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Internet Engineering Task Force (IETF) R. Johnson Request for Comments: 5859 Cisco Systems, Inc. Category: Informational June 2010 ISSN: 2070-1721

               TFTP Server Address Option for DHCPv4


 This memo documents existing usage for the "TFTP Server Address"
 option.  The option number currently in use is 150.  This memo
 documents the current usage of the option in agreement with RFC 3942,
 which declares that any pre-existing usages of option numbers in the
 range 128-223 should be documented, and the Dynamic Host
 Configuration working group will try to officially assign those
 numbers to those options.  The option is defined for DHCPv4 and works
 only with IPv4 addresses.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Johnson Informational [Page 1] RFC 5859 TFTP Server Address June 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 ( in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
 2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
 3.  TFTP Server Address Option Definition . . . . . . . . . . . . . 4
 4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
 6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
   6.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

Johnson Informational [Page 2] RFC 5859 TFTP Server Address June 2010

1. Introduction

 Voice over IP (VoIP) devices, such as IP phones, have a need to
 download their configuration from a configuration server on the
 network.  There are two commonly accepted methods to discover this
 server via DHCP; the "sname" field in the DHCP header [RFC2131] and
 the "TFTP Server Name" option (66) [RFC2132].  Both of these sources
 of information, however, contain the TFTP server's hostname.  That
 hostname must then be translated to an IP address.  The usual method
 to accomplish this would be DNS [RFC1034].  This means the firmware
 in a VoIP device (with possibly limited flash, memory, and/or
 processing resources) would need to implement the DNS protocol in
 order to perform this translation.  This would also introduce an
 additional unnecessary point of failure whereby the device is
 dependent on the DNS server infrastructure in order to boot up and
 communicate with its call agent.
 In order to eliminate DNS as a point of failure and to keep the
 firmware in such a VoIP device to a minimum, the "VoIP Configuration
 Server Address" option (150) was introduced.  This option allows the
 DHCP server to pass one or more IP addresses of the VoIP
 configuration server(s) instead of the hostname, thus making the
 information directly usable by the VoIP device.
 Other reasons for this option are (1) the "siaddr" field is not
 configurable on some DHCP servers; (2) the "siaddr" field only allows
 for one IPv4 address, and it is desirable to have the ability to
 configure multiple IP addresses for redundancy; (3) some DHCP servers
 have been found to fill in their own IPv4 address as siaddr; (4) some
 customers were already using the "siaddr" field for other purposes;
 and finally (5) the configuration server may use a protocol other
 than TFTP to serve configuration files, making the use of the "TFTP
 Server Name" option (66) inappropriate.
 In cases where other download server address information also appears
 in the response packet, such as "sname" and "TFTP Server Name", it is
 left to the device to decide which piece of information to use.

2. Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119 [RFC2119].

Johnson Informational [Page 3] RFC 5859 TFTP Server Address June 2010

3. TFTP Server Address Option Definition

 The TFTP Server Address option is a DHCP option [RFC2132].  The
 option contains one or more IPv4 addresses that the client MAY use.
 The current use of this option is for downloading configuration from
 a VoIP server via TFTP; however, the option may be used for purposes
 other than contacting a VoIP configuration server.
 The format of the option is:
            Code   Len   IPv4 Configuration Server Address(es)
           | 150 |  n  |     IPv4 address      | ...
                               Figure 1
 The option minimum length (n) is 4.
 The "Len" field must specify a length that is an integral multiple of
 4 octets (4, 8, 12, etc.).  If an option is received where this is
 not the case, the option information MUST be ignored, but further
 option processing may continue.  Dividing this "Len" value by 4 will
 give the number of IPv4 VoIP configuration server addresses that are
 specified in the option.
 The option MUST NOT be specified by the DHCP client, as it is
 intended only to be returned from the DHCP server.  If the DHCP
 client wants to receive this information from the server, it needs to
 include the number 150 in the "DHCP Parameter List" option (55).
 Server addresses SHOULD be listed in order of preference, and the
 client SHOULD use the addresses sequentially but may be configured to
 use addresses randomly.  The client may use as many or as few of the
 addresses provided as it likes.  For example, if the client is only
 capable of accepting two configuration server addresses, it may
 ignore any other addresses provided after the second address.
 Each TFTP server address that is being used by the client should be
 tried a total of four times with a 4-second wait time before
 proceeding to the next address.
 When this option appears along with the TFTP Server Name option (66)
 [RFC2132], this option SHOULD have priority over option 66.
 There is currently no defined IPv6 DHCP equivalent for this option.

Johnson Informational [Page 4] RFC 5859 TFTP Server Address June 2010

4. Security Considerations

 A rogue DHCP server could use this option in order to coerce a client
 into downloading configuration data from an alternate configuration
 server, and thus gain control of the device's configuration.  This,
 however, is no more of a security threat than similar attacks using
 other DHCP options that specify server names or addresses, of which
 there are many.  If this is a concern, then DHCP authentication may
 be used, but even secure delivery of an address over DHCP does not
 protect the subsequent insecure download over TFTP.  TFTP itself
 provides no authentication or access control mechanisms, so even if
 DHCP messages were authenticated, downloading the configuration would
 still be insecure, unless some object-level security mechanisms were
 Where security concerns are an issue, it is suggested that
 configuration files should be signed by a trusted agent.
 Configuration files may also be encrypted based on a configuration
 parameter on the DHCP client device.  In other words, there are
 various methods to ensure the integrity of configuration data
 independent from ensuring the integrity of this DHCP option or even
 DHCP itself.  The full extent of such options is far too broad to be
 addressed in this document.
 Message authentication in DHCP for intradomain use where the out-of-
 band exchange of a shared secret is feasible is defined in [RFC3118].
 Potential exposures to attack are discussed in Section 7 of the DHCP
 protocol specification [RFC2131].

5. IANA Considerations

 IANA has assigned DHCP option number 150, in accordance with

6. References

6.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
            RFC 2131, March 1997.
 [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
            Extensions", RFC 2132, March 1997.

Johnson Informational [Page 5] RFC 5859 TFTP Server Address June 2010

 [RFC3942]  Volz, B., "Reclassifying Dynamic Host Configuration
            Protocol version 4 (DHCPv4) Options", RFC 3942,
            November 2004.

6.2. Informative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, November 1987.
 [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
            Messages", RFC 3118, June 2001.

Author's Address

 Richard A. Johnson
 Cisco Systems, Inc.
 170 W. Tasman Dr.
 San Jose, CA  95134
 Phone: +1 408 526 4000

Johnson Informational [Page 6]

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