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Versions: 00 01 02 03 04 05 06                                          
Dynamic Host Configuration Working                            D. Hankins
Group                                                             Google
Internet-Draft                                           October 1, 2011
Updates: 2131 (if approved)
Intended status: Standards Track
Expires: April 3, 2012

 Dynamic Host Configuration Protocol DHCPINFORM Message Clarifications


   The DHCPINFORM message within the DHCPv4 protocol has in operation
   diverged incompatibly from the current defined standard.  This
   document seeks to provide clarification of actual behaviour and
   guidance for some situations that were previously omitted.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted 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."

   This Internet-Draft will expire on April 3, 2012.

Copyright Notice

   Copyright (c) 2011 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . . . 4
   3.  Client Behaviour  . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Server Behaviour  . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   The most recent DHCPv4 Standard [RFC2131] added a new DHCPv4 message:
   DHCPINFORM.  The intent of the DHCPINFORM message was for clients
   that used manually entered fixed IPv4 addresses to still be able to
   get some configuration state dynamically.  Since that time, however,
   we have seen this message used by normal DHCPv4 dynamically addressed
   clients; clients that have previously succeeded in receiving
   configuration through DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, and
   finally DHCPACK messages.

   These clients are attempting DHCPINFORM messages in order to obtain
   additional configuration state that was not present in their lease
   binding.  The discovery is that DHCPINFORM can be used to reach extra
   DHCP servers, other than the one that gave an address, which may have
   more configuration options available but aren't in a position to give
   addresses.  This extra configuration state is often required by
   applications that were not running at system startup, when the DHCP
   client was initialized, and supplied by servers or services bundled
   with a product that cannot easily be integrated with the network's
   existing DHCP infrastructure and so are provided separately.

   Some of these DHCPINFORM clients have surfaced which run with
   stripped down user priveleges, but still perform some network related
   functions.  This software does not have the capacity to determine its
   IPv4 address(es), nor does it know what interface(s) are present on
   the system, or their hardware addresses.  But it can send and receive
   DHCP packets.  Consequently, the 'ciaddr' and 'chaddr' fields have
   been witnessed to be empty, even though they appear to be required to
   be filled by RFC 2131.  Clarification is sought for server behaviour
   when ciaddr is zero.

   Another set of DHCP clients set the 'chaddr' field to a fixed magic
   value, rather than the client's hardware address, identifying them as
   part of a vendor's product.  Although the 'chaddr' contents were
   never defined by any IETF RFC to be a valid place to store 'Vendor
   Identifying Information', their implementors believed this field was
   unused by the DHCP protocol in specific regards to DHCPINFORM because
   a server would determine the client's hardware address through normal
   UDP unicast methods; IP forwarding leading to ARP [RFC0826]
   processing or similar.

   We also wish to clarify a DHCPv4 server's behaviour when it receives
   a DHCPINFORM via a relay (when 'giaddr' is non-zero).  Section 4.1 of
   the DHCPv4 specification [RFC2131] seems to include
   DHCPINFORM->DHCPACK exchanges by describing generic behaviour for all
   DHCPOFFER and DHCPACK replies, and it requires that if 'giaddr' is
   non-zero that it "MUST" be used.  But this advice does not work in

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   practice (due to BOOTP Relay Agent [RFC1542] requirements to use
   'yiaddr' field contents, which MUST be zero as also per [RFC2131]).
   Furthermore, this guidance conflicts with [RFC2131] Section 4.3.5,
   which directs that the server replies directly to the 'ciaddr'
   contents when responding to DHCPINFORM, and makes no other directions
   for other header fields.  As a result, it also does not adequately
   describe current operational deployments of the DHCPINFORM message
   exchange which definitely direct replies directly to 'ciaddr' and may
   (it has not been concretely determined) direct replies to the
   'giaddr' first.

2.  Requirements Language

   In this document, the key words "MAY", "MUST", "SHALL", "MUST NOT",
   "SHOULD", and "SHOULD NOT", are to be interpreted as described in BCP
   14, RFC 2119 [RFC2119].

3.  Client Behaviour

   Clients are still required to fulfill the DHCPv4 requirements for
   DHCPINFORM messages ([RFC2131], Sections 4.4.1 and 4.4.3).  But the
   following are clarified as in addition, or to overlay those

   o  Clients MUST set 'ciaddr' to a working IPv4 address which they can
      use to receive replies.  This address SHOULD be an address that is
      currently assigned to the interface upon which the client is
      transmitting its DHCPINFORM, except in the condition where the
      DHCP client is unable to determine a valid IP address for its
      host, in which case the client MUST set 'ciaddr' to all-zero.

   o  Clients MUST set 'chaddr', 'htype', and 'hlen' to the hardware
      address of the interface upon which the DHCPINFORM message is
      being transmitted, except in the condition where the DHCP client
      is unable to determine this address, in which case all three
      fields MUST be set all-zero.

   o  Clients MUST set the 'flags' field to zero.  This means that the
      client MUST NOT set the 'BROADCAST' flag, and MUST be capable of
      receiving IP unicasts.

   o  Clients SHOULD direct their DHCPINFORM via unicast UDP to the IPv4
      address contained in the Server Identifier [RFC2132] option, if
      they have a currently active binding from previous DHCPREQUEST
      message exchanges.  It MAY be unicast to a known DHCP server, or
      otherwise broadcast to the appropriate IPv4 broadcast address on

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      the interface being configured.

4.  Server Behaviour

   DHCPv4 server behaviour in processing DHCPINFORM messages is a more
   difficult question to answer, due to inconsistent client behaviour
   and conflicting directions in RFC 2131.  The following is intended to
   be a more complete reference.

   First, upon receiving a DHCPINFORM, a DHCPv4 Server MUST determine
   the client's "relevant IPv4 address" according to the following in
   order of priority:

   1.  The Subnet Selection Option [RFC3011], if it is present.

   2.  The 'ciaddr' field, if it is non-zero.

   3.  The Relay Agent Link Selection Sub-Option [RFC3527], if it is
       present in a Relay Agent Information Option [RFC3046].

   4.  The 'giaddr' field, if it is non-zero.

   5.  The IPv4 source address field, if it is non-zero.

   6.  The DHCPv4 Server's address on the interface on which the
       DHCPINFORM was received.

   The DHCPv4 server checks to see if the "relevant IPv4 address" is
   within a range or subnet over which it holds authority, or if it is
   configured to respond.  It will manufacture a DHCPACK response with
   configuration values appropriate for the "relevant IPv4 address".  If
   the "relevant IPv4 address" is from the 'ciaddr' field (because the
   Subnet Selection Option was not provided, and the 'ciaddr' field is
   non-zero), the server MAY also inspect that address's current lease
   in order to source configuration specific to the host, but MUST NOT
   modify the lease in any way.

   In the DHCPACK reply:

   o  The 'htype', 'hlen', 'chaddr', 'ciaddr', 'xid', 'flags' (with the
      exception noted below), and 'giaddr' fields MUST be copied from
      the client's DHCPINFORM.

   o  The 'hops' field MUST be zero.

   o  The 'secs' field MUST be zero.

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   o  The 'yiaddr' field MUST be zero.

   o  The 'siaddr' field MUST be zero.

   o  The 'sname' and 'file' fields MAY be used exclusively for 'option
      overloading', but MUST be all-zero otherwise.

   o  The 'options' field MUST be filled as described in RFC 2131
      Section 4.3.1.

   Next, the DHCPv4 server MUST determine the "reply address and port"
   according to the first of the following conditions it finds a valid
   reply address for, in order:

   1.  If the 'ciaddr' field is non-zero, the server selects its
       contents as an IPv4 address and port 68 ('DHCP client').

   2.  If the 'giaddr' field is non-zero, the server selects its
       contents as an IPv4 address and port 67 ('DHCP server').

   3.  If the IPv4 source address field is non-zero, the server selects
       its contents as an IPv4 address and port 68 ('DHCP client')

   4.  The server selects the limited broadcast address (all-ones) and
       port 68 ('DHCP client').

   At this point, the DHCPv4 server verifies that it holds configuration
   authority over the reply address (or link in case of limited
   broadcast address) it has selected to transmit the reply to.  If the
   server has not been configured to hold authority over this address,
   it MUST NOT reply.  It SHOULD increment a counter visible to the
   operator but SHOULD NOT log an error (unless a mechanism is used to
   suppress repeated log messages).  See the Security section
   (Section 5) for the rationale behind this direction.

   Note very carefully that a DHCPv4 server will send replies directly
   to a DHCPv4 client by way of 'ciaddr' even if the DHCPINFORM message
   was relayed.  Note that this means DHCPINFORM processing is
   intentionally broken in deployments where the client's address space
   is unreachable by the DHCPv4 server.  In such cases, the server
   should probably be configured not to reply to DHCPINFORMs.

   Now, the server performs an exception to assist relay agents.  If it
   selected the 'giaddr' as the destination address and port, then it
   MUST set the 'BROADCAST' bit in the flags field true, no matter what
   its value was in the client's DHCPINFORM message, without altering
   the other bits of the flag field.  Otherwise, the response could not
   be delivered; a BOOTP Relay Agent [RFC1542] is required to direct

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   unicast server replies to the 'chaddr' and 'yiaddr' field contents,
   but 'chaddr' is not reliably filled, and 'yiaddr' is required to be
   all-zero.  Setting the broadcast flag assists the relay agent in
   locating the client by informing it to perform a local limited

   Having selected a destination IPv4 address and port number, the last
   step is to select a destination link layer address.

   For the all-ones limited broadcast address, the DHCPv4 server MUST
   use the all-ones broadcast hardware address.

   For all other (unicast) destination selections, the DHCPv4 server
   MUST use its host operating system's usual methods to determine
   hardware addressing, as by IP forwarding and subsequent address
   resolution (such as through ARP [RFC0826]).  Note that the DHCPv4
   server MAY have seeded its ARP cache from a previous stateful
   exchange with the client (from 'chaddr' contents while processing a
   DHCPREQUEST message, due to the requirement of DHCPv4 servers to
   unicast some replies before clients will process ARP), and some
   DHCPv4 software MAY still use 'chaddr' contents to direct replies to
   directly connected clients.  Consequently, DHCPINFORM can not be
   reasonably expected to instigate an immediate ARP broadcast, nor can
   'chaddr' contents be used for any purpose other than to carry the
   unicast hardware address with which a client might reasonably be

5.  Security Considerations

   As with all DHCP messages, DHCPINFORM and DHCPACK replies contain no
   capacity for encryption, and all packet contents must be presumed
   readable in the clear.  In particular, and as outlined above, in some
   circumstances the packets may be broadcast and so more easily
   intercepted than most other messages.

   Authentication for DHCPv4 Messages [RFC3118] does exist, but is not
   well deployed.  Care should be taken in the degree to which
   configuration parameters provided by DHCPv4 are trusted, as the
   replies can be easily spoofed by any eavesdropper.  Again noting that
   packets may be broadcast under some circumstances, the BOOTP header
   Transaction Id field ("XID") is insufficient protection from man-in-
   the-middle attacks.

   A relay agent receives replies via unicast UDP messages from a DHCP
   server, and may broadcast these packets on the inside-facing network.
   If an outside attacker was aware of this relay agent and its unicast
   address, this facility could be used to produce broadcast storms on

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   the network.  Care should be taken to ensure that the relay agent is
   not open to this kind of attack, possibly making use of Relay Agent
   Authentication [RFC4030] to ensure that a DHCPv4 server can not be
   induced to sending bogus replies to the relay.

   This protocol uses the 'ciaddr' field contents to direct replies,
   which may be set blindly by the client to any value, regardless of IP
   source address validation or related filter restrictions.  If an
   attacker were to identify a number of DHCPv4 servers which reply to
   addresses not under their authority to configure, and those servers
   had enough large DHCPv4 options in configuration to request, it could
   represent a significant amplification vector in straight packet-load
   Denial-of-Service attacks.  For this reason, servers MUST NOT make
   replies to addresses not explicitly configured under their authority
   to configure.

6.  IANA Considerations

   This document has no action for IANA.

7.  Acknowledgements

   This document has been reviewed and improved by the comments of
   several people, but the author would like to take a moment to thank
   Alfred Hoenes, who has submitted revised text for this document.

8.  References

8.1.  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.

   [RFC3011]  Waters, G., "The IPv4 Subnet Selection Option for DHCP",
              RFC 3011, November 2000.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

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   [RFC3527]  Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
              "Link Selection sub-option for the Relay Agent Information
              Option for DHCPv4", RFC 3527, April 2003.

8.2.  Informative References

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [RFC1542]  Wimer, W., "Clarifications and Extensions for the
              Bootstrap Protocol", RFC 1542, October 1993.

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC4030]  Stapp, M. and T. Lemon, "The Authentication Suboption for
              the Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Option", RFC 4030, March 2005.

Author's Address

   David W. Hankins
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: dhankins@google.com

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