Setting up a PXE environment for OS installations

Thursday, September 1, 2011

If you're fooling around with various OSes, installing them by first burning CDs or DVDs gets to be a drag - and you end up with piles of old discs that just go into a landfill. Sure, there are rewritable disks, but they wear out and get scratched eventually. USB memsticks can be painful too - sometimes difficult to create and with different BIOSes having different levels of support.

A slick way to go is to set yourself up to do PXE (Preboot eXecution Environment) installations over a network. Most network cards have had PXE support included for many years now. If you have a machine handy that can act as a simple server, you can have an enviroment where you boot a machine, select the OS you want to install from a menu, and everything will just be pulled over your local network.

There are plenty of writeups on how to PXE install Ubuntu from an Ubuntu server, or FreeBSD from a FreeBSD server - but to make things more interesting and explicit I'll go cross-platform and talk about deploying Ubuntu Server 11.04 from a FreeBSD 8.2 server, and try to make it general enough so that later on we can add other OSes to the menu such as CentOS or OpenBSD.

Requirements

PXE booting a machine requires two basic services be present on your network:

DHCP - to assign the booted machine an IP address and tell it what "network bootstrap program" (NBP) to fetch from a TFTP server
TFTP (Trivial FTP - not to be confused with regular FTP) serves up the initial boot files

OSes such as Ubuntu or CentOS require a third service:

HTTP Server - serves up the bulk of the OS install files.

PXELINUX

For the Network Bootstram Program, we'll use PXELINUX, which is available as part of the SYSLINUX project. The name SYSLINUX is a bit misleading in that it's not actually Linux, but rather a collection of bootloaders that are often used with Linux, and capable of loading other OSes as well. Think of something more along the lines of GRUB, than an actual Linux distro.

To start off with, I'll create a /tftpboot directory, download syslinux-4.04.tar.gz from here, extract and copy two files we want:

mkdir /tftpboot
fetch http://www.kernel.org/pub/linux/utils/boot/syslinux/syslinux-4.04.tar.gz
tar xzvf syslinux-4.04.tar.gz
cp syslinux-4.04/core/pxelinux.0 /tftpboot
cp syslinux-4.04/com32/menu/menu.c32 /tftpboot

We're done with the syslinux download now, so you could clean it up if you want with:

rm -rf syslinux-4.04*

Next, create a configuration directory

mkdir /tftpboot/pxelinux.cfg

and in that directory create a file named default with these initial contents:

DEFAULT menu.c32
PROMPT 0
TIMEOUT 200                           

LABEL local                           
    MENU LABEL Local Boot
    LOCALBOOT 0

That should be enough to get us a barebones menu when we PXE boot a machine, with a single option to boot off the local harddisk (we'll get to Ubuntu later).

Enable TFTP

TFTP is already included in FreeBSD, just need to make sure it's enabled.

In /etc/inetd.conf make sure this line has the default # removed from the front (so it's not commented out)

tftp   dgram   udp     wait    root    /usr/libexec/tftpd      tftpd -l -s /tftpboot

In /etc/rc.conf, make sure inetd is enabled, adding if necessary:

inetd_enable="YES"

Depending on what you had to do above, start, or reload the inetd daemon with:

service inetd start

service inetd reload

Check that the machine is now listing on UDP port 69

sockstat | grep :69

See if you can fetch the NBP using the tftp utility (assuming your server's IPv4 address on the network you'll be doing PXE boots is 10.0.0.1)

cd /tmp
tftp 10.0.0.1
tftp> get /pxelinux.0
tftp> quit
rm pxelinux.0

If it works you should have seen somthing like:

Received 26443 bytes during 0.1 seconds in 53 blocks

Tweak DHCP Server

For this part I'm assuming you're running an ISC dhcpd server (if not, we'll have to cover that in another post). You basically just need to add two lines to /usr/local/etc/dhcpd.conf telling a client what server to use for TFTP and what NBP to fetch:

next-server 10.0.0.1;
filename "/pxelinux.0";

On my server, I just wanted to do this on one particular subnet, so there's a chunk that looks something like this now:

subnet 10.0.0.0 netmask 255.255.255.0 
    {
    range 10.0.0.127 10.0.0.250;
    option routers 10.0.0.1;

    next-server 10.0.0.1;
    filename "/pxelinux.0";
    }

Restart dhcpd

service isc-dhcpd restart

Give it a try

On your client machine, you may have to poke around in the BIOS to enable PXE booting. You'll have to figure out this part for yourself. If you can select your Network Card as the boot device, and everything else is working right, you should see a simple menu something like this:

Initial success

OK! we're at the "Hello World" stage, we know the client and server are doing the bare minimum necessary for PXE to function at all. Time to move on to the good stuff.

Ubuntu Server 11.04

For this next step, I'll assume you've downloaded an ISO into say /foo/ubuntu-11.04-server-amd64.iso The specific version shouldn't matter too much, so if you want to do 10.04 LTS or something else, it should all be about the same.

Mount the ISO image, so we can copy a couple files into /tftpboot and share the rest with a web server.

mkdir -P /iso_images/ubuntu-11.04-server-amd64
mount -t cd9660 /dev/`mdconfig -f /foo/ubuntu-11.04-server-amd64.iso` /iso_images/ubuntu-11.04-server-amd64
mkdir /tftpboot/ubuntu-11.04-server-amd64
cp /iso_images/ubuntu-11.04-server-amd64/install/netboot/ubuntu-installer/amd64/linux /tftpboot/ubuntu-11.04-server-amd64
cp /iso_images/ubuntu-11.04-server-amd64/install/netboot/ubuntu-installer/amd64/initrd.gz /tftpboot/ubuntu-11.04-server-amd64

So now our /tftpboot directory has these five files underneath it:

pxelinux.0
pxelinux.cfg/default
menu.c32
ubuntu-11.04-server-amd64/linux
ubuntu-11.04-server-amd64/initrd.gz

To the /tftpboot/pxelinux.cfg/default file append

LABEL ubuntu-11.04-server-amd64-install             
    MENU LABEL Ubuntu 11.04 Server AMD64 Install
    kernel ubuntu-11.04-server-amd64/linux
    append vga=788 initrd=ubuntu-11.04-server-amd64/initrd.gz

Try PXE booting your client again, this time you'll have "Ubuntu 11.04 Server AMD64 Install" as one of your choices, select that, cross your fingers, and if all goes well in a few seconds you should see:

Initial success

and you can go through and answer the initial questions about the install.

If you're OK with pulling the bulk of the OS over the internet from the official Ubuntu mirrors, it should work although it might be slow. Since we have a nice server sitting on our LAN with a copy of the ISO, we should setup to use that and do a much faster install.

Web Server

For this example, I'll assume nginx has been installed as the webserver (any one will do though, so if you've already got apache installed - that'll work fine too).

The default nginx install uses /usr/local/www/nginx as its docroot, lets put a symlink to our mounted ISO image in there:

ln -s /iso_images/ubuntu-11.04-server-amd64 /usr/local/www/nginx

and also put in a minimal Debian Installer "preseed" file in there that'll help things along by telling the installer to use our webserver for the installation packages. Create a text file named /usr/local/www/nginx/ubuntu-11.04-server-amd64.txt with these contents:

d-i mirror/country string manual
d-i mirror/http/hostname string 10.0.0.1
d-i mirror/http/directory string /ubuntu-11.04-server-amd64
d-i mirror/http/proxy string

Check that you can fetch that file with the URL: http://10.0.0.1/ubuntu-11.04-server-amd64.txt

Edit the /tftpboot/pxelinux.cfg/default file and append

url=http://10.66.0.1/ubuntu-11.04-server-amd64.txt

to the end of the append line of our Ubuntu section, so it now looks like:

LABEL ubuntu-11.04-server-amd64-install             
    MENU LABEL Ubuntu 11.04 Server AMD64 Install
    kernel ubuntu-11.04-server-amd64/linux
    append vga=788 initrd=ubuntu-11.04-server-amd64/initrd.gz url=http://10.66.0.1/ubuntu-11.04-server-amd64.txt

Try PXE booting the Ubuntu install again. You'll still get some initial questions about language and keyboard (we can deal with those in another post), but you shouldn't be asked about mirrors - the installer will know to pull files from your local webserver.

Go through the install on the client, watch the /var/log/nginx-access.log file on the server, you'll see the installer fetching all kinds of files, so you'll know it's all working.

You're in business

So at this point you've got yourself a working PXE installation environment and can do a basic Ubuntu server install.

By adding a few more parameters to your seed file and the PXE configuration you can eliminate some of the installer questions. I'll probably write about that in another post, but if you want to figure it out yourself, check out the Ubuntu Installation Guide - Appendix B. Automating the installation using preseeding

There's so many things you can do with the PXE menus, kernel options, and so on - it can't all be covered in one place. But hopefully you've got a good starting point now, if you know all the basic services are in place and working.