draft-ietf-dhc-dhcpv6-08.txt   draft-ietf-dhc-dhcpv6-09.txt 
Internet Engineering Task Force J. Bound Internet Engineering Task Force J. Bound
INTERNET DRAFT Digital Equipment Corp. INTERNET DRAFT Digital Equipment Corp.
DHC Working Group C. Perkins DHC Working Group C. Perkins
Obsoletes: draft-ietf-dhc-dhcpv6-07.txt IBM Research Obsoletes: draft-ietf-dhc-dhcpv6-08.txt Sun Microsystems
22 November 1996 27 February 1997
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
draft-ietf-dhc-dhcpv6-08.txt draft-ietf-dhc-dhcpv6-09.txt
Status of This Memo Status of This Memo
This document is a submission to the DHC Working Group of the This document is a submission to the DHC Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted Internet Engineering Task Force (IETF). Comments should be submitted
to the dhcp-v6@bucknell.edu mailing list. to the dhcp-v6@bucknell.edu mailing list.
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
skipping to change at page 1, line 72 skipping to change at page 1, line 72
4. DHCP Message Formats and Field Definitions 8 4. DHCP Message Formats and Field Definitions 8
4.1. DHCP Solicit Message Format . . . . . . . . . . . . . . . 8 4.1. DHCP Solicit Message Format . . . . . . . . . . . . . . . 8
4.2. DHCP Advertise Message Format . . . . . . . . . . . . . . 9 4.2. DHCP Advertise Message Format . . . . . . . . . . . . . . 9
4.3. DHCP Request Message Format . . . . . . . . . . . . . . . 10 4.3. DHCP Request Message Format . . . . . . . . . . . . . . . 10
4.4. DHCP Reply Message Format . . . . . . . . . . . . . . . . 12 4.4. DHCP Reply Message Format . . . . . . . . . . . . . . . . 12
4.5. DHCP Release Message Format . . . . . . . . . . . . . . . 13 4.5. DHCP Release Message Format . . . . . . . . . . . . . . . 13
4.6. DHCP Reconfigure Message Format . . . . . . . . . . . . . 14 4.6. DHCP Reconfigure Message Format . . . . . . . . . . . . . 14
5. DHCP Client Considerations 15 5. DHCP Client Considerations 15
5.1. Sending DHCP Solicit Messages . . . . . . . . . . . . . . 15 5.1. Sending DHCP Solicit Messages . . . . . . . . . . . . . . 16
5.2. Receiving DHCP Advertise Messages . . . . . . . . . . . . 16 5.2. Receiving DHCP Advertise Messages . . . . . . . . . . . . 16
5.3. Sending DHCP Request Messages . . . . . . . . . . . . . . 16 5.3. Sending DHCP Request Messages . . . . . . . . . . . . . . 17
5.4. Receiving DHCP Reply Messages . . . . . . . . . . . . . . 18 5.4. Receiving DHCP Reply Messages . . . . . . . . . . . . . . 18
5.5. Sending DHCP Release Messages . . . . . . . . . . . . . . 18 5.5. Sending DHCP Release Messages . . . . . . . . . . . . . . 19
5.6. Receiving DHCP Reconfigure Messages . . . . . . . . . . . 19 5.6. Receiving DHCP Reconfigure Messages . . . . . . . . . . . 19
6. DHCP Server Considerations 19 6. DHCP Server Considerations 20
6.1. Receiving DHCP Solicit Messages . . . . . . . . . . . . . 20 6.1. Receiving DHCP Solicit Messages . . . . . . . . . . . . . 21
6.2. Sending DHCP Advertise Messages . . . . . . . . . . . . . 20 6.2. Sending DHCP Advertise Messages . . . . . . . . . . . . . 21
6.3. DHCP Request and Reply Messages . . . . . . . . . . . . . 21 6.3. DHCP Request and Reply Messages . . . . . . . . . . . . . 21
6.4. Receiving DHCP Release Messages . . . . . . . . . . . . . 22 6.4. Receiving DHCP Release Messages . . . . . . . . . . . . . 23
6.5. Sending DHCP Reconfigure Messages . . . . . . . . . . . . 23 6.5. Sending DHCP Reconfigure Messages . . . . . . . . . . . . 24
7. DHCP Relay Considerations 23 7. DHCP Relay Considerations 24
7.1. DHCP Solicit and DHCP Advertise Message Processing . . . 24 7.1. DHCP Solicit and DHCP Advertise Message Processing . . . 24
7.2. DHCP Request Message Processing . . . . . . . . . . . . . 25 7.2. DHCP Request Message Processing . . . . . . . . . . . . . 25
7.3. DHCP Reply Message Processing . . . . . . . . . . . . . . 26 7.3. DHCP Reply Message Processing . . . . . . . . . . . . . . 25
8. Retransmission and Configuration Variables 26 8. Retransmission and Configuration Variables 26
9. Security Considerations 28 9. Security Considerations 28
10. Acknowledgements 29 10. Acknowledgements 29
A. Related Work in IPv6 29 A. Related Work in IPv6 29
B. Change History 30 B. Change History 30
B.1. Changes from November 95 to February 96 Drafts . . . . . 30 B.1. Changes from November 95 to February 96 Drafts . . . . . 30
B.2. Changes from February 96 to June 96 Drafts . . . . . . . 31 B.2. Changes from February 96 to June 96 Drafts . . . . . . . 31
B.3. Changes from June 96 to August 96 Drafts . . . . . . . . 31 B.3. Changes from June 96 to August 96 Drafts . . . . . . . . 31
B.4. Changes from August 96 to November 96 Drafts . . . . . . 32 B.4. Changes from August 96 to November 96 Drafts . . . . . . 32
B.5. Changes from November 96 to February 97 Drafts . . . . . 33
C. Comparison between DHCPv4 and DHCPv6 33 C. Comparison between DHCPv4 and DHCPv6 34
Chair's Address 37 Chair's Address 38
Author's Address 37 Author's Address 38
1. Introduction 1. Introduction
The Dynamic Host Configuration Protocol (DHCPv6, or in this The Dynamic Host Configuration Protocol (DHCPv6, or in this
document usually DHCP) provides configuration parameters to Internet document usually DHCP) provides configuration parameters to Internet
nodes. DHCP consists of a protocol for delivering node-specific nodes. DHCP consists of a protocol for delivering node-specific
configuration parameters from a DHCP server to a client, and a configuration parameters from a DHCP server to a client, and a
mechanism for allocation of network addresses and other related mechanism for allocation of network addresses and other related
parameters to IPv6 [3] nodes. parameters to IPv6 [3] nodes.
skipping to change at page 2, line 7 skipping to change at page 2, line 7
definitions used for each message. Sections 5, 6, and 7 specify definitions used for each message. Sections 5, 6, and 7 specify
how clients, servers, and relays interact. Appendix A summarizes how clients, servers, and relays interact. Appendix A summarizes
related work in IPv6 that will provide helpful context; it is not related work in IPv6 that will provide helpful context; it is not
part of this specification, but included for informational purposes. part of this specification, but included for informational purposes.
Appendix B itemizes changes between different versions of this Appendix B itemizes changes between different versions of this
protocol specification. Appendix C discusses the differences between protocol specification. Appendix C discusses the differences between
DHCPv4 and DHCPv6. DHCPv4 and DHCPv6.
2. Terminology and Definitions 2. Terminology and Definitions
Relevant terminology from the IPv6 Protocol, IPv6 Addressing Relevant terminology from the IPv6 Protocol [3], IPv6 Addressing
Architecture, and IPv6 Stateless Address Autoconfiguration will be Architecture [4], and IPv6 Stateless Address Autoconfiguration [11]
provided, and then the DHCPv6 terminology. will be provided, and then the DHCPv6 terminology.
2.1. IPv6 Terminology 2.1. IPv6 Terminology
IP Internet Protocol Version 6 (IPv6). The terms IPv4 and IP Internet Protocol Version 6 (IPv6). The terms IPv4 and
IPv6 are used only in contexts where necessary to avoid IPv6 are used only in contexts where necessary to avoid
ambiguity. ambiguity.
node A device that implements IP. node A device that implements IP.
router A node that forwards IP datagrams not explicitly router A node that forwards IP datagrams not explicitly
skipping to change at page 2, line 41 skipping to change at page 2, line 41
link-layer identifier link-layer identifier
a link-layer identifier for an interface. Examples a link-layer identifier for an interface. Examples
include IEEE 802 addresses for Ethernet or Token Ring include IEEE 802 addresses for Ethernet or Token Ring
network interfaces, and E.164 addresses for ISDN links. network interfaces, and E.164 addresses for ISDN links.
link-local address link-local address
An IP address having link-only scope that can be used An IP address having link-only scope that can be used
to reach neighboring nodes attached to the same link. to reach neighboring nodes attached to the same link.
All interfaces have a link-local address. All interfaces have a link-local address.
neighbors neighbor Nodes attached to the same link.
Nodes attached to the same link.
interface interface
A node's attachment to the link. A node's attachment to the link.
address An IP layer identifier for an interface or a set of address An IP layer identifier for an interface or a set of
interfaces. interfaces.
message A unit of data carried in a datagram, exchanged between message A unit of data carried in a datagram, exchanged between
DHCP agents and clients. DHCP agents and clients.
skipping to change at page 3, line 21 skipping to change at page 3, line 21
multicast address multicast address
An identifier for a set of interfaces (typically An identifier for a set of interfaces (typically
belonging to different nodes). A datagram sent to belonging to different nodes). A datagram sent to
a multicast address is delivered to all interfaces a multicast address is delivered to all interfaces
identified by that address. identified by that address.
2.2. DHCPv6 Terminology 2.2. DHCPv6 Terminology
configuration parameter configuration parameter
Any parameter that can be used by a node to Any parameter that can be used by a node to configure
configure its network environment and enable its network environment and enable communication on a
communication on a link or internetwork. link or internetwork.
DHCP client A node that initiates requests on a link to obtain DHCP client A node that initiates requests on a link to obtain
configuration parameters. configuration parameters.
DHCP server A server is a node that responds to requests from DHCP server A server is a node that responds to requests from
clients to provide: addresses, prefix lengths, or clients to provide: addresses, prefix lengths, or
other configuration parameters. other configuration parameters.
DHCP relay A node that acts as an intermediary to deliver DHCP DHCP relay A node that acts as an intermediary to deliver DHCP
messages between clients and servers. messages between clients and servers.
DHCP Agent DHCP Agent
Either a DHCP server or a DHCP relay. Either a DHCP server or a DHCP relay.
agent address agent address
The address of a neighboring DHCP relay or DHCP The address of a neighboring DHCP relay or DHCP server
server on the same link as the DHCP client. on the same link as the DHCP client.
transaction-ID transaction-ID
The transaction-ID is a monotonically increasing The transaction-ID is a monotonically increasing
integer identifier specified by the client and used integer identifier specified by the client or server,
to match a DHCP Reply to a pending DHCP Request. and used to match a response to a pending message.
binding A binding (or, client binding) in DHCP contains the binding A binding (or, client binding) in DHCP contains the
data which a DHCP server maintains for each of its data which a DHCP server maintains for each of its
clients (see Section 6). clients (see Section 6).
2.3. Specification Language 2.3. Specification Language
In this document, several words are used to signify the requirements In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. of the specification. These words are often capitalized.
skipping to change at page 6, line 13 skipping to change at page 6, line 13
this document. this document.
3.2. DHCP Messages 3.2. DHCP Messages
Each DHCP message contains a type, which defines their function Each DHCP message contains a type, which defines their function
within the protocol. Processing details for these DHCP messages are within the protocol. Processing details for these DHCP messages are
specified in Sections 5, 6, and 7. The message types are as follows: specified in Sections 5, 6, and 7. The message types are as follows:
01 DHCP Solicit 01 DHCP Solicit
The DHCP Solicit message is a DHCP message from a client to one The DHCP Solicit message is a DHCP message sent to one or more
or more DHCP Agents. DHCP Agents.
02 DHCP Advertise 02 DHCP Advertise
The DHCP Advertise is an IP unicast message from a DHCP Agent The DHCP Advertise is an IP unicast message from a DHCP Agent
in response to a client DHCP Solicit message. in response to a client DHCP Solicit message.
03 DHCP Request 03 DHCP Request
The DHCP Request is an IP unicast message from a client to The DHCP Request is an IP unicast message from a client to a
a server, when the client knows the IP unicast address of a server to request configuration parameters on a network.
server, to request configuration parameters on a network.
04 DHCP Reply 04 DHCP Reply
The DHCP Reply is an IP unicast message sent by a server to The DHCP Reply is an IP unicast message sent by a server to
respond to a client's DHCP Request. Extensions [7] to the DHCP respond to a client's DHCP Request. Extensions [7] to the DHCP
Reply describe the resources that the DHCP Server has committed Reply describe the resources that the DHCP Server has committed
and allocated to the client, and may contain other information and allocated to the client, and may contain other information
for use by the client. for use by the client.
05 DHCP Release 05 DHCP Release
The DHCP Release message is used by a DHCP client to inform The DHCP Release message is used by a DHCP client to inform
the server that the client is releasing a particular address, the server that the client is releasing a particular address,
or set of addresses or resources, so that the server may or set of addresses or resources, so that the server may
subsequently mark the addresses or resources as invalid in the subsequently mark the addresses as invalid, or release
server's binding for the client. resources in the server's binding for the client.
06 DHCP Reconfigure 06 DHCP Reconfigure
The DHCP Reconfigure message is used by a DHCP server to inform The DHCP Reconfigure message is used by a DHCP server to inform
its client that the server has new configuration information of its client that the server has new configuration information of
importance to the client. The client is expected to initiate a importance to the client. The client is expected to initiate a
new Request/Reply transaction. new Request/Reply transaction.
3.3. Request/Response Processing Model 3.3. Request/Response Processing Model
skipping to change at page 7, line 23 skipping to change at page 7, line 23
The response (DHCP Reply) is from the server (possibly via a DHCP The response (DHCP Reply) is from the server (possibly via a DHCP
Relay). At this point in the flow all data has been transmitted Relay). At this point in the flow all data has been transmitted
and, hopefully, received. To provide a method of recovery if either and, hopefully, received. To provide a method of recovery if either
the client or server do not receive the messages to complete the the client or server do not receive the messages to complete the
transaction, the client is required to retransmit any DHCP Request transaction, the client is required to retransmit any DHCP Request
message until it elicits the corresponding DHCP Reply or Replies, message until it elicits the corresponding DHCP Reply or Replies,
or until it can be reasonably certain that the desired DHCP Server or until it can be reasonably certain that the desired DHCP Server
is unavailable. The timeout and retransmission guidelines and is unavailable. The timeout and retransmission guidelines and
configuration variables are discussed in Section 8. configuration variables are discussed in Section 8.
All DHCP Agents (Servers and Relays) MUST join the All-DHCP-Agents All DHCP Agents (Servers and Relays) MUST join the link-local
multicast group at the well-known multicast address All-DHCP-Agent multicast group at the well-known multicast address
FF02:0:0:0:0:0:1:2. All DHCP Servers MUST, in addition, join FF02:0:0:0:0:0:1:2. All DHCP Servers MUST, in addition, join
the All-DHCP-Servers multicast group at the well-known multicast the site-local All-DHCP-Servers multicast group at the well-known
address FF05:0:0:0:0:0:1:3. All DHCP Relays MUST, on other multicast address FF05:0:0:0:0:0:1:3. All DHCP Relays MUST, on the
hand, join in addition the ALL-DHCP-Relays multicast group at the other hand, join in addition the site-local All-DHCP-Relays multicast
well-known multicast address FF05:0:0:0:0:0:1:4. group at the well-known multicast address FF05:0:0:0:0:0:1:4.
DHCP uses the UDP [9] protocol to communicate between clients DHCP uses the UDP [9] protocol to communicate between clients
and servers. UDP is not reliable, but DHCP has to provide some and servers. UDP is not reliable, but DHCP has to provide some
reliability between clients and servers. If a response is not reliability between clients and servers. If a response is not
received after transmission of a DHCP message, the message MUST be received after transmission of a DHCP message, the message MUST be
retransmitted according to the rules specified in Section 8. The retransmitted according to the rules specified in Section 8. The
DHCP Relays address will be used eventually when DHCP Servers wish to All-DHCP-Relays address will be used eventually when DHCP Servers
automatically configure all site DHCP Relays. wish to automatically configure all site DHCP Relays.
A client MUST transmit all messages over UDP using port 547 as the A client MUST transmit all messages over UDP using port 547 as the
destination port. A client MUST receive all messages from UDP port destination port. A client MUST receive all messages from UDP port
546. 546.
A DHCP Agent MUST transmit all messages to clients over UDP using A DHCP Agent MUST transmit all messages to clients over UDP using
port 546 as the destination port. A DHCP Agent MUST receive all port 546 as the destination port. A DHCP Agent MUST receive all
messages over UDP using port 547. The source port for DHCP messages messages over UDP using port 547. The source port for DHCP messages
is arbitrary. is arbitrary.
skipping to change at page 8, line 19 skipping to change at page 8, line 19
types not defined here (msg-types 0 and 7-255) are reserved. types not defined here (msg-types 0 and 7-255) are reserved.
4.1. DHCP Solicit Message Format 4.1. DHCP Solicit Message Format
A DHCP client (or DHCP relay on behalf of a client) transmits a DHCP A DHCP client (or DHCP relay on behalf of a client) transmits a DHCP
Solicit message to obtain one or more DHCP server addresses. Solicit message to obtain one or more DHCP server addresses.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type |C|L|A|P| RESERVED | | msg-type |C|A| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| link-local address (if present) | | client's link-local address |
| (16 octets) | | (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DHCP relay address (if present) | | relay address |
| (16 octets) | | (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 1 msg-type 1
C If set, the client requests that all servers receiving C If set, the client requests that all servers receiving
the message deallocate the resources associated with the message deallocate the resources associated with
the client. the client.
L If set, the link-local address is present
A If set, the relay's address is present A If set, the relay's address is present
P If set, the client is willing to accept previously reserved 0
cached server addresses from relays
RESERVED 0 client's link-local address
The IP link-local address of the client interface from
which the client issued the DHCP Request message
relay address
If present, the IP address of the interface on which
the relay received the client's DHCP Solicit message
If a DHCP client does not know any DHCP Agent address, or wants If a DHCP client does not know any DHCP Agent address, or wants
to locate a new server to receive configuration parameters, the to locate a new server to receive configuration parameters, the
client SHOULD use, as the destination IP address, the client SHOULD use, as the destination IP address, the well-known All
All-DHCP-Agent multicast address FF02:0:0:0:0:0:1:2. If the DHCP Agents multicast address FF02:0:0:0:0:0:1:2. Any DHCP Relay
'P' bit is not set, any DHCP Relay receiving the solicitation MUST receiving the solicitation MUST forward it to the All-DHCP-Servers
forward it to the All-DHCP-Servers multicast address, to instruct multicast address, to instruct DHCP Servers to send their
DHCP Servers to send their advertisements to the prospective client. advertisements to the prospective client. In that case, the relay
In that case, the relay MUST copy the client's link-local address MUST copy the client's link-local address into the message, copy the
into the message, set the 'L' bit, copy the address of its interface address of its interface from which the client's solicitation was
from which the client's solicitation was received into the agent's received into the agent's address field, and set the 'A' bit.
address field, and set the 'A' bit.
4.2. DHCP Advertise Message Format 4.2. DHCP Advertise Message Format
A DHCP agent sends a DHCP Advertise message to inform a prospective A DHCP agent sends a DHCP Advertise message to inform a prospective
client about the IP address of a DHCP Agent to which a DHCP Request client about the IP address of a DHCP Agent to which a DHCP Request
message may be sent. When the client and server are on different message may be sent. When the client and server are on different
links, the server sends the advertisement back through the DHCP Relay links, the server sends the advertisement back through the DHCP Relay
whence the solicitation came. Relays MAY cache DHCP server addresses whence the solicitation came.
gleaned from DHCP advertisements with nonzero lifetimes, in order to
satisfy possible future client solicitations.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type |S|L| rsvd | server-count | lifetime | | msg-type |S| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address | | agent address |
| (16 octets) | | (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| link-local address (if present) | | client's link-local address |
| (16 octets) | | (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server addresses | | server address |
| (16 octets each) | | (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ... | extensions (variable number and length) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 2 msg-type 2
S If set, the agent address is also a server address. S If set, the agent address is also a server address.
L If set, the link-local address is present reserved 0
rsvd 0
server-count
The number of addresses listed in the server
addresses field.
lifetime Lifetime for the server advertisement, in units of
4096 seconds. If the destination address of the
DHCP Advertisement is a link-local address, then the
Lifetime MUST be 0. A value of 0 means that this
field is not used.
agent address agent address
The IP address of a DHCP Agent interface on the same The IP address of a DHCP Agent interface on the same
link as the client. link as the client.
server addresses client's link-local address
The IP address(es) of the DHCP server(s) The IP link-local address of the client interface
from which the client issued the DHCP Request message
server address
The IP address of the DHCP server
extensions See [7]. extensions See [7].
Suppose that a DHCP server on the same link as a client issues the Suppose that a DHCP server on the same link as a client issues the
DHCP Advertise in response to a DHCP Solicit message sent to the DHCP Advertise in response to a DHCP Solicit message sent to the
All-DHCP-Agents multicast address. Then the agent address will be All-DHCP-Agent multicast address. Then the agent address will be the
the IP address of one of the server's interfaces, the 'S' bit will be IP address of one of the server's interfaces, the 'S' bit will be
set, the agent address will be an address of the server, and there set, the agent address will be an address of the server, and there
may be zero server addresses sent in the DHCP Advertise message. It will be no server address sent in the DHCP Advertise message. It is
is an error for server-count to be zero if the 'S' bit is not set. an error for server-count to be zero if the 'S' bit is not set.
If the DHCP Server is sending the advertisement in response to a
solicitation with the client's link-local address present, the server
MUST copy the link-local address into the advertisement.
The source IP address of the IP header of any DHCP Advertise message The DHCP Server MUST copy the link-local address into the
MUST have sufficient scope to be reachable by the DHCP Client. In advertisement which is sent in response to a DHCP Solicit. The
particular, the source address of any DHCP Advertise message sent source IP address of the IP header of any DHCP Advertise message MUST
by a DHCP relay MUST NOT be a link-local address. In situations have sufficient scope to be reachable by the DHCP Client. Moreover,
where there are no routers sending Router Advertisements, then a DHCP the source address of any DHCP Advertise message sent by a DHCP relay
Server MUST be configured on the same link as prospective clients. MUST NOT be a link-local address. In situations where there are no
routers sending Router Advertisements, then a DHCP Server MUST be
configured on the same link as prospective clients.
4.3. DHCP Request Message Format 4.3. DHCP Request Message Format
In order to request parameters from a DHCP server, a client sends a In order to request parameters from a DHCP server, a client sends a
DHCP Request message, and MAY append the appropriate extensions [7]. DHCP Request message, and MAY append the appropriate extensions [7].
If the client does not know any DHCP server address, it MUST first If the client does not know any DHCP server address, it MUST first
obtain a server address by multicasting a DHCP Solicit message (see obtain a server address by multicasting a DHCP Solicit message (see
Section 4.1). If the client does not have a valid IP address of Section 4.1). If the client does not have a valid IP address of
sufficient scope for the DHCP server to communicate with the client, sufficient scope for the DHCP server to communicate with the client,
the client MUST use the unicast IP address of a local DHCP relay the client MUST use the unicast IP address of a local DHCP relay
as the destination IP address. Otherwise, the client MAY omit the (which then becomes the agent address in the message header) as the
server address in the DHCP Request message; in this case, the client Destination IP address. In this case, the client cannot send the
MUST send the DHCP Request message directly to the server, using the message directly to the DHCP server because the server could not
server address as the IP destination address in the IP header. return any response to the client. Otherwise, the client MAY omit
the server address in the DHCP Request message; in this case, the
client MUST send the DHCP Request message directly to the server,
using the server address as the IP destination address in the IP
header.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type |S|C| reserved | transaction-ID | | msg-type |S|C| reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (if present) | | server address |
| server address (16 octets) | | (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address | | agent address |
| (16 octets) | | (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| link-local address | | client's link-local address |
| (16 octets) | | (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) .... | extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 3 msg-type 3
S If set, the server address is present S If set, the server address is present
C If set, the client requests the server to clear all C If set, the client requests the server to clear all
existing resources and bindings currently associated other existing resources and bindings (not requested
with the client, deallocating as needed. in extensions) currently associated with the client,
deallocating as needed.
reserved 0 reserved 0
transaction-ID transaction-ID
A monotonically increasing number which the client asks A monotonically increasing number used to identify this
the server to copy into its DHCP Reply, so that the Request, and copied into the Reply.
client can match Replies with pending Requests.
server address server address
If present, the IP address of the DHCP server which If present, the IP address of the DHCP server which
should receive the client's DHCP Request message. should receive the client's DHCP Request message.
agent address agent address
The IP address of a relay or server interface, copied The IP address of a relay or server interface, copied
from a DHCP Advertisement message. from a DHCP Advertisement message.
link-local address client's link-local address
The IP link-local address of the client interface from The IP link-local address of the client interface from
which the client issued the DHCP Request message which the client issued the DHCP Request message
extensions See [7]. extensions See [7].
4.4. DHCP Reply Message Format 4.4. DHCP Reply Message Format
The server sends one or more DHCP Reply message in response to every The server sends one DHCP Reply message in response to every DHCP
DHCP Request received. If the request comes with the 'S' bit set, Request or DHCP Release received. If the request comes with the 'S'
the client could not directly send the Request to the server and had bit set, the client could not directly send the Request to the server
to use a neighboring relay agent. In that case, the server sends and had to use a neighboring relay agent. In that case, the server
back the DHCP Reply with the 'L' bit set, and the DHCP Reply is sends back the DHCP Reply with the 'L' bit set, and the DHCP Reply is
addressed to the agent address found in the DHCP Request message. If addressed to the agent address found in the DHCP Request message. If
the 'L' bit is set, then the client's link-local address will also be the 'L' bit is set, then the client's link-local address will also be
present. present.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type |L| error code | transaction-ID | | msg-type |L| error code | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (if present) | | client's link-local address |
| link-local address (16 octets) | | (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) .... | extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 4 msg-type 4
L If set, the link-local address is present L If set, the link-local address is present
error code error code
One of the following values: One of the following values:
0 Success 0 Success
16 Failure, reason unspecified 16 Failure, reason unspecified
17 Authentication failed or nonexistent 17 Authentication failed or nonexistent
18 Poorly formed request 18 Poorly formed Request or Release
19 Resources unavailable 19 Resources unavailable
20 Client record unavailable 20 Client record unavailable
21 Invalid source address in Release 21 Invalid client IP address in Release
22 Unable to honor required extension parameters 23 Relay cannot find Server Address
24 Cannot understand selected Character Set
64 Server unreachable (ICMP error) 64 Server unreachable (ICMP error)
transaction-ID transaction-ID
Copied from the transaction-ID which the DHCP server A monotonically increasing number used to identify this
received in the DHCP Request, to help the client match Reply, and copied from the client's Request.
this reply with an outstanding Request.
link-local address client's link-local address
If present, the IP address of the client interface If present, the IP address of the client interface
which issued the corresponding DHCP Request message. which issued the corresponding DHCP Request message.
extensions extensions
See [7]. See [7].
If the 'L' bit is set, and thus the link-local address is present in If the 'L' bit is set, and thus the link-local address is present in
the Reply message, the Reply is sent by the server to the relay's the Reply message, the Reply is sent by the server to the relay's
address which was specified as the agent address in the DHCP Request address which was specified as the agent address in the DHCP Request
message, and the relay uses the link-local address to deliver the message, and the relay uses the link-local address to deliver
Reply message to the client. Error code 22 MUST be sent only in the the Reply message to the client. If the length in the UDP header
case that the Server could otherwise honor the requested resource, preceding the DHCP message does not match that which is expected in
if the client had not made the parameter values (included in the the DHCP Request, error code 23 MUST be sent.
relevant Extension requesting the resource) required for the server
to obey. If the length in the UDP header preceding the DHCP message
does not match that which is expected in the DHCP Request, error code
18 MUST be sent.
4.5. DHCP Release Message Format 4.5. DHCP Release Message Format
The DHCP Release message is sent without the assistance of any DHCP The DHCP Release message is sent without the assistance of any DHCP
relay. When a client sends a Release message, it is assumed to relay. When a client sends a Release message, it is assumed to
have a valid IP address with sufficient scope to allow access to have a valid IP address with sufficient scope to allow access to
the target server. Only the parameters which are specified in the the target server. Only the parameters which are specified in the
extensions are released. The DHCP server acknowledges the Release extensions are released. The DHCP server acknowledges the Release
message by sending a DHCP Reply (Section 4.4, 6.3). message by sending a DHCP Reply (Section 4.4, 6.3).
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type |D| msg-flags | transaction-ID | | msg-type |D| reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address | | agent address |
| (16 octets) | | (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| link-local address | | client's link-local address |
| (16 octets) | | (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) .... | extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 5 msg-type 5
D If the 'D' ("Direct") flag is set, the client instructs D If the 'D' ("Direct") flag is set, the client instructs
the server to send the DHCP Reply directly back to the the server to send the DHCP Reply directly back to the
client, instead of using the given agent address and client, instead of using the given agent address and
link-local address to relay the Reply message. link-local address to relay the Reply message.
msg-flags 0 reserved 0
transaction-ID transaction-ID
A monotonically increasing number which the client asks A monotonically increasing number used to identify this
the server to use in its DHCP Reply, to help the client Release, and copied into the Reply.
match Replies with outstanding Releases.
agent address agent address
The IP address of the agent interface to which the The IP address of the agent interface to which the
client issued the DHCP Request message client issued the DHCP Request message
link-local address client's link-local address
The IP link-local address of the client interface from The IP link-local address of the client interface from
which the the client issued the DHCP Request message which the the client issued the DHCP Request message
extensions client address
See [7] The IP address of the client interface from which the
the client issued the DHCP Request message. The client
address field is present whenever the 'D' bit is set,
even if it is equal to the link-local address.
Suppose that the client knows that the address it uses as the source extensions See [7]
IP address in its IP header will still be valid after the server
performs the operations requested in the extensions to the DHCP Suppose that the client has an IP address that will still be valid
Release message. In that case, and only then, the client SHOULD then after the server performs the operations requested in the extensions
specify the 'D' flag. When the 'D' flag is set, the server MUST send to the DHCP Release message. In that case, and only then, the client
the DHCP Reply back to the client's address as shown in the source SHOULD then specify the 'D' flag. When the 'D' flag is set, the
address of the IP header of the Release message. Otherwise, when server MUST send the DHCP Reply back to the client's address as shown
in the client address field of the Release message. Otherwise, when
the 'D' bit is not set, the server MUST use the agent address and the 'D' bit is not set, the server MUST use the agent address and
link-local address in its DHCP Reply message to forward the Reply link-local address in its DHCP Reply message to forward the Reply
message back to the releasing client. message back to the releasing client.
4.6. DHCP Reconfigure Message Format 4.6. DHCP Reconfigure Message Format
The DHCP Reconfigure message is sent without the assistance of any The DHCP Reconfigure message is sent without the assistance of any
DHCP relay. When a server sends a Reconfigure message, the client DHCP relay. When a server sends a Reconfigure message, the client
to which it is sent is assumed to have a valid IP address with to which it is sent is assumed to have a valid IP address with
sufficient scope to be accessible by the server. Only the parameters sufficient scope to be accessible by the server. Only the parameters
which are specified in the extensions to the Reconfigure message need which are specified in the extensions to the Reconfigure message need
be requested again by the client. be requested again by the client.
The client SHOULD listen at UDP port 546 to receive possible
DHCP Reconfigure messages, except in cases where the client knows
that no Reconfigure message will ever be issued. In some cases,
the IP address at which the client listens will be a multicast
address sent to the client by the DHCP server in an extension to an
earlier DHCP Reply message. If the client does not listen for DHCP
Reconfigure messages, it is possible that the client will not receive
notification that its DHCP server has deallocated the client's
The client MAY receive an update to the prefix for their addresses
and then MUST use that prefix for their addreses.
IP address and/or other resources allocated to the client. See
discussion in 6.5.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| msg-type | msg-flags | reserved | | msg-type | reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) .... | extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 6 msg-type 6
msg-flags 0
reserved 0 reserved 0
transaction-ID
A monotonically increasing number used to identify
this Reconfigure message, and copied into the
client's Request.
server address
The IP address of the DHCP server issuing the DHCP
Reconfigure message.
extensions See [7] extensions See [7]
5. DHCP Client Considerations 5. DHCP Client Considerations
A DHCP client MUST silently discard any DHCP Solicit, DHCP Request, A DHCP client MUST silently discard any DHCP Solicit, DHCP Request,
or DHCP Release message it receives. or DHCP Release message it receives.
A DHCP client MAY retain its configured parameters and resources A DHCP client MAY retain its configured parameters and resources
across client system reboots and DHCP client program restarts. across client system reboots and DHCP client program restarts.
However, in these circumstances a DHCP client MUST also formulate a However, in these circumstances a DHCP client MUST also formulate a
DHCP Request message to verify that its configured parameters and DHCP Request message to verify that its configured parameters and
resources are still valid. This Request message MUST have the 'C' resources are still valid. This Request message MUST have the 'C'
bit set, to clean up stale client binding information at the server bit set, to clean up stale client binding information at the server
which may no longer be in use by the client; stale information is which may no longer be in use by the client; stale information is
that which the client does not include in extensions to such request that which the client does not include in extensions to such request
messages. messages.
If the server does not respond to the DHCP Request message, the
client may still use any addresses which have not yet expired. In
this case, however, the client MUST begin to search for another
server by multicasting a new DHCP Solicit message, again with the 'C'
bit set, containing its IP address in the appropriate extension.
This also handles the case wherein a client restarts on a new
network, so that its IP address is no longer valid. When the client
multicasts a new DHCP Discover message, servers will respond with
the information needed for the client to release its old address, if
need be, and request an address reachable on the new network. In
this situation, when the client receives a new IP address and the old
IP address is no longer reachable, the client MUST release its old
IP address by issuing a DHCP Release message with the appropriate
extension.
5.1. Sending DHCP Solicit Messages 5.1. Sending DHCP Solicit Messages
If a node wishes to become a new DHCP client, it MUST first If a node wishes to become a new DHCP client, it MUST first
locate a DHCP Server. The client does this by multicasting a DHCP locate a DHCP Server. The client does this by multicasting a DHCP
Solicit message to the All-DHCP-Agents address multicast address Solicit message to the All-DHCP-Agents address multicast address
FF02:0:0:0:0:0:1:2, setting the Hop Limit == 1. If there are no FF02:0:0:0:0:0:1:2, setting the Hop Limit == 1. If there are
DHCP servers on the same link as the node, then a DHCP Relay MUST be no DHCP servers on the same link as the node, then a DHCP Relay
present for further handling of the solicitation. If the node is MUST be present for further handling of the solicitation. The
willing to accept cached server addresses from the DHCP Relay instead prospective client SHOULD wait for ADV_WAIT seconds to get all the
of requesting the Relay to multicast its solicitation over all site DHCP Advertisement messages which may be sent in response to the
networks, then the node MAY set the 'P' bit in its solicitation. solicitation.
By setting the 'C' bit in the solicitation, a DHCP client requests If a DHCP client reboots and does not have a valid IP address,
that all the DHCP Servers that receive the solicitation should clean it MUST set the 'C' bit in the DHCP Solicit message it sends
up their stale client records that match its link-local address. when restarting. By setting the 'C' bit in the solicitation, a
DHCP client requests that all the DHCP Servers that receive the
solicitation should clean up their stale client records that match
its link-local address.
If a client sends a DHCP Solicit message after it reboots, the If a client sends a DHCP Solicit message after it reboots, the
solicitation MUST be delayed after reception of the first Router solicitation SHOULD be delayed after reception of the first Router
Advertisement [6] message, by at least some random amount of time Advertisement [6] message, by at least some random amount of time
between zero and MAX_SOLICIT_DELAY seconds. This delay is intended between MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY seconds. This delay
to help stagger requests to DHCP Servers (and avoid link-layer is intended to help stagger requests to DHCP Servers (and avoid
collisions) after a power outage causes many nodes to reboot all at link-layer collisions) after a power outage causes many nodes to
once. Each subsequent DHCP Solicit message that is issued before reboot all at once. Each subsequent DHCP Solicit message that is
receiving an advertisement MUST be delayed by twice the amount by issued before receiving an advertisement MUST be delayed by twice the
which the previous DHCP Solicit message was delayed. amount by which the previous DHCP Solicit message was delayed.
5.2. Receiving DHCP Advertise Messages 5.2. Receiving DHCP Advertise Messages
When a DHCP client receives a DHCP Advertise message, it may When a DHCP client receives a DHCP Advertise message, it may
formulate a DHCP Request message to receive configuration information formulate a DHCP Request message to receive configuration information
and resources from the DHCP servers listed in the advertisement. If and resources from the DHCP servers listed in the advertisement.
the Advertise message has zero server addresses and does not have the If the Advertise message has no server address field and does
'S' bit set, the client MUST silently discard the message. If the not have the 'S' bit set, the client MUST silently discard the
server's address is shown as a Multicast address, the advertisement message. If the server's address is shown as a Multicast address,
MUST be silently discarded. the advertisement MUST be silently discarded.
If the 'S' bit is set, the DHCP Advertise message was transmitted If the 'S' bit is set, the DHCP Advertise message was transmitted
by a DHCP server on the same link as the client. In this case, the by a DHCP server on the same link as the client. In this case, the
client MUST use the agent address as the destination address for any client MUST use the agent address as the destination address for any
future DHCP message transactions sent to that server. future DHCP message transactions sent to that server.
Advertisements may have extensions; this might allow the DHCP client Advertisements may have extensions; this might allow the DHCP client
to select the configuration that best meets its needs from among to select the configuration that best meets its needs from among
several prospective servers. several prospective servers.
5.3. Sending DHCP Request Messages 5.3. Sending DHCP Request Messages
A DHCP client obtains configuration information from a DHCP server by A DHCP client obtains configuration information from a DHCP server by
sending a DHCP Request message. The client MUST know the server's sending a DHCP Request message. The client MUST know the server's
address before sending the Request message, and client MUST have address before sending the Request message, and client MUST have
acquired a (possibly different) DHCP agent address. If the client acquired a (possibly identical) DHCP agent address. If the client
and server are on the same link, the agent address used by the client and server are on the same link, the agent address used by the client
MUST be the same as the DHCP server's address. A DHCP Request MUST be the same as the DHCP server's address. A DHCP Request
message MUST NOT be sent to any multicast address, since otherwise message MUST NOT be sent to any multicast address, since otherwise
multiple DHCP agents would possibly allocate resources to the client multiple DHCP agents would possibly allocate resources to the client
in response to the same Request, and the client would have no way to in response to the same Request, and the client would have no way to
know which servers had made the allocations, if any datagrams were know which servers had made the allocations, if any datagrams were
lost due to collisions, etc. lost due to collisions, etc.
If the client has no valid IP address of sufficient scope, and the If the client has no valid IP address of sufficient scope, and the
DHCP server is off-link, then the client MUST include the server DHCP server is off-link, then the client MUST include the server
address in the appropriate field of the DHCP Request message and set address in the appropriate field of the DHCP Request message and set
the 'S' bit. In this case, the IP destination address of the Request the 'S' bit. In this case, the IP destination address of the Request
message will be a DHCP relay address. message will be a DHCP relay address.
Otherwise, if the client already has a valid IP address and knows Otherwise, if the client already has a valid IP address of sufficient
the IP address of a candidate IP server, it SHOULD send the Request scope and knows the IP address of a candidate DHCP server, it
message directly to the DHCP server without requiring the services of SHOULD send the Request message directly to the DHCP server without
the local DHCP relay. requiring the services of the local DHCP relay.
If a client wishes to instruct a DHCP server to deallocate all If a client wishes to instruct a DHCP server to deallocate all
unknown previous resources, configuration information, and bindings unknown previous resources, configuration information, and bindings
associated with its agent address and link-local address, it sets the associated with its agent address and link-local address, it sets the
'C' bit in the DHCP Request. A client MAY send in such a Request 'C' bit in the DHCP Request. A client MAY send in such a Request
even when it is no longer attached to the link on which the relay even when it is no longer attached to the link on which the relay
address is attached. address is attached.
In any case, after choosing a transaction-ID which is numerically In any case, after choosing a transaction-ID which is numerically
greater than its previous transaction-ID, and filling in the greater than its previous transaction-ID, and filling in the
skipping to change at page 17, line 38 skipping to change at page 18, line 20
a particular IP address, or request that the server send an update a particular IP address, or request that the server send an update
containing the client's new IP address to a Domain Name Server. When containing the client's new IP address to a Domain Name Server. When
all desired extensions have been applied, the DHCP client unicasts all desired extensions have been applied, the DHCP client unicasts
the DHCP Request to the appropriate DHCP Agent. the DHCP Request to the appropriate DHCP Agent.
For each pending DHCP Request message, a client MUST maintain the For each pending DHCP Request message, a client MUST maintain the
following information: following information:
- The transaction-ID of the request message, - The transaction-ID of the request message,
- The server address, - The server address,
- The agent address, - The agent address (which can be the same as the server address),
- The time at which the next retransmission will be attempted, and - The time at which the next retransmission will be attempted, and
- All extensions appended to the request message. - All extensions appended to the request message.
If a client does not receive all the relevant DHCP Reply messages If a client does not receive a DHCP Reply message (Section 5.4) with
(Section 5.4) with the same transaction-ID as a pending DHCP Request the same transaction-ID as a pending DHCP Request message within
message within REPLY_MSG_INITIAL_TIMEOUT seconds, or if the received REPLY_MSG_INITIAL_TIMEOUT seconds, or if the received DHCP Reply
DHCP Request messages contain DHCP Authentication extensions which message contains a DHCP Authentication extension which fails to
fail to provide the correct authentication information, the client provide the correct authentication information, the client MUST
MUST retransmit the Request with the same transaction-ID and continue retransmit the Request with the same transaction-ID and continue to
to retransmit according to the rules in Section 8. retransmit according to the rules in Section 8.
If the client transmits a DHCP Request in response to a DHCP
Reconfigure message (see Section5.6), the client can continue to
operate with its existing configuration information and resources
until it receives the corresponding DHCP Reply from the server. The
same retransmission rules apply as for any other DHCP Request message
from the client.
5.4. Receiving DHCP Reply Messages 5.4. Receiving DHCP Reply Messages
When a client receives a DHCP Reply message, it MUST check whether When a client receives a DHCP Reply message, it MUST check whether
the transaction-ID in the Reply message matches the transaction-ID the transaction-ID in the Reply message matches the transaction-ID
of a pending DHCP Request message. If no match is found, the Reply of a pending DHCP Request message. If no match is found, the Reply
message MUST be silently discarded. If the transaction-ID matches message MUST be silently discarded.
that of a pending Request, and the 'L' bit is set, but the source
address in the IP header does not match the pending agent address,
the client MUST discard the message, and SHOULD log the event.
Likewise, if the transaction-ID matches that of a pending Request,
and the 'L' bit is not set, but the source address in the IP header
does not match the pending server address, the client MUST discard
the message, and SHOULD log the event.
If the Reply message is acceptable, the client processes each If the Reply message is acceptable, the client processes each
Extension [7], extracting the relevant configuration information Extension [7], extracting the relevant configuration information
and parameters for its network operation. The Error Code found in and parameters for its network operation. The client can determine
the Reply message applies to all extensions found in the Reply. If when all extensions in the Reply have been processed by using the
all expected extensions are not found in the same Reply message, Length field of the Reply. Some extensions in the Reply may have
then they are likely to be located in another Reply, possibly error codes, when the server was unable to honor the request, which
with a different Error Code, but with the same transaction-ID. The will indicate to the client the reason for failure. If the server is
DHCP Client MUST continue processing DHCP Reply messages until all simply unable to honor the request in an extension included by the
requested extensions are accounted for. If some requested extensions client, that extension may simply be omitted from the Reply.
are not accounted for within DHCP Reply messages sent by the server,
the client MUST reissue the entire DHCP Request again, with all
extensions, and the same transaction-ID.
Some configuration information extracted from the extensions to the Some configuration information extracted from the extensions to the
DHCP Reply message MUST remain associated with the DHCP server that DHCP Reply message MUST remain associated with the DHCP server that
sent the message. The particular extensions that require this extra sent the message. The particular extensions that require this extra
measure of association with the server are indicated in the DHCP measure of association with the server are indicated in the DHCP
Extensions document [7]. These "resource-server" associations are Extensions document [7]. These "resource-server" associations are
used when sending DHCP Release messages. used when sending DHCP Release messages.
5.5. Sending DHCP Release Messages 5.5. Sending DHCP Release Messages
skipping to change at page 19, line 13 skipping to change at page 19, line 36
extensions. extensions.
Suppose a client wishes to release resources which were granted to Suppose a client wishes to release resources which were granted to
it on another link. In that case, the client MUST instruct the it on another link. In that case, the client MUST instruct the
server to send the DHCP Reply directly back to the client, instead server to send the DHCP Reply directly back to the client, instead
of performing the default processing of sending the DHCP Reply back of performing the default processing of sending the DHCP Reply back
through the agent-address included in the DHCP Release. This is done through the agent-address included in the DHCP Release. This is done
by setting the 'D' bit in the DHCP Release message. Note that it is by setting the 'D' bit in the DHCP Release message. Note that it is
an error (Error Code 21) to include within the DHCP Release message an error (Error Code 21) to include within the DHCP Release message
both the 'D' bit and an IP address extension which has the IP address both the 'D' bit and an IP address extension which has the IP address
used as the source address of the datagram containing DHCP Release used as the client IP address field of the DHCP Release message
message. header.
5.6. Receiving DHCP Reconfigure Messages 5.6. Receiving DHCP Reconfigure Messages
Each DHCP client MUST listen at UDP port 546 to receive possible
DHCP Reconfigure messages, except in cases where the client knows
that no Reconfigure message will ever be issued. In some cases,
the IP address at which the client listens will be a multicast
address sent to the client by the DHCP server in an extension to
an earlier DHCP Reply message. If the client does not listen for
DHCP Reconfigure messages, it is possible that the client will
not receive notification that its DHCP server has deallocated the
client's IP address and/or other resources allocated to the client.
See discussion in 6.5. The client MAY receive an update to the
prefix for their addresses and then MUST use that prefix for their
addresses.
If a DHCP client receives a DHCP Reconfigure message, it is a request If a DHCP client receives a DHCP Reconfigure message, it is a request
for the client to initiate a new DHCP Request/Reply transaction with for the client to initiate a new DHCP Request/Reply transaction with
the server which sent the Reconfigure message. The server sending the server which sent the Reconfigure message. The server sending
the Reconfigure message MAY be different than the server which sent a the Reconfigure message MAY be different than the server which sent a
DHCP Reply message containing the original configuration information. DHCP Reply message containing the original configuration information.
For each Extension which is present in the Reconfigure message, the For each Extension which is present in the Reconfigure message, the
client appends a matching Extension to its DHCP Request message client appends a matching Extension to its Request message, which
which it formulates to send to the DHCP server which is found in it formulates to send to the server specified in the server address
the IP source address of the message. The client also selects a field of the message. The client also copies a transaction-ID from
transaction-ID numerically greater than its last choice and inserts the Reconfigure message into the Request message. From then on,
it into the Request message. From then on, processing is the same as processing is the same as specified above in Section 5.3.
specified above in Section 5.3.
Note that a client may be requested by its server to join a multicast Resources held by the client which are not identified by Extensions
group for the purpose of receiving DHCP Reconfigure messages. When a in the server's Reconfigure message are not affected.
Note that a server may ask its client to join a multicast group
for the purpose of receiving DHCP Reconfigure messages. When a
Reconfigure message is delivered to the client by way of the selected Reconfigure message is delivered to the client by way of the selected
multicast address, the client MUST delay its further response for multicast address, the client MUST delay its further response for
a random amount of time uniformly distributed within the interval a random amount of time uniformly distributed within the interval
between RECONF_MSG_MIN_RESP and RECONF_MSG_MAX_RESP seconds. This between RECONF_MSG_MIN_RESP and RECONF_MSG_MAX_RESP seconds. This
will minimize the likelihood that the server will be bombarded with will minimize the likelihood that the server will be bombarded with
DHCP Request messages all at the same time. DHCP Request messages all at the same time.
6. DHCP Server Considerations 6. DHCP Server Considerations
A server MUST ignore any DHCP Advertise, DHCP Reply, or DHCP
Reconfigure message it receives.
A server maintains a collection of client records, called A server maintains a collection of client records, called
``bindings''. Each binding is uniquely identifiable by the ordered ``bindings''. Each binding is uniquely identifiable by the ordered
pair <link-local address, agent address>, since the link-local pair <link-local address, agent address>, since the link-local
address is guaranteed to be unique [11] on the link identified address is guaranteed to be unique [11] on the link identified
by the agent address. An implementation MUST support bindings by the agent address. An implementation MUST support bindings
consisting of at least a client's link-local address, agent address, consisting of at least a client's link-local address, agent address,
preferred lifetime and valid lifetime [11] for each client address, preferred lifetime and valid lifetime [11] for each client address,
and the transaction-ID. A client binding may be used to store any and the transaction-ID. A client binding may be used to store any
other information, resources, and configuration data which will be other information, resources, and configuration data which will be
associated with the client. A DHCP server MUST retain its clients' associated with the client. A DHCP server MUST retain its clients'
bindings across server reboots, and, whenever possible, a DHCP client bindings across server reboots, and, whenever possible, a DHCP client
should be assigned the same configuration parameters despite server should be assigned the same configuration parameters despite server
system reboots and DHCP server program restarts. A DHCP server MUST system reboots and DHCP server program restarts. A DHCP server MUST
support fixed or permanent allocation of configuration parameters to support fixed or permanent allocation of configuration parameters to
specific clients. specific clients.
Servers on the same link as the client MUST use the source address Servers on the same link as the client MUST use the source address
in the IP header from the client as the destination address in DHCP in the IP header from the client as the destination address in DHCP
response messages sent by the server to the client. response messages sent by the server to the client.
A server MUST ignore any DHCP Advertise, DHCP Reply, or DHCP
Reconfigure message it receives.
6.1. Receiving DHCP Solicit Messages 6.1. Receiving DHCP Solicit Messages
If the DHCP Solicit message was received at the All-DHCP-Servers If the DHCP Solicit message was received at the All-DHCP-Servers
multicast address, the DHCP Server MUST check to make sure that the multicast address, the DHCP Server MUST check to make sure that the
source address is not a link-local address. In that case, if the source address is not a link-local address. In that case, if the
source address is a link-local address, the server MUST silently source address is a link-local address, the server MUST silently
discard the packet. If any solicitation has the 'L' bit set without discard the packet. If the UDP length disagrees with the length
the 'A' bit also being set, the server MUST discard the packet, and determined by the format of the DHCP Solicit message, the server
SHOULD log the error, in case the packet was sent by a malfunctioning MUST drop the packet and SHOULD log the error. Note that if the
relay agent. If the UDP length disagrees with the length determined client sends a DHCP Solicit message from a link-local address, the
by the format of the DHCP Solicit message, the server MUST drop the multicast destination will be the All-DHCP-Agents address, not the
packet and SHOULD log the error. All-DHCP-Servers address.
6.2. Sending DHCP Advertise Messages 6.2. Sending DHCP Advertise Messages
Upon receiving and verifying the correctness of a DHCP Solicit Upon receiving and verifying the correctness of a DHCP Solicit
message, a server constructs a DHCP Advertise message and transmits message, a server constructs a DHCP Advertise message and transmits
it on the same link as the solicitation was received from. The it on the same link as the solicitation was received from. The
destination address of the advertisement MUST be the source address destination address of the advertisement MUST be the source address
of the solicitation. The DHCP server MUST use an IP address of the of the solicitation. The DHCP server MUST use an IP address of the
interface on which it received the Solicit message as the source interface on which it received the Solicit message as the source
address field of the IP header of the message. address field of the IP header of the message.
skipping to change at page 21, line 21 skipping to change at page 22, line 10
the 'S' bit is set, the server MUST check that the server address the 'S' bit is set, the server MUST check that the server address
matches the destination IP address at which the Request message was matches the destination IP address at which the Request message was
received by the server. If the server address does not match, the received by the server. If the server address does not match, the
Request message MUST be silently discarded. Request message MUST be silently discarded.
If the received agent address and link-local address do not If the received agent address and link-local address do not
correspond to any binding known to the server, then the server MAY correspond to any binding known to the server, then the server MAY
create a new binding for the previously unknown client; otherwise, it create a new binding for the previously unknown client; otherwise, it
SHOULD return a DHCP Reply with an error code of 20. SHOULD return a DHCP Reply with an error code of 20.
Before processing the Request, the server MUST determine whether or While processing the Request, the server MUST first determine whether
not the Request is a retransmission of an earlier DHCP Request from or not the Request is a retransmission of an earlier DHCP Request
the same client. This is done by comparing the transaction-ID to from the same client. This is done by comparing the transaction-ID
all those transaction-IDs received from the same client during the to all those transaction-IDs received from the same client during the
previous XID_TIMEOUT seconds. If the transaction-ID is the same as previous XID_TIMEOUT seconds. If the transaction-ID is the same as
one received during that time, the server MUST take the same action one received during that time, the server MUST take the same action
(e.g., retransmit the same DHCP Reply to the client) as it did after (e.g., retransmit the same DHCP Reply to the client) as it did after
processing the previous DHCP Request with the same transaction-ID. processing the previous DHCP Request with the same transaction-ID.
Otherwise (the transaction-ID has not been recently used), when the Otherwise, if the transaction-ID has not been recently used, the
server has identified and allocated all the relevant information, server identifies and allocates all the relevant information,
resources, and configuration data that is associated with the client, resources, and configuration data that is associated with the client.
it sends that information to its DHCP client by constructing a Then it sends that information to its DHCP client by constructing a
DHCP Reply message and including the client's information in DHCP DHCP Reply message and including the client's information in DHCP
Extensions to the Reply message. The DHCP Reply message uses the Extensions to the Reply message. The DHCP Reply message uses the
same transaction-ID as found in the received DHCP Request message. same transaction-ID as found in the received DHCP Request message.
Note that the reply message MAY (and often will) contain information Note that the reply message MAY contain information not specifically
not specifically requested by the client. requested by the client.
If the DHCP Request message has the 'S' bit set in the message If the DHCP Request message has the 'S' bit set in the message
header, then the Request was sent to the server by a DHCP Relay. In header, then the Request was sent to the server by a DHCP Relay. In
this case, the DHCP server MUST send the corresponding DHCP Reply this case, the DHCP server MUST send the corresponding DHCP Reply
message to the agent address found in the Request (see section 7.2). message to the agent address found in the Request (see section 7.2).
The DHCP Request may contain extensions, which are interpreted The DHCP Request may contain extensions, which are interpreted
(by default) as advisory information from the client about its (by default) as advisory information from the client about its
configuration preferences. For instance, if the IP Address Extension configuration preferences. For instance, if the IP Address Extension
is present, the DHCP server SHOULD attempt to allocate or extend the is present, the DHCP server SHOULD attempt to allocate or extend the
lifetime of the address indicated by the extension. Some extensions lifetime of the address indicated by the extension. Some extensions
may be marked by the client as required. may be marked by the client as required.
The DHCP server may accept some extensions for successful processing The DHCP server may accept some extensions for successful processing
and allocation, while still rejecting others, or the server may and allocation, while still rejecting others, or the server may
reject various extensions for different reasons (and therefore reject various extensions for different reasons. The server sets
different Error Codes). The Error Code found in the Reply message the Error Code appropriately for those extensions which return error
applies to all extensions found in the Reply. The DHCP server can status to the client. The DHCP server sends a single Reply message
send multiple Reply messages in response to the same DHCP Request, in response to each DHCP Request, with the same transaction-ID as the
each possibly with a different Error Code, but all with the same Request.
transaction-ID. The DHCP server MUST send enough DHCP Reply messages
to account for all requested extensions. The DHCP server SHOULD
attempt to put all the extensions that were processed with the same
Error Code into the same DHCP Reply, in the order in which they were
received.
When a client DHCP Request is received that has the 'C' bit set, the
server should check to find out whether the extensions listed in the
Request message match those which it has associated with the client's
binding. Any resources which are not indicated by the client are
presumed to be unknown by the client, and thus possible candidates
for reallocation to satisfy requests from other clients. The DHCP
Server MUST deallocate all resources associated with the client upon
reception of a DHCP Request with the 'C' bit set, except for those
which meet the following two conditions:
- they are requested by the client in extensions to the same
Request message , and
- the server is willing to reallocate them in response to the Whenever it is able to, the server includes an extension in the
client's request. Reply message for every extension sent by the client in the Request
message. If the client requests some extensions that cannot be
supplied by the server, the server can simply fail to provide them,
not including them in the Reply. Other extensions can be rejected by
including them in the Reply with an appropriate error code indicating
failure.
. Wise implementations will not deallocate any resources until after When a client DHCP Request is received that has the 'C' bit set,
the list of extensions to the request have been inspected. the server should check to find out whether the extensions listed
in the Request message match those which it has associated with the
client's binding. Any resources which are not indicated by the
client are presumed to be unknown by the client, and thus possible
candidates for reallocation to satisfy requests from other clients.
The DHCP Server MUST deallocate all resources associated with the
client upon reception of a DHCP Request with the 'C' bit set, except
for those which the server is willing to reallocate response to the
client's request. It may be more efficient to avoid deallocating any
resources until after the list of extensions to the request have been
inspected.
6.4. Receiving DHCP Release Messages 6.4. Receiving DHCP Release Messages
If the server receives a DHCP Release Message, it MUST verify that If the server receives a DHCP Release Message, it MUST verify that
the link-local address field of the message contains an address the link-local address field of the message contains an address
which could be a valid link-local address (i.e., one with the prefix which could be a valid link-local address (i.e., one with the prefix
FE80:00:00:00/64). If not, the message MUST be silently discarded. FE80::0000/64). If not, the message MUST be silently discarded.
In response to a DHCP Release Message with a valid link-local address In response to a DHCP Release Message with a valid client's
and relay address, the DHCP server formulates a DHCP Reply message link-local address and agent address, the DHCP server formulates a
that will be sent back to the releasing client by way of the client's DHCP Reply message that will be sent back to the releasing client by
link-local address. A DHCP Reply message sent in response to a DHCP way of the client's link-local address. A DHCP Reply message sent
Release message MUST be sent to the client's link-local address via in response to a DHCP Release message MUST be sent to the client's
the agent address in the Release message and set the 'L' bit in the link-local address via the agent address in the Release message
Reply, unless the 'D' bit is set in the Release message. and set the 'L' bit in the Reply, unless the 'D' bit is set in the
Release message.
If the received agent address and link-local address do not If the received agent address and link-local address do not
correspond to any binding known to the server, then the server SHOULD correspond to any binding known to the server, then the server SHOULD
return a DHCP Reply with an error code of 20. return a DHCP Reply with an error code of 20.
If the agent address and link-local address indicate a binding Otherwise, if the agent address and link-local address indicate a
known to the server, then the server continues processing the binding known to the server, then the server continues processing the
Release message. If there are any extensions, the server releases Release message. If there are any extensions, the server releases
the particular configuration items specified in the extensions. the particular configuration items specified in the extensions.
Otherwise, if there are no extensions, the server releases all Otherwise, if there are no extensions, the server releases all
configuration information in the client's binding. configuration information in the client's binding.
After performing the operations indicated in the DHCP Release message After performing the operations indicated in the DHCP Release message
and its extensions, the DHCP server formulates a DHCP Reply message, and its extensions, the DHCP server formulates a DHCP Reply message,
copying the transaction-ID, from the DHCP Release message. For copying the transaction-ID, from the DHCP Release message. For
each Extension in the DHCP Release message successfully processed each Extension in the DHCP Release message successfully processed
by the server, a matching Extension is appended to the DHCP Reply by the server, a matching Extension is appended to the DHCP Reply
message. For extensions in the DHCP Release message which cannot be message. For extensions in the DHCP Release message which cannot be
successfully processed by the server, DHCP Reply messages with the successfully processed by the server, a DHCP Reply message containing
appropriate error codes MUST be returned by the server. extensions with the appropriate error codes MUST be returned by the
server.
6.5. Sending DHCP Reconfigure Messages 6.5. Sending DHCP Reconfigure Messages
If a DHCP server needs to change the configuration associated to any If a DHCP server needs to change the configuration associated to any
of its clients, it constructs a DHCP Reconfigure message and sends of its clients, it constructs a DHCP Reconfigure message and sends it
it to each such client. The Reconfigure MAY be sent to a multicast to each such client [7]. The Reconfigure MAY be sent to a multicast
address chosen by the server and sent to each of its clients. address chosen by the server and sent to each of its clients in an
extension to a previous DHCP Reply message.
7. DHCP Relay Considerations 7. DHCP Relay Considerations
The DHCP protocol is constructed so that a relay does not have The DHCP protocol is constructed so that a relay does not have
to maintain any state in order to facilitate DHCP client/server to maintain any state in order to facilitate DHCP client/server
interactions. interactions.
All relays MUST use the IP address of the interface from which the All relays MUST use the IP address of the interface from which the
DHCP request was received as the source address for the IP header of DHCP request was received as the source address for the IP header of
that DHCP message. that DHCP message.
The main purpose of the DHCP Relay is to assist clients and servers The main purpose of the DHCP Relay is to enable clients and servers
to carry out DHCP protocol transactions. DHCP Solicit messages are to carry out DHCP protocol transactions. DHCP Solicit messages are
issued by the relay when initiated by prospective DHCP clients. issued by the relay when initiated by prospective DHCP clients.
By default, the relay discovers local DHCP Servers by use of By default, the relay discovers local DHCP Servers by use of
multicasting DHCP solicitations to the All-DHCP-Servers multicast multicasting DHCP solicitations to the All-DHCP-Servers multicast
address, but relays SHOULD allow this behavior to be configurable. address, but relays SHOULD allow this behavior to be configurable.
The relay SHOULD NOT send such a multicast solicitation on the The relay SHOULD NOT send such a multicast solicitation on the
interface from which it received the solicitation from the client. interface from which it received the solicitation from the client.
The relay MAY update its list of available servers after whenever it
receives an updated advertisement (with a nonzero lifetime) from any
DHCP Servers.
The DHCP Relay SHOULD be able to be configured with additional DHCP
Server IP addresses for its subsequent advertisements in response
to link-local DHCP Solicit messages with the 'P' bit set. Such
configured Server addresses MAY still be updated by way of the
abovementioned multicast solicitations, but on the other hand MAY be
configured with infinite lifetimes.
7.1. DHCP Solicit and DHCP Advertise Message Processing 7.1. DHCP Solicit and DHCP Advertise Message Processing
Upon receiving a DHCP Solicit message from a prospective client, a Upon receiving a DHCP Solicit message from a prospective client, a
relay, by default, forwards the message to all DHCP Servers at a site relay, by default, forwards the message to all DHCP Servers at a site
according to the following procedure: according to the following procedure:
- setting the 'L' bit, - copying the prospective client's solicitation message fields into
the appropriate fields of the outgoing solicitation,
- copying the prospective client's link-local address into the
appropriate field of the outgoing solicitation,
- setting the 'A' bit, - setting the 'A' bit,
- copying the address of its interface from which the solicitation - copying the address of its interface from which the solicitation
was received from the client, and finally, was received from the client into the DHCP Relay address field,
and
- sending the resulting message to the All-DHCP-Servers multicast
address, FF05:0:0:0:0:0:1:3, over all interfaces except that from
which the client's solicitation was received.
When the relay receives a DHCP advertisement with the 'L' and 'A'
bits set, it relays the advertisement to the client at the indicated
link-local address by way of the interface indicated in the agent's
address field. Any datagram with the 'L' bit set and the 'A' bit not
set MUST be silently discarded.
If the relay receives a DHCP solicit message with the 'P' bit
set, the relay MAY construct a DHCP Advertise message and transmit
it to the soliciting client on the same link as the solicitation
was received from. In that case, the destination address of the
advertisement MUST be the source address of the solicitation. When
transmitting such a DHCP Advertise message to a prospective client,
a relay indicates how many server addresses are included in the
advertisement, and includes each address in the DHCP Advertise
message. DHCP Advertise messages constructed by DHCP relays from
cached server addresses MUST NOT include a server address on the same
link as the soliciting client.
If the Relay receives a nonzero lifetime in a DHCP Advertise - finally, sending the resulting message to the All-DHCP-Servers
message from one of the DHCP Servers responding to the solicitation, multicast address, FF05:0:0:0:0:0:1:3, over all interfaces except
the Relay MAY unicast a new solicitation to that server before that from which the client's solicitation was received.
the lifetime expires, even if that occurs before the passing of
RELAY_DISCOVERY_PERIOD seconds. Such a unicast solicitation MUST
have the 'A' bit set, the address of the agent's interface for
which DHCP Server addresses are desired, and finally the 'L' bit
not set (no link-local address present). If the relay receives a
DHCP advertisement with the 'A' bit set, and the 'L' bit not set,
the relay MAY cache the server addresses even though no link-local
address is present.
The Relay MUST strip off all extensions to DHCP Advertise messages When the relay receives a DHCP advertisement with the 'A' bit set, it
before storing them in its cache of DHCP Server addresses, except relays the advertisement to the client at the indicated link-local
where specifically noted in the specification of particular DHCP address by way of the interface indicated in the agent's address
extensions. field.
7.2. DHCP Request Message Processing 7.2. DHCP Request Message Processing
When a relay receives a DHCP Request message, it MUST check that the When a relay receives a DHCP Request message, it MUST check that the
message is received from a link-local address, that the link-local message is received from a link-local address, that the link-local
address matches the link-local address field in the Request message address matches the link-local address field in the Request message
header, and that the agent address field of the message matches an header, and that the agent address field of the message matches an
IP address associated to the interface from which the DHCP Request IP address associated to the interface from which the DHCP Request
message was received. If any of these checks fail, the Relay MUST message was received. If any of these checks fail, the Relay MUST
silently discard the Request message. silently discard the Request message.
The relay MUST also check whether the 'S' bit is set in the message The relay MUST also check whether the 'S' bit is set in the message
header. If not, the datagram is discarded, and and the relay SHOULD header. If not, the datagram is discarded, and and the relay SHOULD
return a DHCP Reply message to the source address of the Request return a DHCP Reply message to the source address of the Request
message with error code 18. message with error code 18.
If the received request message is acceptable, the relay then If the received request message is acceptable, the relay then
transmits the DHCP Request message to the DHCP server found in the transmits the DHCP Request message to the address of the DHCP
Server Address field of the received DHCP Request message. All server found in the Server IP Address field of the received DHCP
of the fields of DHCP Request message header transmitted by the Request message. All of the fields of DHCP Request message header
relay are copied over unchanged from the DHCP Request received from transmitted by the relay are copied over unchanged from the DHCP
the client. Only the fields in the IP header will differ from the Request received from the client. Only the fields in the IP header
datagram received from the client, not the payload. If the Relay will differ from the datagram received from the client, not the
receives an ICMP error, the Relay SHOULD return a DHCP Reply message payload. If the Relay receives an ICMP error, the Relay SHOULD
to the client address (which can be found in the payload of the ICMP return a DHCP Reply message to the client address (which can be found
message [2]), with error code 64. in the payload of the ICMP message [2]), with error code 64.
7.3. DHCP Reply Message Processing 7.3. DHCP Reply Message Processing
When the relay receives a DHCP Reply, it MUST check whether When the relay receives a DHCP Reply, it MUST check whether the
the message has the 'L' bit set. It MUST check whether the message has the 'L' bit set. It MUST check whether the link-local
link-local address field contains an IP address that has prefix address field contains an IP address that has prefix FE80::0000/64.
FE80:00:00:00/64. If all the checks are satisfied, the relay MUST If all the checks are satisfied, the relay MUST send a DHCP Reply
send a DHCP Reply message to the link-local address listed in the message to the link-local address listed in the received Reply
received Reply message. Only the fields in the IP header will differ message. Only the fields in the IP header will differ from the
from the datagram received from the server, not the payload. datagram received from the server, not the payload.
8. Retransmission and Configuration Variables 8. Retransmission and Configuration Variables
When a DHCP client does not receive a DHCP Reply in response to a When a DHCP client does not receive a DHCP Reply in response to a
pending DHCP Request, the client MUST retransmit the identical DHCP pending DHCP Request, the client MUST retransmit the identical DHCP
Request, with the same transaction-ID, to the same server again until Request, with the same transaction-ID, to the same server again
it can be reasonably sure that the DHCP server is unavailable and an until it can be reasonably sure that the DHCP server is unavailable
alternative can be chosen. It is important for the DHCP Server to and an alternative can be chosen. The DHCP Server assumes that the
be sure that its client has received the configuration information client has received the configuration information included with the
included with the extensions to the DHCP Reply message. All the extensions to the DHCP Reply message, and it is up to the client
to continue to try for a reasonable amount of time to complete the
transaction in order to make that assumption hold true. All the
actions specified for DHCP Request in this section hold also for DHCP actions specified for DHCP Request in this section hold also for DHCP
Release messages received by the DHCP Server. Release messages sent by the DHCP Client.
Likewise, but less commonly, when a DHCP server does not receive a Similarly, when a client sends a DHCP Request message in response to
DHCP Request message in response to its DHCP Reconfigure message to a Reconfigure message from the server, the client assumes that the
the client, the server MUST retransmit the identical DHCP Reconfigure DHCP server has received the Request. The server MUST retransmit the
to the client until it is reasonably certain that the client is not identical DHCP Reconfigure to the client for a reasonable amount of
available for reconfiguration. If no corresponding DHCP Request time, to try to elicit the Request message from the client, in order
is ever received by the server, the server MAY erase or deallocate to make the best effort for that assumption to hold true. If no
information as needed from the client's binding. corresponding DHCP Request is ever received by the server, the server
MAY erase or deallocate information as needed from the client's
binding.
These retransmissions occur using the following configuration These retransmissions occur using the following configuration
variables for a DHCP implementation that MUST be configurable by a variables for a DHCP implementation that MUST be configurable by a
client or server: client or server:
ADV_WAIT
The amount of time a client waits to hear DHCP Advertisements
after issuing a DHCP Solicit to the All-DHCP Agents multicast
address.
Default: 5 seconds
REPLY_MSG_INITIAL_TIMEOUT REPLY_MSG_INITIAL_TIMEOUT
The time in seconds that a DHCP client waits to receive a The time in seconds that a DHCP client waits to receive a
server's DHCP Reply before retransmitting a DHCP Request. server's DHCP Reply before retransmitting a DHCP Request.
Default: 2 seconds. Default: 2 seconds.
REPLY_MSG_MIN_RETRANS REPLY_MSG_MIN_RETRANS
The minimum number of DHCP Request transmissions that a DHCP The minimum number of DHCP Request transmissions that a DHCP
skipping to change at page 28, line 5 skipping to change at page 28, line 12
Default: 2 second. Default: 2 second.
RECONF_MSG_MAX_RESP RECONF_MSG_MAX_RESP
The maximum amount of time before a client MUST respond to a The maximum amount of time before a client MUST respond to a
DHCP Reconfigure message sent to a multicast address. DHCP Reconfigure message sent to a multicast address.
Default: 10 seconds. Default: 10 seconds.
RELAY_DISCOVERY_PERIOD MIN_SOLICIT_DELAY
The period of time between successive attempts by the DHCP The maximum amount of time a prospective client is required
Relay to discover available DHCP Servers. to wait, after determining from a Router Discovery message
that the client should perform stateful address configuration,
before sending a DHCP Solicit to a DHCP Server.
Default: 3600 seconds (1 hour). Default: 1 second
MAX_SOLICIT_DELAY MAX_SOLICIT_DELAY
The maximum amount of time a prospective client is required The maximum amount of time a prospective client is required
to wait, after determining from a Router Discovery message to wait, after determining from a Router Discovery message
that the client should perform stateful address configuration, that the client should perform stateful address configuration,
before sending a DHCP Solicit to a DHCP Server. before sending a DHCP Solicit to a DHCP Server.
Default: 5 seconds Default: 5 seconds
MAX_ADV_WAIT XID_TIMEOUT
The amount of time a client waits to hear DHCP Advertisements The amount of time a DHCP server has to keep track of
after issuing a DHCP Solicit to the All-DHCP Agents multicast client transaction-IDs in order to make sure that client
address. retransmissions using the same transaction-ID are idempotent.
Default: 5 seconds Default: 600 seconds
Note that, if a client receives a DHCP message which fails Note that, if a client receives a DHCP message which fails
authentication, it should continue to wait for another message which authentication, it should continue to wait for another message which
might be correctly authenticated just as if the failed message had might be correctly authenticated just as if the failed message had
never arrived; however, receiving such failed messages SHOULD be never arrived; however, receiving such failed messages SHOULD be
logged. logged.
XID_TIMEOUT
The amount of time a DHCP server has to keep track of
client transaction-IDs in order to make sure that client
retransmissions using the same transaction-ID are idempotent.
Default: 10800 seconds
9. Security Considerations 9. Security Considerations
DHCP clients and servers often have to authenticate the messages they DHCP clients and servers often have to authenticate the messages they
exchange. For instance, a DHCP server may wish to be certain that a exchange. For instance, a DHCP server may wish to be certain that a
DHCP Request originated from the client identified by the <link-local DHCP Request originated from the client identified by the <link-local
address, agent address> fields included within the Request message address, agent address> fields included within the Request message
header. Conversely, it is often essential for a DHCP client to header. Conversely, it is often essential for a DHCP client to
be certain that the configuration parameters and addresses it has be certain that the configuration parameters and addresses it has
received were sent to it by an authoritative DHCP server. Similarly, received were sent to it by an authoritative DHCP server. Similarly,
a DHCP server should only accept a DHCP Release message which seems a DHCP server should only accept a DHCP Release message which seems
to be from one of its clients, if it has some assurance that the to be from one of its clients, if it has some assurance that the
client actually did transmit the Release message. At the time of client actually did transmit the Release message. At the time of
this writing, there is no generally accepted mechanism useful with this writing, there is no generally accepted mechanism useful with
DHCPv4 that can be extended for use with DHCP. DHCPv4 that can be extended for use with DHCPv6.
The IPv6 Authentication Header can provide security for DHCP The IPv6 Authentication Header can provide security for DHCPv6
messages when both endpoints have a suitable IP address. However, messages when both endpoints have a suitable IP address. However,
a client often has only a link-local address, and such an address a client often has only a link-local address, and such an address
is not sufficient for a DHCP server which is off-link. In those is not sufficient for a DHCP server which is off-link. In those
circumstances the DHCP relay is involved, so that the DHCP message circumstances the DHCP relay is involved, so that the DHCP message
MUST have the relay's address in the IP destination address field, MUST have the relay's address in the IP destination address field,
even though the client aims to deliver the message to the DHCP even though the client aims to deliver the message to the DHCP
server. The DHCP Client-Server Authentication Extension [7] is server. The DHCP Client-Server Authentication Extension [7] is
intended to be used in these circumstances. intended to be used in these circumstances.
10. Acknowledgements 10. Acknowledgements
Thanks to the DHC Working Group for their time and input into the Thanks to the DHC Working Group for their time and input into the
specification. A special thanks for the consistent input, ideas, specification. A special thanks for the consistent input, ideas,
and review by (in alphabetical order) Brian Carpenter, Ralph Droms, and review by (in alphabetical order) Brian Carpenter, Ralph Droms,
Thomas Narten, Jack McCann, Yakov Rekhter, Matt Thomas, Sue Thomson, Thomas Narten, Jack McCann, Yakov Rekhter, Matt Thomas, Sue Thomson,
and Phil Wells. and Phil Wells.
Thanks to Steve Deering and Bob Hinden, who have consistently Thanks to Steve Deering and Bob Hinden, who have consistently
taken the time to discuss the more complex parts of the IPv6 taken the time to discuss the more complex parts of the IPv6
specifications. specifications. Thanks to Stuart Cheshire for his excellent minutes.
The authors MUST also thank their employers for the opportunity and
funding to work on DHCP as individuals within the IETF.
A. Related Work in IPv6 A. Related Work in IPv6
The related work in IPv6 that would best serve an implementor The related work in IPv6 that would best serve an implementor
to study is the IPv6 Specification [3], the IPv6 Addressing to study is the IPv6 Specification [3], the IPv6 Addressing
Architecture [4], IPv6 Stateless Address Autoconfiguration [11], IPv6 Architecture [4], IPv6 Stateless Address Autoconfiguration [11], IPv6
Neighbor Discovery Processing [6], and Dynamic Updates to DNS [12]. Neighbor Discovery Processing [6], and Dynamic Updates to DNS [12].
These specifications enable DHCP to build upon the IPv6 work to These specifications enable DHCP to build upon the IPv6 work to
provide both robust stateful autoconfiguration and autoregistration provide both robust stateful autoconfiguration and autoregistration
of DNS Host Names. of DNS Host Names.
skipping to change at page 33, line 5 skipping to change at page 33, line 5
Changed definitions for client, server, and relay to be definitions Changed definitions for client, server, and relay to be definitions
for DHCP client, DHCP server, and DHCP relay. for DHCP client, DHCP server, and DHCP relay.
Changed definitions of DHCP multicast addresses to conform to recent Changed definitions of DHCP multicast addresses to conform to recent
IANA allocations. IANA allocations.
Corrected references to "leases", to more accurately refer to IPv6 Corrected references to "leases", to more accurately refer to IPv6
address lifetimes. address lifetimes.
B.5. Changes from November 96 to February 97 Drafts
Clients can continue to use valid addresses, after restarts or any
request triggered by a DHCP Reconfigure message, at least until it
receives the DHCP Request from the server.
All extensions sent in response to a single DHCP Request now must be
part of the same DHCP Reply message. If some requested resources and
configuration parameters are not available or cannot be allocated,
each particular extension will either have the appropriate error code
indicating the particular problem, or simply will not be included
in the Reply. The extensions are to be modified to have fields for
error codes whenever the server might have to indicate to the client
a reason why the information requested in its extension was unable to
be supplied.
If a client receives a DHCP Reconfigure message which does not list
some the client's configuration information, it can continue to
assume that configuration information is valid.
If a client reboots, it MUST set the 'C' bit and transmit a DHCP
Request. If it doesn't have a valid server address, it MUST set the
'C' bit in its DHCP Solicit message.
Relays are no longer allowed to cache server addresses. The
DHC working group decided to ice this plan until there was some
determination that it might be useful. This caused the elimination
of the 'P' bit, and quite a bit of discussion about the 'P' bit and
DHCP server address caching was eliminated. The 'server count' field
of the Advertisement and the lifetime field were eliminated, since
relays never keep track of server addresses and clients have to
solicit again whenever they lose their DHCP server.
The working group decided to make programming as simple as possible,
and therefore to include IP addresses in the appropriate DHCP
message headers whenever those addresses would otherwise have to be
discovered by manipulating the IP header itself. This caused many
changes to the message header formats. The 'L' bit in the DHCP
Solicit and DHCP Advertise messages is no longer necessary, because
the link-local address of the client is always present in the header.
Previously, there was language which required the client to match
pending Requests with Reply messages with the same destination
agent addresses. Those agent addresses were to be determined by
inspecting the IP headers of the DHCP Reply messages. We deleted
the requirement, in preference to loading possibly two more agent
addresses in every DHCP Advertise message and DHCP Reply message.
The DHCP Reconfigure message now has a transaction ID which the
client copies into the corresponding DHCP Request, and then which
subsequently the server copies again into the corresponding DHCP
Reply message.
Clients now use the DHCP server address found in the appropriate
field of the DHCP Reconfigure message header instead of inspecting
the IP header of the Reconfigure message.
C. Comparison between DHCPv4 and DHCPv6 C. Comparison between DHCPv4 and DHCPv6
This appendix is provided for readers who will find it useful to see This appendix is provided for readers who will find it useful to see
a model and architecture comparison between DHCPv4 and DHCPv6. There a model and architecture comparison between DHCPv4 and DHCPv6. There
are three key reasons for the differences: are three key reasons for the differences:
o IPv6 inherently supports a new model and architecture for o IPv6 inherently supports a new model and architecture for
communications and autoconfiguration of addresses. communications and autoconfiguration of addresses.
o DHCPv6 in its design was able to take advantage of the inherent o DHCPv6 in its design was able to take advantage of the inherent
benefits of IPv6. benefits of IPv6.
o New features were added to support the evolution and the o New features were added to support the evolution and the
existence of mature Internet users in the industry. existence of mature Internet users in the industry.
IPv6 Architecuture/Model Changes: IPv6 Architecture/Model Changes:
o The link-local address permits a node to have an address o The link-local address permits a node to have an address
immediately when the node boots, which means all clients have a immediately when the node boots, which means all clients have a
source IP address at all times to locate a server or relay agent source IP address at all times to locate a server or relay agent
on the local link. on the local link.
o The need for bootp compatibility and broadcast flags are removed, o The need for bootp compatibility and broadcast flags are removed,
which permitted a great deal of freedom in designing the new which permitted a great deal of freedom in designing the new
packet formats for the client and server interaction. packet formats for the client and server interaction.
skipping to change at page 36, line 24 skipping to change at page 37, line 24
[3] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6) [3] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883, December 1995. Specification. RFC 1883, December 1995.
[4] R. Hinden and S. Deering. IP Version 6 Addressing Architecture. [4] R. Hinden and S. Deering. IP Version 6 Addressing Architecture.
RFC 1884, December 1995. RFC 1884, December 1995.
[5] J. McCann, S. Deering, and J. Mogul. Path MTU Discovery for IP [5] J. McCann, S. Deering, and J. Mogul. Path MTU Discovery for IP
version 6. RFC 1981, August 1996. version 6. RFC 1981, August 1996.
[6] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for [6] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for
IP Version 6 (IPv6). draft-ietf-ipngwg-discovery-06.txt -- work IP version 6 (IPv6). RFC 1970, August 1996.
in progress, March 1996.
[7] C. Perkins. Extensions to DHCPv6. draft-ietf-dhc-dhcpv6ext-02.txt [7] C. Perkins. Extensions to DHCPv6, February 1997.
-- work in progress, June 1996. draft-ietf-dhc-dhcpv6ext-05.txt, work in progress.
[8] David C. Plummer. An Ethernet Address Resolution Protocol: [8] David C. Plummer. An Ethernet Address Resolution Protocol:
Or Converting Network Protocol Addresses to 48.bit Ethernet Or Converting Network Protocol Addresses to 48.bit Ethernet
Addresses for Transmission on Ethernet Hardware. RFC 826, Addresses for Transmission on Ethernet Hardware. RFC 826,
November 1982. November 1982.
[9] J. B. Postel. User Datagram Protocol. RFC 768, August 1980. [9] J. B. Postel. User Datagram Protocol. RFC 768, August 1980.
[10] J. B. Postel, Editor. Internet Protocol. RFC 791, September [10] J. B. Postel, Editor. Internet Protocol. RFC 791, September
1981. 1981.
[11] S. Thomson and T. Narten. IPv6 Stateless Address [11] S. Thomson and T. Narten. IPv6 stateless address
Autoconfiguration. draft-ietf-addrconf-ipv6-auto-06.txt autoconfiguration. RFC 1971, August 1996.
- work in progress, November 1995.
[12] S. Thomson, Y. Rekhter, and J. Bound. Dynamic Updates in the [12] P. Vixie, S. Thomson, Y. Rekhter, and J. Bound.
Domain Name System (DNS). draft-ietf-dnsind-dynDNS-06.txt -- Dynamic Updates in the Domain Name System (DNS).
work in progress, February 1996. draft-ietf-dnsind-dynDNS-11.txt, November 1996. (work
in progress).
Chair's Address Chair's Address
The working group can be contacted via the current chair: The working group can be contacted via the current chair:
Ralph Droms Ralph Droms
Computer Science Department Computer Science Department
323 Dana Engineering 323 Dana Engineering
Bucknell University Bucknell University
Lewisburg, PA 17837 Lewisburg, PA 17837
Phone: (717) 524-1145 Phone: (717) 524-1145
E-mail: droms@bucknell.edu E-mail: droms@bucknell.edu
Author's Address Author's Address
Questions about this memo can be directed to: Questions about this memo can be directed to:
Jim Bound Charles Perkins Jim Bound Charles Perkins
Digital Equipment Corporation T. J. Watson Research Center Digital Equipment Corporation Netcentricity Group
110 Spitbrook Road, ZKO3-3/U14 IBM Corporation 110 Spitbrook Road, ZKO3-3/U14 Sun Microsystems, Inc.
Nashua, NH 03062 30 Saw Mill River Rd., Rm H3-D34 Nashua, NH 03062 2550 Garcia Avenue.
Hawthorne, NY 10532 Mountain View, CA 94043
Phone: +1-603-881-0400 +1-914-784-7350 Phone: +1-603-881-0400 +1-415-336-7153
Fax: +1-914-784-6205 Fax: +1-415-336-0673
E-mail: bound@zk3.dec.com perk@watson.ibm.com E-mail: bound@zk3.dec.com charles.perkins@corp.sun.com
 End of changes. 

This html diff was produced by rfcdiff 1.23, available from http://www.levkowetz.com/ietf/tools/rfcdiff/