DHCPv6 option for network boot
draft-ietf-dhc-dhcpv6-opt-netboot-01

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Abstract

The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) provides a framework for passing configuration information to nodes on a network. This document describes a new option for DHCPv6 to convey information, required for network booting, to the nodes.

Table of Contents

1.  Introduction
2.  Conventions
3.  Netboot option format
4.  Suboption: Boot file Uniform Resource Locator (URL)
5.  Suboption: Vendor class extension
6.  Appearance of these options
7.  Boot protocol considerations
8.  IANA considerations
9.  Security considerations
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

Network booting means that a node which should be booted fetches the files required for booting via its network device from a server. Network booting is, for example, very useful in environments where the administrators have to maintain a large number of nodes. Since all boot and configuration files are stored on a central server, the maintenance of all nodes can be kept simple this way.

A typical boot file would be, for example, an operating system kernel or a boot loader program. To be able to download such a file, the firmware (BIOS) running on the client node must be provided with information such as: the server on which the boot files can be found, the protocol to be used for the download (for example TFTP (Sollins, K., “The TFTP Protocol (Revision 2),” July 1992.) [RFC1350]) and the name of the boot file. Since some kernels or boot loaders need to be provided with additional parameters, there should also be the possibility to pass additional parameters along with the server address, the protocol and the file name.

DHCPv6 allows client nodes to ask a DHCPv6 server for configuration parameters. Contrary to its IPv4 predecessor, DHCPv6 does not define a way to query network boot options such as the IPv6 address of a boot file server and boot file names. Therefore this document defines a new DHCPv6 option which is required for network booting clients.

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Terminology specific to IPv6 and DHCPv6 are used in the same way as defined in the "Terminology" sections of RFC 3315 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315].

3.  Netboot option format

The netboot option is used as an encapsulation for suboptions which carry the actual information needed to boot a client. This option will be used by clients to request boot information from a server.

 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_NET_BOOT           |           option-len          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      suboption-code 1         |          suboption-len        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    subopt-data 1 (variable length)            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                                                               .
.                       <multiple suboptions>                   .
.                                                               .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      suboption-code n         |          suboption-len        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    subopt-data n (variable length)            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

option-code   OPT_NET_BOOT (tbd).

option-len    Length of the netboot option in octets (not including
              the size of the option-code and option-len fields).

Multiple occurences of each suboption-type can occur within a netboot option (for example when more than one boot server is available). Clients MUST process the suboptions in the order in which they appear in the message sent by the server.

So far, only the suboptions in the following chapters have been defined. Other suboptions might be defined in future RFCs.

4.  Suboption: Boot file Uniform Resource Locator (URL)

This suboption consists of multiple null-terminated strings. It is used to convey an URL to a boot file together with additional parameters for the boot file (e.g. parameters for the kernel or boot loader program).

Since multiple occurrences of SUBOPT_BOOTFILE_URL can be present in a single OPT_NETBOOT message, clients MUST process them in the order in which they appear within the message. For example in the case of a boot file URL the first file should be downloaded and executed. In case of a failure the process should continue with the second one and so on.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       SUBOPT_BOOTFILE_URL     |          suboption-len        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| bootfile-url                                                  |
.                           (variable)                          .
|                                               |     '\0'      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| parameter 1                                                   |
.                           (variable)                          .
|                                               |     '\0'      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                                                               .
.                       <multiple Parameters>                   .
.                                                               .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| parameter n                                                   |
.                           (variable)                          .
|                                               |     '\0'      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Format description:

suboption-code

SUBOPT_BOOTFILE_URL (tbd).

suboption-len

Length of the bootfile suboption in octets (not including the size of the suboption-code and suboption-len fields).

bootfile-url

This NULL-terminated ASCII string is the URL (conforming to [RFC2396] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax,” August 1998.)) to a boot file. This string starts with the protocol which is used for downloading. Separated by '://', the hostname or IPv6 address of the server hosting the boot file (see also the note below), the path, file name and query parts of the URL follow.

parameters 1...n

These NULL-terminated ASCII strings are parameters needed for booting, e.g. kernel parameters. In cases where no parameters are needed, everything but the boot file URL can be omitted. Parameters following the boot file name should be directly related to the boot file (kernel) itself.

Note about the bootfile-url: This string can either contain a hostname or an IPv6 address to specify the server where the boot file should be downloaded from. All clients which implement support for the SUBOPT_BOOTFILE_URL suboption MUST be able to handle IPv6 addresses here. The IPv6 address in the URL then MUST be enclosed in "[" and "]" characters, conforming to [RFC2732] (Hinden, R., Carpenter, B., and L. Masinter, “Format for Literal IPv6 Addresses in URL's,” December 1999.). Clients SHOULD also be able to handle hostnames in the URLs. However, in this case the firmware implementation on the client machine must support DNS, too. Due to size limitations, this might not be possible in all firmware implementations, so support for hostnames in the URLs is only optional.

5.  Suboption: Vendor class extension

With this suboption, vendors can define their own netboot suboptions: It can be used by clients and servers to exchange vendor-specific information which is related to network booting.

This suboption can occur multiple times within a OPT_NET_BOOT option (also with different enterprise-numbers in case a server and client implementation supports different vendor extensions). Clients MUST process them in the order in which they appear within the message. Unsupported vendor extensions MUST be ignored.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      SUBOPT_NETBOOT_VENDOR    |          suboption-len        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       enterprise-number                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                                                               .
.                       vendor-class-data                       .
.                       (variable length)                       .
.                                                               .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Format description:

suboption-code

SUBOPT_NETBOOT_VENDOR (tbd).

suboption-len

Length of the vendor class suboption in octets (not including the size of the suboption-code and suboption-len fields).

enterprise-number

The enterprise number of the vendor as registered with IANA (see [VENDORIDS] (IANA, “Private Enterprise Numbers,” .)).

vendor-class-data

Vendor-specific information. The meaning is defined by the vendor identified by the enterprise-number.

6.  Appearance of these options

The netboot option MUST NOT appear in DHCPv6 messages other than the types Solicit, Advertise, Request, Renew, Rebind, Information-Request and Reply.

The number of the netboot option MAY appear in the Option Request Option in the DHCPv6 message types Solicit, Request, Renew, Rebind, Information-Request and Reconfigure.

The suboptions MUST appear only in the netboot option.

7.  Boot protocol considerations

RFC 906 (Finlayson, R., “Bootstrap Loading using TFTP,” June 1984.) [RFC906] suggests to use TFTP for bootstrap loading. Because it is easy to implement this protocol in firmware (where one has to deal with size and complexity constraints), this is still the recommended protocol for network booting. Every firmware implementation SHOULD at least support this protocol. The boot file URLs then must be specified according to RFC 3617 (Lear, E., “Uniform Resource Identifier (URI) Scheme and Applicability Statement for the Trivial File Transfer Protocol (TFTP),” October 2003.) [RFC3617].

An alternative approach to TFTP network booting is to bootstrap the system with iSCSI. In this case, the URL in the SUBOPT_BOOTFILE_URL suboption MUST be specified according to the "iscsi:" string definition in chapter 5 of [RFC4173] (Sarkar, P., Missimer, D., and C. Sapuntzakis, “Bootstrapping Clients using the Internet Small Computer System Interface (iSCSI) Protocol,” September 2005.). Note that [RFC4173] (Sarkar, P., Missimer, D., and C. Sapuntzakis, “Bootstrapping Clients using the Internet Small Computer System Interface (iSCSI) Protocol,” September 2005.) also suggests that the "iscsi:" string should be specified in the so-called "Root Path" option. However, this option does not exist for DHCPv6 yet, and with the SUBOPT_BOOTFILE_URL it is also not necessary anymore. So for IPv6 iSCSI booting, the "iscsi:" string MUST be specified as URL in the SUBOPT_BOOTFILE_URL suboption instead.

In some different scenarios, it might also be useful to use other protocols like FTP or HTTP for network booting, so a firmware implementation can support these protocols, too. Then it is up to the network administrator to choose the appropriate boot protocol for the network, and to specify the right boot file URLs in the DHCPv6 server configuration file.

8.  IANA considerations

The following option needs to be assigned by the IANA from the option number space defined in the chapter 22 of the DHCPv6 RFC (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315].

The netboot suboptions numbers form a new name space to be defined by the IANA:

9.  Security considerations

The new DHCPv6 option described in this document could be sent in untrusted networks by malicious people with a fake DHCPv6 server to confuse the booting clients. The clients could be provided with a wrong URL so that the boot either fails, or even worse, the client boots the wrong operating system which has been provided by a malicious file server. To prevent this kind of attack, clients SHOULD use authentication of DHCPv6 messages (see chapter 21. in RFC 3315 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [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.

10.  Acknowledgements

The authors would like to thank Ketan P. Pancholi for corrections and suggestions.

Vijayabhaskar Kalusivalingam and Senthil Balasubramanian published a similar draft for IPv6 network booting some years ago (available at http://tools.ietf.org/html/draft-ietf-dhc-dhcpv6-opt-rboot-00), which however was abandoned for unknown reasons.

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses

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