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Network Working Group Yakov Rekhter
Internet Draft Cisco Systems
Expiration Date: August 1998 February 1998
Interaction between DHCP and DNS
draft-ietf-dhc-dhcp-dns-07.txt
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).
2. Abstract
DHCP provides a powerful mechanism for IP host autoconfiguration.
However, the autoconfiguration provided by DHCP does not include
updating DNS, and specifically updating the name to address and
address to name mappings maintained by DNS.
This document specifies how DHCP clients and servers should use the
Dynamic DNS Updates mechanism to update the DNS name to address and
address to name mapping, so that the mappings for DHCP clients would
be consistent with the IP addresses that the clients acquire via
DHCP.
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Internet Draft draft-ietf-dhc-dhcp-dns-07.txt February 1998
3. Terminology
Throughout this document, the words that are used to define the
significance of particular requirements are capitalized. These words
are:
- "MUST"
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification.
- "MUST NOT"
This phrase means that the item is an absolute prohibition of
this specification.
- "SHOULD"
This word or the adjective "RECOMMENDED" means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the
case carefully weighed before choosing a different course.
- "SHOULD NOT"
This phrase means that there may exist valid reasons in
particular circumstances when the listed behavior is acceptable
or even useful, but the full implications should be understood
and the case carefully weighed before implementing any behavior
described with this label.
- "MAY"
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.
4. Interaction between DHCP and DNS
DNS [RFC1034, RFC1035] maintains (among other things) the information
about mapping between hosts' Fully Qualified Domain Names (FQDNs)
[RFC1594] and IP addresses assigned to the hosts. The information is
maintained in two types of Resource Records (RRs): A and PTR. The A
RR contains mapping from a FQDN to an IP address; the PTR RR contains
mapping from an IP address to a FQDN.
DHCP [RFC1541] provides a mechanism by which a host (a DHCP client)
could acquire certain configuration information, and specifically its
IP address(es). However, DHCP does not provide any mechanisms to
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Internet Draft draft-ietf-dhc-dhcp-dns-07.txt February 1998
update the DNS RRs that contain the information about mapping between
the host's FQDN and its IP address(es) (A and PTR RRs). Thus the
information maintained by DNS for a DHCP client may be incorrect - a
host (the client) could acquire its address by using DHCP, but the A
RR for the host's FQDN wouldn't reflect the address that the host
acquired, and the PTR RR for the acquired address wouldn't reflect
the host's FQDN.
Dynamic DNS Updates [RFC2136] is a mechanism that enables DNS
information to be updated DNS over a network.
The Dynamic DNS Update protocol can be used to maintain consistency
between the information stored in the A and PTR RRs and the actual
address assignment done via DHCP. When a host with a particular FQDN
acquires its IP address via DHCP, the A RR associated with the host's
FQDN would be updated (by using the Dynamic DNS Updates protocol) to
reflect the new address. Likewise, when an IP address gets assigned
to a host with a particular FQDN, the PTR RR associated with this
address would be updated (using the Dynamic DNS Updates protocol) to
reflect the new FQDN.
5. Models of operations
When a DHCP client acquires a new address, both the A RR (for the
client's FQDN) and the PTR RR (for the acquired address) have to be
updated. Therefore, we have two separate Dynamic DNS Update
transactions. Acquiring an address via DHCP involves two entities: a
DHCP client and a DHCP server. In principle each of these entities
could perform none, one, or both of the transactions. However, upon
some introspection one could realize that not all permutations make
sense. This document covers the possible design permutations:
(1) DHCP client updates the A RR, DHCP server updates the PTR
RR
(2) DHCP server updates both the A and the PTR RRs
One could observe that the only difference between these two cases is
whether the FQDN to IP address mapping is updated by a DHCP client or
by a DHCP server. The IP address to FQDN mapping is updated by a DHCP
server in both cases.
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5.1. Client FQDN Option
To update the IP address to FQDN mapping a DHCP server needs to know
FQDN of the client to which the server leases the address. To allow
the client to convey its FQDN to the server this document defines a
new option, called "Client FQDN".
The code for this option is 81. Its minimum length is 4.
Code Len Flags RCODE1 RCODE2 Domain Name
+------+------+------+------+------+------+--
| 81 | n | | | | ...
+------+------+------+------+------+------+--
The Flags field allows a DHCP client to indicate to a DHCP server
whether (a) the client wants to be responsible for updating the FQDN
to IP address mapping (if Flags is set to 0), or (b) the client wants
the server to be responsible for updating the FQDN to IP address
mapping (if Flags is set to 1). The Flags field also allows a DHCP
server to indicate to a DHCP client that the server assumes the
responsibility for updating the FQDN to IP address mapping, even if
the client wants to be responsible for this update (if Flags is set
to 3).
The RCODE1 and RCODE2 fields are used by a DHCP server to indicate to
a DHCP client the Response Code from Dynamic DNS Updates.
The Domain Name part of the option carries FQDN of a client.
5.2. DHCP Client behavior
The following describes behavior of a DHCP client that implements the
Client FQDN option.
If a client that owns/maintains is own FQDN wants to be responsible
for updating the FQDN to IP address mapping for the FQDN and
address(es) used by the client, then the client MUST include the
Client FQDN option in the DHCPREQUEST message originated by the
client. The Flags field in the option MUST be set to 0. Once the
client's DHCP configuration is completed (the client receives a
DHCPACK message, and successfully completed a final check on the
parameters passed in the message), the client MUST originate an
update for the A RR (associated with the client's FQDN). The update
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MUST be originated following the procedures described in [RFC2136].
A client that owns/maintains its own FQDN can choose to delegate the
responsibility for updating the FQDN to IP address mapping for the
FQDN and address(es) used by the client to the server. In order to
inform the server of this choice, the client MUST include the Client
FQDN option in the DHCPREQUEST message originated by the client. The
Flags field in the option MUST be set to 1. In this case, the client
MAY supply an FQDN in the Client FQDN option, or it MAY leave that
field empty as a signal to the server to determine an FQDN for the
client in a local to the server manner.
A client that delegates the responsibility for updating the FQDN to
IP address mapping to a server MAY not receive any indications
(either positive or negative) from the server whether the server was
able to perform the update. In this case the client SHOULD use DNS
query to check whether the mapping is updated.
A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option.
If a client releases its address lease prior to the lease expiration
time, and the client is responsible for updating its A RR(s), the
client SHOULD delete the A RR (following the procedures described in
[RFC2136]) associated with the leased address before sending DHCP
RELEASE message.
5.3. DHCP Server behavior
When a server receives a DHCPREQUEST message from a client, if the
message contains the Client FQDN option, and the server replies to
the message with a DHCPACK message, the server SHOULD originate an
update for the PTR RR (associated with the address leased to the
client). The update MUST be originated following the procedures
described in Section 5.4. The server MAY originate the update before
the server sends the DHCPACK message to the client. In this case the
RCODE from the update [RFC2136] MUST be carried to the client in the
RCODE1 field of the Client FQDN option in the DHCPACK message and the
RCODE2 field MUST be set to 0. Alternatively, the server MAY send the
DHCPACK message to the client without waiting for the update to be
completed. In this case the RCODE1 field of the Client FQDN option
in the DHCPACK message MUST be set to 255, and the RCODE2 field MUST
be set to 0. The choice between the two alternatives is a local to a
DHCP server matter.
In addition, if the Client FQDN option carried in the DHCPREQUEST
message has its Flags field set to 1, then the server MUST originate
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an update for the A RR (associated with the FQDN carried in the
option). The update MUST be originated following the procedures
described in Section 5.4. The server MAY originate the update before
the server sends the DHCPACK message to the client. In this case the
RCODE from the update [RFC2136] MUST be carried to the client in the
RCODE2 field of the Client FQDN option in the DHCPACK message.
Alternatively the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the
RCODE2 field of the Client FQDN option in the DHCKACK message MUST be
set to 255. The choice between the two alternatives is a local to the
server matter.
Even, if the Client FQDN option carried in the DHCPREQUEST message
has its Flags field set to 0 (indicating that the client wants to
update the A RR), the server MAY (under configuration control) update
the A RR. The update MUST be originated following the procedures
described in Section 5.4. The server MAY originate the update before
the server sends the DHCPACK message to the client. In this case the
RCODE from the update [RFC2136] MUST be carried to the client in the
RCODE2 field of the Client FQDN option in the DHCPACK message, and
the Flags field in the Client FQND option MUST be set to 3.
Alternatively, the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the
RCODE2 field of the Client FQDN option in the DHCKACK message MUST be
set to 255, and the Flags field in the Client FQDN option MUST be set
to 3. The choice between the two alternatives is a local to the
server matter.
When a server receives a DHCPREQUEST message from a client, and the
message contains the Client FQDN option, the server MUST ignore the
value carried in the RCODE1 and RCODE2 fields of the option.
When a DHCP server sends the Client FQDN option to a client in the
DHCPACK message, the server MUST copy the Domain Name fields from the
Client FQDN option that the client sent to the server in the
DHCPREQUEST message.
If the DHCPREQUST message received by a DHCP server from a DHCP
client doesn't carry the Client FQDN option (e.g., the client doesn't
implement the Client FQDN option), and the DHCP client acquires its
FQDN from a DHCP server (as part of a normal DHCP transaction), then
the server MAY be configured to update both A and PTR RRs. The
updates MUST be originated following the procedures described in
Section 5.4.
If a server originates updates for both the A and PTR RRs, then the
order in which the updates are generated is not significant.
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If a server detects that a lease on an address that the server leases
to a client expires, the server SHOULD delete the PTR RR associated
with the address. In addition, if the client authorized the server to
update its A RR, the server SHOULD also delete the A RR. The deletion
MUST follow the procedures described in [RFC2136].
If a server terminates a lease on an address prior to the lease
expiration time, the server SHOULD delete the PTR RR associated with
the address. In addition, if the client (that leased the address)
authorized the server to update its A RR, the server SHOULD also
delete the A RR. The deletion MUST follow the procedures described in
[RFC2136].
5.4. Procedures for performing DNS updates
When a DHCP server needs to update the PTR RR for a particular IP
address, the server just adds a new PTR RR for that address.
When a DHCP server needs to update the A RR for a particular FQDN,
the server first has to delete all the A RRs associated with that
FQDN, and then add a new A RR for that FQDN. Note that this rule
precludes the ability to support multi-homed hosts in the scenario
where A RRs are updated by a DHCP server. Therefore, multi-homed
hosts SHOULD perform updates to their A RRs by themselves.
Procedures for deleting and adding RRs are described in [RFC2136].
6. Updating other RRs
The procedures described in this document cover updates only to the A
and PTR RRs. Updating other types of RRs is outside the scope of this
document.
7. Security Considerations
Whether the client wants to be responsible for updating the FQDN to
IP address mapping, or whether the client wants to delegate this
responsibility to a server is a local to the client matter. The
choice between the two alternatives may be based on a particular
security model that is used with the Dynamic DNS Update protocol
(e.g., only a client may have sufficient credentials to perform
updates to the FQDN to IP address mapping for its FQDN).
Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping),
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regarless of the wishes of a DHCP client, is a local to the server
matter. The choice between the two alternatives may be based on a
particular security model.
The client SHOULD use some form of data origin authentication
procedures (e.g., DNSSEC [DNSSEC]) when performing DNS updates.
While the DHCP client SHOULD be the one to update the DNS A record,
in certain specialized cases a DHCP server MAY do so instead. In
this case, the DHCP server MUST be sure of both the name to use for
the client, as well as the identity of the client.
In the general case, both of these conditions are not satisfied --
one needs to be mindful of the possibility of MAC address spoofing in
a DHCP packet. It is not difficult for a DHCP server to know
unambiguously the DNS name to use for a client, but only in certain
relatively unusual circumstances will the DHCP server know for sure
the identity of the client. One example of such a circumstance is
where the DHCP client is connected to a network through an MCNS cable
modem, and the CMTS (head-end) of the cable modem ensures that MAC
address spoofing simply does not occur.
Another example where the DHCP server would know the identity of the
client would be in a case where it was interacting with a remote
access server which encoded a client identification into the DHCP
client-id option. In this case, the remote access server as well as
the DHCP server would be operating within a trusted environment, and
the DHCP server could trust that the user authentication and
authorization procedure of the remote access server was sufficient,
and would therefore trust the client identification encoded within
the DHCP client-id.
In either of these cases, a DHCP server would be able to correctly
enter the DNS A record on behalf of a client, since it would know the
name associated with a client (through some administrative procedure
outside the scope of this protocol), and it would also know the
client's identity in a secure manner.
Yakov Rekhter [Page 8]
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8. References
[RFC1034] P. Mockapetris, "Domain names - concepts and facilities",
RFC1034, 11/01/1987
[RFC1035] P. Mockapetris, "Domain names - implementation and
specification", RFC1035, 11/01/1987
[RFC2131] R. Droms, "Dynamic Host Configuration Protocol", RFC2131,
March 1997
[RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and
Answer Answers to Commonly asked ``New Internet User'' Questions",
RFC1594, 03/11/1994
[DNSSEC]
[RFC2136] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC2136, April 1997
9. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford, Edie
Gunter, Kim Kinnear, Stuart Kwan, Ted Lemon, Michael Lewis, Michael
Patton, Mark Stapp, and Glenn Stump for their review and comments.
10. Author Information
Yakov Rekhter
cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 235-2128
email: yakov@cisco.com
Yakov Rekhter [Page 9]
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