Multi-OS Installation: Configuring the PXE Loader Environment

This article is one in a series detailing the implementation of a Multi-OS boot environment. While the article should stand on its own, it may be generally helpful to read the other articles in the series for the highest level of understanding.

The configuration of the PXE environment requires several components:

• dhcpd server
• tftpd server
• PXE bootloader

Each of these items has customized configuration files which will be described below.

At the lowest level there is a dhcpd server listening for initial boot requests. ISC’s dhcpd server, specifically version 3.0.6 which was the current version at the time. On the production RHEL machine, version 3.0.5 from the RPM was used.

The full intricacies of configuring the dhcpd file can be found in the manpage, however at a minimum one needs to specify the following pxelinux entries and create a single subnet stanza for basic functionality.

# pxelinux settings
option space pxelinux;
option pxelinux.magic code 208 = string;
option pxelinux.configfile code 209 = text;
option pxelinux.pathprefix code 210 = text;
option pxelinux.reboottime code 211 = unsigned integer 32;

#and

# Servers subnet
subnet 172.16.32.0 netmask 255.255.254.0 {
authoritative;
option routers 172.16.32.1;
option subnet-mask 255.255.254.0;
option broadcast-address 172.16.33.255;
option domain-name “dhcp.karlmajer.com karlmajer.com”;
range dynamic-bootp 172.16.33.201 172.16.33.251;

ddns-domainname “dhcp.karlmajer.com”;
ddns-rev-domainname “dhcp.33.30.172.in-addr.arpa.”;

next-server 172.16.32.16;

filename “boot/pxelinux.0”;
site-option-space “pxelinux”;
option pxelinux.magic f1:00:74:7e;
option pxelinux.configfile “pxelinux.cfg”;
option pxelinux.pathprefix “/tftpboot/pxelinux/files/”;
option pxelinux.reboottime 30;
}

Several assumptions are made by that configuration snippet that will need updating:

1. *There is no other dhcpd server on the network*
2. The tftpd server is at 172.16.32.16
3. The default router is at 172.16.32.1
4. You’ve placed your pxelinux and configuration files in $TFTPROOT/boot directory
5. Your network spans 2 class Cs, i.e. the 255.255.254 (/23) netmask and 33.255 broadcast.

After making these changes in the dhcpd.conf file, start up the daemon using the method of your choice, i.e., service dhcpd start, svcadm enable dhcpd, or even simply ‘dhcpd’ from the command line.

At this point there should be a functioning dhcpd server on the network. The next thing to configure is the tftpd server. Tftp-hpa version 0.42 was chosen as the tftp server of choice as it supported the remapping of file paths in requests, and offered tcp wrappers as an added bonus. Edit the tftpd.map file, often sought by tftpd at /etc/tftpd.map, and add the following lines to it:

rgi ^pxelinux.0 /boot/pxelinux.0
rgi ^pxelinux.cfg/ /boot/
rgi ^boot/pxelinux.cfg/ /boot/
rgi ^boot// /

This sets up the location of the boot files relative to the tftpd root which is specified on startup. In the environment that was built, all provisioning related items were placed in /export/install and the tftpd daemon was chrooted to this path. Therefore the pxelinux related files were in /export/install/boot, the menu files were in /export/install/hostmenu.

Now the tftpd server can be run from either inetd or as a standalone. Given the low usage of the install system, it was decided that tftpd would run from xinetd on RHEL ignoring the startup/shutdown costs of the daemon which may be noticeable were the install frequency several installs per hour rather than week.

Configuration of the inetd process will differ depending on the flavor of unix employed. On RHEL one need simply add a service definition to the xinetd.d directory and the xinetd daemon will pick it up on the next invocation. A sample RHEL stanza looks as follows:

# default: off
# description: The tftp server serves files using the trivial file transfer \
# protocol. The tftp protocol is often used to boot diskless \
# workstations, download configuration files to network-aware printers, \
# and to start the installation process for some operating systems. service tftp
{
socket_type = dgram
protocol = udp
wait = yes
user = root
server = /usr/sbin/in.tftpd
server_args = -m /etc/tftpd.map -v -v -v -s /export/install -t 10
disable = no
per_source = 11
cps = 100 2
flags = IPv4
}

Note the explicit paths to the tftpd.map as well as the chroot to /export/install.

At this point we have a daemon to respond to our PXE requests, and we have a daemon to serve the content using the tftp protocol for the PXE requests. Now we need to supply the file to be served.

PXELinux from http://syslinux.zytor.com/pxe.php was used as the bootloader. The files were unbundled and placed into /export/install/boot. A menu file was created for pxelinux, the contents of which look like:

#Note, this menu starts the local disk, a memtest utility, and a Windows 2000 RIS #Installation.
default local
prompt 1
timeout 0
display /boot/install.msg
F1 /boot/install.msg

label local
localboot 0

label w50aa
kernel /images/W50AA/boot/w50aa.0

label memtest
kernel /boot/memtest
append –

For safety the default configuration chosen is to always favor the internal drives and only attempt to boot from the w50aa stanza on request. Now copy the menu containing the described data into the /export/install/boot/ (adjusted appropriately for your filesystem layout) and either name the file default, or name the file something else and symlink it to default.

At this point you should have the basic workings of a PXE loader environment. To test it first make sure that dhcpd is running:

ps -ef | grep dhcpd

Next use tftp to connect to localhost and request the file pxelinux:

tftp localhost
get pxelinux

Finally, reboot a host with a PXEboot capable network adapter and hit F12 to netboot.

Please let me know if you have any questions or comments.

>>> Karl