Network Working Group Yakov Rekhter
INTERNET-DRAFT Mark Stapp
Cisco Systems
February 1999
Expires August 1999
Interaction between DHCP and DNS
<draft-ietf-dhc-dhcp-dns-09.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
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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1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. 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 [RFC2136] is a mechanism that enables DNS infor-
mation to be updated 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.
3. Models of operation
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 transac-
tions. 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
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reflection 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.
The reason these two are important, while others are unlikely, has to
do with authority over the respective DNS RRs. A client may be given
authority over mapping its own A RRs, or that may be restricted to a
server to prevent the client from listing arbitrary addresses. In
all cases, the only reasonable place for the authority over the PTR
RRs associated with the address is in the DHCP server that allocates
them.
3.1. 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 map-
ping (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).
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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.
3.2. 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
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 generate an FQDN for the
client in any manner the server chooses.
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 a 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 a DHCP
RELEASE message.
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3.3. 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 3.4. The server MAY complete 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 entirely up
to the DHCP server.
In addition, if the Client FQDN option carried in the DHCPREQUEST
message has its Flags field set to 1, then the server MUST originate
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 3.4. 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 entirely up to
the DHCP server.
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 (at the determination of the local adminis-
trator) update the A RR. The update MUST be originated following the
procedures described in Section 3.4. 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. Whether the DNS update occurs before or after the
DHCPACK is sent is entirely up to the DHCP server.
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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 DHCPRE-
QUEST 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 Sec-
tion 3.4.
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 A RR (of the client) was ini-
tially updated by the server, 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 server (that leased the address)
initially updated the A RR (of the client), the server SHOULD also
delete the A RR. The deletion MUST follow the procedures described in
[RFC2136].
3.4. Procedures for performing DNS updates
There are two principal issues that need to be addressed concerning A
RR DNS updates:
o Name Collisions
If the entity updating the A RR (either the DHCP client or DHCP
server) attempts to perform a DNS update with a DNS name that is
already in use, what should be done? In some scenarios these
name collisions are unlikely, in some scenarios they are very
likely:
1. Client updates A RR, uses DNSSEC: Name collisions in
this scenario are unlikely (though not impossible), since
for the client to use DNSSEC, it has already received
credentials specific to the name it will add. This implies
that the name has already been allocated (through some
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implementation- or organization-specific procedure, and
presumably uniquely) to that client.
2. Client updates A RR, uses some form of TSIG: Name
collisions in this scenario are possible, since the
credentials necessary for the client to update DNS are not
name specific. Thus, for the client to be attempting to
update a unique name requires the existence of some
administrative procedure to ensure client configuration
with unique names.
3. Server updates the A RR, uses a name for the client
which is known to the server: Name collisions in this
scenario are likely unless prevented by the server' name
configuration procedures. See Section 7 for security issues
with this form of deployment.
4. Server updates the A RR, uses a name supplied by the
client: Name collisions in this scenario are highly
likely, even with administrative procedures designed to
prevent them. (This scenario is a popular one in
real-world deployments in many types of organizations.)
See Section 7 for security issues with this type of
deployment.
Scenarios 3 and 4 are much more attractive given some form of
DHCP Authentication, but the difficulties remain.
Scenarios 2, 3, and 4 rely on administrative procedures to
ensure name uniqueness for DNS updates, and these procedures may
break down. Experience has shown that, in fact, these pro-
cedures will break down at least occasionally. The question is
what to do when these procedures break down or, for example in
scenario #4, may not even exist.
In all cases of name collisions, the desire is to offer two
modes of operation to the administrator of the combined DHCP-DNS
capability: first-update-wins (i.e., the first updating entity
gets the name) or most-recent-update-wins (i.e., the last updat-
ing entity for a name gets the name).
o Multiple DHCP servers
If multiple DHCP servers are able to update the same DNS zones,
or if DHCP servers are performing A RR updates on behalf of DHCP
clients, and more than one DHCP server may be able to serve
addresses to the same DHCP clients, the DHCP servers should be
able to provide reasonable DNS name update behavior for DHCP
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clients.
The solution to both of these problems is for the updating entities
(both DHCP clients or DHCP servers) to be able to cooperate when
updating DNS A RRs.
Specifically, a KEY RR, described in [RFC2065] is used to associate
client ownership information with a DNS name and the A RR associated
with that name. When either a client or server adds an A RR for a
client, it also adds a KEY RR which specifies a unique client iden-
tity (based on a "client specifier" created from the client's
client-id or MAC address: see Appendix A). In this model, only one A
RR is associated with a given DNS name at a time.
By associating this ownership information with each A RR, cooperating
DNS updating entities may determine whether their client is the first
or last updater of the name (and implement the appropriately config-
ured administrative policy), and DHCP clients which currently have a
host name may move from one DHCP server to another without losing
their DNS name.
See Appendix A for the details of the use of the KEY RR for this pur-
pose.
The specific algorithms utilizing the KEY RR to signal client owner-
ship are explained below. The algorithms only work in the case where
the updating entities all cooperate -- this approach is advisory only
and does not substitute for DNS security, nor is it replaced by DNS
security.
3.4.1. Adding A RRs to DNS
When a DHCP client or server intends to update an A RR, it first
prepares a DNS UPDATE query which includes as a prerequisite the
assertion that the name does not exist. The update section of the
query attempts to add the new name and its IP address mapping and the
KEY RR with its unique client-identity.
If this update operation succeeds, the updater can conclude that it
has added a new name whose only RRs are the A and KEY RR records.
The A RR update is now complete (and a client updater is finished,
while a server would then proceed to perform a PTR RR update).
If the first update operation fails with YXDOMAIN, the updater can
conclude that the intended name is in use. The updater then attempts
to confirm that the DNS name is not being used by some other host.
The updater prepares a second UPDATE query in which the prerequisite
is that the desired name has attached to it a KEY RR whose contents
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match the client identity (see Appendix A). The update section of
this query deletes the existing A records on the name, and adds the A
record that matches the DHCP binding and the KEY RR with the client
identity.
If this query succeeds, the updater can conclude that the current
client was the last user of this name, and that the name now contains
the updated A RR. The A RR update is now complete (and a client
updater is finished, while a server would then proceed to perform a
PTR RR update).
If the second query fails with NXRRSET, the updater must conclude
that the client's desired name is in use by another host. At this
juncture, the updater can decide (based on some administrative confi-
guration outside of the scope of this document) whether to let the
existing owner of the name keep that name, and to (possibly) perform
some name disambiguation operation on behalf of the current client,
or to 'take-over' the name on behalf of the current client.
DISCUSSION:
The updating entity may be configured to allow the existing owner
to keep the name, and to perform disambiguation on the name of the
current client in order to attempt to generate a similar but
unique name for the current client. In this case, once such a
similar name has been generated, the updating entity should res-
tart the process of adding an A RR as specified in this section.
3.4.2. 2 Adding PTR RR Entries to DNS
The DHCP server submits a DNS query which deletes all of the PTR RRs
associated with the lease IP address, and adds a PTR RR whose data is
the client's (possibly disambiguated) host name. The server also adds
a KEY RR whose data is the client's client-identity as described in
Appendix A.
3.4.3. Removing Entries from DNS
The first rule in removing DNS entries is be sure that an entity
removing a DNS entry is only removing an entry that it added.
When a lease expires or a DHCP client issues a DHCPRELEASE request,
the DHCP server SHOULD delete the PTR RR that matches the DHCP bind-
ing, if one was successfully added. The server's update query SHOULD
assert that the name in the PTR record matches the name of the client
whose lease has expired or been released.
The entity chosen to handle the A record for this client (either the
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client or the server) SHOULD delete the A record that was added when
the lease was made to the client.
In order to perform this delete, the updater prepares an UPDATE query
which contains two prerequisites. The first prerequisite asserts
that the KEY RR exists whose data is the client identity described in
Appendix A. The second prerequisite asserts that the data in the A RR
contains the IP address of the lease that has expired or been
released.
If the query fails, the updater MUST conclude that it cannot delete
the DNS name. It may be that the host whose lease on the server has
expired has moved to another network and obtained a lease from a dif-
ferent server, which has caused the client's A RR to be replaced. It
may also be that some other client has been configured with a name
that matches the name of the DHCP client, and the policy was that the
last client to specify the name would get the name. In this case,
the KEY RR will no longer match the updater's notion of the client-
identity of the host pointed to by the DNS name.
4. Updating other RRs
The procedures described in this document only cover updates to the A
and PTR RRs. Updating other types of RRs is outside the scope of this
document.
5. 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),
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 pro-
cedures (e.g., DNSSEC [DNSSEC]) when performing DNS updates.
While the DHCP client SHOULD be the one to update the DNS A record,
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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 unambi-
guously the DNS name to use for a client, but only in certain rela-
tively 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 authori-
zation 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.
6. Appendix A - Use of the KEY RR
The KEY RR used to hold the DHCP client's identity is formatted as
follows:
The name of the KEY RR is the name of the A or PTR RR which refers to
the client.
The flags field is set to 0x42 - that is, the 1 bit and the 6 bit are
set.
The protocol field is set to 0.
The algorithm field is set to 254.
The first byte in the key field contains the length of the client-
identity, and is followed by that number of bytes. If a DHCP client
sent the client-id option in its request, the client-identity MUST be
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identical to the data in the client-id option. If a client did not
send the client-id option, the client-identity is constructed from
the htype byte, the hlen byte, and hlen bytes of the client's chaddr
from its request message.
7. References
[RFC1034] P. Mockapetris, "Domain names - concepts and facilities",
RFC1034, 11/01/1987
[RFC1035] P. Mockapetris, "Domain names - implementation and specifi-
cation", 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
[RFC2119] Bradner, S. "Key words for use in RFCs to Indicate Require-
ment Levels", RFC 2119.
[RFC2065] D. Eastlake, C. Kaufman, "Domain Name System Security
Extensions", RFC 2065, January 1997.
8. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford,
Edie Gunter, Kim Kinnear, Stuart Kwan, Ted Lemon, Michael Lewis,
Michael Patton, and Glenn Stump for their review and comments.
9. Author information
Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 235-2128
email: yakov@cisco.com
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Mark Stapp
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8498
email: mjs@cisco.com
10. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
explain it or assist in its implmentation may be prepared, copied,
published and distributed, in whole or in part, without restric-
tion of any kind, provided that the above copyright notice and
this paragraph are included on all such copies and derivative
works. However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the
Internet Society or other Internet organizations, except as needed
for the purpose of developing Internet standards in which case
the procedures for copyrights defined in the Internet Standards
process must be followed, or as required to translate it into
languages other than English.
The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
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ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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