PXELINUX

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What is PXELINUX?

PXELINUX is a SYSLINUX derivative, for booting Linux off a network server, using a network ROM conforming to the Intel PXE (Pre-Execution Environment) specification. PXELINUX is not a program that is intended to be flashed or burned into a PROM on the network card; if you want that, check out Etherboot ( http://www.etherboot.org/).

If you want to create PXE-compliant boot PROM for your network card (to use with PXELINUX, for example), check out NetBoot (http://netboot.sourceforge.net/).

How do I Configure PXELINUX?

PXELINUX operates in many ways like SYSLINUX. If you are not familiar with SYSLINUX, read the SYSLINUX FAQ first, since this documentation only explains the differences.

On the TFTP server, create the directory "/tftpboot", and copy pxelinux.0 (from the SYSLINUX distribution) and any kernel or initrd images that you want to boot.

Finally, create the directory "/tftpboot/pxelinux.cfg". The configuration file (equivalent of syslinux.cfg -- see the SYSLINUX FAQ for the options here) will live in this directory. Because more than one system may be booted from the same server, the configuration file name depends on the IP address of the booting machine. PXELINUX will search for its config file on the boot server in the following way:

First, it will search for the config file using the hardware type (using its ARP type code) and address, all in lower case hexadecimal with dash separators; for example, for an Ethernet (ARP type 1) with address 88:99:AA:BB:CC:DD it would search for the filename 01-88-99-aa-bb-cc-dd. 
Next, it will search for the config file using its own IP address in upper case hexadecimal, e.g. 192.0.2.91 -> C000025B (you can use the included progam gethostip to compute the hexadecimal IP address for any host.) 
If that file is not found, it will remove one hex digit and try again. Ultimately, it will try looking for a file named default (in lower case). As an example, if the boot file name is /mybootdir/pxelinux.0, the Ethernet MAC address is 88:99:AA:BB:CC:DD and the IP address 192.0.2.91, it will try: 
       /mybootdir/pxelinux.cfg/01-88-99-aa-bb-cc-dd
       /mybootdir/pxelinux.cfg/C000025B
       /mybootdir/pxelinux.cfg/C000025
       /mybootdir/pxelinux.cfg/C00002
       /mybootdir/pxelinux.cfg/C0000
       /mybootdir/pxelinux.cfg/C000
       /mybootdir/pxelinux.cfg/C00
       /mybootdir/pxelinux.cfg/C0
       /mybootdir/pxelinux.cfg/C
       /mybootdir/pxelinux.cfg/default

... in that order. 
Note that all filename references are relative to the directory pxelinux.0 lives in. PXELINUX generally requires that filenames (including any relative path) are 127 characters or shorter in length. 

PXELINUX does not support MTFTP, and I have no immediate plans of doing so. It is of course possible to use MTFTP for the initial boot, if you have such a setup. MTFTP server setup is beyond the scope of this document.

How Should I Setup my TFTP server?

PXELINUX currently requires that the boot server has a TFTP server which supports the "tsize" TFTP option (RFC 1784/RFC 2349).

Also, please do check out the problematic hardware reference page to see if your PXE stacks need any special workarounds.

Some TFTP servers which have been successfully used with PXELINUX include:

  • The "tftp-hpa" TFTP server, a highly portable update and port of the BSD TFTP server code is available at:
http://www.kernel.org/pub/software/network/tftp/ 
ftp://www.kernel.org/pub/software/network/tftp/ 
... and on any kernel.org mirror (see http://www.kernel.org/mirrors/). 
  • Another TFTP server which supports this is atftp by Jean-Pierre Lefebvre:

ftp://ftp.mamalinux.com/pub/atftp/

atftp is likely going to perform better than tftp-hpa on a large boot server, but may not be quite as widely portable. 
If your boot server runs Windows (and you can't fix that), try tftpd32 by Philippe Jounin: 
http://tftpd32.jounin.net/ 
  • Eric Cook of Intel also reports that the TFTPD server from Win2000 Server RIS can be used:
The trick is to install RIS, but DON'T configure it with the GUI. Instead, do the following: 
In the registry, add the folder  \HKLM\System\CurrentControlSet\Services\TFTPD\Parameters. In the Parameters folder, add a key called Directory, with a value of the TFTP root directory path. With the Services GUI, configure the TFTPD service for Automatic start and start it.  
If you DO configure the RIS in Win2k, you end up with the MS PXE stuff, which is ugly to get rid of. 
However, Christian "Dr. Disk" Hechelmann reports having success with using the Windows RIS as-is, and has sent a nice writeup on how to set it up. See this link. 

How Should I Setup My DHCP server?

The PXE protocol uses a very complex set of extensions to DHCP or BOOTP. However, most PXE implementations -- this includes all Intel ones version 0.99n and later -- seem to be able to boot in a "conventional" DHCP/TFTP configuration. Assuming you don't have to support any very old or otherwise severely broken clients, this is probably the best configuration unless you already have a PXE boot server on your network.

A sample DHCP setup, using the "conventional TFTP" configuration, would look something like the following, using ISC dhcp (2.0 or later) dhcpd.conf syntax:

       allow booting;
       allow bootp;
       # Standard configuration directives...
       option domain-name "domain_name";
       option subnet-mask subnet_mask;
       option broadcast-address broadcast_address;
       option domain-name-servers dns_servers;
       option routers default_router;
       # Group the PXE bootable hosts together
       group {
               # PXE-specific configuration directives...
               next-server TFTP_server_address;
               filename "/tftpboot/pxelinux.0";
               # You need an entry like this for every host
               # unless you're using dynamic addresses
               host hostname {
                       hardware ethernet ethernet_address;
                       fixed-address hostname;
               }
       }

Note that if your particular TFTP daemon runs under chroot (tftp-hpa will do this if you specify the -s (secure) option; this is highly recommended), you almost certainly should not include the /tftpboot prefix in the filename statement.

If this does not work for your configuration, you probably should set up a "PXE boot server" on port 4011 of your TFTP server; a free PXE boot server is available at: http://www.kano.org.uk/projects/pxe/

With such a boot server defined, your DHCP configuration should look the same except for an "option dhcp-class-identifier" (ISC dhcp 2) or "option vendor-class-identifier" (ISC dhcp 3):

       allow booting;
       allow bootp;
       # Standard configuration directives...
       option domain-name "domain_name";
       option subnet-mask subnet_mask;
       option broadcast-address broadcast_address;
       option domain-name-servers dns_servers;
       option routers default_router;
       # Group the PXE bootable hosts together
       group {
               # PXE-specific configuration directives...
               option dhcp-class-identifier "PXEClient";
               next-server pxe_boot_server_address;
               # You need an entry like this for every host
               # unless you're using dynamic addresses
               host hostname {
                       hardware ethernet ethernet_address;
                       fixed-address hostname;
               }
       }

Here, the boot file name is obtained from the PXE server.

If the "conventional TFTP" configuration doesn't work on your clients, and setting up a PXE boot server is not an option, you can attempt the following configuration. It has been known to boot some configurations correctly; however, there are no guarantees:

       allow booting;
       allow bootp;
       # Standard configuration directives...
       option domain-name "domain_name";
       option subnet-mask subnet_mask;
       option broadcast-address broadcast_address;
       option domain-name-servers dns_servers;
       option routers default_router;
       # Group the PXE bootable hosts together
       group {
               # PXE-specific configuration directives...
               option dhcp-class-identifier "PXEClient";
               option vendor-encapsulated-options 09:0f:80:00:0c:4e:65:74:77:6f:72:6b:20:62:6f:6f:74:0a:07:00:50:72:6f:6d:70:74:06:01:02:08:03:80:00:00:47:04:80:00:00:00:ff;
               next-server TFTP_server;
               filename "/tftpboot/pxelinux.0";
               # You need an entry like this for every host
               # unless you're using dynamic addresses
               host hostname {
                       hardware ethernet ethernet_address;
                       fixed-address hostname;
               }
       }

Note that this will not boot some clients that will boot with the "conventional TFTP" configuration; Intel Boot Client 3.0 and later are known to fall into this category.

Can I send information to PXELINUX via special options in the DHCP response?

PXELINUX (starting with version 1.62) supports the following nonstandard DHCP options, which depending on your DHCP server you may be able to use to customize the specific behaviour of PXELINUX:

  • Option 208: pxelinux.magic
Must be set to F1:00:74:7E (241.0.116.126) for PXELINUX to recognize any special DHCP options whatsoever. 
  • Option 209: pxelinux.configfile
Specifies the PXELINUX configuration file name. 
  • Option 210: pxelinux.pathprefix
Specifies the PXELINUX common path prefix, instead of deriving it from the boot file name. This almost certainly needs to end in whatever character the TFTP server OS uses as a pathname separator, e.g. slash (/) for Unix. 
  • Option 211: pxelinux.reboottime
Specifies, in seconds, the time to wait before reboot in the event of TFTP failure. 0 means wait "forever" (in reality, it waits approximately 136 years.) 

ISC dhcp 3.0 supports a rather nice syntax for specifying custom options; you can use the following syntax in dhcpd.conf if you are running this version of dhcpd:

       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;

IMPORTANT: This is a site-option-space, not a vendor-option-space. These two types of option spaces are encoded differently, and using the wrong kind of option space will not work.

Inside your PXELINUX-booting group or class (whereever you have the PXELINUX-related options, such as the filename option), you would add, for example:

       # Always include the following lines for all PXELINUX clients
       site-option-space "pxelinux";
       option pxelinux.magic f1:00:74:7e;
       if exists dhcp-parameter-request-list {
               # Always send the PXELINUX options (specified in hexadecimal)
               option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,d0,d1,d2,d3);
       }
       # These lines should be customized to your setup
       option pxelinux.configfile "configs/common";
       option pxelinux.pathprefix "/tftpboot/pxelinux/files/";
       option pxelinux.reboottime 30;
       filename "/tftpboot/pxelinux/pxelinux.bin";

Note that the configfile is relative to the pathprefix: this will look for a config file called /tftpboot/pxelinux/files/configs/common on the TFTP server.

The "option dhcp-parameter-request-list" statement forces the DHCP server to send the PXELINUX-specific options, even though they are not explicitly requested. Since the DHCP request is done before PXELINUX is loaded, the PXE client won't know to request them.

Using ISC dhcp 3.0 you can create a lot of these strings on the fly. For example, to use the hexadecimal form of the hardware address as the configuration file name, you could do something like:

       site-option-space "pxelinux";
       option pxelinux.magic f1:00:74:7e;
       if exists dhcp-parameter-request-list {
               # Always send the PXELINUX options (specified in hexadecimal)
               option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,d0,d1,d2,d3);
       }
       option pxelinux.configfile =
               concat("pxelinux.cfg/", binary-to-ascii(16, 8, ":", hardware));
       filename "/tftpboot/pxelinux.bin";

If you used this from a client whose Ethernet address was 58:FA:84:CF:55:0E, this would look for a configuration file named "/tftpboot/pxelinux.cfg/1:58:fa:84:cf:55:e".

What Happens When a Boot Fails?

If the boot fails, PXELINUX (unlike SYSLINUX) will not wait forever; rather, if it has not received any input for approximately five minutes after displaying an error message, it will reset the machine. This allows an unattended machine to recover in case it had bad enough luck of trying to boot at the same time the TFTP server goes down.

How do I keep the PXE stack loaded after boot?

Normally, PXELINUX will unload the PXE and UNDI stacks before invoking the kernel. In special circumstances (for example, when using MEMDISK to boot an operating system with an UNDI network driver) it might be desirable to keep the PXE stack in memory. If the option "keeppxe" is given on the kernel command line, PXELINUX will keep the PXE and UNDI stacks in memory. (If you don't know what this means, you probably don't need it.)

What Problems Are There Currently With PXELINUX?

Requires a TFTP server which supports the "tsize" option.

The error recovery routine doesn't work quite right. For right now, it just does a hard reset - seems good enough.

We should probably call the UDP receive function in the keyboard entry loop, so that we answer ARP requests.

Boot sectors don't work yet... they probably need auxilliary information (such as device) to work at all.

If you have additional problems, please contact the SYSLINUX mailing list. See the SYSLINUX FAQ for details. Before you post something, please make sure you have checked that your kernel files aren't named using the extensions which have special meaning:

 .0            PXE bootstrap program (NBP) [PXELINUX only]
 .bin          "CD boot sector" [ISOLINUX only]
 .bs           Boot sector [SYSLINUX only]
 .bss          Boot sector, DOS superblock will be patched in [SYSLINUX only]
 .c32          COM32 image (32-bit COMBOOT)
 .cbt          COMBOOT image (not runnable from DOS)
 .com          COMBOOT image (runnable from DOS)
 .img          Disk image [ISOLINUX only]

PXE stack on a floppy

If your network card doesn't have a PXE boot ROM, there is are a couple of PXE stacks available.

Etherboot is a ROM kit that allows you to create your own PXE boot ROM (version 5.3.7 or later required), as well as make one that can be run from a boot floppy. The Etherbot home page is at: http://www.etherboot.org/

... and you can use ROM-o-matic to automatically create customized boot ROMs for your needs. 

http://rom-o-matic.net/ NetBoot is a ROM kit that may allow you to create your own PXE boot ROM, and possibly also run one from a floppy. It is available at: http://netboot.sourceforge.net/

A multi-hardware boot floppy is included with Windows Server 2000 and 2003. A company called Argon Technology used to offer a free-as-in-beer updated version, but is seems to have gone fully commercial. This floppy (which can also be burned to a CD using El Torito in floppy-emulation mode), is known to work with PXELINUX 2.03 or later only.