Configure reverse ssh tunneling

When you have a need to access your local PC from the internet, you might be interested in this blog.

Requirement:
1. Root access to a public server (or any kind of access as long as you can do things below :).
2. SSH client


Now, let's begin with the server:
1. Edit sshd_config located in /etc/ssh/ and add the line below
GatewayPorts clientspecified

2. Restart sshd
sudo service ssh restart

That's all for the server.


To the local PC we go:
1. Create reverse ssh tunneling using ssh client.
Open a terminal/console to run the ssh client. In windows you might use Putty.
ssh user@server -R server:serverport:localdestination:localport

2. Test the connection by using telnet
telnet server serverport
Expect no connection refused :)


The steps above is very simple but requires some configuration changes in the public server.

There's a more secure way but requires more effort on the client side which means only geek will be able to access it :) I'll discuss it later on a separate blog.

Setting up vnc for ubuntu desktop

If your installation is ubuntu desktop and you need to remote it using vnc, the steps below might help you to do it:
1. Update your package list
sudo apt-get update

2. Install Gnome desktop environment essential component and Gnome session manager
sudo apt-get install gnome-core gnome-session-fallback

3. Install vnc server
sudo apt-get install vnc4server

4. Start the vnc server
vncserver 
Note: find out the console port being assigned to you. There is a way to start up the connection with a pre defined port number (e.g. vncserver :1)

Example: the assigned port is 1 (5901)

New 'server:1 (server)' desktop is server:1

Starting applications specified in /home/user/.vnc/xstartup

Log file is /home/user/.vnc/user:1.log

5. Edit the xstartup file located /home/user/.vnc/ as below.

At first, when you open the vnc connection there will be only a terminal. You will need to start the gnome-session for the vnc connection.

#!/bin/sh

# Uncomment the following two lines for normal desktop:

 unset SESSION_MANAGER

#exec /etc/X11/xinit/xinitrc

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup

[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources

xsetroot -solid grey

vncconfig -iconic &

#x-terminal-emulator -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &

#x-window-manager &

gnome-session --session=gnome-classic &

6. Restart the vncserver by killing it first then starting it up again

vncserver -kill :1

To access vnc from the client, you need to install vnc viewer app. 

The sample url for vnc: server_url:5901

the port number is assigned when you start the vnc connection.

Up to now, for ubuntu server I use Xfce desktop environment instead of gnome. It is lightweight and somehow I still can't setup the gnome env under vnc :)

Reference link:
http://coddswallop.wordpress.com/2012/05/09/ubuntu-12-04-precise-pangolin-complete-vnc-server-setup/

Configuring php for handling large file upload

Cut it short, what I need is to configure my nginx-php to be able to handle around 200M file upload.
So here are the configuration changes I need to make:
1. nginx.conf
    usually the file is placed in /etc/nginx/nginx.conf.
    The parameters to add inside http are:
    - client_max_body_size: specifies the maximum accepted body size of a client request
    - client_body_timeout: sets the read timeout for the request body from client
    http {

        ##
        # Basic Settings
        ##

        client_max_body_size 200m;
        client_body_timeout 600s;
       
        ....
   }

2. php.ini
    Since I use php5-fpm, this file is placed in /etc/php5/fpm/php.ini
    The parameters to change is:
    - post_max_size: Sets max size of post data allowed
    - upload_max_filesize: The maximum size of an uploaded file
    - max_execution_time:This sets the maximum time in seconds a script is allowed to run before it is terminated by the parser.
    - max_input_time:  This sets the maximum time in seconds a script is allowed to parse input data, like POST and GET.


After changing those variables, restart both nginx and php5-fpm and you're done!

Reference links:
http://wiki.processmaker.com/index.php/Nginx_and_PHP-FPM_Installation
https://rtcamp.com/tutorials/php/increase-file-upload-size-limit/
http://www.radinks.com/upload/config.php
http://wiki.nginx.org/HttpCoreModule#client_body_timeout

Struts2 in Websphere 6.1

Just when someone asked you if Java really delivers what it promises, especially about compile once run anywhere. I just deployed a webapp developed using Struts 2, Spring, Hibernate in Websphere 6.1. And as expected, the deployment failed. After a few googling around and friend's advice, it turns out that I have to configure some properties for the server.

Here are the properties:

The new properties added are the last two, and here is a bit explanation on the property:
- com.ibm.ws.webcontainer.invokefilterscompatibility = true
 Quoted from ibm docs (see here)
With this custom property, the Web container calls custom servlet filters before looking for welcome files. Also, if the Web container cannot find a resource, it calls the custom servlet filters before creating a FileNotFoundException exception. This change enables the Web container to verify whether the custom servlet filters modify the path to a resource. 
In short, this property enables custom servlet filter to process http request and Struts 2 utilizes Servlet Filter.

- com.ibm.ws.webcontainer.assumefiltersuccessonsecurityerror = true
Quoted from ibm docs (see here)
When a request is received for a static file which does not exist, the web container calls defined servlet filters. If the filters do no successfully complete, a 404 error code is set. In a situation where application security is enabled, a security check is performed as part of filter invocation. Typically if the security check fails the web container considers the filters to have failed and still sets a 404 error code instead of the 401 error code that indicates the failure of a security check. The 404 error code enables the requester to access the static file without logging on.
 In short, just to be more secure this property is set to true. Anyway, this property doesn't have any thing to do with Struts 2.


To make sure everything works, don't forget to restart the server.

VirtualBox misc

A note for myself on my dealings with VirtualBox, refer to this link for complete command:

• Changing the virtual harddisk UUID (.vdi)
  VBoxManage internalcommands sethduuid /VirtualMachine/virtualmachine.vdi

• Resizing the virtual harddisk
  VBoxManage modifyhd virtualmachine.vdi --resize SIZE_MB

• Disable time synchronization
  VBoxManage setextradata LHBU "VBoxInternal/Devices/VMMDev/0/Config/GetHostTimeDisabled" "1"   

          You need to restart your vbox instance if it is running when you execute the command.
          Unrecommended workaround is:
          Guest Windows: go to Services, find VirtualBox Guest Additions service and stop it. You will find that time synch is no longer working, but I'm not sure whether other functionalities regarding virtual box - host integration are affected or no.
          Guest Linux:  /etc/init.d/vboxadd-timesync stop
          Disclaimer! Never tried the linux version, found it during googling.

Tracking Hibernate SQL query and parameters

Every developer using Hibernate might need to know the SQL it generates and the parameters being sent. Normally to have the sql printed out to console, one only have to configure hibernate property show_sql to true. However the parameters are not printed out.

Luckily for me, I found this post on configuring the necessary log level to be able to print out the sql and parameters. Here are the configuration that I used:

org.hibernate.SQL=DEBUG

org.hibernate.type=TRACE 

Expiring remote desktop certificate

Recently each time I need to connect remotely to one of the servers in my office, I always get certificate expired kind of error. As my curiosity drove me and based on the keyword "expired", I went to the server and accessed the console to check the server's date. It turned out that somehow the date and time of the server went back to 2002 (hey, I write this on 2011 :).
At first, I thought a simple update of the time on the server will fix this. Unfortunately the result was still the same. Googling around and found this link which helped me fixing the issue. Below are the steps which I executed on my computer along with some screenshots:

1. Open mmc.exe

2. On the file menu, click Add/remove snap-in
3. Click certificates, and then click add

4. Choose computer account and click Next

5. On the following dialog, as in my case, I choose another computer and put in the server name. Click Finish to continue

5. Back on the main window, expand the certificate node and choose Remote Desktop certificates.

6. On the middle section, the certificate which expired is displayed. Just delete it by right clicking and choose delete.

In short, we need to *renew* the certificate simply by deleting it and let windows recreate it later.

In case you cannot do it from your computer, you can always go to the server's console and do it there :) .... if you have the permission.....

Oracle query to find out table basic information

Query below helped me to know the current status of my tables and their places in the oracle db. So that I could further decide whether I should move it or not, notwithstanding that I must understand the undergoing code accessing those tables.

select tbl.OWNER || '.' || tbl.TABLE_NAME table_name, tbl.TABLESPACE_NAME, trunc(ds.BYTES/1024/1024) || ' Mb' table_size, tbl.NUM_ROWS, tbl.LAST_ANALYZED from SYS.ALL_TABLES tbl
join SYS.dba_segments ds on ds.SEGMENT_NAME = tbl.TABLE_NAME
where ds.SEGMENT_TYPE = 'TABLE'

The output of the query will have:
- Table name
- Tablespace where the table is stored
- The size of the table in Mb
- Number of rows
- Last analyzed

The above query could be expanded further just as needed.

How to configure the way BeanUtils converts null value

By default BeanUtils.setProperty always assign a default value for null. For an instance, if the property's type being set is Integer then you will get 0. This could be the behavior that many do not expect, for example you use Struts and you have an entry form with a number field. And you're expecting that if the user leave it empty, you would like to display it as empty later on.
BeanUtils utilized by Struts will fill the field as 0 instead of leaving it null.

Underneath BeanUtils is utilizing a ConverterUtils to convert the value before setting it to the property. Therefore to change this behavior all we need to do is to register a new IntegerConverter to ConverterUtils, thereby overriding the default converter used.

e.g.
ConvertUtils.register(new IntegerConverter(null), Integer.class);

The same thing you should do for other types as needed.

If you indeed configure the converter, you might want to read this.

Understanding seam's persistence context configuration

Actually this post is a note for myself regarding how seam could be configured in the web application. The facts are:
- The application is running on JBoss 5.1
- Seam is used to manage the lifecycle of EntityManager instances

Here's the configuration:

1. Datasource configuration file placed in {server}/deploy
   JNDI name is Datasource
2. Persistence unit configuration in persistence.xml
   <persistence-unit name="PersistenceUnit" transaction-type="JTA">
   <provider>org.hibernate.ejb.HibernatePersistence</provider>
   <jta-data-source>Datasource</jta-data-source>
   ......
   <properties>
   ........
   <property name="jboss.entity.manager.factory.jndi.name" value="java:/EntityManagerFactory"/>
   </properties>
   
   
   
3. Declaration of persistence-unit-ref element in web.xml
   <persistence-unit-ref>
   <persistence-unit-ref-name>PersistenceUnit/pu</persistence-unit-ref-name>
   <persistence-unit-name>PersistenceUnit</persistence-unit-name>
   </persistence-unit-ref>
   
4. Configuration in component.properties
   seamBootstrapsPu=false
   seamEmfRef=#{null}
   puJndiName=java:comp/env/PersistenceUnit/pu
5. Configuration of persistence managed context in components.xml
   <persistence:managed-persistence-context name="entityManager" auto-create="true"
   entity-manager-factory="@seamEmfRef@"
   persistence-unit-jndi-name="@puJndiName@"/>
   
   

Here's the short explanation:

1. A new datasource is deployed (step 1)
2. The datasource is referred to in persistence.xml (step 2)
3. Add reference to the new persistence unit name in web.xml (step 3)
4. Add new property in component.properties which refers to the persistence-unit-ref-name declared in web.xml (step 4)
5. The values configured in component.properties is used to configure the managed-persistence-context (step 5)

Note: The entity manager factory is bound to the global JNDI thru "jboss.entity.manager.factory.jndi.name" property configured during step 2, however it is not used... yet :)

Beware of customizing BeanUtils converter

If you ever have to customize BeanUtils converter just like I did, one thing that should be of your concern and that is BeanUtils is also utilized by many open source libraries. And some of the libraries might happened to have the same need as yours to customize the converters. If in any case you are using one of those libraries, it just might override your converters.

In my case, I was using jXLS . It is a great tool and it's my fault that I didn't read the documentation thoroughly. The documentation says:

"jXLS integrates with Jakarta BeanUtils conversion utilities to perform actual conversion from Excel cell values into bean properties...... BeanUtils Converters for primitive types return a default value when a conversion error occurs. jXLS overrides this behaviour in ReaderConfig class registering these classes to throw a ConversionException."

And what I configured for my application is for BeanUtils to return null for empty string instead of the default value.

Now, here's a short story on what truly happened to my application due to the conflict described above:
At first, an empty text field (for number input) in the form is converted to null and after some operations performed on the application "suddenly" it throws NumberFormatException for empty text field in the form. I'm lucky to know the steps to reproduce it and it turns out that things started to act strange after jXLS is utilized (to create/read excel file).

At this point, I'm grateful to have jXLS and BeanUtils source code. I could just debug into the source code and analyze it. And I found out that jXLS overrides BeanUtils's converter configuration as the documentation said. Now all I can do is after every invocation of jXLS I have to reconfigure BeanUtils as my application need it.

One more thing still disturb me, how to handle concurrent processes where one process is still working on jXLS and the other one need to convert the value from the form. And both are utilizing BeanUtils. I guessed at this moment I have to accept that I will get NumberFormatException and tell the user to try to submit the form again (and hoping that the jXLS process is already finished).

Setting up usb modem for ubuntu 10.10

If you want to setup the drivers for your usb modem, firstly check out this link. It shows the list of 3G hardware and the test result along with the necessary information.
In my case, I have Option Wireless iCON 225 (HSDPA) and I'm quite lucky to have mine listed as working and there's enough information for preparing the drivers.

Here's the summary of how I prepare the drivers:
1. Download and compile ozerocdoff --> udev.tar.gz (http://www.pharscape.org/ozerocdoff.html)
2. Skip installing hso driver kernel module since my kernel version is 2.6.35 (which is above or equal 2.6.31) --> http://www.pharscape.org/hso.html
3. Install HSOConnect (http://www.pharscape.org/hsoconnect.html) which requires hsolinkcontrol (http://www.peck.org.uk/hsolinkcontrol.html). And it turns out it just didn't work for me and I opt to use the default Network Manager.
4. Check if all works fine:
- Make sure hso module is already installed
find /lib/modules/`uname -r` -name 'hso.ko'
- Plug in the usb modem device and execute command below
lsmod | grep hso
If you see that hso is listed that's a good sign for you
- Check if the ports have been created
ls /dev/ttyHS*
My output is: /dev/ttyHS0 /dev/ttyHS1 /dev/ttyHS2
If you can see the output that also means ozerocdoff is already working
5. Go to my Network Manager, edit connections and add a new Mobile broadband connection.

6. Go online with my new broadband connection :)


References:

1. Network Manager - Hardware 3G (https://wiki.ubuntu.com/NetworkManager/Hardware/3G)
2. PHARscape iCon 255 (http://www.pharscape.org/icon225.html)
3. ozerocdoff (http://www.pharscape.org/forum/index.php?topic=545.0)
4. HSOConnect (http://www.pharscape.org/forum/index.php/topic,743.0.html)
5. Using Network Manager (http://www.pharscape.org/networkmanager-0.7.0-and-3g-wwan-modems.html)
   

Aspire 4741z battery is not charging

If a battery is not charging then the cause is either the battery is broken or the charging module needs replacement or could be BIOS problem. In my case the cause is neither above, apparently after taking out my battery I just realized that somehow I left the lock switch to unlock position. And indeed that's the cause of the battery is not charging.

Configuring mic in aspire 4741z - ubuntu 10.10

Luckily for me, my installed alsa is already the latest one. All that is needed is just to add 2 lines below to /etc/modprobe.d/alsa-base.conf

alias snd-card-0 snd-hda-intel
options snd-hda-intel model=acer


My mic works now but it seems a bit quiet. Hopefully I can find the way to boost the mic.

A nice tool that might help is alsamixer, run it from a terminal.

Ubuntu 10.10 brightness problem in Acer Aspire 4741

To those who have problem with brightness configuration after installing Ubuntu 10.10 and you are using Acer Aspire 4741, here's a solution for you:

1. sudo gedit /etc/default/grub
2. Change the line GRUB_CMDLINE_LINUX="" into
   GRUB_CMDLINE_LINUX="acpi_osi=Linux"
3. sudo update-grub
4. Restart your linux
   

Hopefully it helps.

Spring security core build path problem

First time trying Grails 1.3.5, creating a new test project. Installing spring-security-core plugin (following the tutorial) and then importing the project to STS 2.5M1 and letting STS know that it's a Grail project (Configure->Convert to a Grails project... something like this). I was hoping for an easy ride here, expecting no errors since I haven't add anything, all are auto generated.

But as usual, with programming world, there ain't no such things as a bug/error free codes. I got compile errors like this in my LoginController.groovy.



I know that I'm missing org.springframework.security.core-3.0.3.RELEASE.jar release in this case and the shortcut to solve this is just to add the jar to the project build path. But, is this going to happen for each plugin jar that I'm going to use? and anyway I don't like the solution to put the jar manually on the build path, it does look an ugly solution.

Fortunately I find a menu to refresh Grails' dependencies and the good news is it works for me, no more compile errors. Here's the context menu that pops up when I right click on my project:



I think this is caused by me adding the plugin from command prompt and then importing the project in STS. Imho, if the plugin is added thru STS' grails command prompt it should works without refreshing.

Hmm... just remember that this is similar to when I add a new file to my project using my file manager. Eclipse can't see the file, until I refresh the project.

Using hibernate generated collection in h:dataTable

It's a bit difficult to pick the correct title for the thing that I'm going to write :) It's about having your POJO generated by hibernate and then feed it to h:dataTable.

When your object has a collection properties and the elements contained are unique within the collection, hibernate will create a Set to contains it. The thing with a Set is h:dataTable just can't accept it. The simplest solution is to change Set to Collection object and h:dataTable can accept it and Hibernate won't complain about it :)

Rolling back transaction (Seam managed transaction)

JSF+Seam is what I'm learning at the moment. And the case is I need to rollback the transaction due to business exception but I don't want to not catch the transaction (when a runtime exception is uncaught the transaction will be rollback automatically). The business exception here is a new exception class extending RuntimeException.

At first try, I add ApplicationException annotation with rollback = true in my BusinessException class. The controller class which invokes the business service class and under certain condition an exception of BusinessException type is thrown. In my scenario my transaction should be rolled back, the thing is... it was not rolled back.

This forced me to dive into seam's code (that's why I love open source) and find out how to handle the transaction. And I came into Transaction class, which I can use to do what I want.
Here's how to use it to mark the transaction as rollback:

Transaction.instance().rollback();

Actually all I wish is when the exception is thrown by the service class (already registered to seam) the transaction will be rolled back automatically, unfortunately for me it doesn't work :(

Entity inheritance-single table per class hierarchy strategy with JPA + Hibernate

I happened to have the chance to implement entity inheritance using single table per class hierarchy strategy. At first I though I would sail smooth and arrive at the promised land easily :) but as usual with programming, it is a rare thing a code would sail smooth at the first time.
Persisting entity works like charm, retrieving it makes me speechless. Here's an example:

The base class (ItemReference)


Subclass#1 (ItemReferenceAge)


Subclass#2 (ItemReferenceSex)


DDL


One class that I don't include here is the class which has a collection of ItemReferenceSex and a collection of ItemReferenceAge, let's call this class Wrapper.

The scenario is I add 2 instances of ItemReferenceAge to wrapper collection and persisted it. What happened when I try to load the same wrapper instance is both collections are having elements in them. Remember, I only add ItemReferenceAge instance which means the collection of ItemReferenceSex should be empty.
Looking at the query generated shows that the discriminator column is not used in the where clause of the select query. What I can think of the design reasoning is that it is assumed that there will not be two or more type of instances loaded in the same parent object which in my case it is.
Luckily for me, there's a solution for this and unfortunately it's hibernate specific annotation (is it maybe because of JPA doesn't state how to handle my case or is it hibernate implementation?). All I need to do is to add ForceDiscriminator annotation in my base class and the problem is solved!

Eclipse fails to start?

Your eclipse fails to start? actually we could say your workspace fails to start. I've gone through it several times and my solution was to create a new workspace and import the existing projects into the new workspace.
However, this time I'm trying to solve it once for all (as though it's possible :). Googled around and I found, imho, a rough solution that works for me. It's as simple as deleting org.eclipse.core.resources folder inside {workspace}/.metadata/.plugins and then run eclipse -clean.
The result is eclipse is working but I still have to import the existing projects back. The good thing is I don't lost all the preferences and server setting (though I could just import the preferences file I once exported).

Check the link on the title and also this one, tools from eclipse team that maybe could help with the broken workspace (I never tried it yet... my workspace already fixed :)