Tuesday, 22 November 2016

Linux: Verify a burned CD/DVD with the image file used for burning

So I just burned a CD sometime ago and I am not sure if it still works. A quick and easy way to test it's against the original ISO would be to something like this:

thanassis:Downloads/ $ dd if=/dev/sr1 | md5sum                                                                                                                                     [20:46:28]
1366016+0 records in
1366016+0 records out
699400192 bytes (699 MB, 667 MiB) copied, 265,145 s, 2,6 MB/s
d2d939ca0e65816790375f6826e4032f  -
thanassis:Downloads/ $ md5sum ubuntu-16.04.1-server-amd64.iso                                                                                                                     [20:57:07]
d2d939ca0e65816790375f6826e4032f  ubuntu-16.04.1-server-amd64.iso

... and I 'm good to go.

Note: All credit for this solution goes to theSuperusr Community.

Friday, 3 June 2016

Using a raspberry pi (or any Debian based computer) to set up an open VPN server


There are many howto's explaining the set-up of an OpenVPN server. The majority of what I have seen, provide instructions about setting up a tunnel device. This allows you to gain access to the OpenVPN server itself, but requires a lot more customisation if you want to access other computers on the remote LAN.

Using a tap device is much easier, and allows the OpenVPN client to be able to access not only the server but all other computers connected to the same LAN.

The basic idea is that you create an Ethernet bridge on the VPN server. Each time clients connect, the server taps them to that bridge and so clients behave as if they are physically connected to the remote LAN.


I would advice starting your setup with a clean fresh install. Before proceeding to follow the instructions below, issue:

pi@gabriella:~ $ sudo apt-get update && sudo apt-get dist-upgrade 

Install the bridge

The first step is to set up out bridge:

pi@gabriella:~ $ sudo apt-get install bridge-utils

After the bridge-utils package is installed, we need to change the server's network configuration file in order to activate the bridge.The actual file is:/etc/network/interfaces and the definition of the bridge is :

# interfaces(5) file used by ifup(8) and ifdown(8)

# Please note that this file is written to be used with dhcpcd
# For static IP, consult /etc/dhcpcd.conf and 'man dhcpcd.conf'

# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto lo
iface lo inet loopback

face eth0 inet manual

# Our bridge initially contains just the ethernet interface and is configured to use DHCP
auto br0
  iface br0 inet dhcp
  bridge_ports eth0

# You can delete these lines if you do not whish to use Wi-fi 
allow-hotplug wlan0
iface wlan0 inet manual
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

allow-hotplug wlan1
iface wlan1 inet manual
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

This setup uses DHCP for the bridge. Setting up a static IP is explained in the Network Configuration section of the Debian Wiki. The same pages provide more information and details about bridging. The actual link is here.

At this point a server reboot is required in order to verify the bridge setup. When the server is up again you can check that the bridge is working by examining the outcome of the ip addr command which should be similar to this:

pi@gabriella:/etc/openvpn $ ip addr
1: lo:  mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0:  mtu 1500 qdisc pfifo_fast master br0 state UP group default qlen 1000
    link/ether b8:27:eb:b2:d8:f2 brd ff:ff:ff:ff:ff:ff
    inet brd scope global eth0
       valid_lft forever preferred_lft forever
3: br0:  mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether b8:27:eb:b2:d8:f2 brd ff:ff:ff:ff:ff:ff
    inet brd scope global br0
       valid_lft forever preferred_lft forever
    inet brd scope global secondary br0
       valid_lft forever preferred_lft forever
    inet6 fe80::ba27:ebff:feb2:d8f2/64 scope link 
       valid_lft forever preferred_lft forever

install openvpn

With the bridge working the next step will be installation and configuration of the OpenVPN server itself. Before we proceed with the server installation we will lay out the necessary fact about the network the server is on:

  • Our network is a class C net with a broadcast address of
  • Default gateway is
  • Network DNS is
  • The target network uses DHCP. We have set aside the range between and to be used by our clients,

Having clear our theses fact we may begin our installation.

pi@gabriella:~ $ sudo apt-get install openvpn easy-rsa 

Since all the commands that we will type from now on will require root privileges, it is wise to do a :

pi@gabriella:~ $ sudo su -
root@gabriella:~# cd /etc/openvpn/

We will create a new server configuration file. This file is /etc/openvpn/server.conf with the following contents

# --------------------------------------------------------------------------------
# OpenVPN bridging configuration file.
# Adopted from the Ubuntu Community Page at https://help.ubuntu.com/community/OpenVPN
# Date may -15th 2016
# --------------------------------------------------------------------------------
mode server

local       ## ip/hostname of server
port 1194               ## default openvpn port
proto udp

# --------------------------------------------------------------------------------
# Bidging directive
# --------------------------------------------------------------------------------
dev tap0 

script-security 2       ## allow calling up.sh and down.sh
up "/etc/openvpn/up.sh br0 tap0 1500"
down "/etc/openvpn/down.sh br0 tap0"


# --------------------------------------------------------------------------------
# certificates and encryption
# --------------------------------------------------------------------------------
ca ca.crt
cert server.crt
key server.key          # This file should be kept secret
dh dh2048.pem
tls-auth ta.key 0       # This file is secret

cipher BF-CBC           # Blowfish (default)

# --------------------------------------------------------------------------------
# DHCP Information
# --------------------------------------------------------------------------------
ifconfig-pool-persist ipp.txt
# The first address is the bridge address of the server the second is the netmask to use
# and the next twoo addresses are the range that should be assigned to clients
push "dhcp-option DNS"
push "dhcp-option DOMAIN simsons"
max-clients 8           ## set this to the max number of clients that should be connected at a time

# --------------------------------------------------------------------------------
# log and security
# --------------------------------------------------------------------------------
user nobody
group nogroup
keepalive 10 120
status openvpn-status.log
verb 3

Next we will need to create the two scripts that hook and unhook our tap device to the server bridge. These files should be called up.sh and down.sh (These are the names in server.conf file) with the following contents.

# File up.sh adds a device to a bridge with a specified MTU

BR=$1    # bridge to add device to
DEV=$2   # actual device to add
MTU=$3   # MTU value

/sbin/ip link set "$DEV" up promisc on mtu "$MTU"
/sbin/brctl addif $BR $DEV
# file down.sh remove a device from a bridge


/sbin/brctl delif $BR $DEV
/sbin/ip link set "$DEV" down

Do not forget, to make both scripts executable

root@gabriella:/etc/openvpn# chmod +x up.sh down.sh 

Having done all that, it's time for a break. We will create our own DH 2048 bit file.

root@gabriella:/etc/openvpn# openssl dhparam -out dh2048.pem 2048
Generating DH parameters, 2048 bit long safe prime, generator 2
This is going to take a long time

Even on a four core Raspberry-PI2 model B board, this takes well over 40 minutes, so sit back and relax

Create the Certificates

With the dh2048.pem file in place, we will start creating our server and client certificates by first copying the easy-rsa scripts to our servers setup directory and next creating the folder that will host our keys.

In order to make things a bit more secure we will create an additional tls-auth directive and the related key

root@gabriella:/etc/openvpn# openvpn --genkey --secret ta.key
root@gabriella:/etc/openvpn# cp -r /usr/share/easy-rsa/ /etc/openvpn
root@gabriella:/etc/openvpn# cd easy-rsa/
root@gabriella:/etc/openvpn/easy-rsa# mkdir keys

Next step is the creation of the actual server certificates. Oprn the /etc/openvpn/easy-rsa/vars file and fill in the details accordingly.

root@gabriella:/etc/openvpn# vim /etc/openvpn/easy-rsa/vars 

Change the lines (first one is line 64 in my version) that refer to the certificate creation parameters, so they look similar to this:

# These are the default values for fields
# which will be placed in the certificate.
# Don't leave any of these fields blank.
export KEY_COUNTRY="gr"
export KEY_PROVINCE="Rodope"
export KEY_CITY="Komotini"
export KEY_ORG="Aryballos"
export KEY_EMAIL="admin@aryballos.gr"
export KEY_OU="Aryballos-IT-Services"

# X509 Subject Field
export KEY_NAME="server"

Save, exist, run the file ... and do a clean-all to start fresh.

root@gabriella:/etc/openvpn/easy-rsa# . ./vars 
NOTE: If you run ./clean-all, I will be doing a rm -rf on /etc/openvpn/easy-rsa/keys
root@gabriella:/etc/openvpn/easy-rsa# ./clean-all

The next command will build the required certificate authorities for the server (Hit ENTER when there is a prompt).

root@gabriella:/etc/openvpn/easy-rsa# ./build-ca
Generating a 2048 bit RSA private key
writing new private key to 'ca.key'
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
Country Name (2 letter code) [gr]:
State or Province Name (full name) [Rodope]:
Locality Name (eg, city) [Komotini]:
Organization Name (eg, company) [Aryballos]:
Organizational Unit Name (eg, section) [Aryballos-IT-Services]:
Common Name (eg, your name or your server's hostname) [Aryballos CA]:
Name [server]:
Email Address [admin@aryballos.gr]:

Now it's time to build the actual server key.

root@gabriella:/etc/openvpn/easy-rsa# ./build-key-server server
Generating a 2048 bit RSA private key
writing new private key to 'server.key'
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
Country Name (2 letter code) [gr]:
State or Province Name (full name) [Rodope]:
Locality Name (eg, city) [Komotini]:
Organization Name (eg, company) [Aryballos]:
Organizational Unit Name (eg, section) [Aryballos-IT-Services]:
Common Name (eg, your name or your server's hostname) [server]:
Name [server]:
Email Address [admin@aryballos.gr]:

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:
Using configuration from /etc/openvpn/easy-rsa/openssl-1.0.0.cnf
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
countryName           :PRINTABLE:'gr'
stateOrProvinceName   :PRINTABLE:'Rodope'
localityName          :PRINTABLE:'Komotini'
organizationName      :PRINTABLE:'Aryballos'
commonName            :PRINTABLE:'server'
name                  :PRINTABLE:'server'
emailAddress          :IA5STRING:'admin@aryballos.gr'
Certificate is to be certified until May 13 14:09:17 2026 GMT (3650 days)
Sign the certificate? [y/n]:y

1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated

Copy the certificates to their designated place.

root@gabriella:/etc/openvpn/easy-rsa# cp /etc/openvpn/easy-rsa/keys/{server.crt,server.key,ca.crt} /etc/openvpn

Yes we are ready fire up the openvpn server!

root@gabriella:/etc/openvpn/easy-rsa# service openvpn start
root@gabriella:/etc/openvpn/easy-rsa# service openvpn status
● openvpn.service - OpenVPN service
   Loaded: loaded (/lib/systemd/system/openvpn.service; enabled)
   Active: active (exited) since Sun 2016-05-15 17:13:53 EEST; 4s ago
  Process: 2009 ExecStart=/bin/true (code=exited, status=0/SUCCESS)
 Main PID: 2009 (code=exited, status=0/SUCCESS)

May 15 17:13:53 gabriella systemd[1]: Started OpenVPN service

At this point it would be very wise to switch to your ISP's router or firewall setup page and enable port 1194 forwarding between the external IP address of your router and the internal IP address of the OpenVPN server.

The final step will be to generate keys for the client. OpenVPN will not allow two clients with the same key to log on simultaneously, unless you add a line with duplicate-cn in the server.conf parameter file. Client certificates are build the same way as the server's, only this time you get to use the build-key script

root@gabriella:/etc/openvpn/easy-rsa# ./build-key client
Generating a 2048 bit RSA private key
writing new private key to 'client.key'
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
Country Name (2 letter code) [gr]:
State or Province Name (full name) [Rodope]:
Locality Name (eg, city) [Komotini]:
Organization Name (eg, company) [Aryballos]:
Organizational Unit Name (eg, section) [Aryballos-IT-Services]:
Common Name (eg, your name or your server's hostname) [client]:
Name [server]:
Email Address [admin@aryballos.gr]:

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:
Using configuration from /etc/openvpn/easy-rsa/openssl-1.0.0.cnf
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
countryName           :PRINTABLE:'gr'
stateOrProvinceName   :PRINTABLE:'Rodope'
localityName          :PRINTABLE:'Komotini'
organizationName      :PRINTABLE:'Aryballos'
commonName            :PRINTABLE:'client'
name                  :PRINTABLE:'server'
emailAddress          :IA5STRING:'admin@aryballos.gr'
Certificate is to be certified until May 13 14:31:07 2026 GMT (3650 days)
Sign the certificate? [y/n]:y

1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated

You may create a different key for each of the clients you want to allow access.

Client setup

Each client must have the openvpn package installed. After installation move to the /etc/openvpn folder create a file named client.conf and enter the following:

### Client configuration file for OpenVPN

# Specify that this is a client

# Same setting as on server
script-security 3

# Bridge device setting
dev tap
proto udp

# Host name and port for the server (default port is 1194)
# note: replace with the correct values your server set up
remote  extennal_ip.of.openvpn.server 1194

# Client does not need to bind to a specific local port

user nobody
group nogroup

# Keep trying to resolve the host name of OpenVPN server.
## The windows GUI seems to dislike the following rule. 
##You may need to comment it out.
resolv-retry infinite

# Preserve state across restarts

# SSL/TLS parameters - files created previously
ca /etc/openvpn/ca.crt
cert /etc/openvpn/client.crt
key /etc/openvpn/diaamath/client.key
ns-cert-type server

# Since we specified the tls-auth for server, we need it for the client
# note: 0 = server, 1 = client
# tls-auth /etc/openvpn/diaamath/ta.key 1

# Specify same cipher as server
cipher BF-CBC

# Use compression

# Log verbosity (to help if there are problems)
verb 3

up /etc/openvpn/update-resolv-conf
down /etc/openvpn/update-resolv-conf

Next, copy the filesta.key, ca.crt, client.crt and client.key from the the server to the clients /etc/openvpn. directory.

Disable autostart of the OpenVPN service.

sudo update-rc.d openvpn disable

Reboot the client. (The OpenVPn setup usually fires up the OpenVPN service) And finally connect

sudo openvpn /etc/openvpn.client.conf

That should do it!


This howto has been assembled using various sources. Most important being:

  1. The Ubuntu Community OpenVPN Wiki
  2. The Digital Ocean Tutorail about setting up an Open VPN Server on Debian 9
  3. The OpenVPN howto on the Debian Wiki

Friday, 26 February 2016

Java Swing GUI's default and cancel buttons

None of these are mine. I just copied and re-factored them from here and here in stackoverflow.com.

The code comes from a login form that needs to have it's login button clicked when enter is pressed and to be closed when user hits escape.

Other frameworks like good old VCL and WindowsForms allow this to be done by visually managing properties, but with Java you have to type something like this :

        // set default botton on ENTER

        // set escape to close this dialog
        KeyStroke escapeKeyPressed = KeyStroke.getKeyStroke(KeyEvent.VK_ESCAPE, 0);        
        this.getRootPane().registerKeyboardAction( e -> this.dispose(), escapeKeyPressed, JComponent.WHEN_IN_FOCUSED_WINDOW);


Saturday, 13 June 2015

JPA Queries containing count

A simple howto and reference regarding the usage of the count() function in your JPA code.

First thing is correct syntax. Counting the number of rows is not done using the syntax count(*) but rather count(entity). So a proper named query declaration would be something like :

            name = "PixelStation.pixelStationExists",
            query = "SELECT COUNT(p) FROM PixelStation p WHERE p.pixel = :pixel AND p.station = :station"

Next thing to remember is that the class returned by the query is the Java Long. So getting a value would be like:

        Long count = _entityManager.createNamedQuery("PixelStation.pixelStationExists", Long.class)
                .setParameter("station", station)
                .setParameter("pixel", pixel)

        // record exists no need to do anything
        if (count >= 1)

        // insert new record

Friday, 10 April 2015

IP tables forward traffic between ethernet and wi-fi

Recently, I was asked to create a Linux based WI-fi access point using the new Raspberry Pi 2 Model B. This project had a .. happy ending, thanks to the numerous bloggers of the π community. Here, there and there just to mention a few.

What I wish to keep as a reference to this blog post is the iptables setup that allowed the device to forward IP v4 traffic between the Ethernet and the Wi-fi ports.

To get started create a file like /etc/iptables.ip-v4.nat containing the following:

# Generated by iptables-save v1.4.14 on Mon Mar 23 18:48:53 2015
:INPUT ACCEPT [121:10892]
-A FORWARD -i eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i wlan0 -o eth0 -j ACCEPT
# Completed on Mon Mar 23 18:48:53 2015
# Generated by iptables-save v1.4.14 on Mon Mar 23 18:48:53 2015
:INPUT ACCEPT [20:2899]
# Completed on Mon Mar 23 18:48:53 2015

Next, edit /etc/network/interfaces and place the following after the wi-fi setup at the very end:

# Configure firewall to allow traffic between wlan0 and eth0
up iptables-restore < /etc/iptables.ip-v4.nat

Wednesday, 18 March 2015

MySQL Cross database foregin keys

Here is a small query to display all foreign keys across different databases. May come in very handy when deciding which database to restore first :)

USE information_schema;


Thursday, 12 March 2015

Last modified date/time for CakePHP projects

What is the last modification date of a CakePHP project? To answer that you would probably have to ask what is the latest date and time that any file was modified in your CakePHP project sub-folders. These sub-folders are usually Model, View, Controller, webroot and perhaps Vendor or Console.

In order to solve this in Linux, I crafted the following bash script:


BASE_PATH="/var/www/html/MyProject/app"           # replace with your own
SUB_COMPONENTS="Model View Controller webroot"    # add folders as needed

  RESULT=`find $SUB_PATH -type f -exec stat --format '%z' "{}" \; | sort -r | head --bytes=16`
  echo $RESULT
 done | sort -r | head --lines=1 )

exit 0

the idea is to get all files in each of the paths, contained in the $SUB_COMPONENTS variable, using the find command. Then execute stat to retrieve the last modification datetime, sort the results in reverse order and finally use head to retrieve the first 16 characters which make up the first YYYY-mm-dd HH:ii characters of the timestamp.

The entire result of the for loop is then fed to sort once more to create the list with the highest time stamp being first. Finally we use head again to throw away all output lines apart from the first.

An example of using this in CakePHP controller code would be something like:

    const LAST_MODIFIED_COMMAND = '/opt/bin/cakeLastModified';


    public function about()
        $this->set('lastModified', exec(self::LAST_MODIFIED_COMMAND));

... and then display the value of the $lastModified variable in the corresponding about view.

Note: Once again, I would like to give credit to the good people at stackoverflow.com for providing answers to all the minor problems that made this one up. So this code would not be here if l0b0 had not provided such a concise and clear answer to Sorting in shell script and if How to recursively find and list the latest modified files in a directory with subdirectories and times? did not have a working answer.

Saturday, 14 February 2015

Java: Read String from console

Once again, I was out there trying to find the neatest solution for reading a string from the IDE console. It turns out that the shortest piece of code that one can write to achieve this would be :

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;


     public static String getString() throws IOException
        try (BufferedReader br = new BufferedReader(new InputStreamReader(System.in))) {;
            return br.readLine();

Wednesday, 15 October 2014

Listbox mnipulation with jQuery

This is my easy to paste reference code for list-box manipulation using jQuery. By the way, the HTML <input "type"="select"> element, will create what desktop programmers usually refer as a combo-box. To make the input display as a list box, we need to specify an additional size="N" attribute where N refers to the number of visible items of the list.

So, suppose that we have a select input somewhere in an HTML page and that we need to add a button that removes the selected item from the input's options list.

       // remove the selected item from the list box
            jQuery("#myListBox option:selected").remove();
            return false;

By the same token, we can also create an other button that empties the entire options list

        // remove all items from list box
            // check that there is something to remove
            if (jQuery("#myListBox option").length == 0)
                return false;

            // verify that user really wants to do this
            if (!confirm(''))
                return false;

            // empty list box
            jQuery("#myListBox option").remove();
            return false;

Finally, to add a new item at the end of the options box we can use a code fragment that looks more or less like this:

                    var newOptionhtml ='<option value="'
                        + response.id
                        + '">'
                        + response.name
                        + ' </option>';


Needles to say that all the above have been collected from various Stack Overflow questions and answers.

Saturday, 13 September 2014

GNOME3: How to enable desktop icons and right click menu

This is the third time I am looking this up, so I am making a quick howto note here.

So in order to enable desktop icons on a newly installed gnome3 desktop perform the following steps:

  1. Open the dconf-editor program.
  2. Locate the page following path org → gnome → desktop → background
  3. Check the Show desktop icons option.

Monday, 14 July 2014

CakePHP Find out the link that brough us to the current page

Most web applications today have some kind of menu build with anchor tags. The problem that the following script -- courtesy of my friend Anestis from alfasoftware.gr -- solves, is how to determine the actual link that was used to bring the user to the currently displayed page.

To use it, place it inside a script tag towards the end of your app/Views/Layout/default.ctp file (just before the closing body tag, will do just fine) and a little bit of CSS code in your appropriate style file (again app/webroot/css/cake.generic.css is a good candidate).

The CSS code is pretty easy :

a.active {
    background-color: yellow; // add anything you like

and the javascript :

        <script type="text/javascript">
            // add the "active" class to the navigation link that brought us to this page
            jQuery( function() {
                // retrieve the relative url of the current page
                var curUrl = "<?php echo (Router::url( NULL, FALSE)); ?>";
                // in a cake application a relative URL is usually like
                // /application/controller/action/param1/param2 ...
                // so what we really need is the first three pieses of the URL
                var tokens = curUrl.split("/").slice(0,4);
                curUrl = tokens.join("/");
                // if there is any pagination information then the word "index" also appears in the URL
                // in this case we need to remove it, so it can match generated URL
                var indexIndex = curUrl.indexOf('index');
                if ( indexIndex > -1)
                    curUrl = curUrl.substring( 0, indexIndex - 1);

                // for each page anchor tag
                $('a').each( function() {
                    // retrieve the arnchor's target
                    var ref = $(this).attr("href");
                    // so if that anchor points to the current page
                    if(ref === curUrl){



Sunday, 18 May 2014

How to remove a particular host key from SSH's ./ssh/known_hosts file

I will leave the sed solution for the real gurus. As far as I am concerned the easy way to get the job down is use the ssh-keygen -R your_host command.

If you are working in a environment with a DNS suffix, then you will probably need a second invocation like: ssh-keygen -R your_host.your_domain.

Friday, 4 April 2014

Setting up Environment variables before Apache starts

That was the third time I had to look this up, so I am putting it down for reference.

On Red Hat machines, apachectl sources /etc/sysconfig/httpd, so any variables you set there will be available to the apache runtime environment. Debian based systems use /etc/apache2/envvars for the same purpose.

Thursday, 3 April 2014

Linux (EL6) Required packages for installing the 64 bit Oracle 12c Client

The Oracle documentation has a list of the packages required in order to install the database client software for Enterprise Linux 6.

Since the minimum versions requirements are satisfied on an updated EL6 system, all one has to do is just copy and paste the names into a single yum command in order to complete the step. Did it once this morning and I hope that if I ever have to repeat it, I will just copy and paste the following:

yum -y install binutils compat-libcap1 compat-libstdc++-33.i686 compat-libstdc++-33.x86_64 \
gcc gcc-c++ glibc.i686 glibc.x86_64 glibc-devel.x86_64 glibc-devel.i686 \
ksh libgcc.i686 libgcc.x86_64 \
libstdc++.x86_64 libstdc++.i686 libstdc++-devel.x86_64 libstdc++-devel.i686 \
libaio.x86_64 libaio.i686 libaio-devel.x86_64 libaio-devel.i686 \
libXext.x86_64 libXext.i686 libXtst.x86_64 libXtst.i686 \
libX11.x86_64 libX11.i686 \
libXau.x86_64 libXau.i686 \
libxcb.x86_64 libxcb.i686 \
libXi.x86_64 libXi.i686 \
make sysstat

PS: If not running this as root, remember to prefix this with sudo :)

Thursday, 20 March 2014

CakePHP-2 and AJAX The dependent list boxes problem (remake)


Back in 2010, I had written a post regarding the case of dependent list boxes in a CakePHP view. Back at those days CakePHP was at version 1.2 and support for Javascript and AJAX was very limited. Today I shall revise this using JQuery and JSON encoding which will make things simpler and easier to understand, implement and maintain.

The long story short

Suppose you have a page with two list boxes. One contains a standard set of values while the second one's list of values must be dynamically updated depending on the actual selected value of the first.

In our (sort of) real life example we have a list of commissions (aka production orders) that produce a series of products of varying lengths. The list of lengths that each commission is allowed to produce is available via a detail table and our goal is write an addProduct action and view that allows the user to specify the commission based on which the actual product was produced and the actual length of the product that should be one of the assigned commission lengths. The Product model has a commission_id and an actual_length fields. So each time the commission combo box changes the actual length field input options should also change in order to contain the commissions list of allowed lengths.

Getting Started

To begin with our tutorial make sure that your standard layout references the jQuery library. The easiest was to do this would be to open APP/View/Layout/default.ctp and make sure that a line like

<script src="http://code.jquery.com/jquery-1.10.1.min.js"></script>

can be found somewhere in your html's head section.

The next thing that needs to be done is to add the RequestHandler component in our AppController. Add or modify your existing source so it looks more or less like this.

class AppController extends Controller {
    public $components = array(


Next we need to inform our routing system that it should also parse json URLs. So the line


should be added to the APP/Config/routes.php file right below the other Router::xxx() commands.

Create the AJAX method and view to return the JSON encoded list of lengths

The next thing to do is create the action that will return the list of lengths to be sent back to the view given commission id. The best place to put it will be the commissions controller. The code for the function looks a lot like the one baked by view. We just need to make sure that the data returned "contain" the correct detail information

    public function getRequestedLengths($id)
        if (!$this->Commission->exists($id)) 
            throw new NotFoundException(__('Invalid record'));
        $options = array(
            'conditions' => array(
                'Commission.' . $this->Commission->primaryKey => $id
            'contain' => array(
        $this->set('commission', $this->Commission->find('first', $options));

The view for the method should be placed in: APP/View/Commissions/json/get_requested_lengths.ctp.

 * Create and echo a json encoded list of the allowed lengths
$output = array();
if (!empty($commission['CommissionProduct']))
    foreach ($commission['CommissionProduct'] as $commissionProduct)
        $output[] = $commissionProduct['actual_length'];

echo json_encode($output);

So now if we point our browser towards http://yourserver/projectPath/Commissions/getRequestedLengths/7.json, we will receive a response with a JSON array containing all allowed lengths for commission id 7. The actual response will be something like ["45900","23400"].

Building the addProduct view

So far we have the mechanism to retrieve the required data. The final step will be to use it in the actual add product view, the PHP part of which should look more or less like this:

<div class="products form">
    <?php echo $this->Form->create('Product'); ?>
            <legend>Add Product</legend>
            <?php echo $this->Form->input('commission_id', array('empty' => __('Please select a commission'))); ?>
            <?php echo $this->Form->input('operator_id'); ?>
            <?php echo $this->Form->input('shift'); ?>
                echo $this->Form->input(
                    'type' => 'select',
                    'options' => $statusList
            <?php echo $this->Form->input('actual_length', array('type' => 'select')); ?>
            <?php echo $this->Form->input('gross_weight'); ?>
    <?php echo $this->Form->end('Save'); ?>

The final part will be the adding of Javascript code to make our form responsive.

<script type="text/javascript">
    var commissionsCombo;
    var allowedLengthsCombo;

    jQuery(function() {
        commissionsCombo = jQuery('#ProductCommissionId');
        allowedLengthsCombo = jQuery('#ProductActualLength');

        commissionsCombo.change( function() {
            var selectedCommission = this.value;  // or $(this).val()
            // build the url that contained the selected commission code
            var ajaxUrl =
                    '<?php echo Router::url(array('controller' => 'Commissions', 'action' => 'getRequestedLengths', 'admin' => FALSE), TRUE)?>'
                    + '/'
                    + selectedCommission
                    + '.json';

            // do a "synchronous" AJAX call
                    async: false,
                    cache: false,
                    url: ajaxUrl,
                    success: function(response) {
                        // remove all options from the allowed metres per bobbin

                        // add all returned values
                        for (var i = 0; i < response.length; i++) {
                            var currentKey = response[i];
                            var currentKeyDescr = response[i] + ' metres';
                            var optionText = '<option value="'
                                    + currentKey
                                    + '">'
                                    + currentKeyDescr
                                    + '</option>';

As a last statement The code for manipulating the select input options comes from the very consise post from StackOverflow.com

Thursday, 13 March 2014

CentOS 6.5 does not like my handware or their combionation

Spent the whole morning trying to install CentOS 6.5 on a relatively new machine using a Gigabyte GA-B85-HD3 motherboard, with a NVIDIA GeForce GT 620. The message I got was that the hardware (or a combination of what) I 'm using is incompatible with CentOS.

As I had Fedora 20 already running on this very machine, I figured that the problem was related to UEFI and wasted a lot of time trying to the get rid of the EFI partition on the Linux disk. Eventually after updating the BIOS and trying all sorts of magic. I disabled the on board Intel graphics adapter and everything worked like a charm.

Friday, 31 January 2014

CakePHP 2.x: Saving paging data in the session

Quite some time ago I wrote a blog post about saving CakePHP 1.x paging data in the session so that they can be available at next page visit. The basic idea was that you could store the page, sort and direction named parameters in the session and restore them back when no paging parameters were available.

When I tried applying the same technique in CakePHP 2.x, I run into a very serious obstacle and that was the fact that the Paginator::numbers() function does not anymore include the page:1 named parameter when creating the link for the first (or previous) page. This created a phenomenon that when someone visited a page without any paging parameters, it was impossible to know whether they were there because of a link to the first page or as a result of a redirect from somewhere else, in which case the session had to be checked and paging data to be restored.

The thing had been puzzling me for some days now. I tried different remedies without any luck or results until I cried for help at stackoverflow.com. It was there that Ilie Pandia shook things a bit and then I managed, thanks to his advice, to create the following component that mimics the original Cake 1.x PaginationRecallComponent by mattc found in the Bakery, from which I borrowed the name and structure.

This version is new (April-2015) and hopefully includes the bug-fixes pointed out by all comments. The tests where made using CakePHP version 2.6.3.

App::uses('Component', 'Controller');

 * Pagination Recall CakePHP Component
 * CakePHP 2.x version. Thanassis Bakalidis abakalidis.blogspot.com=
 * @author  Thanassis Bakalidis
 * @version  2.2
 * @license  MIT
 * @property SessionComponent $Sesion Session handler to save paging data into
class PaginationRecallComponent extends Component {
    const PREV_DATA_KEY = 'Paginaion-PrevData';

    public $components = ['Session'];
    private $_controller = NULL;
    private $_action = NULL;
    private $_previousUrl;

    public function initialize(\Controller $controller)
        $this->_controller = $controller;
        $this->_action = $controller->params['action'];

    public function startup(Controller $controller)
        if ($this->_controller->name === 'CakeError')


        // save the current controller and action for the next time
                'controller' => $this->_controller->name,
                'action' => $this->_action

    private function _restorePagingParams()
        $sessionKey = "Pagination.{$this->_controller->name}.{$this->_action}";

        // extract paging data from the request parameters
        $pagingParams = $this->_extractPagingParams();

        // if paging data exist write them in the session
        if (!empty($pagingParams)) {
            $this->Session->write( $sessionKey, $pagingParams);

        // no paging data.
        // construct the previous URL
        $this->_previousUrl = $this->Session->check(self::PREV_DATA_KEY)
            ? $this->Session->read(self::PREV_DATA_KEY)
            : [
                'controller' => '',
                'action' => ''

        // and check if the current page is the same as the previous
        if ($this->_previousUrl['controller'] === $this->_controller->name &&
            $this->_previousUrl['action'] === $this->_action) {
            // in this case we have a link from our own paging::numbers() function
            // to move to page 1 pf the current page, delete any paging data

        // we are comming from a different page so if we have any session data
        if ($this->Session->check($sessionKey))
            // then restore and use them
            $this->_controller->request->params['named'] = array_merge(

    private function _extractPagingParams()
        $pagingParams = $this->_controller->request->params['named'];
        $vars = ['page', 'sort', 'direction'];
        $keys = array_keys($pagingParams);
        $count = count($keys);

        for ($i = 0; $i < $count; $i++)
            if (!in_array($keys[$i], $vars))

        return $pagingParams;

Note: It turns out that the components shutdown method is not called when the owner controller's action returns a redirect(). Hence, this last update was about getting rid of any shutdown() functionality and performing everything in the components startup() moethod code. This hopefully, fixes the bug of loosing paging data after a call to the delete() method which returns an immediate redirect..

Sunday, 12 January 2014

Enable MP3 on Fedora 20

A very short how to enable mp3 playback on your brand new fedora installation.

  1. Enable PRM Fusion repository.
  2. Install the vlc phonon-backend. (Fedora comes with GStreamer).
    . yum install phonon-backend-vlc
  3. Goto system settings → Multimedia → Audio and Video Settings. Select the backend tab. There should be two entries there: GStreamer and VLC. Select the VLC entry and click the Prefer buttom below the list. Finally click Apply. The final result should be something like this:
  4. Log out and back in for the changes to take effect.

After that you can play and kind of context directly from Dolphin, you can install additional music players like amarok, or vlc and enjoy the full music, audio and video experence.

yum install vlc
yum install amarok amarok-doc

Thursday, 7 November 2013

CakePHP New application database config (never forget the enconding)

Today I will put down the way to begin a new web application project using CakePHP.

The first part is database creation. Database charset must be utf-8 and collation: utf8_unicode_ci. The difference between general and unicode collations is explained in an excellent manner in this post on stackoverlow.com

Next, when you start a new CakePHP project the database config class looks like this

public $default = array(
  'datasource' => 'Database/Mysql',
  'persistent' => false,
  'host' => 'localhost',
  'login' => 'user',
  'password' => 'password',
  'database' => 'test_database_name',
  'prefix' => '',
  // 'encoding' => 'utf8',

My lesson today is: Before changing anything else, uncomment the last line!.

... in case you do not then Unicode (Greek in my case) text will still appear correctly in the web application, but phpMyAdmin and sqldump will display garbage. In addition searches with non-latin text will always fail. To make matters worse when I discovered the case of the problem, I realised that I would have to re-enter all my test data.

Friday, 1 November 2013

SchemaSync: An effective and simple way to synchronize MySQL databases

I have spend the last couple of days trying to find out a solution for migrating changes from a development database to the productive. I was looking for a clean, simple and reliable solution that (ideally) runs on Linux.

My first attempt was to to use the official MySQL mysqldiff command. It turned out that in my case, the only suggestion I got was to recreate indexes and drop primary keys every time there was a record number mismatch between tables. I also run into Windows GUI's with a 30 day evaluation period and a cost ranging from $90-$150.

My lack changes when I run into Schema Sync written in Python by Mitch Matuson. The page has everything you need and the setup (if you already have python) takes less that a minute.

Quoting from the utility's examples page, Sync the production db with the changes from the development db

 schemasync mysql://user:pass@dev-host:3306/dev_db mysql://user:pass@prod-host:3306/production_db

This will give you two files in your home directory, one to do the patching and one to revert back the changes if requested. A excellent piece of work, that I am very happy to use. Thumbs up for mr Matuson.