Category: Ubuntu Server

How to test if Bacula-fd is running

james 2 Comments

I just installed a new Bacula client but for some reason the server could not connect to it. I found this out by trying to get the status of the client on the server with the following command:

sudo bconsole
status client=my.client-rd

Here is the things that I checked on the server.

Step 1 – Check that the service bacula-fd is started:

sudo service bacula-fd status

Step 2 – Check the bacula-fd process is bound to the port 9102

sudo netstat -an|grep 9102

Step 3 – Check that you can telnet into the client port from the server.

telnet ip.ad.dr.ess 9102

I found the the client configuration did have the correct client ipaddress but each of these checks enabled me to narrow down the problem.

How to repair / restore Grub on Ubuntu using Live CD

james Leave a comment

I just upgraded Ubuntu and selected the wrong location to install Grub onto. Basically, I installed Grub on /dev/sda1 and not /dev/sda and so I need to repair and installing grub.

I got out an Ubuntu installer cd which is essential a live cd and booted in it up. Then mounted the sda1 partition:

sudo mount /dev/sda1 /mnt

Grub then needs to have access to /dev so:

sudo mount --bind /dev /mnt/dev

To install Grub on sda then:

grub-install /dev/sda --root-directory=/mnt/ /dev/sda

Note: we are using the –root-directory option rather than changing the root directory using the chroot command which sometimes doesn’t work on a live CD.

For a tutorial with more words these two pages might helped you learn more:

How to remove exif metadata from jpegs using php or command line

james 1 Comment

There is a method of hacking php using a security flaw in php in how it handles exif meta data embedded in jpeg, png or gif images. You describe this as an EXIF injection attack. As a precaution it is a good idea to strip all EXIF information from any images that are uploaded to your website.

I have found these two tools really useful to do this and this is a quick outline of how we used them:

Removing EXIF meta data from .jpg on Ubuntu / PHP
There is a really good tool called exiftool. There is a version in the Ubuntu repositories – so it is super easy to install.

apt-get install libimage-exiftool-perl

You can then strip exif meta from a jpeg using the command:

exiftool -all=  filename.jpg

So in php this would look like this:

$output = exec(sprintf("exiftool -all= %s", escapeshellarg($_image_path)));

Removing EXIF meta data from .png images on Ubuntu / PHP
The best tool for removing exif information from .pngs is Optipng. You can install optipng on Ubuntu from source using this recipe.

apt-get install optipng

and so again in php it would look something like this:

$output = exec(sprintf('optipng -strip all %s', escapeshellarg($_image_path)));

I hope this helps someone.

a2ensite ERROR: Site www.example.com does not exist! Ubuntu 13.10

james 4 Comments

On our Ubuntu 13.10 server, I had created an apache configuration file for a virtual host however, I could not enable it using a2ensite. The file was here:

/etc/apache2/sites-available/www.example.com

I tried to enable it:

a2ensite www.example.com

However, the command complained that ERROR: Site www.example.com does not exist! The files clearly exists. It turns out the this command only works if the file ends with .conf. So

mv www.example.com www.example.com.conf
a2ensite www.example.com.conf

Voila it now work. Why or why doesn’t the developer give useful error messages. For example they should have write. Error – the files doesn’t have a .conf on the end!

How to detect multiple dhcp servers on network using wireshark and Ubuntu

james 2 Comments

We just had a problem with our DHCP server and there seems to be another dhcp server on the network. To find this I used Wireshark on my Ubuntu machine to find the problem.

1 – Install wireshark

apt-get install wireshark

2 – Launch wireshark with permissions to read the network interfaces.
You can either do this by runnning Wireshark as root (which is really not recommended but a quick hack if you need to get the job done). Or you can give your user permission to read the interfaces which is much better in the longterm.

2.1 – To give your user permission to capture network interfaces do the following:

sudo dpkg-reconfigure wireshark-common

This will ask you the following question. Answer Yes.

giving_wireshark_permission_to_read_network_interfaces

This creates a group called wireshark and anyone in this group can capture network data on the interface.

2.2 – Add you user the wireshark group.

sudo adduser $USER wireshark

2.3 – Logout and log back in
For you group permissions to change – you need to log out and then log in again. Alternatively, you can just restart your computer.

3 – Start Capturing With Wireshark
Launch Wireshark either from your launcher or using terminal “wireshark”. On the start screen you should see a list of interfaces on the left hand side. Select the interface you would like to capture data from and press the start button.

The Wireshark Network Analyzer [Wireshark 1.10.2 (SVN Rev 51934 from -trunk-1.10)]_010

4 – Filtering For “bootp” messages
You can then filter Wireshark just to show dhcp messages by filtering for bootp message but typing “bootp” and clicking apply.

Capturing from eth0 [Wireshark 1.10.2 (SVN Rev 51934 from -trunk-1.10)]_011

5 – Find all the offer packets
If you have multiple DHCP servers – you will have multiple offer packets. You can filter the messages by bringing up the packet details

click View -> Packet Details

This will show the packet details below the message list like so.

Capturing from eth0 [Wireshark 1.10.2 (SVN Rev 51934 from -trunk-1.10)]_012

You should then go into “Bootstrap Protocol” -> “Options: DHCP Message Type” and right click on “DHCP: Offer” and select “Apply As Filter”.

This will then filter all dhcp offers and you will be able to see what servers are responding on the system.

Great YouTube Video Tutorial
There is also a good wireshark dhcp tutorial on youtube which shows this in action. It is a Windows focused tutorial but explains the other general concepts really well.

I hope this helps someone. Let me know if you have any suggestions of how this tutorial can be improved.