CentOS 6 Linux Server Cookbook: An all-in-one guide to installing, configuring, and running a Centos 6 server. Ideal for newbies and old-hands alike, this practical tutorial ensures you get the best from this popular, enterprise-class free server solution.

To complete this recipe it is assumed that you are using a typical Windows-based computer (Windows 7, Windows Vista, or similar) with full administration rights. You will need an Internet connection to download the required installation files and access to a standard DVD/CD disk burner with the appropriate software in order to create the relevant installation disks.

Regardless as to what type of installation files you download, the following techniques can be applied to all image files supplied by the CentOS project:

The act of downloading an installation image and validating the file's integrity is just the first step towards building the perfect server, and although the process of downloading CentOS is very simple, many do forget the need to confirm the checksum.

So what have we learned from this experience?

We began this recipe by showing you how to download CentOS by outlining the various options available to you. It is not expected that you will need to do this often, but during the course of this recipe you also learned how to navigate the CentOS Project website and select the best download location based on your current location.

Having done this, our next step was to obtain a copy of md5sum.txt before downloading the preferred image files.

The time required to complete the task of downloading the relevant files will depend largely on the speed of your Internet connection, but it was important that we obtained a copy of md5sum.exe in order to confirm file integrity and compare the value given against those listed in md5sum.txt.

The DOS-based tool was obtained from http://mirror.centos.org/centos/dostools/md5sum.exe.

Having completed these steps, we then proceed to open a command prompt session and navigate to the expected destination of all the files by typing:

cd downloads

We then proceeded to view the contents of C:\Users\<username>\Downloads by typing the following command:

dir

This action lists the filenames concerned, thereby enabling us to modify the following command in order to validate the appropriate ISO files:

md5sum.exe your_image_file_name_here.iso

The resulting response provides us with the checksum value of the file in question in order that we can compare it with the corresponding value found in md5sum.txt.

For example, the content of md5sum.txt may look like this:

087713752fa88c03a5e8471c661ad1a2  CentOS-6.X-x86_64-minimal.iso

Whereas the display in command prompt may show:

087713752fa88c03a5e8471c661ad1a2  *CentOS-6.X-x86_64-minimal.iso

You should repeat this process for all image files, and if these values match, then you can be confident that your download is verified. A fact that not only reduces the chance of wasting optical disks, but with security in mind, it also implies that what you downloaded is what you were expecting to download (and not something else).

So remember, with every new release the CentOS team will update the main website and enable you to download the required image files. Unless the CentOS project states otherwise, you will only need to do this once, as all minor updates can be managed like any other operating system (discussed later in this book). However, if you do intend to download them again, then it is always a good idea to check the integrity of your image files with the md5sum checksum tool.

Before we begin, it is assumed that you have followed a previous recipe in which you were shown how to download CentOS, confirm the checksum of the relevant image files, and create the relevant installation disks.

Depending on which installation image(s) were downloaded, in order to complete this recipe you will be expected to have completed the initial stages of a network install or that you are using one of the following DVD suites:

In addition to this, you should be confident that your server can meet the following minimum specifications:

Of course, it should be said that the preceding specification acts as guidance only and further details concerning the minimum system requirements can be found at the end of this recipe.

To begin this recipe, start by booting your computer from the first CentOS installation DVD and wait for the welcome screen:

In this recipe you have discovered how to install the CentOS 6 operating system, and having covered the typical approach to the graphical installation process you are now in a position to develop the server with additional configuration changes and packages that will suit the role you intend the server to fulfill.

So what have we learned from this experience?

We started by initializing the main installer and determining whether we would like to check the installation media for possible corruption and defects. Of course, validating the data on your disk is always a useful process and it can save time in the long run, but you should be aware that it will not necessarily guarantee a trouble-free installation. For this reason, most users who have already validated the checksum tend to skip this process.

The next steps then welcomed us to CentOS and invited us to confirm the appropriate language, keyboard, location, hostname, root password, network, and storage settings.

For example, any changes to the language setting were immediate, whereas the configuration of your network settings was simply a matter of ensuring the Ethernet device would connect automatically and modifying the appropriate IPv4/IPv6 settings. You were able to create a static or dynamic IP address, while the setting of a root password was simply a matter of choosing an appropriate series of alpha-numeric values that did not consist of known words, typical phrases, or contain any whitespace. Whatever password you decide on, it should consist of no less than six characters to avoid any warnings issued by the installer.

During the next stage, we then considered the need to partition the hard disks by selecting the default option known as Use All Space. By choosing this option, you were allowing the installer to determine the relative sizes of each partition and to build a logical volume that would assign GRUB to a default location. Of course, diving in to the depths of partition management was beyond the purpose of this recipe, but you did see that the various options that enable you to build your own custom layout should this be the preferred option.

You were then given the option to change the location of GRUB. This is not always deemed to be necessary, as moving GRUB can be an awkward process for even the most experienced users but ignoring this, you did discover an option that would enable you to password-protect the boot loader and provide additional system-wide protection.

Finally we came upon the package group selection screen where we discovered that CentOS not only enables us to choose a minimal install, but it also provides for a significant number of package selection groups that are designed for convenience.

The sole purpose of these groups is to simplify the process of selecting the correct packages for your installation. I think you would agree that most of the group titles are self-explanatory, but it is generally accepted that a minimal install is by far the most efficient method of installing any server because it is far easier to add and configure packages when we need them rather than simply installing everything at once. This rule of thumb not only reduces your initial workload, but it also ensures that your server remains safe and secure which is of a particular concern if it is located in the public domain.

So in conclusion, I think it would be appropriate to say that there is always more to discover about the installation process and that there are many different ways to install your server, but at this early stage you should now take the time to sit back and relax. Well done, you have now completed the recommend installation process and yes, you can now install CentOS.

Before we begin, it is assumed that you have followed a previous recipe in which you downloaded the netinstall image of Centos 6, confirmed the checksum values of the image file, and that you have already created the relevant installation disk.

To complete this recipe you will need an active Internet connection, and because this recipe will serve as a starting point to the graphical installation process, you should be confident that your server can meet the following minimum requirements:

To begin this recipe, insert the netinstall disc, boot your computer from the optical drive, and wait for the welcome screen to appear:

The purpose of this recipe was to introduce you to the concept of the CentOS network installation process in order to show you just how simple this approach can be. By completing this recipe you have not only saved time by limiting your initial download to those files required by the installation process, but you have been able to take advantage of the full graphical installation method without the need for a complete DVD suite.

So what have we learned from this experience?

We have learned how to boot from the netinstall image, initialize the main installer, test the optical media for errors, provide the relevant network settings for the host computer, and provide a URL in order to download the relevant files.

The URL in this case was web-based and dependent on the preferred architecture, but as an alternative you could have modified this instruction to use a local mirror site or even implement an installation from a local repository from your local network.

So in conclusion, it must be said that if you are intending to install more than one server at a time (with a full array of packages), then this may not be the best approach, but for a single server or multiple servers using a minimal install, then the netinstall image represents a far more flexible approach. Rather than waiting to download a complete set of installation files you may never need, the installer will download the required files directly from the chosen location thereby ensuring that you will not only reduce the overall time it takes to get your server up and running, but you will be able to take advantage of the full installation without the need for a DVD drive.

Before we begin, it is assumed that you have followed a previous recipe in which you were shown how to download CentOS, confirm the checksum values of the image files, and that you have created and labeled the relevant installation disk(s).

For the purpose of this recipe, it will be assumed that you are using the twin DVD-based installation disks. In many respects, installing your system by Text Mode is similar to using the minimal installation image file and for this reason some configuration screens may not be available unless used in the graphical mode.

Text Mode is automatically activated if the system is running less than 652 MB of RAM and for this reason, you should be confident that your system meets the following minimum requirements:

To begin this recipe you should boot your computer from the CentOS installation media of your choice and wait for the welcome screen to appear:

In this recipe, you have discovered how to install the CentOS 6 operating system using Text Mode. In many respects it is very similar to using the minimal installation file and if you have read the official documentation supplied by the CentOS project, they advise that it may not be the recommended approach. However, depending on the type of hardware you have at your disposal you may not have an alternative.

So what have we learned from this experience?

We started by initializing Text Mode at the welcome screen by pressing the Tab key to reveal the boot instruction, adding a single white space, and entering the following:

text

So in practice, your screen instruction will look like this:

>vmlinuz initrd=initrd.img text

When finished, pressing the Return key enables us to continue the installation process and check the installation media for possible corruption and defects.

Validating the data on your disk is a quick and painless process but remember, at this stage the installer will only validate the data contained on your installation disk(s). It will not reliably detect any hardware issues, so in effect merely completing this process does not necessarily guarantee a trouble-free installation. For this reason, most confident users who have already confirmed the checksum will generally skip this step.

The following stages then welcome us to CentOS and invite us to review and confirm the appropriate language settings, keyboard settings, and time zone location. Completing these configuration screens is again very simple and this is one of the benefits of streamlining. However, for veterans of other installation methods, you will quickly notice that the number of options have been limited.

If you were using a new hard disk, then it is quite possible that you were asked to initialize the disk by choosing re-initialize all. Initializing the hard disk is a process that is generally confined to new hard disks only; it is common to all operating systems and may have taken just a few seconds to complete. Once complete, you were then invited to create a root password before choosing the preferred partition scheme: use entire drive.

Again, veterans of the graphical installation method would have noticed that there were no options regarding the configuration of advanced storage methods such as LVM, RAID and iSCSI, customizing the partition layout, customizing the boot loader layout, and the selection of packages during the final phase of the installation. In fact, streamlining the CentOS 6 text installer has removed all of this and instead simply provided you a fast-track route to installing the necessary files that would make a complete minimal install.

You could argue that this approach has made it simpler, whereas you could also argue that the lack of such options can be a hindrance. It is not what the official documentation would call the recommended approach by any means, but this method does have its advantages. Text Mode is used for convenience, and it represents just another tool in your kit-bag, so regardless of whether you benefit from this streamlined approach or not, you now know that CentOS is accessible to even more computers than you originally thought.

To complete this recipe, you will require a standard installation disk of the CentOS 6 operating system. You will also require the use of a computer display, a standard keyboard, and an optical disk drive.

To begin this recipe you should boot your computer from the CentOS installation DVD and wait for the welcome screen to appear.

GRUB, otherwise known as the GRand Unified Bootloader, is a boot loader package that provides access to the CentOS system. It is a very important file, but accidents can happen and on occasion it has been known to suffer damage or even be removed.

If this happens it is very possible that your server will not boot and for this reason this recipe serves to invoke a process that is known as Rescue Mode in order to re-install this file.

So what have we learned from this experience?

We started by learning how to access Rescue Mode and configuring the necessary language and keyboard preferences in order that we can interact with the session.

In the following stages, Rescue Mode then enabled us to mount the relevant disk partition with full read/write capability in order that we could target the correct location and re-install GRUB.

So in conclusion, we could say that this was a very simple recipe, but it is also a very important recipe; and as damage to GRUB is typically associated with an installation procedure it was felt that such a recipe should remain close at hand. In many respects, it is always a good idea to keep a backup of your current device.map, grub.conf, and menu.lst files but without the benefit of hindsight, and with the assistance of this recipe, a typical disaster may have been averted as the result of following a very straightforward process.

Of course, it may not save you on every occasion and experience may prove that deciding to back up the contents of all the major files in /boot/grub will provide additional insurance, but at this moment, just rest easy, the boot loader has been restored and both you and your server are ready to get back to work.

To complete this recipe, you will require a minimal installation of the CentOS 6 operating system with root privileges with a connection to the Internet in order to facilitate the download of additional packages.

We will begin this recipe by updating the system.

The purpose of this recipe is to enhance a minimal installation of the CentOS 6 operating system and by doing this you have not only introduced yourself to the Yum Package Manager (something to which we will return to later on in this book), but you now have a system that is capable of running a vast amount of applications right out of the box.

So what have we learned from this experience?

We started the recipe by updating the system in order to ensure that our system was up to date:

yum –y update

At this stage it is often a good idea to reboot the system. The reason behind this is typically based on the desire to take advantage of a new kernel or revised security updates (that is, SELinux or IPTables). It is not expected that we will do this very often, but rebooting the server was simply a matter of using the following command:

reboot

In the next phase the recipe showed you how to add a series of package groups that may prove to be more than useful in the future. To save time we wrapped the instruction to install three main package groups in a single command like so:

yum –y groupinstall "Base" "Development Libraries" "Development Tools"

The preceding action alone installs over 200 individual packages thereby giving your server the ability to compile code and run a vast array of applications out of the box. This list includes Perl, Python, GCC, Make, nslookup, ping, and a vast array of other necessary and important packages you may need over the life time of your server.

For example, you can run nslookup as follows:

nslookup www.packtpub.com

Moreover, instead of using the Vi text editor, you now have the opportunity to use the Nano text editor and many more. Which, given its lack of complexity, usually makes the process of managing your server that much simpler.

For example, if you are ever asked to edit or create a new a file. Instead of typing the following:

vi /path/to/some_file_name

You could type:

nano /path/to/some_file_name

You will be happy to know that this form of text editor replacement will work for all the examples shown throughout this book.

On top of this, you will also have the chance to use vim-enhanced , another fantastic text editor that supports scripts and color coding, but as a tutorial on this subject is beyond the purpose of this recipe I have included some links at the end of this section that will get you started.

The final part of this recipe was to action another reboot by typing:

reboot

Again this is not always required, but given the number of new packages you have just installed, it is often advisable in order for you to enjoy the benefits of your new server.

To complete this recipe, you will require a minimal installation of the CentOS 6 operating system with root privileges and a connection to the Internet in order to facilitate the download of additional packages.

On a minimal installation Firstboot is not installed by default and for this reason we will begin this recipe by installing the necessary package and its related dependencies.

Firstboot is a simple package with the sole purpose of activating the Setup agent. It is not intended to be complicated and for this reason you probably found that it was both easy to install and even easier to activate.

So what did we learn from this experience?

We start the recipe by installing the necessary packages. In this case, the target package was called Firstboot and so we issued the following command to the server:

yum –y install firstboot

The system then proceeded to install a number of different dependencies that includes many setup utilities, but as Firstboot runs as a service, to access these utilities you should ensure that the service is running. So our next step was to return to the console and type:

chkconfig firstboot on

chkconfig --level 5 firstboot on

Having done this, we then proceeded to reboot the system like so:

reboot

On a successful reboot, you are then invited to run the various tools associated with this service. Of course, a discussion regarding the actual settings you should employ is a subject we will return to later on in this book, but having successfully completed this recipe you now understand that this small addition does have specific uses that serve to simplify the process of finalizing the installation of any CentOS server.

On a console-based server, the act of running Firstboot will enable you to run a screen by screen setup process that covers the main aspects of a first-time installation. For experienced users, this can prove to be a welcome reminder, but for new users Firstboot can prove to be a useful introduction to CentOS by offering a one-stop, tried and tested starting point. It may not be a necessary feature, but this small tool does go some way to make CentOS a very easy experience.

As the name implies, Firstboot will only run once. However, should you ever wish to return to this screen in the future, simply log in to your server as the root user and ensure the service is set to run at boot by typing:

chkconfig firstboot on

Now remove the following file by typing:

rm /etc/sysconfig/firstboot

You will be asked if removing this file is intended, so type y (yes) and press the Return key to confirm your decision like so:

rm: remove regular file 'etc/sysconfig/firstboot' ? y

Having done this Firstboot is now active and will be ready to run on your next reboot.

To complete this recipe, you will require a minimal installation of the CentOS 6 operating system with root privileges, a console-based text editor of your choice, and a connection to the Internet in order to facilitate the download of additional packages. The system should also have a computer display, a keyboard, and a mouse.

Before we begin, 64-bit users should be aware that the desktop environment will result in your system using a mixture of 32-bit and 64-bit software. This may change over time, but at the time of writing this book, most desktop applications are still 32-bit.

During the course of this recipe, you have not only learned how to install the GNOME desktop environment, but have also successfully reconfigured the runlevel of your server and installed some additional software packages that will enhance your overall enjoyment of CentOS.

So what have we learned from this experience?

We began by installing the GNOME desktop environment by introducing ourselves to the Yum Package Manager's ability to install package groups. Yum is something we will return to later in this book, but in this instance we were required to install all the necessary components needed by a typical desktop, and we achieved this by typing:

yum groupinstall "Desktop" "X Window System" "Fonts" "Desktop Platform"

Having completed this initial step, we were then asked to reconfigure the runlevel of the server in order that the desktop environment would be initiated during the boot process. Without completing this step, the server will continue to boot in console mode.

As described in this recipe, a runlevel is a preset operating state that determines which programs are executed at system startup. In this case, we are only considering the needs of X Window System and where a console-based server will typically start at runlevel 3, we need to modify this in order to activate the desktop environment that functions at runlevel 5.

To do this, we opened the main configuration file by typing:

vi /etc/inittab

You were then asked to scroll down towards the bottom of this file and find the following line:

id:3:initdefault:

Having found it, you were then asked to replace it with the following:

id:5:initdefault:

Finally, and last but not least, with a desktop environment everyone wants software, and by typing the following command, you were given the opportunity to enhance your system with a variety of tools that would make you feel at home:

yum groupinstall "General Purpose Desktop" "Graphical Administration Tools"

To complete this recipe, you were then required to reboot the server and having accessed your desktop environment, you can now navigate to System | Administration | Add/Remove Software to manage new and existing packages.