DHCPv4 Relay Flags: The Dynamic Host Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags Suboption
A one-byte flag from the DHCP relay that tells the server whether the client's request arrived by unicast or broadcast.
Kinnear, K., Normoyle, M., and M. Stapp, "The Dynamic Host Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags Suboption", RFC 5010, DOI 10.17487/RFC5010, September 2007, <https://www.rfc-editor.org/info/rfc5010>.
@misc{rfc5010,
series = {Request for Comments},
number = 5010,
howpublished = {RFC 5010},
publisher = {RFC Editor},
doi = {10.17487/RFC5010},
url = {https://www.rfc-editor.org/info/rfc5010},
author = {K. Kinnear and M. Normoyle and M. Stapp},
title = {{The Dynamic Host Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags Suboption}},
pagetotal = 7,
year = 2007,
month = sep,
} RFC text
Contents
Network Working Group K. Kinnear
Request for Comments: 5010 M. Normoyle
Category: Standards Track M. Stapp
Cisco Systems, Inc.
September 2007
The Dynamic Host Configuration Protocol Version 4 (DHCPv4)
Relay Agent Flags Suboption
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.
Abstract
This memo defines a new suboption of the Dynamic Host Configuration
Protocol (DHCP) relay agent information option that allows the DHCP
relay to specify flags for the forwarded packet. One flag is defined
to indicate whether the DHCP relay received the packet via a unicast
or broadcast packet. This information may be used by the DHCP server
to better serve clients based on whether their request was originally
broadcast or unicast.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 2
3. The Flags Suboption . . . . . . . . . . . . . . . . . . . . . . 3
4. DHCP Relay Agent Behavior . . . . . . . . . . . . . . . . . . . 3
5. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . . 5
1. Introduction
Any time a client's DHCP packet is broadcast, a local DHCP relay will
process its request and forward it on to the DHCP server. When the
DHCP relay performs this function, it can be configured to use the
DHCP relay agent information option to forward additional information
to the DHCP server, which the server may then use to alter its
processing algorithms. Once the lease has been granted, however,
future DHCP DHCPREQUEST/RENEWAL messages are unicast directly to the
DHCP Server [RFC2131] [RFC2132] [RFC3046].
In general, DHCP servers may also make subtle (and sometimes not so
subtle) changes in their processing algorithms depending on whether
or not the DHCP server received the message as a unicast packet from
the DHCP client directly, a broadcast packet from the DHCP client on
a locally connected network, or a unicast packet from a DHCP Relay
Agent, which has forwarded on a packet broadcast from a DHCP client
connected to a network local to the DHCP Relay Agent.
In some situations, DHCP Clients may unicast their DHCPREQUEST/RENEW
packets to the DHCP Relay Agent, which will forward the packet on to
the DHCP server. In these cases, the DHCP server cannot tell whether
the packet was broadcast or unicast by the DHCP client, and so it may
be unable to process the DHCP client packets in the manner that it
would if it knew whether the original DHCP packet was broadcast or
unicast. For example, a server might be willing to NAK a client in
the REBINDING state based on a determination that the client's
address does not match its location in the network, but might not be
willing to do so if the client is in the RENEWING state.
The purpose of the suboption described in this document is to allow
the DHCP relay to specify flags for the forwarded packet. These
flags can be used to describe DHCP client attributes that are useful
to the DHCP server, but can only be detected by the local DHCP relay.
The DHCP server can use the information provided by the DHCP relay to
improve its processing algorithms.
One flag is defined to indicate whether the DHCP relay received the
packet via a unicast or broadcast packet. This allows the DHCP
server to know if a packet forwarded on by a DHCP Relay Agent was
broadcast or unicast to the DHCP Relay Agent.
2. Requirements Terminology
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 [RFC2119].
3. The Flags Suboption
The Flags suboption provides an extensible suboption definition for
several possible flags. The first flag defined is the unicast flag.
The format of the suboption is:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code The suboption code (10).
Length The suboption length, 1 octet.
Flags The Relay Agent flags for this forwarded packet.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|U| MBZ |
+-+-+-+-+-+-+-+-+
U: UNICAST flag
unicast = 1
broadcast = 0
MBZ: MUST BE ZERO (reserved for future use)
4. DHCP Relay Agent Behavior
A DHCP relay agent that claims to conform to this specification MUST
include this suboption in every Relay Agent Information Option
[RFC3046] it adds to a forwarded DHCP request. In this way, the DHCP
server can distinguish a request forwarded from a DHCP relay agent
that does not support the relay-agent-flags suboption from a request
forwarded by a DHCP relay agent that supports the relay-agent-flags
suboption, and which received the request that is being forwarded in
a broadcast packet.
To put this another way, A DHCP relay agent that supports the relay-
agent-flags suboption MUST always include it in every relay-agent-
information-option that it inserts into packets that it forwards on
to the DHCP server, whether the packet it is forwarding was received
as a broadcast or as a unicast. This is because the DHCP server will
be dealing with DHCP relay agents that support the relay-agent-flags
suboption as well as DHCP relay agents that do not support the relay-
agent-flags suboption.
5. DHCP Server Behavior
This option provides additional information to the DHCP server. The
DHCP server MAY use this information to make processing decisions
regarding the DHCP Client's packet that it is processing. For
instance, knowledge of the broadcast or unicast reception of a packet
by a DHCP relay agent could be used when making the processing
decisions required to implement Load Balancing [RFC3074]. A load-
balancing server may be willing to respond to a REBINDING client, but
the server cannot determine the client's state without this
additional indication.
The option length is one octet. If the DHCP server receives a relay-
agent-flags suboption that is longer than one octet, it MUST evaluate
the first octet.
Note to implementors: In specifying the behavior of new flags bits in
the future, careful attention must be paid to compatibility with
earlier implementations. If additional flags values are defined in
the future, it will not always be possible to distinguish between
messages from relay agents who understand the new value and set its
value to 'zero', and relay agents who are simply setting a series of
unassigned bits to 'zero'. It would be a mistake to specify
significant behavior changes based on 'zero' values of flags
specified in the future.
6. Security Considerations
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 in [RFC2131].
The DHCP Relay Agent option depends on a trusted relationship between
the DHCP relay agent and the server, as described in Section 5 of
[RFC3046]. While the introduction of fraudulent relay-agent options
can be prevented by a perimeter defense that blocks these options
unless the relay agent is trusted, a deeper defense using the
authentication option for relay agent options [RFC4030] SHOULD be
deployed as well.
7. IANA Considerations
IANA has assigned a suboption number (10) for the Flags Suboption
from the DHCP Relay Agent Information Option [RFC3046] suboption
number space.
8. Acknowledgements
Thanks to David Hankins for realizing the problems created by the
server-id-override option document and for helping us understand the
value of finally solving this problem in a way that has general
applicability.
9. References
9.1. 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.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
[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.
9.2. Informative References
[RFC3074] Volz, B., Gonczi, S., Lemon, T., and R. Stevens, "DHC Load
Balancing Algorithm", RFC 3074, February 2001.
Authors' Addresses
Kim Kinnear
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
US
Phone: +1 978 936 0000
EMail: kkinnear@cisco.com
Marie Normoyle
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
US
Phone: +1 978 936 0000
EMail: mnormoyle@cisco.com
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
US
Phone: +1 978 936 0000
EMail: mjs@cisco.com
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AI ELI5
On this page
§1Why the Server Needs to Know
A DHCP relay agent forwards a client's DHCP packet to a server on another network. It can add extra information using the relay agent information option from RFC 3046. After a lease is granted, later DHCPREQUEST and renewal messages are normally unicast straight to the DHCP server.
Servers may change how they process a message depending on whether they got it directly by unicast, directly by broadcast, or by unicast from a relay that forwarded a client broadcast. The trouble is that some clients unicast their DHCPREQUEST or renewal packets to the relay, which forwards them on. The server then cannot tell whether the client broadcast or unicast the original.
The RFC gives an example: a server might be willing to NAK a client in the REBINDING state because its address does not match its place in the network, but not be willing to do so for a client in the RENEWING state.
Imagine a mail clerk passing your letter to the boss. The boss acts differently if you shouted your request to the whole room or quietly handed it over, but the clerk never says which it was.
§3The Flags Suboption Format
The Flags suboption is an extensible way to carry several possible flags. It has three one-octet fields: Code (the suboption code, 10), Length (1 octet) and Flags.
Only the first flag is defined so far. In the Flags octet, the high bit U is the UNICAST flag: 1 means the relay received the packet by unicast and 0 means broadcast. The remaining bits are MBZ, short for MUST BE ZERO, and are reserved for future use.
It is a tiny note with a label, a size and one switch. The switch is up if the request was handed over directly and down if it was shouted to everyone.
§4What the Relay Agent Does
A relay agent that claims to conform to this specification MUST include the suboption in every relay agent information option it adds to a forwarded DHCP request. That holds whether the packet it is forwarding arrived by broadcast or by unicast.
The reason is that the server will meet both relays that support the suboption and relays that do not. If supporting relays always include it, the server can tell a request from a non-supporting relay apart from a request from a supporting relay that received a broadcast.
A missing suboption means the relay does not support it. A present suboption with U set to 0 means the relay does support it and saw a broadcast.
The helper has to attach the note every single time, even when the answer is 'shouted'. Otherwise the boss could not tell 'shouted' from 'this helper never writes notes'.
§5What the DHCP Server Does
The server MAY use the flag to make processing decisions about the client's packet. The RFC's example is load balancing as in RFC 3074: a load-balancing server may be willing to respond to a REBINDING client, but cannot determine the client's state without this additional indication.
The suboption length is one octet. If the server receives a suboption longer than one octet, it MUST evaluate the first octet.
The RFC also warns implementors about future flags. It will not always be possible to tell relays that understand a new flag and set it to zero from relays that simply set unassigned bits to zero. So it would be a mistake to specify significant behavior changes based on zero values of flags defined in the future.
The boss may use the note when deciding what to do. If the note is longer than expected, the boss reads only the first part. And nobody should assume that a blank spot on a new switch means anything.
§6Security and IANA Considerations
The relay agent option depends on a trusted relationship between the relay agent and the server, as described in RFC 3046. A perimeter defense that blocks these options unless the relay is trusted can prevent fraudulent relay-agent options, but the RFC says a deeper defense using the authentication option for relay agent options (RFC 4030) SHOULD be deployed as well.
For message authentication within a domain where a shared secret can be exchanged out of band, it points to RFC 3118. Potential exposures are discussed in Section 7 of RFC 2131.
On the IANA side, suboption number 10 was assigned for the Flags suboption from the relay agent information option suboption number space.
The boss has to trust the helper's notes. Blocking strangers at the door helps, and having the helper sign each note helps even more.