DHCP relay source port: Generalized UDP Source Port for DHCP Relay
Lets a DHCP relay agent send from any UDP source port and still get replies routed back to it.
Shen, N. and E. Chen, "Generalized UDP Source Port for DHCP Relay", RFC 8357, DOI 10.17487/RFC8357, March 2018, <https://www.rfc-editor.org/info/rfc8357>.
@misc{rfc8357,
series = {Request for Comments},
number = 8357,
howpublished = {RFC 8357},
publisher = {RFC Editor},
doi = {10.17487/RFC8357},
url = {https://www.rfc-editor.org/info/rfc8357},
author = {N. Shen and E. Chen},
title = {{Generalized UDP Source Port for DHCP Relay}},
pagetotal = 10,
year = 2018,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) N. Shen
Request for Comments: 8357 E. Chen
Category: Standards Track Cisco Systems
ISSN: 2070-1721 March 2018
Generalized UDP Source Port for DHCP Relay
Abstract
This document defines an extension to the DHCP protocols that allows
a relay agent to use any available source port for upstream
communications. The extension also allows inclusion of a DHCP option
that can be used to statelessly route responses back to the
appropriate source port on downstream communications.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8357.
Copyright Notice
Copyright (c) 2018 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
(https://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
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
3. Changes to DHCP Specifications . . . . . . . . . . . . . . . 4
3.1. Additions to DHCPv4 in RFC 2131 . . . . . . . . . . . . . 4
3.2. Additions to DHCPv6 in RFC 3315 . . . . . . . . . . . . . 4
4. Relay Source Port Sub-option and Option . . . . . . . . . . . 4
4.1. Source Port Sub-option for DHCPv4 . . . . . . . . . . . . 5
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
5.4. Deployment Considerations . . . . . . . . . . . . . . . . 7
6. Example of an IPv6-Cascaded Relay . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Normative References . . . . . . . . . . . . . . . . . . . . 9
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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-
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 (547) and the client port is (546).
The fixed UDP port combinations for the 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 of 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 deployments, the decision to split the DHCP
functionality into multiple processes on a node may not be purely
based on DHCP relay computational load. Rather, DHCP relay could
just be one of the functions in a multi-process implementation.
Although assigning a different IPv4/IPv6 source address for each DHCP
relay process can be a solution, this would introduce operational and
network management complexities, especially given the scarceness of
IPv4 addresses.
This document defines 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 IPv4/IPv6 address. When sending back replies, the DHCP
server 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.
2. Terminology
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Definitions
Downstream Device: In the DHCP relay context, this refers to the
next relay agent that forwards Relay-reply messages.
Upstream Device: In the DHCP relay context, this refers to the next
relay agent or DHCP server that forwards 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 and non-zero UDP port other than port
67 for DHCPv4 and port 547 for DHCPv6.
3. Changes to DHCP Specifications
3.1. Additions to DHCPv4 in RFC 2131
Section 4.1 of RFC 2131 [RFC2131] specifies that:
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).
Relay agents implementing this specification may be configured
instead to 1) use a source port number other than 67 when relaying
messages toward servers and 2) receive responses toward clients on
that same port. This will only work when the DHCP server or relay
agent to which such a relay agent is forwarding messages is upgraded
to support this extension.
3.2. Additions 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.
Relay agents implementing this specification may be configured
instead to 1) use a source port number other than 547 when relaying
messages toward servers and 2) receive responses toward clients on
that same port. This will only work when the DHCP server or relay
agent to which such a relay agent is forwarding messages is upgraded
to support this extension.
4. Relay Source Port Sub-option and Option
Relay agents do not maintain state. To return a message to its
source, the relay agent must include all the required information in
the Relay-forward message. When a relay in a sequence of cascaded
relays does not use the standard source port, that source port must
be included along with the source address. This option allows the
relay agent to do so.
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:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SubOpt Code | Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
SubOpt Code: SUBOPT_RELAY_PORT. 8-bit value, 19.
Len: 8-bit 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 by either 1) a DHCPv6 relay agent that uses a non-DHCP UDP port
(not 547) communicating with the IPv6 server and the upstream relay
agent or 2) an 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_PORT | Option-Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Source Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
Option-Code: OPTION_RELAY_PORT. 16-bit value, 135.
Option-Len: 16-bit value to be set to 2.
Downstream Source Port: 16-bit 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).
5. Relay Agent and Server Behavior
5.1. DHCPv4
When a relay agent uses a non-DHCP UDP port (not 67) to communicate
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 the port is not
being used by a downstream relay agent, the 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.
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 relay agents to specify a source port 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.
5.4. Deployment Considerations
During deployment, the operator and/or user of the new DHCP play port
implementation should upgrade the DHCP server before the relay
implementations are deployed. This would ensure that the erroneous
case noted in Section 5.3 is not encountered. If the upstream relay
agent or server does not support this extension, this DHCP relay port
feature needs to be disabled.
When the DHCP relay port implementation is deployed, the default
relay agent behavior should use the DHCP UDP port, and it is
recommended that the configuration be set up to allow for the mode of
operation where a non-DHCP port can be used for the DHCP relay
agents.
If the network uses a 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 with this extension. The firewall rules need to be modified
to match only the DHCP server side of the UDP port number and, if
necessary, IP addresses and other attributes.
6. Example of an IPv6-Cascaded Relay
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 that 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 the
Relay-forward message. Relay1 sets the "Downstream Source Port"
field in the option to zero. Relay2 notices that 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.
When Relay2 receives the Relay-reply message with the "Relay Source
Port Option", it finds that 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.
This DHCP extension works with any combination of IPv6-cascaded relay
agents, as long as the relay agent that uses a non-DHCP UDP port (not
547) and its upstream relay device support this generalized UDP
source port extension.
Similar to the above example, but now assume that Relay2 uses the UDP
source port of 2000 instead of 547 as in the diagram. The Relay3
device needs to support this DHCP extension and it will set 2000 in
its "Downstream Source Port" field of the option in the Relay-forward
message. When the DHCP server sends the DHCP Relay-reply to Relay3,
Relay3 finds that its own relay option has this "Downstream Source
Port" with the value of 2000. Relay3 will use this UDP port when
sending the Relay-reply message to Relay2. Relay2 finds its own
relay option also has this "Downstream Source Port" with the value of
1000. Relay2 will use this UDP port when sending the Relay-reply
message to Relay1.
7. IANA Considerations
IANA has registered "DHCPv4 Relay Source Port Sub-option" (19) for
the IPv4 "Relay Agent Information" option within the "DHCP Relay
Agent Sub-Option Codes" registry <https://www.iana.org/assignments/
bootp-dhcp-parameters> [RFC3046].
IANA has registered "OPTION_RELAY_PORT" (135) for the DHCPv6 "Relay
Source Port Option" within the DHCPv6 "Option Codes" registry
<http://www.iana.org/assignments/dhcpv6-parameters"> [RFC3315].
8. Security Considerations
[RFC3118] and [RFC3315] described many of the threats in using DHCP.
This extension does not raise additional security issues.
9. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, DOI 10.17487/RFC3046, January 2001,
<https://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,
<https://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, <https://www.rfc-editor.org/info/rfc3315>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Acknowledgments
The authors would like to thank Peter Arberg, Luyuan Fang, Bhanu
Gopalasetty, Scott Kelly, Andre Kostur, Victor Kuarsingh, Ted Lemon,
Adam Roach, 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".
Authors' Addresses
Naiming Shen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
United States of America
Email: naiming@cisco.com
Enke Chen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
United States of America
Email: enkechen@cisco.com
AI ELI5
On this page
§1Why a Fixed Relay Port Is a Problem
RFC 2131 and RFC 3315 run DHCP over UDP and fix the port numbers. For IPv4 the server port is 67 and the client port is 68. For IPv6 the server port is 547 and the client port is 546.
That fixed pairing causes trouble in large relay deployments. A single switch may split its DHCP relay work across several processes that all share one IP address. If the UDP source port must be a fixed number, all DHCP packets would have to pass through one central process, which defeats the point of distributing the work. Giving each process its own IP address is possible but adds operational and management complexity, especially given the scarcity of IPv4 addresses.
This RFC relaxes the fixed source port rule for relay agents. The server must remember the UDP port of each incoming packet along with the address, and reply to that port instead of the fixed DHCP port.
Imagine every helper in a post office had to use the very same mailbox slot. They would queue up and get in each other's way. This lets each helper use their own slot.
§2Terms Used for Relay Ports
The RFC defines a few port terms for chains of relays. An upstream device is the next relay agent or server that receives Relay-forward messages. A downstream device is the next relay agent that receives Relay-reply messages.
- Relay Source Port: the UDP port a relay agent uses to receive Relay-forward messages from an upstream device.
- Downstream Source Port: the UDP port the downstream device used when forwarding to this relay. This relay uses it when sending Relay-reply messages back to that device.
- Non-DHCP UDP Port: any valid, non-zero UDP port other than
67for DHCPv4 and547for DHCPv6.
Upstream is the way toward the boss, and downstream is the way back toward the person who asked. A port is just the number of the door a message uses.
§3The Sub-option and the Option
Relays keep no state, so a relay that does not use the standard port must put the needed port information into the message itself. Two new items carry it.
For DHCPv4, the Source Port Sub-option lives inside the Relay Agent Information option from RFC 3046. It has an 8-bit code of 19 and an 8-bit length that is set to 0, so it carries no data.
For DHCPv6, the Relay Source Port Option has option code 135 (OPTION_RELAY_PORT) and a length of 2. Its one field, Downstream Source Port, is 16 bits. The relay sets it to the downstream relay's UDP source port, or to zero if only the local relay uses a non-DHCP port.
The DHCPv4 version is a tiny flag that says I am using a different door. The DHCPv6 version is a note that also writes down the door number.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
OPTION-CODE | OPTION_RELAY_PORT | 2 bytes | 135 | Option code. |
OPTION-LEN | Option-Len | 2 bytes | 2 | Length of the option data. |
DOWNSTREAM-PORT | Downstream Source Port | 2 bytes | Downstream relay's UDP source port, or zero if only the local relay uses a non-DHCP port. |
§4Relay Agent and Server Behavior
In DHCPv4, a relay that uses a non-DHCP port (not 67) MUST include the Source Port Sub-option in the messages it forwards. A server that sees the sub-option MUST remember the UDP source port of the message and use it as the destination port when replying to that relay.
In DHCPv6, a relay MUST include the Relay Source Port Option in three cases: the local relay uses a non-DHCP port (not 547), the downstream relay does, or both do. When only the local relay uses such a port, Downstream Source Port is zero. Otherwise it holds the port the downstream relay used.
The DHCPv6 server MUST copy the option into the Relay-reply chain, MUST check and use the UDP source port of the Relay-forward packet when replying, and the option MUST NOT appear in any message other than Relay-forward or Relay-reply. A relay that gets a Relay-reply with a non-zero Downstream Source Port MUST use that port when forwarding to the downstream relay.
The reply goes back to the same door the question came from. If there are several helpers in a chain, each one leaves a note saying which door the previous helper used.
§5A Cascaded IPv6 Relay Example
The RFC walks through three chained relays and a server. Relay1 is the only one using a non-DHCP port, 1000. Relay2 is upstream of Relay1 and supports the extension. Relay3 does not support it.
Relay1 includes the option with Downstream Source Port set to zero. Relay2 notices the option and sees the packet came from port 1000, so it includes its own option with the field set to 1000. The server copies the option into its Relay-reply. Relay2 reads 1000 and sends the reply to Relay1 on that port. Relay1 sees zero and uses the normal port 547 toward a downstream relay, or 546 toward a client.
The RFC states that this works with any combination of cascaded relays, as long as the relay using the non-DHCP port and its upstream device support the extension.
Relay2 writes on a sticky note that the question came through door 1000. When the answer returns, the note tells Relay2 where to hand it back.
- Relay-forward, option = 0Sent from UDP source port 1000.
- Relay-forward, option = 1000Relay2 records the port it saw.
- Relay-reply, option copied
- Relay-reply to port 1000Relay2 uses the recorded port.
§6Compatibility and Deployment
Sites that want relays with custom source ports need new software on both the DHCP server and the relay agents. If only the relay is upgraded, the server cannot talk back to it. Operators should therefore upgrade the server first. If the upstream relay or server lacks the extension, the relay port feature needs to be disabled.
The default relay behavior should remain the DHCP UDP port, and the RFC recommends that configuration allow a mode where a non-DHCP port may be used.
Firewalls that match both a static source and destination port may stop matching the new traffic. Their rules need to match only the DHCP server side port number and, if necessary, IP addresses and other attributes.
Both sides need to learn the new trick before it works, so teach the receiver first. Also, any guard who checks door numbers needs new instructions.
§7Security and IANA Registrations
The security section is short. It points to RFC 3118 and RFC 3315 for the threats of using DHCP and says this extension does not raise additional security issues.
IANA has registered the DHCPv4 Relay Source Port Sub-option (19) in the Relay Agent Information option sub-option registry, and OPTION_RELAY_PORT (135) in the DHCPv6 option codes registry.
The authors say the new trick does not open any new holes, and the new numbers have been written in the official list.