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Update: gPXE has been forked to a new development called iPXE. iPXE is being actively developed by the same team that worked on gPXE and have made many new code changes while gPXE has remained relatively static (look at the respective changelogs for confirmation). The iPXE project has the same features as before with active bug fixes and features being added all the time. Please be sure to double check the commands referenced in this article as they might have changed in name from gPXE to iPXE.

You can turn that PXE enabled network card into an iSCSI enabled HBA for free. Save yourself a couple of bucks on an iSCSI HBA, and boot your server/workstation diskless via iSCSI.

Here is how you turn your card into an HBA.

gPXE iPXE is a PXE compatible bootloader that provides some great functionality including AoE (ATA Over Ethernet), HTTP (loading boot scripts and boot images from HTTP), and the one we are most interested in, iSCSI which allows us to boot from an iSCSI target.

We start off with a working PXE enviroment; DHCP server (to provide IP and PXE settings) and TFTP server (to provide the PXE files we need to load). Now in order to get PXE to load the gPXE iPXE firmware we need to do what is called “chainloading”. This means that our network card will do its standard PXE boot up, and when it loads, we will then load the gPXE iPXE loader and use gPXE iPXE for the rest of the boot process.

Here is how we do that:

In our DHCPd server we need to add some specific settings to enable us to detect wether or not the DHCP request is coming from PXE, or gPXE.

/etc/dhcpd.conf:

allow booting;
allow bootp;
default-lease-time 600;
max-lease-time 7200;
authoritative;
option space gpxe;
option gpxe-encap-opts code 175 = encapsulate gpxe;
option gpxe.bus-id code 177 = string;
ddns-update-style ad-hoc;
subnet 192.168.2.0 netmask 255.255.255.0 {
    use-host-decl-names on;
    range 192.168.2.20 192.168.2.200;
    option subnet-mask 255.255.255.0;
    option broadcast-address 192.168.2.255;
    default-lease-time 1800;
    max-lease-time 86400;
    option domain-name-servers 192.168.1.10;
    next-server 192.168.2.1;
    if not exists gpxe.bus-id {
        filename "undionly.kpxe";
    } else {
        filename "http://192.168.2.1/default/install.gpxe";
    }
}

The important lines are:

option space gpxe;
option gpxe-encap-opts code 175 = encapsulate gpxe;
option gpxe.bus-id code 177 = string;

and

if not exists gpxe.bus-id {
    filename "undionly.kpxe";
} else {
    filename "http://192.168.2.1/default/install.gpxe";
}

This conditional statement allows us to load either the gPXE chainloader when we are called from a standard PXE request (the if not exists gpxe.bus-id) or a gPXE compatiable script when we are called from gPXE.

We are currently using this setup to handle new server OS installations, hence the install.gpxe file.

The contents of that file are rather simple.

install.gpxe:

#!gpxe
kernel http://192.168.2.1/default/centos5 askmethod
initrd http://192.168.2.1/default/centos5.img
boot

This loads the CentOS 5 PXE installation image and initrd to handle OS installation on the server.

Once the server has its OS installed, we then need to add the server’s MAC address to the DHCPd server so that it will chain load gPXE and then load the server’s root disk via iSCSI.

Here is how we accomplish that:

/etc/dhcpd.conf (added in the subnet 192.168.2.0 section above):

host server01 {
    hardware ethernet 00:xx:xx:xx:xx:xx;
    fixed-address 192.168.2.21;
    if not exists gpxe.bus-id {
        filename "undionly.kpxe";
    } else {
        filename "";
        option root-path "iscsi:192.168.2.1::::iqn.2001-04.com.server:server01.vg00.lun0";
    }
}

This, again, chainloads the gPXE iPXE chainloader from PXE, and on the next DHCP request from gPXE iPXE we provide the iSCSI target to load the root for the server. This brings up the normal GRUB screen and the system boots as normal.

And that is how you turn your PXE enabled network card, into an iSCSI HBA.

Gotchas:
I had originally wanted to use gPXE/iSCSI to host the root drive for a Xen based Dom0, however I have discovered that the Xen hypervisor does not support this feature. I have done some searching on the internet and it seems that the problem does lie with Xen’s hypervisor kernel and its inability to read the iBFT (Iscsi Boot Firmware Table). gPXE does support and utilize the iBFT, however the Xen Hypervisor kernel only recognizes the iBFT from certain iSCSI HBAs (listed in thier HCL).