Network Working Group Yakov Rekhter
Internet Draft Cisco Systems
Expiration Date: November 1997 May 1997
Interaction between DHCP and DNS
draft-ietf-dhc-dhcp-dns-04.txt
1. Status of this Memo
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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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3. 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
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 [DynDNS] 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.
4. 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
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(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.
4.1. 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.
4.2. DHCP Client behavior
If a client 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 shall include the Client FQDN option in the DHCPREQUEST
message originated by the client. The Flags field in the option shall
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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 shall
originate an update for the A RR (associated with the client's FQDN).
The update shall be originated following the procedures described in
[DynDNS].
If a client does not want to be responsible for updating the FQDN to
IP address mapping for the FQDN and address(es) used by the client,
then the client shall include the Client FQDN option in the
DHCPREQUEST message originated by the client. The Flags field in the
option shall be set to 1.
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 may use DNS query
to check whether the mapping is updated.
A client should set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option.
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).
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
[DynDNS]) associated with the leased address before sending DHCP
RELEASE message.
4.3. 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 may originate an
update for the PTR RR (associated with the address leased to the
client). The update shall be originated following the procedures
described in Section 4.4. The server may originate the update before
the server sends the DHCPACK message to the client. In this case the
RCODE from the update [DynDNS] shall be carried to the client in the
RCODE1 field of the Client FQDN option in the DHCPACK message and the
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RCODE2 field shall 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 shall be set to 255, and the RCODE2
field shall 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 shall originate
an update for the A RR (associated with the FQDN carried in the
option). The update shall be originated following the procedures
described in Section 4.4. The server may originate the update before
the server sends the DHCPACK message to the client. In this case the
RCODE from the update [DynDNS] shall 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 shall
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 could (under configuration control)
update the A RR. The update shall be originated following the
procedures described in Section 4.4. The server may originate the
update before the server sends the DHCPACK message to the client. In
this case the RCODE from the update [DynDNS] shall 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 shall 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
shall be set to 255, and the Flags field in the Client FQDN option
shall be set to 3. The choice between the two alternatives is a local
to the server matter.
Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping),
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.
When a server receives a DHCPREQUEST message from a client, and the
message contains the Client FQDN option, the server shall 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
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DHCPACK message, the server shall 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, 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. In this scenario the DHCPOFFER message originated by the
server shall carry the Domain Name option, and the client
acknowledges the use of the FQDN carried in this option by including
the option (with the FQDN) in the DHCPREQUEST originated by the
client. The updates shall be originated following the procedures
described in Section 4.4.
If a server originates updates for both the A and PTR RRs, then the
order in which the updates are generated is not significant.
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
should follow the procedures described in [DynDNS].
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 should follow the procedures described
in [DynDNS].
4.4. Procedures for performing DNS updates
When a DHCP server needs to update the PTR RR for a particular IP
address, the server first has to delete all the PTR RRs associated
with the address, and then add 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 [DynDNS].
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5. 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.
6. Security Considerations
Security issues are not discussed in this document.
7. 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
[RFC1541] R. Droms, "Dynamic Host Configuration Protocol", RFC1541,
10/27/1993
[RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and
Answer Answers to Commonly asked ``New Internet User'' Questions",
RFC1594, 03/11/1994
[DynDNS] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates
in the Domain Name System (DNS UPDATE)", RFC2136, April 1997
8. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford, Edie
Gunter, Stuart Kwan, Ted Lemon, Michael Lewis, Michael Patton, Mark
Stapp, and Glenn Stump for their review and comments.
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9. Author Information
Yakov Rekhter
cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 528-0090
email: yakov@cisco.com
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