Fix iptables error “Loading additional iptables modules: ip_conntrack_netbios_n[FAILED]” in XenServer 6 (and others)

As part of setting up and testing routing rules in XenServer 6 I used the built in “lokkit” tool to temporarily turn off the firewall. Unfortunately, just opening the tool overwrote our custom “/etc/sysconfig/iptables” rules and cleared the file. This wasn’t a huge problem as we had a backup and just recreated it (you shouldn’t really be editing iptables manually anyway). On restarting iptables using “/etc/init.d/iptables restart” we received the error:

Loading additional iptables modules: ip_conntrack_netbios_n[FAILED]

This is very easy to fix and is due to a setting in “/etc/sysconfig/iptables-config” which was set by “lokkit” by default. The issue is that iptables is trying to load the “ip_conntrack_netbios_ns” kernel module, which doesn’t exist by default in XenServer (and other linux distributions).

Find the following line at the top of “/etc/sysconfig/iptables-config”:


And set to:


A few people have said to also set “IPTABLES_MODULES_UNLOAD” to =”no”:


But I found that “/etc/init.d/iptables restart” still failed so I left it as “yes”. You may be able to set to “no” so try this first.

This will stop the missing kernel module being loaded and allow iptables to start properly.

If you get any other errors about loading modules when restarting iptables, check “/etc/sysconfig/iptables-config” isn’t trying to load something in “IPTABLES_MODULES=” that you don’t have installed.

Encrypt a USB drive in linux and automatically mount it on startup using a keyfile and dm_crypt

The easiest way of doing this is to use dm_crypt‘s “cryptsetup” on your USB drive, create a keyfile then set the options in “/etc/fstab” and “/etc/crypttab”. By using a keyfile you can get the drive to automatically mount without having to type in your encryption password. I was doing this on a bare install of CentOS 6.3 but the steps should be similar on other distros with “cryptsetup” installed.

I needed to back up some important (and confidential) files to a USB portable drive that I wanted to encrypt with full disk encryption. You can do this in a variety of ways but the method here was the easiest I found. More information can be found at Brad’s Blog and HowtoForge.

Encrypting and mounting your USB drive

First you need to physically plug in your USB drive to the machine and then unmount it if it automatically mounts. I performed all the commands here using the root user. In my case, when I plugged in the USB drive it was found as “/dev/sdb” and automatically mounted by CentOS. To unmount:

umount /dev/sdb

Now the USB drive needs to be formatted using “cryptsetup” and the “luksFormat” command:

cryptsetup luksFormat /dev/sdb

The tool will give you a warning about overwriting data, which you need to confirm by typing an uppercase “YES”. You then type in and confirm your LUKS passphrase, which will be used to unlock the drive in future. This passphrase is also used later when creating the keyfile.

Now you can create a device mapper for the drive using “cryptsetup” and the “luksOpen” command. I called my mapper “secretvol” in this example so the drive will be mapped to “/dev/mapper/secretvol”. You will be prompted for the passphrase:

cryptsetup luksOpen /dev/sdb secretvol

Now before you can mount your newly mapped device you need to format the file system (I used ext3):

mkfs.ext3 /dev/mapper/secretvol

Now you can mount the USB drive. Make sure you have created the mount point (in my case “/mnt/encrypteddrive”) first then mount it with:

mkdir /mnt/encrypteddrive
mount /dev/mapper/secretvol /mnt/encrypteddrive

To test this all works properly reboot your machine before unlocking and mounting your USB drive manually (requiring entry of the passphrase):

cryptsetup luksOpen /dev/sdb secretvol
mount /dev/mapper/secretvol /mnt/encrypteddrive

To unmount and lock the drive by closing the device mapper with the “luksClose” command:

umount /dev/mapper/secretvol
cryptsetup luksClose secretvol

Creating a keyfile to avoid entering your passphrase manually

A keyfile is good as it means you can unlock your USB drive without having to manually type the passphrase. To create a keyfile “/root/keyfile” for your device using “cryptsetup” and the “luksAddKey” command enter the following (you will need to enter your passphrase). The first command creates a random 4096 byte file, the second makes it read only to root and the third stores your passphrase in the keyfile using “luksAddKey”:

dd if=/dev/urandom of=/root/keyfile bs=1024 count=4
chmod 0400 /root/keyfile
cryptsetup luksAddKey /dev/sdb /root/keyfile

Now you can unlock your previously created drive without manually entering the passphrase using:

cryptsetup luksOpen –key-file /root/keyfile /dev/sdb secretvol

And mount with:

mount /dev/mapper/secretvol /mnt/encrypteddrive

Automatically unlock and mount your encrypted USB drive at system startup

Now that you have a keyfile you can set up your linux install to automatically unlock and mount the USB drive by editing a couple of files.

Edit your “/etc/crypttab” file:

nano /etc/crypttab

Add the line below to add the “/dev/mapper/secretvol” device:

secretvol /dev/sdb /root/keyfile luks

NOTE: You can also use the UUID of your drive in “/etc/crypttab” to make sure that the right disk as detected by the kernel is used. In cases where you may be adding or removing disks this is really important as you may have “sdb” or “sdc” or “sdX” depending on what order the disks are detected by your linux install. To find the right UUID type:

ls -l /dev/disk/by-uuid

Which in my case told me that my UUID for “sdb” (my USB drive) was “6858274d-2370-4377-9426-d786c3e7a410″. The line in “/etc/crypttab” that you should use in this case to add “/dev/mapper/secretvol” is:

secretvol /dev/disk/by-uuid/6858274d-2370-4377-9426-d786c3e7a410 /root/keyfile luks

Now edit your “/etc/fstab” file:

nano /etc/fstab

Add the line below to automatically mount the device to “/mnt/encrypteddrive”:

/dev/mapper/secretvol /mnt/encrypteddrive ext3 defaults 0 2

Now to test this, reboot your machine and navigate to “/mnt/encrypteddrive” where your USB drive will be mounted automatically for you. Easy!

Simple setup of Oracle 11g Release 2 on CentOS 6.3, including pdksh and all dependencies, in VirtualBox

I’ve installed Oracle Database 11g Release 2 a few times on various Linux installs and apart from a few quirks it is a pretty similar process on most. The absolute bare bones default install, as described here, is easy to set up and doesn’t take that long. You can see more detail, including all the recommended steps if you follow the instructions in the Oracle install guide. I will describe installing 32bit Oracle Database 11g Release 2 on CentOS 6.3 32bit with the UI installed so we can use the Oracle installer directly. My computer’s name was “localhost.localdomain” as I was testing this in a development VirtualBox install.

First download Oracle 11g Release 2 from their website. For a linux install it comes as 2 zip files which you must first accept the license for before downloading. The exact version I downloaded was “Oracle Database 11g Release 2 ( for Linux x86″.

Now you need to prepare your CentOS install by adding the required users and user groups for the install process. In my setup I am following oracle and running the following commands to add the “oinstall” and “dba” user groups:

groupadd oinstall
groupadd dba

Now add the “oracle” user, who we will be using to run the Oracle 11g install and give the user the correct group membership:

useradd -g oinstall -G dba oracle

Now create a directory and set the appropriate permissions where you are going to install Oracle. In my case I have installed it in the “oracle” user’s home directory under “/home/oracle/app”:

mkdir -p /home/oracle/app
chown -R oracle:oinstall /home/oracle/app/
chmod -R 775 /home/oracle/app/

Now extract the Oracle ZIP files downloaded earlier into somewhere sensible. I chose “/home/oracle/database”. Navigate to the directory and run the install script as your new oracle user:

su oracle

cd /home/oracle/database

NOTE: In my case, because this CentOS install was a VirtualBox virtual machine I needed to explicitly set the $DISPLAY variable to the local machine before the UI for the installer would run. This is done by running the following command and restarting my shell:

export DISPLAY=:0.0

Now the installer will start up. You can ignore entering your email in the first step “Configure Security Updates” and leave the default setting of “Create and configure a database” in the second step “Installation Option”.

For the “System Class” step of the install I just left it as the default “Desktop Class” and in the “Typical Installation” step I left everything as default apart from setting the Administrative password. The default settings puts the oracle base in “/home/oracle/app/oracle” with a global database name of “orcl.localdomain”. For the “Create Inventory” step I left the default folder of “/home/oracle/app/oraInventory” and the group name “oinstall”.

Now we get on to the interesting part of the install, which is the “Prerequisite Checks” stage. If you are running the install on a brand new copy of CentOS you will need to set a few system variables and install a set of prerequisites.

NOTE: You may not need to, but I needed to add more swap space to my CentOS install this time around in order to meet the prerequisites. Run the following commands as root to create a 2048mb swap file called “/swapfile” on your harddrive and set CentOS to use it for swap space:

dd if=/dev/zero of=/swapfile bs=1024 count=2097152
mkswap /swapfile
swapon /swapfile

Now set CentOS to always use this swap space at boot by editing your “/etc/fstab” file using the command:

nano /etc/fstab

And add the following line:

/swapfile  swap  swap  defaults  0  0

So if you have passed the swap space test in the “Prerequisite Checks” in the Oracle install you can start to fix all those “Failed” messages. Click on the button “Fix & Check Again” and a window will pop up to tell you about the handy “” script that will be placed in “/tmp/CVU_11.”. So in your shell, navigate to the directory as root and run the script:

cd /tmp/CVU_11.

The “” script will fix all the system variables for you so you don’t need to set them manually. Now all that remains is to fix the dependencies, most of which can be installed using “yum” with the following command:

yum install gcc gcc-c++ compat-libstdc++-33 elfutils-libelf-devel libaio-devel libstdc++-devel unixODBC unixODBC-devel

Now the only remaining prerequisite that causes a “Failed” message is “pdksh-5.2.14″ which has been removed from the CentOS repositories after CentOS 5 (see here). The replacement is “ksh” but if you install this package using “yum install ksh” you will get the same dependency check “Failed” in the Oracle install for “pdksh-5.2.14″ and “ksh” will conflict with “pdksh” if you then go to install it.

The solution is to install “pdksh” manually from RPM, which can be found at a variety of mirrors. I used the following command to install the “pdksh” package:

rpm -q

Now Oracle should pass all the prerequisite checks and you will see the “Summary” step of the install where you can click the “Finish” button. It may take a while but Oracle Database will install with all the required settings ready for you to use out of the box.

The final step is to execute the configuration scripts as root, which will pop up after you have unlocked any users you might need other than the defaults (you don’t need to though at this stage). The two scripts can be run as follows:

cd /home/oracle/app/oraInventory/

cd /home/oracle/app/oracle/product/11.2.0/dbhome_1/

To test your install worked you can log in to the web based management interface for your computer “localhost.localdomain” with the user name “SYS” connecting as “SYSDBA” and using the password you set during the install of Oracle. Remember to open port 1158 on your firewall if you need to:


Now you can start to use Oracle. I highly recommend looking through the documentation from Oracle themselves to help get yourself used to the Oracle way of doing things. There are loads of client applications that can help, like the command line based Oracle Instant Client and the Oracle SQL Developer UI program. Oracle have a lot of good walkthroughs for working with their tools which are available as part of their Learning LIbrary.

Quickly enable NTFS support in CentOS 6.3 using EPEL, yum and ntfs-3g

Enabling NTFS support in CentOS 6.3 is only 2 commands in a shell script and can be done in seconds by installing the EPEL repository and the “ntfs-3g” package.

I needed to transfer some files from my USB drive to CentOS 6.3 (using the USB device option in VMware) and got an error message about an unknown filesystem “ntfs”. The drive I was using was formatted in Windows using the NTFS filesystem and couldn’t be read by my CentOS 6.3 install by default.

First you need to install the Extra Packages for Enterprise Linux (EPEL) repository, which is done by typing the following into the shell as root assuming you are using 32bit CentOS 6.3:

rpm -Uvh

If you are using 64bit CentOS use:

rpm -Uvh

Accept all the prompts to install the repository to get access to a large number of extra packages for CentOS. You can add the “ntfs-3g” package with the following command:

yum install ntfs-3g

Accept all the prompts and you are done, your NTFS formatted drives can now be read from and written to using CentOS.

Upgrade PHP from 5.1.6 to 5.2.17 on CentOS

The default install of PHP on our CentOS 5.5 box was 5.1.6, which is very out of date (we are currently using PHP 5.3 elsewhere while we figure out how to get around some very serious problems with 5.4). Unfortunately, we needed to upgrade to PHP 5.2 and no further as 5.3 meant upgrading MySQL and potentially breaking compatibility with our web application.

It used to be that you could add the CentOS testing repositories and just update PHP but as PHP 5.2 is depreciated this option is no longer available. The solution is to use the Atomic repositories which can be added to your CentOS install by typing:

wget -q -O – | sh

This will add a new repository file “/etc/yum.repos.d/atomic.repo” which means we can use their packages as well as those from CentOS. Now we need to make sure that we don’t upgrade our PHP beyond 5.2 so we add a single line to “/etc/yum.conf” under the [main] section:


The exclusion means we will include packages from all repositories other than anything that matches “php-*5.3*” so PHP 5.3 won’t be installed as part of an upgrade.

Now just upgrade PHP and restart Apache:

yum update php

service httpd restart

You can check which PHP version you have using:

php -v

Now obviously you want to use a more recent version of PHP than 5.2 but in the rare case where you have to, the previous commands make things very easy.