Networking Working Group N. Shen
Internet-Draft E. Chen
Intended status: Standards Track Cisco Systems
Expires: October 27, 2017 April 25, 2017
Generalized UDP Source Port for DHCP Relay
draft-ietf-dhc-relay-port-05
Abstract
This document proposes an extension to the DHCP protocols that allows
a relay agent to receive packets from a server or an upstream relay
agent on any UDP port, not just the default port 67 for IPv4 or
default port 547 for IPv6.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Changes to DHCP Specifications . . . . . . . . . . . . . . . 3
3.1. Changes to DHCPv4 in RFC 2131 . . . . . . . . . . . . . . 3
3.2. Changes to DHCPv6 in RFC 3315 . . . . . . . . . . . . . . 4
4. Relay Source Port Sub-option and Option . . . . . . . . . . . 4
4.1. Source Port Sub-option for DHCPv4 . . . . . . . . . . . . 4
4.2. Relay Source Port Option for DHCPv6 . . . . . . . . . . . 5
5. Relay Agent and Server Behavior . . . . . . . . . . . . . . . 6
5.1. DHCPv4 . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. DHCPv6 . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Compatibility . . . . . . . . . . . . . . . . . . . . . . 7
6. An IPv6 Cascaded Relay Example . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
10. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
RFC 2131 [RFC2131] and RFC 3315 [RFC3315] specify the use of UDP as
the transport protocol for DHCP. They also define both the server
side and client side port numbers. The IPv4 server port is UDP
number (67) and the client port is UDP number (68); for IPv6 the
server port is (546) and the client port is (547).
This fixed UDP port of DHCP protocol scheme creates challenges in
certain DHCP relay operations. For instance, in a large scale DHCP
relay implementation on a single switch node, the DHCP relay
functionality may be partitioned among multiple relay processes. All
these DHCP relay processes may share the same IP address of the
switch node. If the UDP source port has to be a fixed number as
currently specified, the transport socket operation of DHCP packets
would need to go through a central entity or process which would
defeat the purpose of distributing DHCP relay functionality.
In some large-scale deployment, the decision to split the DHCP
functionality into multiple processes on a node may not be purely
based on DHCP relay computational load. But rather DHCP relay could
just be one of the functions in a multi-process implementation.
Although assigning a different IP/IPv6 source address for each DHCP
relay process can be a solution, it would introduce operational and
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network management complexities, especially given the scarceness of
the IPv4 addresses.
This document proposes an extension to relax the fixed UDP source
port requirement for the DHCP relay agents. This extension requires
a DHCP server to remember the inbound packet's UDP port number along
with the IP/IPv6 address. The DHCP server when sending back replies
MUST use the UDP port number that the incoming relay agent uses
instead of the fixed DHCP port number. In the case of IPv6 cascaded
relay agents [RFC3315], the upstream relay agent needs to use the
"Relay Source Port Option" to record the downstream source port and
it MUST use this recorded port number instead of the fixed DHCP port
number when replaying the reply messages.
1.1. Requirements Language
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 [RFC2119].
2. Terminology
Downstream Device: In the DHCP relay context, it refers to the next
relay agent for forwarding Relay-reply Messages.
Upstream Device: In the DHCP relay context, it refers to the next
relay agent or DHCP server for forwarding Relay-forward
Messages.
Relay Source Port: This is the UDP port that a relay agent uses to
receive Relay-forward Messages from an upstream device.
Downstream Source Port: This is the UDP port that the downstream
device uses when forwarding Relay-forward Messages to this
relay agent device. This UDP port is to be used by this
relay agent device when forwarding the Relay-reply Messages
to that downstream device.
Non-DHCP UDP Port: Any valid UDP port other than port 67 for DHCPv4
and port 547 for DHCPv6.
3. Changes to DHCP Specifications
3.1. Changes to DHCPv4 in RFC 2131
Section 4.1 of RFC 2131 [RFC2131] specifies that:
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DHCP uses UDP as its transport protocol. DHCP messages from a
client to a server are sent to the 'DHCP server' port (67), and
DHCP messages from a server to a client are sent to the 'DHCP
client' port (68).
This specification adds the following extension to the above
paragraph.
DHCP messages from a relay agent to a server are sent to the 'DHCP
server' port (67), and the UDP source port it uses can be any
valid UDP port available in the relay system, including the DHCP
port 67. The default port number is 67 if there is no explicit
configuration for the generalized source UDP port extension for
DHCP relay.
3.2. Changes to DHCPv6 in RFC 3315
Section 5.2 of RFC 3315 [RFC3315] specifies that:
Clients listen for DHCP messages on UDP port 546. Servers and
relay agents listen for DHCP messages on UDP port 547.
This specification adds the following extension to the above
paragraph.
A DHCP relay agent can listen for DHCP messages from a server or
another upstream relay agent device on any valid UDP port
available in the relay system including the DHCP UDP port 547.
The default UDP port is 547 if there is no explicit configuration
for the generalized UDP source port extension for DHCP relay.
4. Relay Source Port Sub-option and Option
Although a DHCP server can implicitly determine the UDP source port
when it receives a message from a relay agent, this sub-option makes
the request explicit for the server to use a non-DHCP UDP port in the
reply message. When IPv6 cascaded relay agents are involved, the
downstream non-DHCP UDP port needs to be recorded using the option.
4.1. Source Port Sub-option for DHCPv4
The Relay Agent "Source Port Sub-option" is a new option, and it is
part of the relay-agent-information option for DHCPv4 [RFC3046].
The format of the "Source Port Sub-option" is shown below:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SubOpt Code | Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
SubOpt Code: SUBOPT_RELAY_PORT. 8 bits value, to be assigned by
IANA.
Len: 8 bits value to be set to 0.
4.2. Relay Source Port Option for DHCPv6
The "Relay Source Port Option" is a new DHCPv6 option. It MUST be
used either by a DHCPv6 relay agent that uses a non-DHCP UDP port
(not 547) communicating with the IPv6 server and the upstream relay
agent, or by a IPv6 relay agent that detects the use of a non-DHCP
UDP port (not 547) by a downstream relay agent.
The format of the "Relay Source Port Option" is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_RELAY_RELAY_PORT | Option-Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Source Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
Option-Code: OPTION_RELAY_RELAY_PORT. 16 bits value, to be
assigned by IANA.
Option-Len: 16 bits value to be set to 2.
Downstream Source Port: 16 bits value. To be set by the IPv6
relay either to the downstream relay agent's UDP source
port used for the UDP packet, or to zero if only the
local relay agent uses the non-DHCP UDP port (not 547).
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5. Relay Agent and Server Behavior
5.1. DHCPv4
When a relay agent uses a non-DHCP UDP port (not 67) communicating
with the DHCP server, it MUST include the "Source Port Sub-option" in
Relay-forward messages to indicate that.
When an IPv4 server receives a message from a relay agent with the
"Source Port Sub-option", it MUST remember the UDP source port of the
message and use that port number as the UDP destination port when
sending the reply message to the same relay agent.
5.2. DHCPv6
The IPv6 relay agent MUST include the "Relay Source Port Option" when
it uses a non-DHCP UDP port (not 547) to communicate to a DHCPv6
server or an upstream IPv6 relay agent. Also when an IPv6 relay
agent detects that a downstream relay agent uses a non-DHCP UDP port
in the packet, it MUST record the port number in the "Downstream
Source Port" field of this option. If this option is included to
indicate only the local non-DHCP UDP port usage and there is no
downstream relay agent's non-DHCP UDP port usage, the field
Downstream Source Port field MUST be set to zero.
The IPv6 relay agent MUST include this option in the following three
cases:
1) The local relay agent uses a non-DHCP UDP port (not 547).
2) the downstream relay agent uses a non-DHCP UDP port (not 547).
3) the local relay agent and the downstream relay agent both use
non-DHCP UDP ports (not 547).
In the first case, the value of the "Downstream Source Port" field is
set to zero. In the other two cases, the value of the field is set
to the UDP port number that the downstream relay agent uses.
When an IPv6 server receives a Relay-forward message with the "Relay
Source Port Option", it MUST copy the option when constructing the
Relay-reply chain in response to the Relay-forward message. This
option MUST NOT appear in any message other than a Relay-forward or
Relay-reply message. Additionally, the IPv6 server MUST check and
use the UDP source port from the UDP packet of the Relay-forward
message in replying to the relay agent.
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When a relay agent receives a Relay-reply message with the "Relay
Source Port Option" from a server or from an upstream relay agent, if
the "Downstream Source Port" field in the option is non-zero, it MUST
use this UDP port number to forward the Relay-reply message to the
downstream relay agent.
5.3. Compatibility
Sites that need for relay agents to specify a source port will need
to install new DHCP server and DHCP relay agent software with this
feature. If a site installs only DHCP relay agent software with this
feature, there is no possibility that the DHCP server will be able to
communicate to the relay agent.
The implementation is advised to allow configuration for relay agent
specifying a DHCP relay port number. It can be used to allow the
relay agent either using a normal DHCP UDP port or non-DHCP UDP port.
6. An IPv6 Cascaded Relay Example
An example of IPv6 cascaded relay agents with the "Relay Source Port
Option" is shown below.
(forward) (forward) (forward)
Relay1 ----------> Relay2 ----------> Relay3 ----------> Server
(1000) (547) (547)
(reply) (reply) (reply)
<---------- <---------- <----------
In the above diagram, all the IPv6 devices support this generalized
UDP source port extension except for Relay3. Relay1 is the only
relay agent device uses a non-DHCP UDP port (not 547). Relay2 is the
upstream device of Relay1.
Both Relay1 and Relay2 include the "Relay Source Port Option" in
Relay-forward message. Relay1 sets the "Downstream Source Port"
field in the option to zero. Relay2 notices the "Relay Source Port
Option" is included in the message from Relay1, and it determines
that the UDP source port used by Relay1 is 1000. Relay2 will include
the "Relay Source Port Option" and it sets the "Downstream Source
Port" field in the option to 1000. The IPv6 server copies the "Relay
Source Port Option" when replying with the Relay-reply message.
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When Relay2 receives the Relay-reply message with the "Relay Source
Port Option", it finds the "Downstream Source Port" field has the
value of 1000. Relay2 then uses this port number in the UDP packet
when sending the Relay-reply message to Relay1.
When Relay1 receives the Relay-reply message with the "Relay Source
Port Option", it finds that the "Downstream Source Port" field has
the value of zero. Relay1 then uses the normal IPv6 port 547 in the
packet sending the Relay-reply message to its downstream relay agent
or uses UDP port 546 to an IPv6 client.
7. IANA Considerations
A new sub-option, DHCPv4 Relay Source Port, is defined in this
document within the IPv4 Relay Agent Information Option. It needs to
be assigned by IANA in the "DHCP Relay Agent Sub-Option Codes"
registry, http://www.iana.org/assignments/bootp-dhcp-parameters as
specified in [RFC3046].
A new option, DHCPv6 Relay Source Port, is defined in this document
for DHCPv6 and it needs to be assigned by IANA for the DHCPv6 option
code, in the "Option Codes" registry for DHCPv6,
http://www.iana.org/assignments/dhcpv6-parameters as specified in
[RFC3315].
8. Security Considerations
[RFC3118] and [RFC3315] described many of the threats in using DHCP.
This extension does not raise addition security issues.
Although if the network uses firewall to block or allow DHCP packets
with both static UDP source and destination port numbers, this may no
longer match the packets from new DHCP relay agent and server
software. The firewall rules need to be modified only to match the
DHCP server side of the UDP port number, and if necessary, IP
addresses and other attributes.
9. Acknowledgments
The authors would like to thank Peter Arberg, Luyuan Fang, Bhanu
Gopalasetty, Andre Kostur, Ted Lemon, Kishore Seshadri and Jackelyn
Shen for their review and comments of this document.
The authors would like to thank Bernie Volz for discussions that led
to the definition of The Relay Source Port sub-option and DHCPv6
Relay Source Port Option.
The RFC text was produced using Marshall Rose's xml2rfc tool.
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10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, DOI 10.17487/RFC3046, January 2001,
<http://www.rfc-editor.org/info/rfc3046>.
[RFC3118] Droms, R., Ed. and W. Arbaugh, Ed., "Authentication for
DHCP Messages", RFC 3118, DOI 10.17487/RFC3118, June 2001,
<http://www.rfc-editor.org/info/rfc3118>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
Authors' Addresses
Naiming Shen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
US
Email: naiming@cisco.com
Enke Chen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
US
Email: enkechen@cisco.com
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