world program using Tkinter. In this program we will create a simple window with “Hello world” text in it. And a button that say “Kill Me”. Clicking that button will cause the python program to exit.

tkinter hello world

That import * line saves your some typing.

This line instantiates Tk and creates main window of the program. Next we create a Label (a widget which displays text ) and call it’s pack method. pack is one three available layout methods provided by Tkinter (rest of the two being grid and place). The widget will not show up until you call pack.  Next, we create a Button widget and pack it, similar to that of Label. The interesting point to note while creating button widget is, we provide command argument to it with a reference of sys.exit function. A newbie mistake that is often made is, passing function with () next to it. That will cause the target function to be invoked immediately. You are only supposed to pass the reference of function and Tkinter will call it when button is pressed. So, when this “Kill Me” button is pressed sys.exit will be called and program exits. One, last important line to notice is root.mainloop() . This line sets the event monitoring loop in motion and Tkinter take control of the program from here.


How to create ascii art using VIM plugin DrawIT

Installing VIM plugin DrawIT

Install the plugin if it is no available in your machine

This script/plugin is distributed as a vimball

This can be downloaded here

To download source go to /usr/local/src Create a directory VIM. go to the directory /usr/local/src/VIM (NOTE: it isn’t manadetory to keep source in this location, it’s upto you)

Dowload the source with following wget

wget -O DrawIt.vba.gz

To install follow the instructions

Open file vim DrawIT.vba.gz In command mode enter :so %

quit :q

Quick start with DrawIt

That’s all you have to do to install a plug in. Now to test, open a new file

and type the following in command mode


If you see [DrawIt], the plugin is enabled an you can draw you art.

Here is how you will start and stop DrawIt

\di start DrawIt \ds stop DrawIt

User your arrow keys to draw whatever you want

To get help on drawit and to find handy key to draw, type the following in command mode in VIM :help drawit.




Common constructs of GUI programs

In this post we will go over some of the basic and common constructs that you come across while building GUI programs almost in any language or library. The following are some of the common items that I can think of

  1. Loop
  2. Widgets
  3. Events
  4. Layout


Loop or Mainloop as it is called in some GUI libraries is one of the most common element of all GUI programs. In a GUI program this is usually called at the at end of constructing the all the items (widgets) in program. When you call the mainloop, it takes over the control and keeps running until you terminate the GUI.

The main purpose of loop is to poll for user input (mouse and keyboard actions) and fire events which your program can handle in asynchronous way. Following diagram explains loop in simple terms.

GUI main loop
GUI main loop

As you can see from the above diagram the GUI loop keeps polling for user input. If there is any event via mouse or keyboard or other input devices (joysticks, game pads ) when input is available loop gives control to callback functions that are bound to input events.  Loop keeps running until the main window (also called parent window) exits. Once the main window is destroyed the loop exits and gives control back to your code. In most cases end of the mainloop would be end of many GUI programs.


GUIs are collection of widgets. Example of widgets are button, scroll bar, check box etc. Widget is an element of user interaction. Their main purpose is to display or collect information from user. A label displays text to user. A text input widget collects text from user. One way to decide the richness of GUI tool kits is to look at how many widgets they offer. The more widgets a library offers you the better it is for you. So, that you don’t have to write extra code to implement custom widgets.  Widgets usually are sub classes of a Widget parent class. So, most of the widgets inherit properties of widget class in respective GUI libraries. In some GUI libraries main window also has properties of a widget. gui window


When ever user presses a button or enters text using keyboard those actions generate events in GUI libraries. GUI libraries provide a way to hook into these events so that your program can respond to user actions. The event hooking/binding style differs from library to library. But, the aim is same across all libs, that is to provide programmer a way to respond to user input.


Layout facilities provided by GUI libs helps to make programmer’s life easier when placing widgets in main window. Layouts are simple rules you use to convey to GUI library on how you would like your widgets to be laid out on screen. Instead of manually placing widgets on screen by specifying distance units and placement pixel positions, once you construct your widgets you hand them over to certain layout scheme to let GUI library handle the placement. The benefits are, layouts will take care of expansions, size proportions and other nitty-gritty detail of placing your widgets in orderly way.

The layout features provided by libraries vary greatly. You often have to pick the one that suits your needs. These layouts can be mixed in order to achieve the placement and look you want.


Desktop GUI libraries in Python

If you want to build desktop GUIs, Python is right candidate to pick. It offers you plethora of GUI libraries compared to any other language. So, there are high number of chances that, some library in there fits your bill which is readily available, in whole of lot of GUI libs python has to offer. Following is a list of some of the popular GUI libs

  1. Tkinter 
  2. wxPython
  3. PyQt / PySide
  4. PyGTK

(Complete list of available libraries can be found on python wiki page.)

All of the above mentioned ones are cross-platform capable libraries. So, that you don’t have to rewrite any part of your GUI and gladly run it across all the main contenders Windows, Linux and Mac . There are other platform specific ones like win32gui , WinForms using IronPython. But, it helps to stick with cross-platform capable libs to avoid extra work when building your app for other platforms.

Over the next few posts I’ll present you with examples of simple hello world programs using above mentioned list of libraries.

Usually the best way to build your GUI application is to write compute intensive part (if there is any ) of your business logic in low level languages like C/C++ and then write GUI code using the above mentioned Python libs. This saves you lot of time and energy. As, compared to low level languages, in Python you get the benefit of automatic memory management and you only have to write less number of lines of code.

Desktop GUIs are on decline with the advent of web. More and more programs are being coded as services using web technologies ( HTML, CSS, JavaScript ). It has gotten so far, now UI/GUI by default means web UI.  Each passing day web technologies are filling up  the gap in terms of number of features (Dynamic content, Media capabilities ) . But, desktop GUIs still have their place in certain areas and there are numerous programs already written using desktop GUI technologies that needs to be managed.

How to implement PayPal payment gateway

The PayPal REST APIs are supported in two environments. Use the Sandbox environment for testing purposes, then move to the live environment for production processing.

The following endpoints address are two environments:

A complete REST operation is formed by combining an HTTP method with the full URI to the resource you’re addressing. For example, here is the operation to create a new payment:

The PayPal REST sdk can be obtained through pip

OAuth Request / Response

Client Id and Secret Id can be obtained from the application created in the paypal account.

For the following each API call, you’ll need to set request headers, including the access token.

Creating a WebProfile:


The name of the web experience profile which should be unique among the profiles for a given merchant.


It contains the parameters for style and presentation.


Parameters for input fields customization:

  1. allow_note : It enables the buyer to enter a note to the merchant on the paypal page during checkout.

  2. no_shipping : Determines whether or not PayPal displays shipping address fields on the experience pages.

    • 0 – PayPal displays the shipping address on the PayPal pages.
    • 1 – PayPal does not display shipping address fields whatsoever.
    • 2 – If you do not pass the shipping address, PayPal obtains it from the buyer’s account profile.
  3. address_override : Determines if the PayPal pages should display the shipping address supplied in this call, rather than the shipping address on file with PayPal for this buyer.

    • 0 – PayPal pages should display the address on file
    • 1 – PayPal pages should display the addresses supplied in this call instead of the address from the buyer’s PayPal account.

Parameters for flow configuration

  1. landing_page_type : Type of PayPal page to be displayed when a user lands on the PayPal site for checkout.

    • Billing – The Non-PayPal account landing page is used
    • Login – The paypal account login page is used.

Creating a Payment:



Payment intent. Allowed values are:

  • “sale” – For immediate payment
  • “authorize” – To authorize a payment for capture later
  • “order” – To create an order

Id that will be obtained from the response of web profile request


Source of the funds for this payment represented by a PayPal account or a credit card.

  • payment_method : Payment method used. Must be either credit_card or paypal.
  • funding_instruments : A list of funding instruments for the current payment
  • payer_info : Information related to the payer
  • status : Status of the payer’s PayPal account. VERIFIED or UNVERIFIED

Transactional details including the amount and item details.


Set of redirect URLs you provide only for PayPal-based payments.

  • return_url : The payer is redirected to this URL after approving the payment.
  • cancel_url : The payer is redirected to this URL after canceling the payment.

Execute an approved PayPal payment:

Use this call to complete a PayPal payment that has been approved by the payer.

The payment_id and payer_id are passed in the return_url. Once the payment is executed,it returns an array of payment object.

In the response state of the payment is obtained as : created approved failed, canceled, expired or pending.

The transaction details in the response contains state of the sale which is obtained as: pending, completed, refunded or partially_refunded

If the payment state is approved and the sale state is completed, the payment is successfully executed.