I’ve been playing with PXE booting servers on Brightbox cloud over the last few days, which is rather cool! It’s also incredibly useful for building OS images which can be snapshotted and registered for later use. It’s quite simple really, here’s a quick guide.
If you haven’t already, go ahead and register for the cloud beta, read through the docs about getting started and make yourself familiar with the basics of the command line client.
First let’s build a new server to PXE boot using the command line client.
ben@spud:~$ brightbox-servers create -n "PXE Boot" img-2ab98 INFO: client_id: ben Creating a nano (typ-4nssg) server with image Ubuntu Lucid 10.04 server (img-2ab98) id status type zone created_on image_id cloud_ips name ------------------------------------------------------------------------------ srv-m1tgj creating nano gb1-b 2011-04-18 img-2ab98 PXE Boot ------------------------------------------------------------------------------
It doesn’t really matter which image you choose at this point, if you’re PXE booting an installer it’s likely you’ll be erasing and repartitioning the disk anyway (there’ll be some blank images available soon). If you’re intending to snapshot the server for registration as a machine image later, it’s best to use the smallest disk available, as this will be the minimum required for your image later on. In this case we’re using a nano, the default, with a 20GB disk.
Once the server has finished creating, activate its web-based console so we can get to the POST screen.
ben@spud:~$ brightbox-servers activate_console srv-m1tgj INFO: client_id: ben Activating console for server srv-m1tgj url token expires ----------------------------------------------------------------------------- https://srv-m1tgj.console.gb1.brightbox.com mk6rr2z8 2011-04-18T23:33:12Z -----------------------------------------------------------------------------
Fire up your web browser and login to the console. Once logged in hit the “Send CtrlAltDel” button in the right hand corner and wait for the machine to reboot. During the POST, hit Ctrl-B when you see “Press Ctrl-B to configure gPXE” to start gPXE.
gPXE is a GPL’d replacement for proprietary PXE boot ROMs that has lots of nice features, including the ability to boot from HTTP, which we’re going to make use of here.
There are a number of choices when it comes to deciding exactly what we’d like to boot from, I’ll cover three options that are useful for image building.
- Booting a preprepared initrd and kernel image
- Booting an ISO with memdisk
- Chain loading with netboot.me
Before we do anything else though, we need to get network connectivity inside the gPXE environment. We can make use of dhcp to automatically configure everything for us.
gPXE> dhcp net0
Now we’re ready to boot something.
Booting a preprepared initrd and kernel image
A number of Linux distributions provide initrd and kernel images that are preprepared for PXE booting into an installer or rescue environment including Debian, Ubuntu, RHEL, the System Rescue CD and many others. This is particularly easy with Red Hat derived distributions, here we’ll boot the Scientific Linux 6.0 installer though the same procedure could easily be used for CentOS, Fedora, RHEL etc.
Most Red Hat derivatives provide the required kernel and initrd on their download mirrors, which can usually be found under os/images/pxeboot/ in the desired version and architecture directory. Go ahead and browse the mirror of your favourite RH clone and locate the vmlinuz and initrd.img for your preferred version and architecture and make a not of their URLs, in this case
http://ftp.scientificlinux.org/linux/scientific/6.0/i386/os/images/pxeboot/vmlinuz
http://ftp.scientificlinux.org/linux/scientific/6.0/i386/os/images/pxeboot/initrd.img
Next, we need to tell gPXE where to find our kernel and initrd.
gPXE> kernel http://ftp.scientificlinux.org/linux/scientific/6.0/i386/os/images/pxeboot/vmlinuz > http://ftp.scientificlinux.org/linux/scientific/6.0/i386/os/images/pxeboot/vmlinuz.......... gPXE> initrd http://ftp.scientificlinux.org/linux/scientific/6.0/i386/os/images/pxeboot/initrd.img > http://ftp.scientificlinux.org/linux/scientific/6.0/i386/os/images/pxeboot/initrd.img............... ..................................................
It can take quite a long time for them to download, particularly if the initrd.img is large. gPXE’s TCP stack isn’t tuned for performance. It’s generally a lot quicker if you use a mirror that’s geographically close (to the UK in this case), as latency has a significant impact on performance due to gPXE’s lack of TCP window scaling.
Anyway, once the downloads have finished, we’re ready to boot!
gPXE> boot
If everything has gone to plan, you should seen the kernel boot messages scrolling by then the anaconda installer starting up. Success!
Now it’s just a case of completing the installation procedure and rebooting into your new install. In the case of RH derivatives, anaconda will ask a few simple questions on language and keyboard layout before asking “What type of media contains the installation image?” To keep the PXE boot initrd small, it doesn’t include everything required to complete the installation so we need to tell anaconda where to find the rest of what it needs. Choose the URL option then complete the network configuration options, when asked for the URL, enter the path of the os/ directory from the mirror we used previously, in this case http://ftp.scientificlinux.org/linux/scientific/6.0/i386/os.
That’s it! Go and make a coffee while you wait for the installation to complete.
Booting an ISO with memdisk
Not all operating systems are kind enough to provide native PXE boot support. Often, the only available installation media for an OS is a cd/dvd or ISO image, particularly in the case of Microsoft Windows. Unfortunately PXE does not support booting from an ISO natively (boo!).
Never fear, memdisk to the rescue! Memdisk, from the SYSLINUX project, is designed to allow booting legacy operating systems. “MEMDISK can boot floppy images, hard disk images and some ISO images.” Unfortunately memdisk’s support for booting ISOs is somewhat limited so YMMV. Here, we’ll boot the Debian netboot installer ISO.
It’s important to make note of a few issues here when choosing an ISO to boot. Memdisk will load the entire ISO into a ram disk before booting, so it’s important you use a server with enough ram to contain the entire ISO and still leave enough free memory for the installation to complete successfully. A large DVD ISO will require lots of memory! Also, as previously mentioned, gPXE’s TCP stack is not tuned for performance, downloading a large ISO can take a very long time! Using a fast, geographically close mirror will help.
First though, we need a PXE bootable memdisk! The nice folks at SliTaz GNU/Linux provide just such a thing on their download mirror at http://distro.ibiblio.org/slitaz/pxe/memdisk, which we’ll make use of.
First, we need to tell gPXE to use the memdisk image as the kernel it should boot from.
gPXE> kernel http://distro.ibiblio.org/slitaz/pxe/memdisk > http://distro.ibiblio.org/slitaz/pxe/memdisk..............
Next, we tell gPXE to use the ISO we’d like to boot as the initrd, in this case we’re using the Debian netboot iso from http://ftp.nl.debian.org/debian/dists/squeeze/main/installer-i386/current/images/netboot/mini.iso
gPXE> initrd http://ftp.nl.debian.org/debian/dists/squeeze/main/installer-i386/current/images/netboot/mini.iso > http://ftp.nl.debian.org/debian/dists/squeeze/main/installer-i386/current/images/netboot/mini.iso....... ...........................................................................
Now is probably a good time to go make coffee, while you wait for the ISO to download. Once it’s finished, we’re ready to boot!
gPXE> boot
If the ISO is supported and everything has worked correctly, it should now be booting!
Chain loading with netboot.me
As ever, I’ve saved the easiest option until last! :) netboot.me is an awesome service that “allows you to boot nearly any operating system or utility on any computer with a wired internet connection.” They provide a custom built PXE environment which includes an easy-to-navigate menu system offering the ability to boot a number of games, OS installers, live OSes and system tools over the internet with little effort.
The best thing is, we can chain load the netboot.me PXE environment over HTTP directly from gPXE with a single command! The PXE environment is available via HTTP at http://static.netboot.me/gpxe/netbootme.kpxe so all we have to do it tell gPXE to chain load it.
gPXE> chain http://static.netboot.me/gpxe/netbootme.kpxe > http://static.netboot.me/gpxe/netbootme.kpxe.................
gPXE will download and boot the netboot.me environment, after waiting a few seconds for netboot.me to start you should be presented with the main menu, which is fairly self-explanatory. Time to reward yourself with a game of nethack! :)
Finishing off
So, you’ve finished installing your shiny new OS, but what to do now?! Before customising anything on the fresh installation, you might want to make a snapshot of the server to use for building new servers in future, certainly a lot quicker than PXE booting!
(NOTE: You may need to make some alterations to the image after installing and before snapshotting if you want to use the snapshot for building new servers later. Often things like networking config, hostname etc. are configured statically during the installation procedure, these will need to be changed/removed.)
First, we’ll take a snapshot of the server
ben@spud:~$ brightbox-servers snapshot srv-m1tgj INFO: client_id: ben Snapshotting server srv-m1tgj
Next, need to find the id of the newly created snapshot.
ben@spud:~$ brightbox-images list ... img-o031h acc-vhba2 snapshot 2011-04-19 creating 20480 Snapshot of srv-m1tgj 19 Apr 13:16 (i686)
Once the snapshot is finished creating, the /images directory of your account’s FTP library will contain a gzipped copy of the snapshot, in this case img-o031h.gz. You can go ahead and start building images from the snapshot using its id right away!
ben@spud:~$ brightbox-servers create img-o031h
Conclusion
I’ve covered some of the basic aspects of PXE booting using gPXE on the Brightbox cloud here, which should be enough to use for OS installation and system rescue. There are also lots of things I haven’t covered, it’s possible to do many weird and wonderful things with gPXE that are beyond the scope of this post, perhaps I’ll cover them in the future. ;)