Dynamic Host Configuration Working D. Hankins
Group ISC
Internet-Draft May 2006
Expires: November 2, 2006
PXELINUX Use of 'Site Local' Option Space
draft-dhankins-pxelinux-02
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Copyright (C) The Internet Society (2006).
Abstract
This document is in response to RFC3942 [1], and describes the use by
PXELINUX of some DHCP Option Codes [2] numbering from 208-211. These
codes were designated 'Site Local' [3] prior to this action, and are
redefined by RFC3942 as available for allocation as standard DHCP
Options.
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Table of Contents
1. PXELINUX in a Nutshell . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. MAGIC Option . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Description . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Response to RFC3942 . . . . . . . . . . . . . . . . . . . 5
4. Configuration File Option . . . . . . . . . . . . . . . . . . 5
4.1. Description . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Response to RFC3942 . . . . . . . . . . . . . . . . . . . 6
4.5. Client and Server Behaviour . . . . . . . . . . . . . . . 6
5. Path Prefix Option . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Description . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Response to RFC3942 . . . . . . . . . . . . . . . . . . . 7
5.5. Client and Server Behaviour . . . . . . . . . . . . . . . 8
6. Option 211 - Reboot Time . . . . . . . . . . . . . . . . . . . 8
6.1. Description . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 9
6.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 9
6.4. Response to RFC3942 . . . . . . . . . . . . . . . . . . . 9
6.5. Client and Server Behaviour . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. PXELINUX in a Nutshell
PXE, the Preboot eXecution Environment, is a first-stage network
bootstrap agent. PXE is loaded out of firmware on the client host,
and performs DHCP queries to obtain an IP Address.
Once on the network, it loads a second-stage bootstrap agent as
configured by DHCP header and option contents.
PXELINUX is one such second-stage bootstrap agent. Once PXE has
passed execution to it, PXELINUX seeks its configuration from a cache
of DHCP Options supplied to the PXE first-stage agent, and then takes
action based upon those options.
Most frequently, this implies loading via TFTP [4] one or more images
which are decompressed into memory and executed to pass execution to
the final Host Operating System.
PXELINUX uses DHCP Options 208-211 to govern parts of this bootstrap
process, but these options are not requested by the PXE DHCP Client
at the time it acquires its lease...at that time, the PXE bootloader
has no knowledge that PXELINUX is going to be in use, and even so
would have no way to know what option(s) PXELINUX might digest.
Local installations that serve this PXELINUX image to its clients
must also configure their DHCP Servers to provide these options even
though they are not on the DHCP Parameter Request List.
These options are:
o "MAGIC" - 208 - An option whose presence and content verifies to
the PXELINUX bootloader that the options numbered 209-211 are for
the purpose as described herein.
o "ConfigFile" - 209 - Configures the file location of the
configuration file this bootloader should use to configure itself.
o "Pathprefix" - 210 - Configures a value to be prepended to the
ConfigFile, to discern the directory location of the file.
o "Reboottime" - 211 - Configures a timeout after which the
bootstrap program will reboot the system (most likely returning it
to PXE).
2. Terminology
o "first-stage bootloader" - Although a given boot loading order may
have many stages, such as where a BIOS boots a DOS Boot Disk which
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then loads a PXE executable, it is in this example only the PXE
executable that this document describes as the "first-stage
bootloader" - in essence, this is the first stage of booting at
which DHCP is involved.
o "second-stage bootloader" - This describes a program loaded by the
first-stage bootloader at the behest of the DHCP Server.
o "bootloader" and "network bootstrap agent" - These are synonyms,
excepting that "bootloader" is intentionally vague in that its
next form of bootstrapping may not in fact involve network
resources.
The key words "MAY", "MUST", "MUST NOT", "SHOULD", and "SHOULD NOT"
in this document are to be interpreted as described in RFC2119 [5].
3. MAGIC Option
3.1. Description
If this option is provided to the PXE bootloader, then the value is
checked to match the octet string f1:00:74:7e. If this matches, then
PXELINUX bootloaders will also consume options 209-211, as described
below. Otherwise, they are ignored.
This measure was intended to ensure that, as the site-local option
space is not allocated from a central authority, no conflict would
result in a PXELINUX bootloader improperly digesting options intended
for another purpose.
3.2. Packet Format
The MAGIC Option format is as follows:
Code Length m1 m2 m3 m4
+--------+--------+--------+--------+--------+--------+
| 208 | 4 | 0xF1 | 0x00 | 0x74 | 0x7E |
+--------+--------+--------+--------+--------+--------+
The code for this option is 208. The length is always four.
3.3. Applicability
This option is absolutely inapplicable to any other purpose.
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3.4. Response to RFC3942
No action will be taken. A collision of the use of this option is
harmless (at least from PXELINUX' point of view) by design: if it
does not match the aforementioned magic value, the PXELINUX
bootloader will take no special action.
The PXELINUX project will deprecate the use of this option, future
versions of the software will not evaluate its contents.
It is not only reasonable to utilize this option code for another
purpose, it is recommended, except that it is undesirable for any
future consumer of this option code to have to suffer potential
collisions in legacy userbases.
4. Configuration File Option
4.1. Description
Once the PXELINUX executable has been entered from the PXE
bootloader, it evaluates this option and loads a file of that name
via TFTP. The contents of this file serve to configure PXELINUX in
its next stage of bootloading (specifying boot image names,
locations, boot-time flags, text to present the user in menu
selections, etc).
In the absence of this option, the PXELINUX agent will search the
TFTP Server (as determined by PXE prior to this stage) for a config
file of several default names.
4.2. Packet Format
The Configuration File Option format is as follows:
Code Length Config-file...
+--------+--------+--------+--------+--------+--------+
| 209 | n | c1 | c2 | ... | c(n) |
+--------+--------+--------+--------+--------+--------+
The code for this option is 209. The Config-file (c1..c(n)) is an
NVT-ASCII printable string, it is not terminated by a zero or any
other value.
4.3. Applicability
Any bootloader, PXE or otherwise, that makes use of a separate
configuration file rather than containing all configuration within
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DHCP options (which may be impossible due to the limited space
available for DHCP options) may conceivably make use of this option.
4.4. Response to RFC3942
The code 209 will be adopted for this purpose.
4.5. Client and Server Behaviour
The Config File Option MUST be supplied by the DHCP Server if it
appears on the Parameter Request List, but MUST also be supplied if
the server administrator believed it would later be useful to the
client (such as because the server is configured to offer a second-
stage boot image which they know will make use of it). The option
MUST NOT be supplied if no value has been configured for it, or if a
value of zero length has been configured.
The DHCP Client MUST only cache this option in a location the second-
stage bootloader may access it.
The second-stage bootloader MUST, in concert with other DHCP Options
and fields, use this option's value as a filename to be loaded via
TFTP and read for further second-stage-loader-specific configuration
parameters. The format and content of such a file is specific to the
second-stage bootloader, and as such is out of scope of this
document.
5. Path Prefix Option
5.1. Description
In PXELINUX' case, it is often the case that several different
environments would have the same TFTP path prefix, but would have
different filenames (for example: hosts' bootloader images and config
files may be kept in a directory structure derived from their MAC
Address). Consequently, it was deemed worthwhile to deliver a TFTP
path prefix configuration option, so that these two things could be
configured separately in DHCP Server configuration: the prefix and
the possibly-host-specific file location.
The actual filename that PXELINUX requests from its TFTP server is
derived by prepending this value to the Config File Option above.
Once this config file is loaded and during processing, any TFTP file
paths specified within it are similarly processed - prepending the
contents of this option.
The contents of the Path Prefix option are also prepended to all
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configured filename locations within the PXELINUX configuration file.
5.2. Packet Format
The Path Prefix Option format is as follows:
Code Length Path-Prefix...
+--------+--------+--------+--------+--------+--------+
| 210 | n | p1 | p2 | ... | p(n) |
+--------+--------+--------+--------+--------+--------+
The code for this option is 210. The Path Prefix is an NVT-ASCII
printable string, it is not terminated by zero or any other value.
5.3. Applicability
This option came into existence because server administrators found
it useful to configure the prefix and suffix of the config file path
separately. A group of different PXE booting clients may use the
same path prefix, but different filenames, or vice versa.
The 'shortcut' this represents is worthwhile, but it is questionable
wether that needs to manifest itself on the protocol wire.
It only becomes interesting from a protocol standpoint if other
options are adopted which prefix this value as well - performing a
kind of string compression is highly beneficial to the limited
available DHCP option space.
But it's clearly inapplicable to any current use of eg the FILENAME
header contents, or the DHCP Boot File Name option (#67). Use of
these fields is encoded on firmware of thousands of devices which
can't or are not likely to be upgraded. Altering any behaviour here
is likely to cause severe compatibility problems.
Although compression of the TFTP-loaded configuration file contents
is not a compelling factor, contrived configurations using these
values may also exist: Where each of a large variety of different
clients load the same configuration file, with the same contents, but
due to a differently configured path prefix actually load different
images. Wether this sort of use is truly needed remains unproven.
5.4. Response to RFC3942
The code 210 will be adopted for this purpose.
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5.5. Client and Server Behaviour
The Path Prefix option MUST be supplied by the DHCP Server if it
appears on the Parameter Request List, but MUST also be supplied if
the server administrator believed it would later be useful to the
client (such as because the server is configured to offer a second-
stage boot image which they know will make use of it). The option
MUST NOT be supplied if no value has been configured for it, or if a
value of zero length has been configured.
The DHCP Client MUST only cache this option in a location where the
second-stage bootloader may access it.
The second-stage bootloader MUST prepend this option's value, if any,
to the contents of the ConfigFile option prior to obtaining the
resulting value via TFTP, or the default 'Config File Search Path'
which the second-stage bootloader iterates in the absence of a Config
File Option. The client MAY prepend the value to other configuration
directives within that file once it has been loaded. The client MUST
NOT prepend this option's value to any other DHCP option contents or
field, unless explicitly stated in a document describing that option
or field.
6. Option 211 - Reboot Time
6.1. Description
Should PXELINUX be executed, and then for some reason be unable to
reach its TFTP server to continue bootstrapping, the client will by
default reboot itself after 300 seconds have passed. This may be too
long, too short, or inappropriate behaviour entirely, depending on
the environment.
By configuring a non-zero value in this option, admins can inform
PXELINUX of what specific timeout is desired. The client will reboot
itself if it fails to acheive its configured network resources within
the specified number of seconds.
This reboot will run through the system's normal boot-time execution
path, most likely leading it back to PXE and therefore PXELINUX. So,
in the general case, this is akin to returning the client to the DHCP
INIT state.
By configuring zero, the feature is disabled, and instead the client
chooses to remove itself from the network and wait indefinitely for
operator intervention.
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It should be stressed that this is in no way related to configuring a
lease-time. The perceived transition to INIT state is due to client
running state - reinitializing itself - not due to lease timer
activity. That is, it is not safe to assume that a PXELINUX client
will abandon its lease when this timer expires.
6.2. Packet Format
The Reboot Time Option format is as follows:
Code Length
+--------+--------+--------+--------+--------+--------+
| 211 | 4 | Reboot Time |
+--------+--------+--------+--------+--------+--------+
The code for this option is 211. The length is always four. The
Reboot Time is a 32-bit (4 byte) integer in network byte order.
6.3. Applicability
Any network bootstrap program in any sufficiently complex networking
environment could conceivably enter into such a similar condition.
Either due to having its IP address stolen out from under it by a
rogue client on the network, by being moved between networks where
its PXE-derived DHCP lease is no longer valid, or any similar means.
It seems desirable for any network bootstrap agent to implement an
ultimate timeout for it to start over.
The client may, for example, get different, working configuration
parameters from a different DHCP server upon restarting.
6.4. Response to RFC3942
The code 211 will be adopted for this purpose.
6.5. Client and Server Behaviour
The Reboot Time Option MUST be supplied by the DHCP Server if it
appears on the Parameter Request List, but MUST also be supplied if
the server administrator believed it would later be useful to the
client (such as because the server is configured to offer a second-
stage boot image which they know will make use of it). The option
MUST NOT be supplied if no value has been configured for it, or if it
contains a value of zero length.
The DHCP Client MUST only cache this option in a location the second-
stage bootloader may access it.
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If the value of this option is nonzero, the second-stage bootloader
MUST schedule a timeout: after a number of seconds equal to this
option's value have passed, the second-stage bootloader MUST reboot
the system, ultimately returning the path of execution back to the
first-stage bootloader. It MUST NOT reboot the system once the
thread of execution has been passed to the host operating system (at
which point this timeout is effectively obviated).
If the value of this option is zero, the second-stage bootloader MUST
NOT schedule such a timeout at all. Any second-stage bootloader that
finds it has encountered excessive timeouts attempting to obtain its
host operating system SHOULD disconnect itself from the network to
wait for operator intervention, but MAY continue to attempt to
acquire the host operating system indefinitely.
7. Security Considerations
PXE and PXELINUX allow any entity acting as a DHCP server to execute
arbitrary code upon a system. At present, no PXE implementation is
known to implement Authentication mechanisms so that PXE clients can
be sure they are receiving configuration information from the
correct, authoritative DHCP Server.
The use of TFTP by PXE and PXELINUX also lacks any form of
cryptographic signature - so a 'Man in the Middle' attack may lead to
an attacker's code being executed on the client system. Since this
is not an encrypted channel, any of the TFTP loaded data may also be
exposed (such as in loading a "RAMDISK" image, which contains /etc/
passwd or similar information).
The use of the Ethernet MAC Address as the client's unique identity
may allow an attacker who takes on that identity to gain
inappropriate access to a client system's network resources by being
given by the DHCP Server whatever 'keys' are required to in fact be
the target system (to boot up as though it were the target).
Great care should be taken to secure PXE and PXELINUX installations,
such as by using IP Firewalls, to reduce or eliminate these concerns.
The use of these options present no additional security risk.
8. IANA Considerations
IANA is requested to:
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1. Move DHCPv4 Option code 208 from 'Tentatively Assigned' to
'Unassigned, Last Resort'. It is hoped that Unassigned DHCP
Option Codes (that had never been Tentatively Assigned) SHOULD be
allocated prior to assigning this option code, but otherwise
SHOULD be allocated before any option code that has been
Tentatively Assigned, or Assigned.
2. Move DHCPv4 Option code 209 from 'Tentatively Assigned' to
'Assigned', referencing this document.
3. Move DHCPv4 Option code 210 from 'Tentatively Assigned' to
'Assigned', referencing this document.
4. Move DHCPv4 Option code 211 from 'Tentatively Assigned' to
'Assigned', referencing this document.
9. Acknowledgements
These options were designed and implemented for the PXELINUX project
by H. Peter Anvin, and he was instrumental in producing this
document. Shane Kerr has also provided feedback which has improved
this document.
10. References
[1] Volz, B., "Reclassifying Dynamic Host Configuration Protocol
version 4 (DHCPv4) Options", RFC 3942, November 2004.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[4] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350,
July 1992.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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Author's Address
David W. Hankins
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
US
Phone: +1 650 423 1307
Email: David_Hankins@isc.org
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