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Bootstrapping Clients using the Internet Small Computer System Interface (iSCSI) Protocol

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4173.
Authors Prasenjit Sarkar , Duncan Missimer , Constantine Sapuntzakis
Last updated 2020-07-29 (Latest revision 2004-03-18)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
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IESG IESG state Became RFC 4173 (Proposed Standard)
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Responsible AD Allison J. Mankin
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IP Storage Working Group                              Prasenjit Sarkar
Internet Draft                                                     IBM
Document: draft-ietf-ips-iscsi-boot-12.txt             Duncan Missimer
Category: Standards Track                                      Brocade
                                                Constantin Sapuntzakis
                                                   Stanford University
                                                         18 March 2004

             Bootstrapping Clients using the iSCSI Protocol

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.
   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or made obsolete by other documents at
   any time. It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."  The list
   of current Internet-Drafts can be accessed at The list of Internet-Draft
   Shadow Directories can be accessed at

   The words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   documents are to be interpreted as described in RFC 2119.


   iSCSI is a proposed transport protocol for SCSI that operates on top
   of TCP.  This memo describes a standard mechanism to enable clients
   to bootstrap themselves using the iSCSI protocol.  The goal of this
   standard is to enable iSCSI boot clients to obtain the information to
   open an iSCSI session with the iSCSI boot server.

1. Introduction

   The Small Computer Systems Interface (SCSI) is a popular family of
   protocols for communicating with I/O devices, especially storage
   devices.  SCSI can be characterized as a request/response messaging
   protocol with a standard architecture and componentized command sets
   for different device classes.

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   iSCSI is a proposed transport protocol for SCSI that operates on top
   of TCP.  The role of iSCSI is necessitated by the evolution of the
   system interconnect from a shared bus to a switched network.  IP
   networks meet the architectural and performance requirements of
   transporting SCSI, paving the way for the iSCSI protocol.

   Many diskless clients sometimes bootstrap off remote SCSI devices.
   Such diskless entities are lightweight, space-efficient and power-
   conserving, and are increasingly popular in various environments.

   This memo describes a standard mechanism to enable clients to
   bootstrap themselves using the iSCSI protocol.  The goal of this
   standard is to enable iSCSI boot clients to obtain the information to
   open an iSCSI session with the iSCSI boot server. It is possible that
   all the information is not available at the very outset, so the memo
   describes steps to obtain the information required to bootstrap
   clients off an iSCSI boot server.

2. Requirements

   1. There must be no restriction of network topology between the iSCSI
   boot client and the boot server other than those in effect for
   establishing the iSCSI session. Consequently, it is possible for an
   iSCSI boot client to boot from an iSCSI boot server behind gateways
   or firewalls as long as it is possible to establish an iSCSI session
   between the client and the server.

   2. The following represents the minimum information required for an
   iSCSI boot client to contact an iSCSI boot server: (a) the client's
   IP address (IPv6 or IPv4); (b) the server's iSCSI Target Name; and
   (c) mandatory iSCSI initiator capability.

   The above assumes that the default LUN for the boot process is 0 and
   the default port for the iSCSI boot server is the well-known iSCSI
   port [Satran02].  However, both may be overridden at the time of

   Additional information may be required at each stage of the boot

   3. It is possible for the iSCSI boot client to have none of the above
   information or capability on starting.

   4. The client should be able to complete boot without user
   intervention (for boots that occur during an unattended power-up).
   However, there should be a mechanism for the user to input values so
   as to bypass stages of the boot protocol.

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   5. Additional protocol software (for example, BOOTP or DHCP) may be
   necessary if the minimum information required for an iSCSI session is
   not provided.

3. Related Work

   The Reverse Address Resolution Protocol (RARP) [Finlayson84] through
   the extensions defined in the Dynamic RARP (DRARP)) [Brownell96]
   explicitly addresses the problem of network address discovery, and
   includes an automatic IP address assignment mechanism.  The Trivial
   File Transfer Protocol (TFTP) [Sollins81] provides for transport of a
   boot image from a boot server. BOOTP [Croft85, Reynolds93, Wimer93]
   is a transport mechanism for a collection of configuration
   information.  BOOTP is also extensible, and official extensions have
   been defined for several configuration parameters.  DHCPv4 [Droms97,
   Droms93] and DHCPv6 [Droms02] are standards for hosts to be
   dynamically configured in an IP network.  The Service Location
   Protocol (SLP) provides for location of higher level services

4. Software stage

   Some iSCSI boot clients may lack the resources to boot up with the
   mandatory iSCSI initiator capability. Such boot clients may choose to
   obtain iSCSI initiator software from a boot server.  Currently, there
   are many established protocols that allow such a service to enable
   clients to load software images. For example, BOOTP and DHCP servers
   have the capability to provide the locations of servers that can
   serve software images on requests from boot clients.

   It is to be noted that this document does not recommend any of the
   above protocols, and the final decision of which boot protocol is to
   be used to load iSCSI initiator software is left to the discretion of
   the implementor.

5. DHCP stage

   In order to use an iSCSI boot server, the following pieces of
   information are required for an ISCSI boot client.

   - The IP address of the iSCSI boot client (IPv4 or IPv6)

   - The IP transport endpoint for the iSCSI Target Port for the iSCSI
   boot server.  If the transport is TCP, for example, this has to
   resolve to an IP address and a TCP port number. TCP is currently the
   only transport approved for iSCSI.

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   - The eight-byte LUN structure identifying the Logical Unit within
   the iSCSI boot server.

   At boot time, all or none of this information may be stored in the
   iSCSI boot client. This section describes techniques for obtaining
   the required information via the DHCP stage. Otherwise, if the iSCSI
   boot client has all the information, the boot client may proceed
   directly to the Boot stage.

   An iSCSI boot client which does not know its IP address at power-on
   may acquire its IP address via BOOTP or DHCP (v4 or v6). Please note
   that DHCP settings (such as the RA settings in DHCPv6) may prohibit
   the use of DHCP in distributing iSCSI boot information -- in this
   case, the DHCP stage cannot be used.

   Unless otherwise specified here, BOOTP or DHCP fields such as the
   client ID and gateway information are used in an identical way as
   applications other than iSCSI do.

   A BOOTP or DHCP server (v4 or v6) MAY instruct an iSCSI client how to
   reach its boot device. This is done using the variable length option
   named Root Path. The use of the option field is reserved for iSCSI
   boot use by prefacing the string with "iscsi:". The Root Path option
   is not currently defined for DHCPv6; if the option is defined for
   DHCPv6 in the future, the use of the option as defined for iSCSI boot
   will apply.

   The option field consists of an UTF-8 [Yergeau98] string.  The string
   has the following composition:


   The fields "servername", "port", "protocol" and "LUN" are OPTIONAL
   and should be left blank if there are no corresponding values. The
   "targetname" field is not optional and MUST be provided.

   The "servername" is the name of iSCSI server and contains either a
   valid domain name, a literal IPv4 address, or a literal IPv6 address.
   The servername must follow the specifications outlined in Section
   3.2.2 of the URI Specification[Lee04]. The characters allowed must
   also conform to Section 2.2 of the same specification.  Servername
   compression MUST NOT be used in this field.

   The "protocol" field is the decimal representation of the IANA-
   approved string for the trasport protocol to be used for iSCSI. If
   the protocol field is left bank, the default value is assumed to be
   "6" for TCP.  The transport protocol MUST have been approved for use
   in iSCSI; currently, the only approved protocol is TCP.

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   The "port" is the decimal representation of the port on which the
   iSCSI boot server is listening. If not specified, the port defaults
   to the well-known iSCSI port [Satran02].

   The "LUN" field is a hexadecimal representation of the LU number.  If
   the LUN field is blank, then LUN 0 is assumed. If the LUN field is
   not blank, the representation MUST be divided into four groups of
   four hexadecimal digits, separated by "-". Digits above 9 may be
   either lower or upper case.  An example of such a representation
   would be 4752-3A4F-6b7e-2F99.  For the sake of brevity, at most three
   leading zero ("0") digits MAY be omitted in any group of hexadecimal
   digits. Thus, the "LUN" representation 6734-9-156f-127 is equivalent
   to 6734-0009-156f-0127.  Furthermore, trailing groups containing only
   the "0" digit MAY be omitted along with the preceding "-". So, the
   "LUN" representation 4186-9 is equivalent to 4186-0009-0000-0000.
   Other concise representations of the LUN field MUST NOT be used.

   Note that SCSI targets are allowed to present different LU numberings
   for different SCSI initiators, so that to our knowledge nothing
   precludes a SCSI target from exporting several different LUs to
   several different SCSI initiators as their respective LUN 0s.

   The "targetname" field is an iSCSI Name that is defined by the iSCSI
   standard [Satran02] to uniquely identify an iSCSI target. The
   approved characters in the targetname field are stated in the iSCSI
   String Profile document[Bakke04].

   If the "servername" field is provided by BOOTP or DHCP, then that
   field is used in conjunction with other associated fields to contact
   the boot server in the Boot stage (Section 7).  However, if the
   "servername" field is not provided, then the "targetname" field is
   then used in the Discovery Service stage in conjunction with other
   associated fields.  (Section 6).

6. Discovery Service stage

   This stage is required if the BOOTP or DHCP server (v4 or v6) is
   unaware of any iSCSI boot servers or if the BOOTP or DHCP server is
   unable to provide the minimum information required to connect to the
   iSCSI boot server other than the targetname.

   The Discovery Service may be based on the SLP protocol [Guttman99,
   Bakke02] and is an instantiation of the SLP Service or Directory
   Agent.  Alternatively, the Discovery Service may be based on the iSNS
   protocol [Tseng03] and is an instantiation of the iSNS Server.

   The iSCSI boot client may have obtained the targetname of the iSCSI

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   boot server in the DHCP stage (Section 5). In that case, the iSCSI
   boot client queries the SLP Discovery Service using query string 1 of
   the iSCSI Target Concrete Service Type Template as specified in
   Section 6.2 of the iSCSI SLP interaction document [Bakke02] to
   resolve the targetname to an IP address and port number.
   Alternatively, the iSCSI boot client may query the iSNS Discovery
   Service with a Device Attribute Query with the targetname as the
   query parameter [Tseng03].  Once this is obtained, the iSCSI boot
   client proceeds to the Boot stage (Section 7).

   It is possible that the port number obtained from the Discovery
   Service may conflict with the one obtained from the DHCP stage. In
   such a case, the implementor has the option to try both port numbers
   in the Boot stage.

   If the iSCSI boot client does not have any targetname information,
   the iSCSI boot client then may query the SLP Discovery Service with
   query string 4 of the iSCSI Target Concrete Service Type Template as
   specified in Section 6.2 of the iSCSI SLP interaction document
   [Bakke02]. In response to this query, the SLP Discovery Service
   provides the boot client with a list of iSCSI boot servers the boot
   client is allowed to access.  Alternatively, the iSCSI boot client
   can query the iSNS Discovery Service to verify if the targets in
   particular Discovery Domain are bootable [Tseng03].

   If the list of iSCSI boot servers is empty, subsequent actions are
   left to the discretion of the implementor. Otherwise, the iSCSI boot
   client may contact any iSCSI boot server in the list. Moreover, the
   order in which iSCSI boot servers are contacted is also left to the
   discretion of the implementor.

7. Boot stage

   Once the iSCSI boot client has obtained the minimum information to
   open an iSCSI session with the iSCSI boot server, the actual booting
   process can start.

   The actual sequence of iSCSI commands needed to complete the boot
   process is left to the implementor. This was done because of varying
   requirements from different vendors and equipment, making it
   difficult to specify a common subset of the iSCSI standard that would
   be acceptable to everybody.

   The iSCSI session established for boot may be taken over by the
   booted software in the iSCSI boot client.

8. Security Considerations

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   The security discussion is centered around securing the communication
   involved in the iSCSI boot process.

   However, the issue of applying credentials to a boot image loaded
   through the iSCSI boot mechanism is outside the scope of this
   document. One key difference between the iSCSI boot mechanism and
   BOOTP-based image loading is the fact that the identity of a boot
   image may not be known when the Boot stage starts. The identity of
   certain boot images and their locations are known only after
   examining the contents of a boot disk exposed by the iSCSI boot
   service. Furthermore, images themselves may recursively load other
   images based on both hardware configurations and user input.
   Consequently, a practical way to verify loaded boot images is to make
   sure that each image loading software verifies the image to be loaded
   using a mechanism of their choice.

   The considerations involved in designing a security architecture for
   the iSCSI boot process include configuration, deployment and
   provisioning issues apart from typical security considerations.
   Enabling iSCSI boot creates a critical operational dependence on an
   external system with obvious security implications, and thus
   administrator awareness of such enablement is extremely important.
   Therefore, iSCSI boot SHOULD NOT be enabled, or put high in the boot
   order, without an explicit administrative action.

   In all phases of the boot process, a client must ensure that a server
   is authorized to send it certain information. This means that the
   authenticated identity of a server must have an authorization
   indication.  A list of authorized servers can be pre-configured into
   a client, or the list can be downloaded in an authenticated form from
   a prior stage in the boot process.

   The software stage SHOULD NOT be involved in a secure iSCSI boot
   process as this would add the additional complexity of trying to
   secure the process of loading the software necessary to run the later
   stages of iSCSI boot. Authentication and integrity protection of
   downloaded boot software has proven to be difficult and complex due
   to administrative issues and limitations of the BIOS environment.  It
   is therefore assumed that all the necessary software is resident on
   the iSCSI boot client.

   If the DHCP stage is implemented using the DHCP protocol, the iSCSI
   boot client SHOULD implement the DHCP authentication ([Droms01],
   [Droms02] for IPv6). In this case an administration interface SHOULD
   be provided for the configuration of the DHCP authentication
   credentials, both when the network interface is on the motherboard,
   and when it is removable. Note that DHCP authentication
   ([Droms01],[Droms02] for IPv6) is focused on intra-domain

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   authentication, which is assumed to be enough for iSCSI boot
   scenarios. In the context of the secure iSCSI boot process, the reply
   from the DHCP server in the DHCP stage SHOULD include the serverName
   in IPv4 (or IPv6) format to avoid reliance on a DNS server (for
   resolving names) or a Discovery Service entity (to look up
   targetnames). This reduces the number of entities involved in the
   secure iSCSI boot process.

   If the Discovery Service stage is implemented using SLP, the iSCSI
   boot client SHOULD provide IPsec support (OPTIONAL to use) for the
   SLP protocol, as defined in [Bakke02] and [Aboba03]. If the Discovery
   Service stage is implemented using iSNS, the iSCSI boot client SHOULD
   provide IPsec support (OPTIONAL to use) for the iSNS protocol, as
   defined in [Tseng03] and [Aboba03]. When iSNS or SLP are used to
   distribute security policy or configuration information, at a
   minimum, per-packet data origin authentication, integrity and replay
   protection SHOULD be used to protect the discovery protocol.

   For the final communication between the iSCSI boot client and the
   iSCSI boot server in the Boot stage, IPsec and in-band authentication
   SHOULD be implemented according to the guidelines in the main iSCSI
   draft [Satran02] and [Aboba03]. Due to memory constraints, it is
   expected that iSCSI boot clients will only support the pre-shared key
   authentication in IKE. Where the host IP address is assigned
   dynamically, IKE main mode SHOULD NOT be used, as explained in
   [Satran02] and [Aboba03]. Regardless of the way parameters in
   previous stages (DHCP, SLP, iSNS) were obtained (securely or not),
   the iSCSI boot session is vulnerable as any iSCSI session (see
   [Satran02] and [Aboba03] for iSCSI security threats). Therefore
   security for this session SHOULD be configured and used according to
   [Satran02] and [Aboba03] guidelines.

   Another point to be noted is that if a boot image inherits an iSCSI
   session from a previously loaded boot image, the boot image also
   inherits the security properties of the iSCSI session.


   We wish to thank John Hufferd for taking the initiative to form the
   iSCSI boot team. We also wish to thank Doug Otis, Julian Satran,
   Bernard Aboba, David Robinson, Mark Bakke, Ofer Biran and Mallikarjun
   Chadalapaka for helpful suggestions and pointers regarding the draft

Normative References

   [Aboba03] Aboba, S. et al. "Securing Block Storage Protocols over

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   IP", Work in Progress, January 2003.

   [Alexander93] Alexander, S., and R. Droms, "DHCP Options and BOOTP
          Extensions", RFC 2132, Lachman Technology, Inc., Bucknell
          University, October 1993.

   [Bakke02] Bakke, M., et al. "Finding iSCSI Targets and Name Servers
   using SLP", Work in Progress, March 2002.

   [Bakke04] Bakke, M., "String Profiles for iSCSI Names", Work in
   Progress, February 2004.

   [Braden89] Braden, R., "Requirements for Internet Hosts - Application
   and Support", RFC 1123, October 1989.

   [Bradner96] Bradner, S., "The Internet Standards Process --
         Revision 3", RFC 2026, October 1996.

   [Bradner97] Bradner, S. "Key Words for use in RFCs to indicate
   Requirement Levels", RFC 2119, Harvard University, March 1997.

   [Croft85] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)",
   RFC 951,
          Stanford and SUN Microsystems, September 1985.

   [Droms93] Droms, D., "Interoperation between DHCP and BOOTP" RFC
          Bucknell University, October 1993.

   [Droms97] R. Droms, "Dynamic Host Configuration Protocol", RFC 2131,
          Bucknell University, March 1997.

   [Droms01] Droms, R., Arbaugh, W., "Authentication for DHCP Messages",
   RFC 3118, June 2001.

   [Droms02] Droma, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
   Carney, M., "Dynamic Host Configuration Protocol for IPv6", Work in
   Progress, November 2002.

   [Guttman99] Guttman, E., Perkins, C., Verizades, J., Day, M.,
   "Service Location Protocol v2", RFC 2608, June 1999.

   [Lee04] Lee, T., Fielding, R., Masinter, L., "Uniform Resource
   Identifier (URI): General Syntax", Work in Progress, February 2004.

   [Reynolds93] Reynolds, J., "BOOTP Vendor Information Extensions", RFC

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Standards Track         iSCSI BootStrapping Draft          18 March 2004

          USC/Information Sciences Institute, August 1993.

   [Satran02] Satran, J. et al., "iSCSI", Work in Progress, September

   [Tseng03] Tseng, J., Gibbons, K., Travostino, F., Du Laney, C.,
   Souza, J., "Internet Storage Name Service", Work in Progress, June

   [Yergeau98] Yergeau, F., "UTF-8: A Transformation Format for
   ISO-10646", RFC 2279, January 1998.

   [Wimer93] Wimer, W., "Clarifications and Extensions for the Bootstrap
   Protocol", RFC 1532, Carnegie Mellon University, October 1993.

Informative References

   [Brownell96] Brownell, D, "Dynamic RARP extensions for Automatic
   Network Adress
          Acquisition", RFC 1931, SUN Microsystems, April 1996.

   [Finlayson84] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A
          Address Resolution Protocol", RFC 903, Stanford, June 1984.

   [Sollins81] Sollins, K., "The TFTP Protocol (Revision 2)",  RFC 783,
          June 1981.

Authors' Addresses

   Prasenjit Sarkar
   IBM Almaden Research Center
   650 Harry Road
   San Jose, CA 95120, USA
   Phone: +1 408 927 1417

   Duncan Missimer
   Brocade Communication Systems
   1745 Technology Drive,
   San Jose, CA 95110, USA

   Constantine Sapuntzakis
   Stanford University
   353 Serra Hall #406

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   Stanford, CA 94306, USA
   Phone: +1 650 520 0205

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