DHCPv6 option for network boot

This draft describes DHCPv6 options that can 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 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 (BIOS) running on the client node must perform the following two steps (see ): First get all information which is required for downloading and executing the boot file. Second, download the boot file and execute it.

Information that is required for booting over the network can include information about the server on which the boot files can be found, the protocol to be used for the download (for example HTTP or TFTP), the name of the boot file and additional parameters which should be passed to the OS kernel or boot loader program respectively. DHCPv6 allows client nodes to ask a DHCPv6 server for configuration parameters. This document provides new options which a client can request from the DHCPv6 server to satisfy its requirements for booting.

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. Terminology specific to IPv6 and DHCPv6 are used in the same way as defined in the "Terminology" sections of RFC 3315.

Option formats comply with DHCPv6 options per (section 6).

This option consists of an UTF-8 string. It is used to convey an URL to a boot file.

Format description: OPT_BOOTFILE_URL (TBD1). Length of the Boot File URL option in octets (not including the size of the option-code and option-len fields). This UTF-8 string is the URL for the boot file, as defined in [RFC3986]. The string is not NUL-terminated. If 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 . Clients that have DNS implementations should support the use of domain names in the URL.

This option consists of multiple UTF-8 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 where a second stage boot loader can download its configuration file from.

. . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | param-len n | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ parameter n . . (variable length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ]]> Format description: OPT_BOOTFILE_PARAM (TBD2). Length of the Boot File Parameters option in octets (not including the size of the option-code and option-len fields). This is a 16-bit integer which specifies the length of the following parameter in octets (not including the parameter-length field). These UTF-8 strings are parameters needed for booting, e.g. kernel parameters. The strings are not NUL-terminated. The firmware MUST pass these parameters in the order they appear in the OPT_BOOTFILE_PARAM option to the boot file which has been specified in the OPT_BOOTFILE_URL option.

This option provides parity with the Client System Architecture Type Option defined for DHCPv4 in section 2.1. The format of the option is:

OPTION_CLIENT_ARCH_TYPE (TBD3). Length of the "processor architecture type" field in octets (not including the option-code and option-len fields). It MUST be an even number greater than zero. See section 2.1 for details. A list of one or more architecture types, as specified in section 2.1.

The Client Network Interface Identifier option is sent by a DHCP client to a DHCP server to provide information about its level of Universal Network Device Interface (UNDI) support (see also and ). This option provides parity with the Client Network Interface Identifier Option defined for DHCPv4 in section 2.2. The format of the option is:

OPTION_NII (TBD4). 3 As specified in section 2.2. As specified in section 2.2. As specified in section 2.2.

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.

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 mininum, implementors of network boot loaders implement the well-known and established hypertext transfer protocol (HTTP, see ) for downloading. Please note that for IPv6, this supersedes which recommended to use TFTP for downloading (see for TFTP URL definition). When using iSCSI for booting, the "iscsi:"-URI is formed as defined in . The functionality attributed in RFC4173 to a root path option is provided for IPv6 by the Boot File URL option instead.

The following options need to be assigned by the IANA from the option number space defined in the chapter 22 of the DHCPv6 RFC. Option name Value Specified in OPT_BOOTFILE_URL TBD1 OPT_BOOTFILE_PARAM TBD2 OPTION_CLIENT_ARCH_TYPE TBD3 OPTION_NII TBD4 This document also introduces a new IANA registry for processor architecture types. The name of this registry shall be "Processor Architecture Type". 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 section 2.1. The assignment policy for values shall be Expert Review (see ), and any requests for values must supply the descriptive name for the processor architecture type.

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 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 can use authentication of DHCPv6 messages (see chapter 21. in ). 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. 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 protocol like HTTPS (see and ) be used to prevent spoofing, or that the boot loader implementation implement a mechanism for signing boot images and a configurable signing key in memory, so that if a malicious image is provided, it can be detected and rejected.

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.