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RFC 5970

Proposed STD
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DHCPv6 Options for Network Boot


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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
                                                               D. Thaler
                                                          September 2010

                    DHCPv6 Options for Network Boot


   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

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
   ( 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.

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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

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   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",
   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)              .
   |                                                               |

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   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

   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)      |

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   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

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

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                       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

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].

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   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

   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.

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   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

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9.  References

9.1.  Normative References

   [PXE21]    Johnston, M., "Preboot Execution Environment (PXE)
              Specification", September 1999,

   [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,

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.

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   [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.

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Authors' Addresses

   Thomas H. Huth
   IBM Germany Research & Development GmbH
   Schoenaicher Strasse 220
   Boeblingen  71032

   Phone: +49-7031-16-2183

   Jens T. Freimann
   IBM Germany Research & Development GmbH
   Schoenaicher Strasse 220
   Boeblingen  71032

   Phone: +49-7031-16-1122

   Vincent Zimmer
   2800 Center Drive
   DuPont  WA 98327

   Phone: +1 253 371 5667

   Dave Thaler
   One Microsoft Way
   Redmond  WA 98052

   Phone: +1 425 703-8835