Network Working Group A.K. Vijayabhaskar
Internet-Draft B. Senthil Kumar
Expires: May 12, 2004 Hewlett-Packard
11 Nov 2003
The Extended Remote Boot Option for DHCPv4
draft-vijay-dhc-opt-extrboot-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
Single TFTP [2] server for huge number of diskless clients is prone
to single point of failure. So, Multiple TFTP servers are needed for
high availability. Moreover, some of the clients need multiple
bootfiles for boot up. This document provides a new DHCPv4 option
for clients to obtain information about multiple TFTP [2] servers and
bootfiles.
1. Introduction
DHCPv4 (Dynamic Host Configuration Protocol Version for IPv4)
provides a framework for passing configuration information to hosts
on an IPv4 network. However, DHCPv4 does not provide a way to send
more than one TFTP server address and bootfile names. This document
defines a new option to provide more than one TFTP server and
bootfile names. This option is required for clients, which are
booting over a network and require more than one file to be
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downloaded and executed. The multiple TFTP servers are needed for
high availability. Network booting is widely used mechanism for
booting up of the clients, because of their advantages; softwares
will be in central server and requires maintenance at only one
location rather than maintaining individual systems separately.
Also, switching between different operating systems becomes easy when
network booting is being used. The additional boot files may be used
as supporting software for the boot image. Different Operating
System vendors use different way of handling this.
2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [7]
3. Terminology
This document uses terminology specific to DHCPv4 as defined in
"Terminology" section of the DHCPv4 specification [1].
4. Extended Remote Boot Option
The Extended Remote Boot Option is used to carry the parameters
needed for remote boot of the DHCPv4 [1] clients. Using the
information provided by this option, the clients will be able to
bootp up.
The format of the Remote Boot Option is as shown below:
Code Len Extended Remote Boot Information Field
+-------+------+------+------+------+------+--...-+------+
| TBD | N | r1 | r2 | r3 | r4 | | rN |
+-------+------+------+------+------+------+--...-+------+
The length N gives the total number of octets in the Extended Remote
Boot Information Field. The length N should be at least 4 bytes.
r1, r2 .. rN are Remote Boot Information suboptions which contain
information needed for boot up of the clients. They should be
listed in the increasing order of preferences.
The Remote Boot Information suboption is explained in the Section 5.
5. Remote Boot Information suboption
The DHCP server uses the Remote Boot Information suboption to convey the
client about the TFTP Server [3] names and list of boot files needed for
booting of the clients. The clients are supposed to contact the TFTP
Server, obtain the boot files one by one and boot up using these files.
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The format of the Remote Boot Information suboption is as shown below:
Code Len Remote Boot Information Field
+-------+------+------+------+------+------+--...-+------+
| 1 | N | ts | f1 | f2 | f3 | | fN |
+-------+------+------+------+------+------+--...-+------+
The length N gives the total number of octets in the Remote
Boot Information Field. The length N should be at least 2 bytes.
'ts' field consists of either TFTP server name (option 66) [4]
or the TFTP Server Address suboption suboption represented in the
Opt/Length/Value tuples. The format of the TFTP Server Address
suboption is given below:
Code Len TFTP Server address
+-----+-----+-----+-----+-----+-----+
| 1 | n | a1 | a2 | a3 | a4 |
+-----+-----+-----+-----+-----+-----+
The TFTP Server Address suboption gives IPv4 address of the one of
the TFTP Server available for the client. a1, a2, a3 and a4 refer
the quadrants of the IPv4 address of TFTP Server address in the
network byte order. The length of the option will be 4 octets.
f1, f2 ... fN are sequence of Bootfile name (option 67) [4]
represented in the Opt/Length/Value tuples.
If Bootfile name option is not following the TFTP Server name/address
suboption, then, bootfiles corresponding to the TFTP server specified
in the 'ts' field defaults to the boot file names from the previous
Remote Boot Information suboption which has non empty 'fn' fields.
Thus, TFTP server name/address and Bootfile name are sent as
suboption to Remote Boot Information option here.
If multiple boot files are provided by the server, then, they should
appear in the order of their execution in the client. The first
appearing Bootfile name should be downloaded and executed first for
boot up, then the next and so on.
6. Precedence of the options
This specification recommends the following precedence for the
various remote boot options, including the ones specified in [4].
Option name/Field name Precedence
---------------------- ----------
Extended Remote Boot Option Highest
options 66 and 67 [4] Medium
'sname' and 'fname' [1] Lowest
Thus, if Extended Remote Boot Option is received by the client and
'sname' and 'file' fields are not overloaded, the client MUST ignore
the 'sname' and 'file' fields. If TFTP server name/address and/or
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Bootfile name are received in the reply the server, along with the
Extended Remote Boot Option, then, the client MUST ignore TFTP server
name/address and/or Bootfile name options.
7. Server behavior
If the server receives the request for TFTP server name and/or
Bootfile name along with the Extended Remote Boot Option, the server
SHOULD ignore the TFTP server name and/or Bootfile name option and
reply back with Extended Remote Boot Option.
When the DHCP server is replying back with Extended Remote Boot
Option, the 'sname' and 'file' field SHOULD be used to overload the
options.
If the length of any of these options exceed the maximum permissible
within a single option (254 octets), then they MUST be represented in
the DHCP message as specified in [2].
8. Client behavior
The client MUST NOT request for TFTP server name and/or Bootfile name
along with the Extended Remote Boot Option.
9. Security Considerations
The Remote Boot Option may be used by an intruder DHCPv4 server to
provide to cause DHCPv4 clients to contact rogue TFTP server (or) to
send invalid file names. This will make booting up of DHCP clients
to fail.
To avoid attacks through this option, the DHCP client SHOULD use
authentication mechanism for DHCP [5].
10. IANA Considerations
IANA is requested to assign an option code to the following options
from the option-code space defined for public DHCP Options in
RFC 2939 [6].
Option Name Value Described in
Extended Remote Boot Option tbd Section 4
11. Normative References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
1997.
[2] T. Lemon, S. Cheshire, Encoding Long Options in the Dynamic Host
Configuration Protocol (DHCPv4), RFC 3396, November 2002.
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12. Informative References
[3] K. Sollins, The TFTP Protocol (Revision 2), RFC 1350, July 1992.
[4] Alexander, S. and R. Droms, "DHCP options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[5] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[6] R. Droms, Procedures and IANA Guidelines for Definition of New
DHCP Options and Message Types, RFC 2939, September 2000.
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Vijayabhaskar A K
Hewlett-Packard STSD-I
29, Cunningham Road
Bangalore - 560052
India
Phone: +91-80-2053085
E-Mail: vijayak@india.hp.com
Senthil Kumar B
Hewlett-Packard STSD-I
29, Cunningham Road
Bangalore - 560052
India
Phone: +91-80-2053103
E-Mail: ksenthil@india.hp.com
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