Compile Erlang using Kerl with crypto module

When working with erlang, we can use old version to run our erlang app which are not yet upgraded.

This multiple erlang version run like python env if you come from python.

So in erlang we using Kerl which can switch to multiple erlang version at runtime. So here we go.

We are using Debian 10.x buster. Some prerequisite before able to finish are install the dev tools.

# apt-get install build-essential autoconf libncurses5-dev openssl libssl-dev fop xsltproc unixodbc-dev libz-dev

Then get the openssl version supported on your erlang version. Here i use 17.5.x release which require openssl-1.0.xx to compile properly with crypto module.

1 $ git clone git@github.com:openssl/openssl.git --branch OpenSSL_1_0_2-stable

2 $ cd openssl

3 $ mkdir __result

4 $ ./config --prefix="${HOME}/openssl" shared zlib -fPIC

5 $ make depend

6 $ make

7 $ make install INSTALL_PREFIX="/home/me/openssl/__result"

Then we can start the kerl compilation.

Get the kerl 

1 $ curl -0 https://raw.githubusercontent.com/kerl/kerl/master/kerl

2 $ chmod a+x kerl

Make the ssl available for kerl compiler

1 $ export KERL_CONFIGURE_OPTIONS="--with-ssl=/home/me/openssl-OpenSSL_1_0_2-stable/__result/home/me/openssl-OpenSSL_1_0_2-stable/openssl/"

2 $ mkdir .kerl

3 $ kerl build 17.5.3 17.5.3

This should be show which indicate SSL included.

 

Then continue with installation:

$ kerl install 17.5.3 ~/.kerl/17.5.3

To test run erlang.

$ . ~/.kerl/17.5.3/activate

$ erl

> crypto:start().

Should no error if install successfully.

And make that fault tolerant app with 0 downtime.

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panic cannot login to linux

Panic ! Thats what occurs when a critical systems you manage cannot login , but the service was running properly.

Recently i do some changes on the /etc/security/limits.conf and it make all lockout. No SSH can be done, but all the service was running properly.

To the rescue, we need to login to the systems and revert back changes, and to do that we need to go into single mode.

We do on Centos 7.x systems which is the grub boot loader is different than the old version.

So how to do it step by step, and no panic. 

• Reboot your machine and immidiately go to rescue mode.
• Chose one boot options and press "e" to change the entry
• Look for below entry :
  linux16 /boot/vmlinuz-3.10.0-123.el7.x86_64 root=UUID=act2884249823928928392 ro  xxxxxx
• Change the ro to below 
  rw init=/sysroot/bin/sh
• Then continue the boot, press ctrl+x to do that. No worries, after finish you can reboot and the grub boot loader will be still the old one.
• once boot to single mode you need to mount the filesystem. do
  # chroot /sysroot/
• After this step you can revert back any settings you made to restore it to working state.
• On our side, its /etc/security/limits.conf
• After finish we can reboot the machine to production mode
  # reboot -f

That was no panic attack anymore if you know what you are doing.

Always make sure you have backup of the OS.

Or better way use Container these days. 

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Compiling GoLang application

When using Golang basic setup need to be done to able compile all application based on golang in github.

To do that install golang then set in your home directory.

let say /home/geek/go/

inside the folder create a src folder, and put all the go app you want to compile.

Then remember to set env variable GOPATH.

in ~/.bashrc set :

GOPATH = /home/geek/go

Example you have a killer-app download from github with go source code.

Put it inside /home/geek/go/src/killer-app/

Then do this :

$ cd /home/geek/go/src/killer-app/

$ go get ./...

$ go build

Then there will be a file created from the compile called killer-app 

Then profit.

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Stafull and Stateless widget in Flutter

In flutter we have 2 type of widget, Statefull and stateless. Difference are the stateless widget will never changes after it rendered. Meantime statefull widget can be change in the future after the widget rendered.

There are difference on how to use it. Because flutter using inheritance of the widget, we will always override some of the function / properties of the widget. There will be a lot of inheritance method in flutter.

Lets go for the stateless widget first. On the stateless widget, the one we override are the build function.

Below is the example code.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {

  @override

  Widget build(BuildContext context) {

    return MaterialApp (

      title: 'Measures Converter' ,

      home: Scaffold(

        appBar: AppBar(

          title: Text('Measures Converter'),

        ),

        body: Center(

          child: Text('Measures Converter'),

        ),

      ),

    );

  }

}

And for statefull widget, we override the createState method.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp (
      title: 'Measures Converter' ,
      home: Scaffold(
        appBar: AppBar(
          title: Text('Measures Converter'),
        ),
        body: Center(
          child: Text('Measures Converter'),
        ),
      ),
    );
  }
}

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() => MyAppState();


}

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Basic Flutter Scaffolding

Flutter makes the development enjoyable and fast.

Here are some of the template for create a basic layout.

We create an app with App bar and a body.

import 'package:flutter/cupertino.dart';
import "package:flutter/material.dart";

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
@override
  Widget build(BuildContext context){
    return MaterialApp(
      title: 'My Work Timer',
      theme: ThemeData(
        primarySwatch: Colors.blueGrey,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('My Killer App'),
        ),
        body: Center(
          child:
            Text('My Killer APp'),
        ),
      ),
    );
  }
}

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Mobile Development with Flutter

Nowdays, mobile development not need to be tricky and hard.

Old timers for mobile dev will use java fro android and object C for ios development, which means 2 code for 2 platform Android and IOS.

Other path was hybrid using web development technique which just load a Web view in the application, so developer can use Web development tools and style to develop mobile app.

Now, we have Google who develop Flutter on top of Dart which will make the Java slogan "code once run on any mobile platform (Android / IOS) but now with happiness.

All in flutter just a widget and tooling all free and available and it also beautifull on the app generated because the template also provided like Material Design or Cupertino for IOS.

My interest on Flutter become increasing and after watching some time on the progress of Flutter. Now as on my writing, Flutter version 2.7.0 is been used on my development tools.

You will have my Flutter blogging on their feature and also this will be my flutter docs on my joourney to flutter.

Hope you enjoy it.

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Securing your Web services using Nginx

If we have an API service and want to publish to public, better use a reverse proxy like nginx to handle all the dirty traffic trying to taken down your services.

With Nginx you will make your API server live prosper and not minding the dirty request which should not coming on your server if not using Nginx in front of it.

So lets do the Nginx configuration for securing your backend API server.

    add_header Cache-Control public;

    add_header X-frame-Options "DENY";

    add_header X-Xss-Protection "1; mode=block" always;

    add_header Content-Security-Policy-Report-Only "script-src https://skyway.shineapi.net";

    add_header X-Content-Type-Options "nosniff" always;

    add_header Strict-Transport-Security 'max-age=31536000;includeSubDomains;preload;' always;

    add_header Referrer-Policy no-referrer-when-downgrade;

 
   if ($http_referer = "") {  return 403; }


With above configuration, any request incoming your API will be handled by Nginx and protected by the header config which is web security standards for securing any web app in the wild.

For the explanation i will put on another blog post for details.

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