Network Working Group Yakov Rekhter
Internet Draft Cisco Systems
Expiration Date: August 1996 February 1996
Interaction between DHCP and DNS
draft-ietf-dhc-dhcp-dns-00.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 to update
DNS information over a network.
Use of the Dynamic DNS Updates protocol enables 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 restricts the possible design permutations to
the following cases:
(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 TBD. Its minimum length is 2.
Code Len Flags Domain Name
+-----+-----+-----+-----+-----+-----+--
| TBD | n | 0/1 | d1 | d2 | d3 | ...
+-----+-----+-----+-----+-----+-----+--
The Flags field allows a DHCP client to indicate to a DHCP server
whether the client wants the server to be responsible for updating
the FQDN to IP address mapping (if Flags is set to 1), or whether the
client wants to take this responsibility (if Flags is set to 0).
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
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].
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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 does not receive any indications
(either positive or negative) from the server whether the server was
able to perform the update. The client may use DNS query to check
whether the mapping is updated.
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.
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 shall originate an
update for the PTR RR (associated with the address leased to the
client). The server shall originate the update only after the server
sends the DHCPACK message to the client. The update shall be
originated following the procedures described in [DynDNS].
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 server shall originate the update only after the server
sends the DHCPACK message to the client. The update shall be
originated following the procedures described in [DynDNS].
If a server originates updates for both the A and PTR RRs, then the
order in which the updates are generated is not significant.
[Discussion: should it be possible to configure a server to perform
updates for the FQDN to IP address mapping, even when a client
indicates to the server that the client wants to update this mapping
?]
[Discussion: how should the duration of the lease be reflected in the
DNS updates ? At the minimum we can set TTL on the A and PTR RRs to
the value of the lease time. What else ?]
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[Discussion: when a server detects that a lease on an address that
the server leases to a client expires, should the server delete the
PTR RR associated with the address ?]
[Discussion: if a server terminates a lease prior to the lease
expiration time, should the server update the associated PTR RR ?
Should the A RR be updated, and if yes, then by whom ? ]
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. Applicability to IPv6
The procedures described above are directly applicable to an IPv6
client. The only difference is that instead of updating its A RR(s)
the client has to update its AAAA RR(s).
7. Security Considerations
Security issues are not discussed in this document.
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
[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)", draft-ietf-dnsind-dynDNS-
06.txt, Feb 1996
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9. Acknowledgements
Many thanks to Ralph Droms for his review and comments.
10. Author Information
Yakov Rekhter
cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 528-0090
email: yakov@cisco.com
Yakov Rekhter [Page 6]
