Request for Comments: 4173
Category: Standards Track
Bootstrapping Clients using
the Internet Small Computer System Interface (iSCSI) Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2005).
Internet Small Computer System Interface (iSCSI) is a proposed transport protocol for Small Computer Systems Interface (SCSI) that operates on top of TCP. This memo describes a standard mechanism for enabling 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.
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.
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 for enabling 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.
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 [Bradner97].
- There must be no restriction of network topology between the iSCSI boot client and the boot server other than that 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.
- The following represent 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 assume that the default LUN for the boot process is 0 and that the default port for the iSCSI boot server is the well-known iSCSI port [Satran02]. However, both may be overridden at the time of configuration.
Additional information may be required at each stage of the boot process.
- It is possible for the iSCSI boot client to have none of the above information or capability on starting.
- 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.
- 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 by which hosts are to be dynamically configured in an IP network. The Service Location Protocol (SLP) provides for location of higher-level services [Guttman99].
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, many established protocols allow such a service in order 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.
Note 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.
- 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 that does not know its IP address at power-on may acquire it via BOOTP or DHCP (v4 or v6), or via IPv6 address autoconfiguration. 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 specified otherwise here, BOOTP or DHCP fields such as the client ID and gateway information are used in an identical way as applications other than iSCSI.
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 [Alexander93, Reynolds93]. 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 [Lee98] [Hinden99]. 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 transport 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.
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 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 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 may then 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 that are 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
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 the contents of a boot disk exposed by the iSCSI boot service are examined. 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 this 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 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.
Note that if a boot image inherits an iSCSI session from a previously loaded boot image, it 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 document.
[Aboba03] Aboba, B., Tseng, J., Walker, J., Rangan, V., and F. Travostino, "Securing Block Storage Protocols over IP", RFC 3723, April 2004. [Alexander93] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [Bakke02] Bakke, M., Hufferd, J., Voruganti, K., Krueger, M., and T. Sperry, "Finding Internet Small Computer Systems Interface (iSCSI) Targets and Name Servers by Using Service Location Protocol version 2 (SLPv2)", RFC 4018, April 2005. [Bakke04] Bakke, M., "String Profile for Internet Small Computer Systems Interface (iSCSI) Names", RFC 3722, April 2004. [Bradner97] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [Croft85] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951, September 1985. [Droms93] Droms, R., "Interoperation Between DHCP and BOOTP", RFC 1534, October 1993. [Droms97] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [Droms01] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [Droms02] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [Guttman99] Guttman, E., Perkins, C., Veizades, J., and M. Day, "Service Location Protocol, Version 2", RFC 2608, June 1999. [Hinden99] Hinden, R., Carpenter, B., and L. Masinter, "Format for Literal IPv6 Addresses in URL's", RFC 2732, December 1999. [Lee98] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. [Reynolds93] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497, August 1993. [Satran02] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M., and E. Zeidner, "Internet Small Computer Systems Interface (iSCSI)", RFC 3720, April 2004. [Tseng03] Tseng, J., Gibbons, K., Travostino, F., Du Laney, C., and J. Souza, "Internet Storage Name Service (iSNS)", RFC 4171, April 2005. [Yergeau98] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [Wimer93] Wimer, W., "Clarifications and Extensions for the Bootstrap Protocol", RFC 1542, October 1993.
[Brownell96] Brownell, D., "Dynamic RARP Extensions for Automatic Network Address Acquisition", RFC 1931, April 1996. [Finlayson84] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "Reverse Address Resolution Protocol", STD 38, RFC 903, June 1984. [Sollins81] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350, July 1992.
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