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====== PXE chainloading ======

{{ :screenshots:gpxe-chaining.png?361x201|PXE chainloading in VMware}}

If you have a large number of machines which already have a legacy PXE implementation (e.g. network cards containing an Intel PXE ROM), then you may want to avoid having to reflash each machine's network card.  You can achieve this by placing gPXE on your TFTP server.  The PXE-capable machines will download gPXE via TFTP, and instantly become gPXE-capable machines.

===== Setting up PXE chainloading =====

Start by [[download|downloading]] the source tree, then build the PXE-chainloadable gPXE image using

    cd src
    make bin/undionly.kpxe

Copy //bin/undionly.kpxe// to your TFTP server, and configure your DHCP server to hand out //undionly.kpxe//; if you are using ISC dhcpd then you need to edit ///etc/dhcpd.conf// to contain

    next-server X.X.X.X;
    filename "undionly.kpxe";

where X.X.X.X is the IP address of your TFTP server.  At this point, you should be able to boot one of your PXE-capable machines, and see it download gPXE from the TFTP server.  If everything has worked, then you should see the gPXE startup banner appear:

    gPXE 0.9.7 -- Open Source Boot Firmware -- http://etherboot.org

===== Breaking the infinite loop =====

When the chainloaded gPXE starts up, it will issue a fresh DHCP request and boot whatever the DHCP server hands out.  The DHCP server is currently set up to hand out the gPXE image, which means that you will be stuck in an infinite loop: PXE will load gPXE which will load gPXE which will load gPXE which will load gPXE...

You can break this infinite loop by configuring the DHCP server to hand out gPXE only for the first DHCP request; the second DHCP request will return the "real" boot filename.

==== Using ISC dhcpd ====

If you are running ISC dhcpd as your DHCP server, then edit ///etc/dhcpd.conf// to contain

    if not ( option user-class = "gPXE" ) {
        filename "undionly.kpxe";
    }
    
    filename "http://my.web.server/real_boot_script.php";

This will ensure that the gPXE image (//undionly.kpxe//) is handed out only when the DHCP request comes from a legacy PXE client.  Once gPXE has been loaded, the DHCP server will direct it to boot from //%%http://my.web.server/real_boot_script.php%%//.

==== Using the Windows DHCP server ====

If you are using the Windows DHCP server, then you must first define a user class for gPXE.  Right-click on your DHCP server and choose "Define User Classes...":

{{ :screenshots:winnt_dhcp_define_user_class.png?666x479 |Defining a DHCP User Class in Windows}}

Click on //Add...// to add a new user class.  Set the display name to "gPXE", the description to "gPXE Clients", and the ID to "gPXE":

{{ :screenshots:winnt_dhcp_define_gpxe_user_class.png?369x379 |Defining the gPXE DHCP User Class in Windows}}

Right-click on //Server Options// and choose "Configure Options...".  On the //Advanced// tab, select "gPXE" from the //User class// drop-down list.  Scroll down to option "067 Bootfile Name" and set it to "%%http://my.web.server/real_boot_script.php%%":

{{ :screenshots:winnt_dhcp_set_gpxe_filename.png?408x448 |Defining the filename for gPXE clients in Windows}}

If everything has worked, then you should see two settings for "067 Bootfile Name" in your DHCP configuration: one containing "undionly.kpxe" and one containing "%%http://my.web.server/real_boot_script.php%%":

{{ :screenshots:winnt_dhcp_loop_broken.png?666x242 |Working PXE-chaining DHCP configuration in Windows}}

This will ensure that the gPXE image (//undionly.kpxe//) is handed out only when the DHCP request comes from a legacy PXE client.  Once gPXE has been loaded, the DHCP server will direct it to boot from //%%http://my.web.server/real_boot_script.php%%//.

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