Network Working Group D. Hankins
Request for Comments: 5071 ISC
Category: Informational December 2007
Dynamic Host Configuration Protocol Options Used by PXELINUX
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document describes the use by PXELINUX of some DHCP Option Codes
numbering from 208-211.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. MAGIC Option . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Description . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Response to RFC 3942 . . . . . . . . . . . . . . . . . . . 5
4. Configuration File Option . . . . . . . . . . . . . . . . . . 5
4.1. Description . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 6
4.4. Response to RFC 3942 . . . . . . . . . . . . . . . . . . . 6
4.5. Client and Server Behaviour . . . . . . . . . . . . . . . 6
5. Path Prefix Option . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Description . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Response to RFC 3942 . . . . . . . . . . . . . . . . . . . 8
5.5. Client and Server Behaviour . . . . . . . . . . . . . . . 8
6. Reboot Time Option . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Description . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Packet Format . . . . . . . . . . . . . . . . . . . . . . 9
6.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 10
6.4. Response to RFC 3942 . . . . . . . . . . . . . . . . . . . 10
6.5. Client and Server Behaviour . . . . . . . . . . . . . . . 10
7. Specification Conformance . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . . 12
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1. Introduction
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 [3] 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 Trivial File Transfer
Protocol (TFTP) [6] one or more images that are decompressed into
memory, then 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 [4].
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 path/filename component of the
configuration file's location, which 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).
Historically, these option codes numbering from 208-211 were
designated 'Site Local', but after publication of RFC3942 [8], they
were made available for allocation as new standard DHCP options.
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This document marks these codes as assigned.
This direct assignment of option code values in the option
definitions below is unusual as it is not mentioned in DHCP Option
Code assignment guidelines [5]. This document's Option Code
assignments are done within RFC 3942's provisions for documenting
prior use of option codes within the new range (128-223 inclusive).
2. Terminology
o "first-stage bootloader" - Although a given bootloading order may
have many stages, such as where a BIOS boots a DOS Boot Disk,
which 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 RFC 2119 [2].
3. MAGIC Option
3.1. Description
If this option is provided to the PXE bootloader, then the value is
checked by PXELINUX 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.
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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.
3.4. Response to RFC 3942
The option code 208 will be adopted for this purpose and immediately
deprecated. Future standards action may return this option to an
available status should it be necessary.
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 reasonable to utilize this option code for another purpose, but
it is recommended to do this at a later time, given the desire to
avoid potential collisions in legacy user bases.
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.
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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 [1] 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 configurations within
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 RFC 3942
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.
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.
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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 Media
Access Control (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 a 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.
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
whether that needs to manifest itself on the protocol wire.
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It only becomes interesting from a protocol standpoint if other
options are adopted that 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, e.g., the
FILENAME header contents or the DHCP Boot File Name option (#67).
Use of these fields is encoded on firmware of thousands of devices
that 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. Whether this sort of use is truly needed remains unproven.
5.4. Response to RFC 3942
The code 210 will be adopted for this purpose.
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 that 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.
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RFC 5071 PXELINUX Options December 2007
6. Reboot Time Option
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 which specific timeout is desired. The client will
reboot itself if it fails to achieve 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.
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.
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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 RFC 3942
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 that 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.
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.
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RFC 5071 PXELINUX Options December 2007
7. Specification Conformance
To conform to this specification, clients and servers MUST implement
the Configuration File, Path Prefix, and Reboot Time options as
directed.
The MAGIC option MAY NOT be implemented, as it has been deprecated.
8. 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 [7] 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, in fact, to 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.
A nearby attacker might feed a "Reboot Time" option value of 1 second
to a mass of unsuspecting clients, to effect a Denial Of Service
(DoS) upon the DHCP server, but then again it may just as easily
supply these clients with rogue second-stage bootloaders that simply
transmit a flood of packets.
This document in and by itself provides no security, nor does it
impact existing DCHP security as described in RFC 2131 [3].
9. IANA Considerations
IANA has done the following:
1. Moved DHCPv4 Option code 208 from 'Tentatively Assigned' to
'Assigned', referencing this document. IANA has marked this same
option code, 208, as Deprecated.
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2. Moved DHCPv4 Option code 209 from 'Tentatively Assigned' to
'Assigned', referencing this document.
3. Moved DHCPv4 Option code 210 from 'Tentatively Assigned' to
'Assigned', referencing this document.
4. Moved DHCPv4 Option code 211 from 'Tentatively Assigned' to
'Assigned', referencing this document.
10. 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 that has improved
this document.
11. References
11.1. Normative References
[1] Postel, J. and J. Reynolds, "Telnet Protocol Specification",
STD 8, RFC 854, May 1983.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[4] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[5] Droms, R., "Procedures and IANA Guidelines for Definition of New
DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.
11.2. Informative References
[6] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350,
July 1992.
[7] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[8] Volz, B., "Reclassifying Dynamic Host Configuration Protocol
version 4 (DHCPv4) Options", RFC 3942, November 2004.
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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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RFC 5071 PXELINUX Options December 2007
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