DHCPv6 network boot: DHCPv6 Options for Network Boot
Four DHCPv6 options that tell a computer where to fetch its boot file and how to run it.
Huth, T., Freimann, J., Zimmer, V., and D. Thaler, "DHCPv6 Options for Network Boot", RFC 5970, DOI 10.17487/RFC5970, September 2010, <https://www.rfc-editor.org/info/rfc5970>.
@misc{rfc5970,
series = {Request for Comments},
number = 5970,
howpublished = {RFC 5970},
publisher = {RFC Editor},
doi = {10.17487/RFC5970},
url = {https://www.rfc-editor.org/info/rfc5970},
author = {T. Huth and J. Freimann and V. Zimmer and D. Thaler},
title = {{DHCPv6 Options for Network Boot}},
pagetotal = 11,
year = 2010,
month = sep,
} RFC text
Contents
Internet Engineering Task Force (IETF) T. Huth
Request for Comments: 5970 J. Freimann
Category: Standards Track IBM Germany R&D GmbH
ISSN: 2070-1721 V. Zimmer
Intel
D. Thaler
Microsoft
September 2010
DHCPv6 Options for Network Boot
Abstract
The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) provides a
framework for passing configuration information to nodes on a
network. This document describes new options for DHCPv6 that SHOULD
be used for booting a node from the network.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5970.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Conventions .....................................................3
3. Options .........................................................3
3.1. Boot File Uniform Resource Locator (URL) Option ............3
3.2. Boot File Parameters Option ................................4
3.3. Client System Architecture Type Option .....................5
3.4. Client Network Interface Identifier Option .................6
4. Appearance of the Options .......................................7
5. Download Protocol Considerations ................................7
6. IANA Considerations .............................................7
7. Security Considerations .........................................8
8. Acknowledgements ................................................8
9. References ......................................................9
9.1. Normative References .......................................9
9.2. Informative References .....................................9
1. Introduction
This document describes DHCPv6 options that SHOULD be used to provide
configuration information for a node that must be booted using the
network rather than from local storage.
Network booting is used, for example, in some environments where
administrators have to maintain a large number of nodes. By serving
all boot and configuration files from a central server, the effort
required to maintain these nodes is greatly reduced.
A typical boot file would be, for example, an operating system kernel
or a boot-loader program. To be able to execute such a file, the
firmware running on the client node must perform the following two
steps (see Figure 1): First get all information that is required for
downloading and executing the boot file. Second, download the boot
file and execute it.
+------+
_______________________\| DHCP |
/ 1 Get boot file info /|Server|
+------+ +------+
| Host |
+------+ +------+
\_______________________\| File |
2 Download boot file /|Server|
+------+
Figure 1: Network Boot Sequence
The information that is required for booting over the network MUST
include at least the details about the server on which the boot files
can be found, the protocol to be used for the download (for example,
HTTP [RFC2616] or TFTP [RFC1350]), and the path and name of the boot
file on the server. Additionally, the server and client MAY exchange
information about the parameters that should be passed to the OS
kernel or boot-loader program, respectively, or information about the
supported boot environment.
DHCPv6 allows client nodes to ask a DHCPv6 server for configuration
parameters. This document provides new options that a client can
request from the DHCPv6 server to satisfy its requirements for
booting. It also introduces a new IANA registry for processor
architecture types that are used by the OPTION_CLIENT_ARCH_TYPE
option (see Section 3.3).
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Terminology specific to IPv6 and DHCPv6 are used in the same way as
is defined in the "Terminology" sections of [RFC3315].
3. Options
Option formats comply with DHCPv6 options per [RFC3315] (Section 6).
The boot-file-url option (see Section 3.1) is mandatory for booting,
all other options are optional.
3.1. Boot File Uniform Resource Locator (URL) Option
The server sends this option to inform the client about a URL to a
boot file.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPT_BOOTFILE_URL | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. boot-file-url (variable length) .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Format description:
option-code OPT_BOOTFILE_URL (59).
option-len Length of the boot-file-url in octets.
boot-file-url This string is the URL for the boot file. It MUST
comply with STD 66 [RFC3986]. The string is not
NUL-terminated.
If the host in the URL is expressed using an IPv6 address rather than
a domain name, the address in the URL then MUST be enclosed in "["
and "]" characters, conforming to [RFC3986]. Clients that have DNS
implementations SHOULD support the use of domain names in the URL.
3.2. Boot File Parameters Option
This option is sent by the server to the client. It consists of
multiple UTF-8 ([RFC3629]) strings. They are used to specify
parameters for the boot file (similar to the command line arguments
in most modern operating systems). For example, these parameters
could be used to specify the root file system of the OS kernel, or
the location from which a second-stage boot-loader program can
download its configuration file.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPT_BOOTFILE_PARAM | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| param-len 1 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ parameter 1 .
. (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. <multiple Parameters> .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| param-len n | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ parameter n .
. (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Format description:
option-code OPT_BOOTFILE_PARAM (60).
option-len Length of the Boot File Parameters option in octets
(not including the size of the option-code and
option-len fields).
param-len 1...n This is a 16-bit integer that specifies the length
of the following parameter in octets (not including
the parameter-length field).
parameter 1...n These UTF-8 strings are parameters needed for
booting, e.g., kernel parameters. The strings are
not NUL-terminated.
When the boot firmware executes the boot file that has been specified
in the OPT_BOOTFILE_URL option, it MUST pass these parameters, if
present, in the order that they appear in the OPT_BOOTFILE_PARAM
option.
3.3. Client System Architecture Type Option
This option provides parity with the Client System Architecture Type
option defined for DHCPv4 in Section 2.1 of [RFC4578].
The format of the option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_CLIENT_ARCH_TYPE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. architecture-types (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_CLIENT_ARCH_TYPE (61).
option-len Length of the "architecture-types" field in
octets. It MUST be an even number greater than
zero. See Section 2.1 of [RFC4578] for details.
architecture-types A list of one or more architecture types, as
specified in Section 2.1 of [RFC4578]. Each
architecture type identifier in this list is a
16-bit value that describes the pre-boot runtime
environment of the client machine. A list of
valid values is maintained by the IANA (see
Section 6).
The client MAY use this option to send a list of supported
architecture types to the server, so the server can decide which boot
file should be provided to the client. If a client supports more
than one pre-boot environment (for example, both 32-bit and 64-bit
executables), the most preferred architecture type MUST be listed as
first item, followed by the others with descending priority.
If the client used this option in the request, the server SHOULD
include this option to inform the client about the pre-boot
environments that are supported by the boot file. The list MUST only
contain architecture types that have initially been queried by the
client. The items MUST also be listed in order of descending
priority.
3.4. Client Network Interface Identifier Option
If the client supports the Universal Network Device Interface (UNDI)
(see [PXE21] and [UEFI23]), it may send the Client Network Interface
Identifier option to a DHCP server to provide information about its
level of UNDI support.
This option provides parity with the Client Network Interface
Identifier option defined for DHCPv4 in Section 2.2 of [RFC4578].
The format of the option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_NII | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Major | Minor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_NII (62).
option-len 3
Type As specified in Section 2.2 of [RFC4578].
Major As specified in Section 2.2 of [RFC4578].
Minor As specified in Section 2.2 of [RFC4578].
The list of valid Type, Major, and Minor values is maintained in the
Unified Extensible Firmware Interface specification [UEFI23].
4. Appearance of the Options
These options MUST NOT appear in DHCPv6 messages other than the types
Solicit, Advertise, Request, Renew, Rebind, Information-Request, and
Reply.
The option-codes of these options MAY appear in the Option Request
option in the DHCPv6 message types Solicit, Request, Renew, Rebind,
Information-Request, and Reconfigure.
5. Download Protocol Considerations
The Boot File URL option does not place any constraints on the
protocol used for downloading the boot file, other than that it MUST
be possible to specify it in a URL. For the sake of administrative
simplicity, we strongly recommend that, at a minimum, implementers of
network boot loaders implement the well-known and established
HyperText Transfer Protocol (HTTP) [RFC2616] for downloading. Please
note that for IPv6, this supersedes [RFC906], which recommended using
TFTP for downloading (see [RFC3617] for the 'tftp' URL definition).
When using the Internet Small Computer System Interface (iSCSI) for
booting, the 'iscsi' URI is formed as defined in [RFC4173]. The
functionality attributed in RFC 4173 to a root path option is
provided for IPv6 by the Boot File URL option instead.
6. IANA Considerations
The following options have been assigned by the IANA from the option
number space defined in Section 24 of the DHCPv6 RFC [RFC3315].
+-------------------------+-------+--------------+
| Option name | Value | Specified in |
+-------------------------+-------+--------------+
| OPT_BOOTFILE_URL | 59 | Section 3.1 |
| OPT_BOOTFILE_PARAM | 60 | Section 3.2 |
| OPTION_CLIENT_ARCH_TYPE | 61 | Section 3.3 |
| OPTION_NII | 62 | Section 3.4 |
+-------------------------+-------+--------------+
This document also introduces a new IANA registry for processor
architecture types. The name of this registry is "Processor
Architecture Types". Registry entries consist of a 16-bit integer
recorded in decimal format and a descriptive name. The initial
values of this registry can be found in [RFC4578], Section 2.1.
The assignment policy for values is through Expert Review (see
[RFC5226]), and any requests for values must supply the descriptive
name for the processor architecture type.
7. Security Considerations
In untrusted networks, a rogue DHCPv6 server could send the new
DHCPv6 options described in this document. The booting clients could
then be provided with a wrong URL so that either the boot fails or,
even worse, the client boots the wrong operating system that has been
provided by a malicious file server. To prevent this kind of attack,
clients SHOULD use authentication of DHCPv6 messages (see Section 21
in [RFC3315]).
Note also that DHCPv6 messages are sent unencrypted by default. So
the boot file URL options are sent unencrypted over the network, too.
This can become a security risk since the URLs can contain sensitive
information like user names and passwords (for example, a URL like
"ftp://username:password@servername/path/file"). At the current
point in time, there is no possibility to send encrypted DHCPv6
messages, so it is strongly RECOMMENDED not to use sensitive
information in the URLs in untrusted networks (using passwords in
URLs is deprecated anyway, according to [RFC3986]).
Even if the DHCPv6 transaction is secured, this does not protect
against attacks on the boot file download channel. Consequently, we
recommend that either (a) implementers use protocols like HTTPS
[RFC2818] or Transport Layer Security (TLS) within HTTP [RFC2817] to
prevent spoofing or (b) the boot-loader software implement a
mechanism for signing boot images and a configurable signing key.
The latter is done so that if a malicious image is provided, it can
be detected and rejected.
8. Acknowledgements
The authors would like to thank Ruth Li, Dong Wei, Kathryn Hampton,
Phil Dorah, Richard Chan, and Fiona Jensen for discussions that led
to this document.
The authors would also like to thank Ketan P. Pancholi, Alfred
Hoenes, Gabriel Montenegro, and Ted Lemon for corrections and
suggestions.
9. References
9.1. Normative References
[PXE21] Johnston, M., "Preboot Execution Environment (PXE)
Specification", September 1999,
<http://www.pix.net/software/pxeboot/archive/pxespec.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4173] Sarkar, P., Missimer, D., and C. Sapuntzakis,
"Bootstrapping Clients using the Internet Small Computer
System Interface (iSCSI) Protocol", RFC 4173,
September 2005.
[RFC4578] Johnston, M. and S. Venaas, "Dynamic Host Configuration
Protocol (DHCP) Options for the Intel Preboot eXecution
Environment (PXE)", RFC 4578, November 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[UEFI23] UEFI Forum, "Unified Extensible Firmware Interface
Specification, Version 2.3", May 2009,
<http://www.uefi.org/>.
9.2. Informative References
[RFC906] Finlayson, R., "Bootstrap Loading using TFTP", RFC 906,
June 1984.
[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
RFC 1350, July 1992.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3617] Lear, E., "Uniform Resource Identifier (URI) Scheme and
Applicability Statement for the Trivial File Transfer
Protocol (TFTP)", RFC 3617, October 2003.
Authors' Addresses
Thomas H. Huth
IBM Germany Research & Development GmbH
Schoenaicher Strasse 220
Boeblingen 71032
Germany
Phone: +49-7031-16-2183
EMail: thuth@de.ibm.com
Jens T. Freimann
IBM Germany Research & Development GmbH
Schoenaicher Strasse 220
Boeblingen 71032
Germany
Phone: +49-7031-16-1122
EMail: jfrei@de.ibm.com
Vincent Zimmer
Intel
2800 Center Drive
DuPont WA 98327
USA
Phone: +1 253 371 5667
EMail: vincent.zimmer@intel.com
Dave Thaler
Microsoft
One Microsoft Way
Redmond WA 98052
USA
Phone: +1 425 703-8835
EMail: dthaler@microsoft.com
AI ELI5
On this page
§1Network Boot with DHCPv6
Some nodes must boot over the network rather than from local storage. The RFC notes this helps where administrators maintain a large number of nodes, because all boot and configuration files can be served from a central server. A typical boot file is an operating system kernel or a boot-loader program.
The client firmware does two things: first it gets the information needed to download and execute the boot file, then it downloads and runs it. The information MUST include at least the server holding the files, the download protocol (for example HTTP or TFTP) and the path and name of the file. The server and client MAY also exchange parameters for the kernel or boot loader and information about the supported boot environment.
First the computer asks where its instructions are kept. Then it goes there, picks them up and starts following them.
- 1 Get boot file info
- 2 Download boot file
§3The Boot File URL and Parameters Options
Option formats follow RFC 3315. The boot-file-url option is mandatory for booting, and all other options are optional.
The Boot File URL option (OPT_BOOTFILE_URL, code 59) is sent by the server and carries a URL for the boot file. The string MUST comply with STD 66 (RFC 3986) and is not NUL-terminated. If the host is an IPv6 address, it MUST be enclosed in square brackets. Clients that have DNS implementations SHOULD support domain names in the URL.
The Boot File Parameters option (OPT_BOOTFILE_PARAM, code 60) is sent by the server and holds several UTF-8 strings, each preceded by a 16-bit length. They work like command line arguments, for example naming the root file system of a kernel. When the firmware runs the boot file it MUST pass these parameters, in the order they appear.
One option is the street address of the instructions. The other is a list of notes to take along, handed over in the same order they were written.
§3.3Architecture and Interface Options
Two options give parity with DHCPv4 options in RFC 4578. The Client System Architecture Type option (OPTION_CLIENT_ARCH_TYPE, code 61) carries a list of 16-bit architecture types describing the client's pre-boot runtime environment. Its length MUST be an even number greater than zero. The client MAY send its supported types so the server can choose a suitable boot file, with the most preferred first. If the client used the option, the server SHOULD include it in reply, listing only types the client asked about, again in descending priority.
The Client Network Interface Identifier option (OPTION_NII, code 62) lets a client that supports the Universal Network Device Interface (UNDI) tell the server its level of support. It has a length of 3 and three one-octet fields, Type, Major and Minor, defined in RFC 4578. Valid values are maintained in the UEFI specification.
The computer can say what kind of machine it is, so the server sends a program that fits it, and the best fit is listed first.
§4Where Options May Appear and How Files Download
These options MUST NOT appear in DHCPv6 messages other than Solicit, Advertise, Request, Renew, Rebind, Information-Request and Reply. Their option codes MAY appear in the Option Request option in Solicit, Request, Renew, Rebind, Information-Request and Reconfigure messages.
The URL option places no constraint on the download protocol, other than that it can be written in a URL. The RFC strongly recommends that boot loaders implement HTTP at a minimum, and notes that for IPv6 this supersedes RFC 906, which recommended TFTP. For iSCSI booting, the functionality that RFC 4173 gave to a root path option is provided by the Boot File URL option instead.
There are rules for which messages may carry these notes. The boot file itself can be fetched by any method that fits in an address, but the web is the one everyone should support.
§6IANA Registrations
IANA assigned four option numbers from the DHCPv6 option space: 59 (OPT_BOOTFILE_URL), 60 (OPT_BOOTFILE_PARAM), 61 (OPTION_CLIENT_ARCH_TYPE) and 62 (OPTION_NII).
The RFC also creates a new registry called "Processor Architecture Types". Entries are a 16-bit integer in decimal plus a descriptive name, the initial values come from RFC 4578, and new values are assigned through Expert Review.
The numbers 59 to 62 are reserved for these notes, and there is a public list of computer types that anyone can ask to add to.
- 59OPT_BOOTFILE_URL — URL of the boot file; mandatory for booting.
- 60OPT_BOOTFILE_PARAM — Parameters to pass to the boot file.
- 61OPTION_CLIENT_ARCH_TYPE — Supported processor architecture types.
- 62OPTION_NII — UNDI support level of the client network interface.
§7Security Considerations
On an untrusted network a rogue DHCPv6 server could send these options and point clients at a wrong URL. The boot would then fail or, worse, the client would boot the wrong operating system from a malicious file server. Clients SHOULD use DHCPv6 message authentication to prevent this.
DHCPv6 messages are unencrypted by default, so URLs can leak sensitive information such as a user name and password. The RFC strongly recommends not putting sensitive information in URLs on untrusted networks.
Securing the DHCPv6 exchange does not protect the download itself. The RFC recommends either HTTPS or TLS within HTTP to prevent spoofing, or boot-loader software that verifies signed boot images with a configurable signing key, so a malicious image can be detected and rejected.
A trickster could hand out a fake address and the computer would wake up with the wrong instructions. So the computer should check who is talking and check the package it receives.