Automate CentOS and RHEL installation with PXE


MAC Trick

You can approach this problem in several ways. Networking is a tricky subject, because it has several potential solutions. Conceivably, for example, you could pack configured systems into their own VLAN at the switch level and have the system configured accordingly by the Anaconda autoinstaller. Another DHCP server on the network could then be set up for this VLAN so that it assigns suitable IP addresses.

Also conceivable would be to store a suitable Kickstart configuration for each host containing the desired network configuration. A trick comes in handy in this case: Once the system launches, the GRUB bootloader does not first search for the grub.cfg file, but for a file named grub.cfg-01-<MAC_for_requesting_NIC> (e.g., grub.cfg-01-0c-42-a1-06-ab-ef). With the inst.ks parameter, you define which Kickstart file the client sees after starting the installer.

In the example here, the GRUB configuration could contain the parameter:


In this case, Anaconda will no longer download the generic ks.cfg, but the variant specifically geared for the host. You can then follow the Anaconda manual if you want to configure static IP addresses.

In this context it is helpful to think of a mechanism to put both the required grub.cfg files and the Kickstart files in place. Ansible's template functions can help enormously here; then, you can build your own role containing a template for grub.cfg and ks.cfg and store the parameters to use for each host in your configuration in the two templates. Afterward, you would use the template function in Ansible to put the populated files in the right place on the filesystem. For each host stored in the Ansible configuration, you would always have a functional, permanently stored autoinstallation configuration. You will find more details of this in the Anaconda documentation [6].

Postinstallation Process

The main actor in the process (Anaconda) has only been implicitly mentioned thus far, but the credit goes to Anaconda, which is part of the CentOS and RHEL initramfs files and automatically appears as soon as you specify an Anaconda parameter like method or inst.ks in the kernel command line (Figure 4).

Figure 4: Anaconda either runs at the command line or graphically, but it does not affect the installed system.

If you have prepared an Anaconda configuration, you can look forward to finding a system with the appropriate parameters afterward. The Anaconda template I looked at here by no means exhausts all of the program's possibilities. You could also configure sudo in Anaconda or store an SSH key for users so that a login without a password works. However, I advise you to design your Kickstart templates to be as clear-cut and simple as possible and leave the rest to an automation expert.

Extending the Setup Retroactively

Up to this point, this article has worked out how you can convert bare metal into an installed base system. Although automation is good, more automation is even better. What you therefore need is to implement the last small piece of the installed basic system up to the step where the system automatically becomes part of the automation. In Ansible-speak, this objective would be met once a host automatically appeared in the Ansible inventory after the basic installation.

Netbox, a tool that has already been discussed several times in ADMIN  [7], is a combined datacenter inventory management (DCIM) and IP address management (IPAM) system. Netbox (Figure 5) lists the servers in your data center, as well as the IPs of your various network interface cards (NICs). If you take this to its ultimate conclusion, you can generate the configuration file for the DHCP server from Netbox, as well as the Kickstart files for individual systems.

Figure 5: Netbox is a combination of DCIM and IPAM and complements the combination of DHCP, TFTP, and HTTP.

Extracting individual values from Netbox is not at all complicated because the service comes with a REST API that can be queried automatically. What's more, a library acts as a Netbox client especially for Python so that values can be queried from Netbox and processed directly in a Python script. Anyone planning a really big leap in automation will be looking to move in this direction and will ultimately use tags in Netbox to assign specific tasks to certain systems.

A Netbox client that regularly queries the parameters of the servers in Netbox can then generate specific Kickstart files according to these details and ensure that, for example, a special set of packages is rolled out on one system but not on another. The overhead is rewarded with a workflow in which you simply bolt a computer into the rack, enter it in Netbox, and tag it appropriately to complete the installation in a completely automated process.

If you want things to be a little less complicated, you might be satisfied with an intermediate stage. Many automators, including Ansible, can now generate their inventory directly from Netbox. As soon as a system appears in Netbox, it is also present in the inventory of an environment, and the next Ansible run would include that new server.

Buy this article as PDF

Express-Checkout as PDF
Price $2.95
(incl. VAT)

Buy ADMIN Magazine

Get it on Google Play

US / Canada

Get it on Google Play

UK / Australia

Related content

comments powered by Disqus
Subscribe to our ADMIN Newsletters
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs

Support Our Work

ADMIN content is made possible with support from readers like you. Please consider contributing when you've found an article to be beneficial.

Learn More”>


		<div class=