draft-ietf-dhc-dhcp-dns-06.txt   draft-ietf-dhc-dhcp-dns-07.txt 
Network Working Group Yakov Rekhter Network Working Group Yakov Rekhter
Internet Draft Cisco Systems Internet Draft Cisco Systems
Expiration Date: July 1998 January 1997 Expiration Date: August 1998 February 1998
Interaction between DHCP and DNS Interaction between DHCP and DNS
draft-ietf-dhc-dhcp-dns-06.txt draft-ietf-dhc-dhcp-dns-07.txt
1. Status of this Memo 1. Status of this Memo
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
working documents as Internet-Drafts. working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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could acquire certain configuration information, and specifically its could acquire certain configuration information, and specifically its
IP address(es). However, DHCP does not provide any mechanisms to IP address(es). However, DHCP does not provide any mechanisms to
update the DNS RRs that contain the information about mapping between 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 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 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 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 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 acquired, and the PTR RR for the acquired address wouldn't reflect
the host's FQDN. the host's FQDN.
Dynamic DNS Updates [DynDNS] is a mechanism that enables DNS Dynamic DNS Updates [RFC2136] is a mechanism that enables DNS
information to be updated DNS over a network. information to be updated DNS over a network.
The Dynamic DNS Update protocol can be used to maintain consistency The Dynamic DNS Update protocol can be used to maintain consistency
between the information stored in the A and PTR RRs and the actual 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 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 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 FQDN would be updated (by using the Dynamic DNS Updates protocol) to
reflect the new address. Likewise, when an IP address gets assigned reflect the new address. Likewise, when an IP address gets assigned
to a host with a particular FQDN, the PTR RR associated with this to a host with a particular FQDN, the PTR RR associated with this
address would be updated (using the Dynamic DNS Updates protocol) to address would be updated (using the Dynamic DNS Updates protocol) to
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the client wants to be responsible for this update (if Flags is set the client wants to be responsible for this update (if Flags is set
to 3). to 3).
The RCODE1 and RCODE2 fields are used by a DHCP server to indicate to The RCODE1 and RCODE2 fields are used by a DHCP server to indicate to
a DHCP client the Response Code from Dynamic DNS Updates. a DHCP client the Response Code from Dynamic DNS Updates.
The Domain Name part of the option carries FQDN of a client. The Domain Name part of the option carries FQDN of a client.
5.2. DHCP Client behavior 5.2. DHCP Client behavior
If a client wants to be responsible for updating the FQDN to IP The following describes behavior of a DHCP client that implements the
address mapping for the FQDN and address(es) used by the client, then Client FQDN option.
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
[DynDNS].
If a client does not want to be responsible for updating the FQDN to If a client that owns/maintains is own FQDN wants to be responsible
IP address mapping for the FQDN and address(es) used by the client, for updating the FQDN to IP address mapping for the FQDN and
then the client MUST include the Client FQDN option in the address(es) used by the client, then the client MUST include the
DHCPREQUEST message originated by the client. The Flags field in the Client FQDN option in the DHCPREQUEST message originated by the
option MUST be set to 1. 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 determine an FQDN for the
client in a local to the server manner.
A client that delegates the responsibility for updating the FQDN to A client that delegates the responsibility for updating the FQDN to
IP address mapping to a server MAY not receive any indications IP address mapping to a server MAY not receive any indications
(either positive or negative) from the server whether the server was (either positive or negative) from the server whether the server was
able to perform the update. In this case the client SHOULD use DNS able to perform the update. In this case the client SHOULD use DNS
query to check whether the mapping is updated. query to check whether the mapping is updated.
A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option. option to 0 when sending the option.
If a client releases its address lease prior to the lease expiration 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 time, and the client is responsible for updating its A RR(s), the
client SHOULD delete the A RR (following the procedures described in client SHOULD delete the A RR (following the procedures described in
[DynDNS]) associated with the leased address before sending DHCP [RFC2136]) associated with the leased address before sending DHCP
RELEASE message. RELEASE message.
5.3. DHCP Server behavior 5.3. DHCP Server behavior
When a server receives a DHCPREQUEST message from a client, if the When a server receives a DHCPREQUEST message from a client, if the
message contains the Client FQDN option, and the server replies to message contains the Client FQDN option, and the server replies to
the message with a DHCPACK message, the server SHOULD originate an the message with a DHCPACK message, the server SHOULD originate an
update for the PTR RR (associated with the address leased to the update for the PTR RR (associated with the address leased to the
client). The update MUST be originated following the procedures client). The update MUST be originated following the procedures
described in Section 4.4. The server MAY originate the update before 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 the server sends the DHCPACK message to the client. In this case the
RCODE from the update [DynDNS] MUST be carried to the client in 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 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 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 DHCPACK message to the client without waiting for the update to be
completed. In this case the RCODE1 field of the Client FQDN option 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 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 be set to 0. The choice between the two alternatives is a local to a
DHCP server matter. DHCP server matter.
In addition, if the Client FQDN option carried in the DHCPREQUEST In addition, if the Client FQDN option carried in the DHCPREQUEST
message has its Flags field set to 1, then the server MUST originate 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 an update for the A RR (associated with the FQDN carried in the
option). The update MUST be originated following the procedures option). The update MUST be originated following the procedures
described in Section 4.4. The server MAY originate the update before 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 the server sends the DHCPACK message to the client. In this case the
RCODE from the update [DynDNS] MUST be carried to the client in 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. RCODE2 field of the Client FQDN option in the DHCPACK message.
Alternatively the server MAY send the DHCPACK message to the client Alternatively the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the 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 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 set to 255. The choice between the two alternatives is a local to the
server matter. server matter.
Even, if the Client FQDN option carried in the DHCPREQUEST message Even, if the Client FQDN option carried in the DHCPREQUEST message
has its Flags field set to 0 (indicating that the client wants to has its Flags field set to 0 (indicating that the client wants to
update the A RR), the server MAY (under configuration control) update update the A RR), the server MAY (under configuration control) update
the A RR. The update MUST be originated following the procedures the A RR. The update MUST be originated following the procedures
described in Section 4.4. The server MAY originate the update before 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 the server sends the DHCPACK message to the client. In this case the
RCODE from the update [DynDNS] MUST be carried to the client in 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 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. the Flags field in the Client FQND option MUST be set to 3.
Alternatively, the server MAY send the DHCPACK message to the client Alternatively, the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the 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 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 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 to 3. The choice between the two alternatives is a local to the
server matter. server matter.
When a server receives a DHCPREQUEST message from a client, and the When a server receives a DHCPREQUEST message from a client, and the
message contains the Client FQDN option, the server MUST ignore the message contains the Client FQDN option, the server MUST ignore the
value carried in the RCODE1 and RCODE2 fields of the option. 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 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 DHCPACK message, the server MUST copy the Domain Name fields from the
Client FQDN option that the client sent to the server in the Client FQDN option that the client sent to the server in the
DHCPREQUEST message. DHCPREQUEST message.
If the DHCPREQUST message received by a DHCP server from a DHCP If the DHCPREQUST message received by a DHCP server from a DHCP
client doesn't carry the Client FQDN option, and the DHCP client client doesn't carry the Client FQDN option (e.g., the client doesn't
acquires its FQDN from a DHCP server (as part of a normal DHCP implement the Client FQDN option), and the DHCP client acquires its
transaction), then the server MAY be configured to update both A and FQDN from a DHCP server (as part of a normal DHCP transaction), then
PTR RRs. In this scenario the DHCPOFFER message originated by the the server MAY be configured to update both A and PTR RRs. The
server MUST 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 MUST be originated following the procedures described in updates MUST be originated following the procedures described in
Section 4.4. Section 5.4.
If a server originates updates for both the A and PTR RRs, then the If a server originates updates for both the A and PTR RRs, then the
order in which the updates are generated is not significant. order in which the updates are generated is not significant.
If a server detects that a lease on an address that the server leases 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 to a client expires, the server SHOULD delete the PTR RR associated
with the address. In addition, if the client authorized the server to 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 update its A RR, the server SHOULD also delete the A RR. The deletion
MUST follow the procedures described in [DynDNS]. MUST follow the procedures described in [RFC2136].
If a server terminates a lease on an address prior to the lease If a server terminates a lease on an address prior to the lease
expiration time, the server SHOULD delete the PTR RR associated with expiration time, the server SHOULD delete the PTR RR associated with
the address. In addition, if the client (that leased the address) the address. In addition, if the client (that leased the address)
authorized the server to update its A RR, the server SHOULD also authorized the server to update its A RR, the server SHOULD also
delete the A RR. The deletion MUST follow the procedures described in delete the A RR. The deletion MUST follow the procedures described in
[DynDNS]. [RFC2136].
5.4. Procedures for performing DNS updates 5.4. Procedures for performing DNS updates
When a DHCP server needs to update the PTR RR for a particular IP 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. 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, 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 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 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 precludes the ability to support multi-homed hosts in the scenario
where A RRs are updated by a DHCP server. Therefore, multi-homed where A RRs are updated by a DHCP server. Therefore, multi-homed
hosts SHOULD perform updates to their A RRs by themselves. hosts SHOULD perform updates to their A RRs by themselves.
Procedures for deleting and adding RRs are described in [DynDNS]. Procedures for deleting and adding RRs are described in [RFC2136].
6. Updating other RRs 6. Updating other RRs
The procedures described in this document cover updates only to the A 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 and PTR RRs. Updating other types of RRs is outside the scope of this
document. document.
7. Security Considerations 7. Security Considerations
Whether the client wants to be responsible for updating the FQDN to Whether the client wants to be responsible for updating the FQDN to
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this case, the DHCP server MUST be sure of both the name to use for 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. the client, as well as the identity of the client.
In the general case, both of these conditions are not satisfied -- In the general case, both of these conditions are not satisfied --
one needs to be mindful of the possibility of MAC address spoofing in 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 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 unambiguously the DNS name to use for a client, but only in certain
relatively unusual circumstances will the DHCP server know for sure relatively unusual circumstances will the DHCP server know for sure
the identity of the client. One example of such a circumstance is 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 where the DHCP client is connected to a network through an MCNS cable
modem, and the CMTS (headend) of the cable modem ensures that MAC modem, and the CMTS (head-end) of the cable modem ensures that MAC
address spoofing simply does not occur. address spoofing simply does not occur.
Another example where the DHCP server would know the identity of the 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 client would be in a case where it was interacting with a remote
access server which encoded a client identification into the DHCP access server which encoded a client identification into the DHCP
client-id option. In this case, the remote access server as well as 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 would be operating within a trusted environment, and
the DHCP server could trust that the user authentication and the DHCP server could trust that the user authentication and
authorization procedure of the remote access server was sufficient, authorization procedure of the remote access server was sufficient,
and would therefore trust the client identification encoded within and would therefore trust the client identification encoded within
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client's identity in a secure manner. client's identity in a secure manner.
8. References 8. References
[RFC1034] P. Mockapetris, "Domain names - concepts and facilities", [RFC1034] P. Mockapetris, "Domain names - concepts and facilities",
RFC1034, 11/01/1987 RFC1034, 11/01/1987
[RFC1035] P. Mockapetris, "Domain names - implementation and [RFC1035] P. Mockapetris, "Domain names - implementation and
specification", RFC1035, 11/01/1987 specification", RFC1035, 11/01/1987
[RFC1541] R. Droms, "Dynamic Host Configuration Protocol", RFC1541, [RFC2131] R. Droms, "Dynamic Host Configuration Protocol", RFC2131,
10/27/1993 March 1997
[RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and [RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and
Answer Answers to Commonly asked ``New Internet User'' Questions", Answer Answers to Commonly asked ``New Internet User'' Questions",
RFC1594, 03/11/1994 RFC1594, 03/11/1994
[DNSSEC] [DNSSEC]
[DynDNS] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates [RFC2136] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
in the Domain Name System (DNS UPDATE)", RFC2136, April 1997 Updates in the Domain Name System (DNS UPDATE)", RFC2136, April 1997
9. Acknowledgements 9. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford, Edie Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford, Edie
Gunter, Kim Kinnear, Stuart Kwan, Ted Lemon, Michael Lewis, Michael Gunter, Kim Kinnear, Stuart Kwan, Ted Lemon, Michael Lewis, Michael
Patton, Mark Stapp, and Glenn Stump for their review and comments. Patton, Mark Stapp, and Glenn Stump for their review and comments.
10. Author Information 10. Author Information
Yakov Rekhter Yakov Rekhter
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