draft-ietf-dhc-ddns-resolution-10.txt   draft-ietf-dhc-ddns-resolution-11.txt 
Dynamic Host Configuration M. Stapp Dynamic Host Configuration M. Stapp
Internet-Draft B. Volz Internet-Draft B. Volz
Expires: March 6, 2006 Cisco Systems, Inc. Expires: August 28, 2006 Cisco Systems, Inc.
September 2, 2005 February 24, 2006
Resolution of FQDN Conflicts among DHCP Clients Resolution of FQDN Conflicts among DHCP Clients
<draft-ietf-dhc-ddns-resolution-10.txt> <draft-ietf-dhc-ddns-resolution-11.txt>
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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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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This Internet-Draft will expire on March 6, 2006. This Internet-Draft will expire on August 28, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
DHCP provides a mechanism for host configuration that includes DHCP provides a mechanism for host configuration that includes
dynamic assignment of IP addresses and fully qualified domain names. dynamic assignment of IP addresses and fully qualified domain names.
To maintain accurate name to IP address and IP address to name To maintain accurate name to IP address and IP address to name
mappings in the DNS, these dynamically assigned addresses and fully mappings in the DNS, these dynamically assigned addresses and fully
qualified domain names require updates to the DNS. This document qualified domain names require updates to the DNS. This document
identifies situations in which conflicts in the use of fully identifies situations in which conflicts in the use of fully
qualified domain names may arise among DHCP clients and servers, and qualified domain names may arise among DHCP clients and servers, and
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resolving those conflicts. resolving those conflicts.
Table of Contents Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Issues with DNS Update in DHCP Environments . . . . . . . . . 3 3. Issues with DNS Update in DHCP Environments . . . . . . . . . 3
3.1. Client Misconfiguration . . . . . . . . . . . . . . . . . 4 3.1. Client Misconfiguration . . . . . . . . . . . . . . . . . 4
3.2. Multiple DHCP Servers . . . . . . . . . . . . . . . . . . 5 3.2. Multiple DHCP Servers . . . . . . . . . . . . . . . . . . 5
4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . . 5 4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . . 5
5. DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Procedures for Performing DNS Updates . . . . . . . . . . . . 6
6. Procedures for Performing DNS Updates . . . . . . . . . . . . 7 5.1. Error Return Codes . . . . . . . . . . . . . . . . . . . . 6
6.1. Error Return Codes . . . . . . . . . . . . . . . . . . . . 7 5.2. Dual IPv4/IPv6 Client Considerations . . . . . . . . . . . 6
6.2. Dual IPv4/IPv6 Client Considerations . . . . . . . . . . . 7 5.3. Adding A and/or AAAA RRs to DNS . . . . . . . . . . . . . 7
6.3. Adding A and/or AAAA RRs to DNS . . . . . . . . . . . . . 7 5.3.1. Initial DHCID RR Query . . . . . . . . . . . . . . . . 7
6.3.1. Initial DHCID RR Query . . . . . . . . . . . . . . . . 8 5.3.2. DNS UPDATE When FQDN in Use . . . . . . . . . . . . . 7
6.3.2. DNS UPDATE When FQDN in Use . . . . . . . . . . . . . 8 5.3.3. FQDN in Use by another Client . . . . . . . . . . . . 8
6.3.3. FQDN in Use by another Client . . . . . . . . . . . . 9 5.4. Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . 8
6.4. Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . 9 5.5. Removing Entries from DNS . . . . . . . . . . . . . . . . 9
6.5. Removing Entries from DNS . . . . . . . . . . . . . . . . 9 5.6. Updating Other RRs . . . . . . . . . . . . . . . . . . . . 9
6.6. Updating Other RRs . . . . . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12 8.2. Informative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
1. Terminology 1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
This document assumes familiarity with DNS terminology defined in RFC This document assumes familiarity with DNS terminology defined in RFC
1035 [4] and DHCP terminology defined in RFC 2131 [5] and RFC 3315 1035 [6] and DHCP terminology defined in RFC 2131 [4] and RFC 3315
[6]. [5].
FQDN, or Fully Qualified Domain Name, is the full name of a system, FQDN, or Fully Qualified Domain Name, is the full name of a system,
rather than just its hostname. For example, "venera" is a hostname rather than just its hostname. For example, "venera" is a hostname
and "venera.isi.edu" is an FQDN. See RFC 1983 [7]. and "venera.isi.edu" is an FQDN. See RFC 1983 [7].
DOCSIS, or Data-Over-Cable Service Interface Specifications, is DOCSIS, or Data-Over-Cable Service Interface Specifications, is
defined by CableLabs (www.cablelabs.com). defined by CableLabs (www.cablelabs.com).
2. Introduction 2. Introduction
"The Client FQDN Option" [8] includes a description of the operation "The Client FQDN Option" [8] includes a description of the operation
of DHCPv4 [5] clients and servers that use the DHCPv4 client FQDN of DHCPv4 [4] clients and servers that use the DHCPv4 client FQDN
option. And, "The DHCPv6 Client FQDN Option" [9] includes a option. And, "The DHCPv6 Client FQDN Option" [9] includes a
description of the operation of DHCPv6 [6] clients and servers that description of the operation of DHCPv6 [5] clients and servers that
use the DHCPv6 client FQDN option. Through the use of the client use the DHCPv6 client FQDN option. Through the use of the client
FQDN option, DHCP clients and servers can negotiate the client's FQDN FQDN option, DHCP clients and servers can negotiate the client's FQDN
and the allocation of responsibility for updating the DHCP client's A and the allocation of responsibility for updating the DHCP client's A
and/or AAAA RRs. This document identifies situations in which and/or AAAA RRs. This document identifies situations in which
conflicts in the use of FQDNs may arise among DHCP clients and conflicts in the use of FQDNs may arise among DHCP clients and
servers, and describes a strategy for the use of the DHCID DNS servers, and describes a strategy for the use of the DHCID DNS
resource record [2] in resolving those conflicts. resource record [2] in resolving those conflicts.
In any case, whether a site permits all, some, or no DHCP servers and In any case, whether a site permits all, some, or no DHCP servers and
clients to perform DNS updates (RFC 2136 [3], RFC 3007 [10]) into the clients to perform DNS updates (RFC 2136 [3], RFC 3007 [10]) into the
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client may encounter inconsistent behavior or may not be able to client may encounter inconsistent behavior or may not be able to
obtain access to network resources. Whether each DHCP client is obtain access to network resources. Whether each DHCP client is
configured with a FQDN by its administrator or whether the DHCP configured with a FQDN by its administrator or whether the DHCP
server is configured to distribute the clients' FQDN, the consistency server is configured to distribute the clients' FQDN, the consistency
of the DNS data is entirely dependent on the accuracy of the of the DNS data is entirely dependent on the accuracy of the
configuration procedure. Sites that deploy Secure DNS [11] may configuration procedure. Sites that deploy Secure DNS [11] may
configure credentials for each client and its assigned FQDN in a way configure credentials for each client and its assigned FQDN in a way
that is more error-resistant, as both the FQDN and credentials must that is more error-resistant, as both the FQDN and credentials must
match. match.
Consider an example in which two DHCP clients in the "org.nil" Consider an example in which two DHCP clients in the "example.com"
network are both configured with the hostname "foo". The clients are network are both configured with the hostname "foo". The clients are
permitted to perform their own DNS updates. The first client, client permitted to perform their own DNS updates. The first client, client
A, is configured via DHCP. It adds an A RR to "foo.org.nil", and its A, is configured via DHCP. It adds an A RR to "foo.example.com", and
DHCP server adds a PTR RR corresponding to its assigned IP address. its DHCP server adds a PTR RR corresponding to its assigned IP
When the second client, client B, boots, it is also configured via address. When the second client, client B, boots, it is also
DHCP, and it also begins to update "foo.org.nil". configured via DHCP, and it also begins to update "foo.example.com".
At this point, the "org.nil" administrators may wish to establish At this point, the "example.com" administrators may wish to establish
some policy about DHCP clients' FQDNs. If the policy is that each some policy about DHCP clients' FQDNs. If the policy is that each
client that boots should replace any existing A RR that matches its client that boots should replace any existing A RR that matches its
FQDN, Client B can proceed, though Client A may encounter problems. FQDN, Client B can proceed, though Client A may encounter problems.
In this example, Client B replaces the A RR associated with In this example, Client B replaces the A RR associated with
"foo.org.nil". Client A must have some way to recognize that the RR "foo.example.com". Client A must have some way to recognize that the
associated with "foo.org.nil" now contains information for Client B, RR associated with "foo.example.com" now contains information for
so that it can avoid modifying the RR. When Client A's assigned IP Client B, so that it can avoid modifying the RR. When Client A's
address expires, for example, it should not remove a RR that reflects assigned IP address expires, for example, it should not remove a RR
Client B's DHCP assigned IP address. that reflects Client B's DHCP assigned IP address.
If the policy is that the first DHCP client with a given FQDN should If the policy is that the first DHCP client with a given FQDN should
be the only client associated with that FQDN, Client B needs to be be the only client associated with that FQDN, Client B needs to be
able to determine if it is not the client associated with able to determine if it is not the client associated with
"foo.org.nil". It could be that Client A booted first, and that "foo.example.com". It could be that Client A booted first, and that
Client B should choose another FQDN. Or it could be that B has Client B should choose another FQDN. Or it could be that B has
booted on a new subnet, and received a new IP address assignment, in booted on a new subnet, and received a new IP address assignment, in
which case B should update the DNS with its new IP address. It must which case B should update the DNS with its new IP address. It must
either retain persistent state about the last IP address it was either retain persistent state about the last IP address it was
assigned (in addition to its current IP address) or it must have some assigned (in addition to its current IP address) or it must have some
other way to detect that it was the last updater of "foo.org.nil" in other way to detect that it was the last updater of "foo.example.com"
order to implement the site's policy. in order to implement the site's policy.
3.2. Multiple DHCP Servers 3.2. Multiple DHCP Servers
It is possible to arrange for DHCP servers to perform A and/or AAAA It is possible to arrange for DHCP servers to perform A and/or AAAA
RR updates on behalf of their clients. If a single DHCP server RR updates on behalf of their clients. If a single DHCP server
manages all of the DHCP clients at a site, it can maintain a database manages all of the DHCP clients at a site, it can maintain a database
of the FQDNs in use, and can check that database before assigning a of the FQDNs in use, and can check that database before assigning a
FQDN to a client. Such a database is necessarily proprietary, FQDN to a client. Such a database is necessarily proprietary,
however, and the approach does not work once more than one DHCP however, and the approach does not work once more than one DHCP
server is deployed. server is deployed.
When multiple DHCP servers are deployed, the servers require a way to When multiple DHCP servers are deployed, the servers require a way to
coordinate the identities of DHCP clients. Consider an example in coordinate the identities of DHCP clients. Consider an example in
which DHCP Client A boots, obtains an IP address from Server S1, which DHCPv4 Client A boots, obtains an IP address from Server S1,
presenting the hostname "foo" in a Client FQDN option [8] in its presenting the hostname "foo" in a Client FQDN option [8] in its
DHCPREQUEST message. Server S1 updates the FQDN "foo.org.nil", DHCPREQUEST message. Server S1 updates the FQDN "foo.example.com",
adding an A RR containing the IP address assigned to A. The client adding an A RR containing the IP address assigned to A. The client
then moves to another subnet, served by Server S2. When Client A then moves to another subnet, served by Server S2. When Client A
boots on the new subnet, Server S2 will assign it a new IP address, boots on the new subnet, Server S2 will assign it a new IP address,
and will attempt to add an A RR containing the newly assigned IP and will attempt to add an A RR containing the newly assigned IP
address to the FQDN "foo.org.nil". At this point, without some address to the FQDN "foo.example.com". At this point, without some
communication mechanism which S2 can use to ask S1 (and every other communication mechanism which S2 can use to ask S1 (and every other
DHCP server that updates the zone) about the client, S2 has no way to DHCP server that updates the zone) about the client, S2 has no way to
know whether Client A is currently associated with the FQDN, or know whether Client A is currently associated with the FQDN, or
whether A is a different client configured with the same FQDN. If whether A is a different client configured with the same FQDN. If
the servers cannot distinguish between these situations, they cannot the servers cannot distinguish between these situations, they cannot
enforce the site's naming policies. enforce the site's naming policies.
4. Use of the DHCID RR 4. Use of the DHCID RR
A solution to both of these problems is for the updater (a DHCP A solution to both of these problems is for the updater (a DHCP
client or DHCP server) to be able to determine which DHCP client has client or DHCP server) to be able to determine which DHCP client has
been associated with a FQDN, in order to offer administrators the been associated with a FQDN, in order to offer administrators the
opportunity to configure updater behavior. opportunity to configure updater behavior.
For this purpose, a DHCID RR, specified in [2], is used to associate For this purpose, a DHCID RR, specified in [2], is used to associate
client identification information with a FQDN and the A, AAAA, and client identification information with a FQDN and the A, AAAA, and
PTR RRs associated with that FQDN. When either a client or server PTR RRs associated with that FQDN. When either a client or server
adds A, AAAA, or PTR RRs for a client, it also adds a DHCID RR that adds A, AAAA, or PTR RRs for a client, it also adds a DHCID RR that
specifies a unique client identity, based on data from the client's specifies a unique client identity, based on data from the client's
DHCPREQUEST message. In this model, only one client is associated DHCP message. In this model, only one client is associated with a
with a given FQDN at a time. given FQDN at a time.
By associating this ownership information with each FQDN, cooperating By associating this ownership information with each FQDN, cooperating
DNS updaters may determine whether their client is currently DNS updaters may determine whether their client is currently
associated with a particular FQDN and implement the appropriately associated with a particular FQDN and implement the appropriately
configured administrative policy. In addition, DHCP clients which configured administrative policy. In addition, DHCP clients which
currently have FQDNs may move from one DHCP server to another without currently have FQDNs may move from one DHCP server to another without
losing their FQDNs. losing their FQDNs.
The specific algorithm utilizing the DHCID RR to signal client The specific algorithm utilizing the DHCID RR to signal client
ownership is explained below. The algorithm only works in the case ownership is explained below. The algorithm only works in the case
where the updating entities all cooperate -- this approach is where the updating entities all cooperate -- this approach is
advisory only and is not a substitute for DNS security, nor is it advisory only and is not a substitute for DNS security, nor is it
replaced by DNS security. replaced by DNS security.
5. DNS RR TTLs 5. Procedures for Performing DNS Updates
RRs associated with DHCP clients may be more volatile than statically
configured RRs. DHCP clients and servers that perform dynamic
updates should attempt to specify resource record TTLs which reflect
this volatility, in order to minimize the possibility that answers to
DNS queries will return records that refer to DHCP IP address
assignments that have expired or been released.
The coupling among primary, secondary, and caching DNS servers is
'loose'; that is a fundamental part of the design of the DNS. This
looseness makes it impossible to prevent all possible situations in
which a resolver may return a record reflecting a DHCP assigned IP
address that has expired or been released. In deployment, this
rarely, if ever, represents a significant problem. Most DHCP-managed
clients are infrequently looked-up by name in the DNS, and the
deployment of IXFR (RFC 1995 [12]) and NOTIFY (RFC 1996 [13]) can
reduce the latency between updates and their visibility at secondary
servers.
We suggest these basic guidelines for implementers. In general, the
TTLs for RRs added as a result of DHCP IP address assignment activity
SHOULD be less than the initial lease time or lifetime. The RR TTL
on a DNS record added SHOULD NOT exceed 1/3 of the lease time or
lifetime, and SHOULD be at least 10 minutes. We recognize that
individual administrators will have varying requirements: DHCP
servers and clients SHOULD allow administrators to configure TTLs and
upper and lower bounds on the TTL values, either as an absolute time
interval or as a percentage of the lease time or lifetime.
While clients and servers MAY update the TTL of the records as the
lease or lifetime is about to expire, there is no requirement that
they do so as this puts additional load on the DNS system with likely
little benefit.
6. Procedures for Performing DNS Updates
6.1. Error Return Codes 5.1. Error Return Codes
Certain RCODEs defined in RFC 2136 [3] indicate that the destination Certain RCODEs defined in RFC 2136 [3] indicate that the destination
DNS server cannot perform an update: FORMERR, SERVFAIL, REFUSED, DNS server cannot perform an update: FORMERR, SERVFAIL, REFUSED,
NOTIMP. If one of these RCODEs is returned, the updater MUST NOTIMP. If one of these RCODEs is returned, the updater MUST
terminate its update attempt. Because these errors may indicate a terminate its update attempt. Because these errors may indicate a
misconfiguration of the updater or of the DNS server, the updater MAY misconfiguration of the updater or of the DNS server, the updater MAY
attempt to signal to its administrator that an error has occurred, attempt to signal to its administrator that an error has occurred,
e.g. through a log message. e.g. through a log message.
6.2. Dual IPv4/IPv6 Client Considerations 5.2. Dual IPv4/IPv6 Client Considerations
At the time of publication of this document, a small minority of DHCP At the time of publication of this document, a small minority of DHCP
clients support both IPv4 and IPv6. We anticipate, however, that a clients support both IPv4 and IPv6. We anticipate, however, that a
transition will take place over a period of time, and more sites will transition will take place over a period of time, and more sites will
have dual-stack clients present. IPv6 clients require updates of have dual-stack clients present. IPv6 clients require updates of
AAAA RRs; IPv4 client require updates of A RRs. The administrators AAAA RRs; IPv4 client require updates of A RRs. The administrators
of mixed deployments will likely wish to permit a single FQDN to of mixed deployments will likely wish to permit a single FQDN to
contain A and AAAA RRs from the same client. contain A and AAAA RRs from the same client.
Sites that wish to permit a single FQDN to contain both A and AAAA Sites that wish to permit a single FQDN to contain both A and AAAA
RRs MUST make use of DHCPv4 clients and servers that support using RRs MUST make use of DHCPv4 clients and servers that support using
the DHCP Unique Identifier (DUID) for DHCPv4 client identifiers such the DHCP Unique Identifier (DUID) for DHCPv4 client identifiers such
that this DUID is used in computing the RDATA of the DHCID RR by both that this DUID is used in computing the RDATA of the DHCID RR by both
DHCPv4 and DHCPv6 for the client, see Node-Specific Client DHCPv4 and DHCPv6 for the client, see [12]. Otherwise, a dual-stack
Identifiers for DHCPv4 [14]. Otherwise, a dual-stack client that client that uses older-style DHCPv4 client identifiers (see [4] and
uses older-style DHCPv4 client identifiers (see [5] and [15]) will [13]) will only be able to have either its A or AAAA records in DNS
only be able to have either its A or AAAA records in DNS under a under a single FQDN because of the DHCID RR conflicts that result.
single FQDN because of the DHCID RR conflicts that result.
6.3. Adding A and/or AAAA RRs to DNS 5.3. Adding A and/or AAAA RRs to DNS
When a DHCP client or server intends to update A and/or AAAA RRs, it When a DHCP client or server intends to update A and/or AAAA RRs, it
starts with the update query in Section 6.3.1. starts with the update query in Section 5.3.1.
As the update sequence below can result in loops, implementers SHOULD As the update sequence below can result in loops, implementers SHOULD
limit the total number of attempts for a single transaction. limit the total number of attempts for a single transaction.
6.3.1. Initial DHCID RR Query 5.3.1. Initial DHCID RR Query
The updater prepares a DNS UPDATE query that includes as a The updater prepares a DNS UPDATE query that includes as a
prerequisite the assertion that the FQDN does not exist. The update prerequisite the assertion that the FQDN does not exist. The update
section of the query attempts to add the new FQDN and its IP address section of the query attempts to add the new FQDN and its IP address
mapping (A and/or AAAA RRs) and the DHCID RR with its unique client mapping (A and/or AAAA RRs) and the DHCID RR with its unique client
identity. identity.
If the update operation succeeds, the A and/or AAAA RR update is now If the update operation succeeds, the A and/or AAAA RR update is now
complete (and a client updater is finished, while a server would then complete (and a client updater is finished, while a server would then
proceed to perform a PTR RR update). proceed to perform a PTR RR update).
If the update returns YXDOMAIN, the updater can now conclude that the If the update returns YXDOMAIN, the updater can now conclude that the
intended FQDN is in use and proceeds to Section 6.3.2. intended FQDN is in use and proceeds to Section 5.3.2.
If any other status is returned, the updater SHOULD NOT attempt an If any other status is returned, the updater SHOULD NOT attempt an
update (see Section 6.1). update (see Section 5.1).
6.3.2. DNS UPDATE When FQDN in Use 5.3.2. DNS UPDATE When FQDN in Use
The updater next attempts to confirm that the FQDN is not being used The updater next attempts to confirm that the FQDN is not being used
by some other client by preparing an UPDATE query in which there are by some other client by preparing an UPDATE query in which there are
two prerequisites. The first prerequisite is that the FQDN exists. two prerequisites. The first prerequisite is that the FQDN exists.
The second is that the desired FQDN has attached to it a DHCID RR The second is that the desired FQDN has attached to it a DHCID RR
whose contents match the client identity. The update section of the whose contents match the client identity. The update section of the
UPDATE query contains: UPDATE query contains:
1. A delete of any existing A RRs on the FQDN if this is an A update 1. A delete of any existing A RRs on the FQDN if this is an A update
or an AAAA update and the updater does not desire A records on or an AAAA update and the updater does not desire A records on
the FQDN. the FQDN or this update is adding an A and the updater only
desires a single IP address on the FQDN.
2. A delete of the existing AAAA RRs on the FQDN if the updater does 2. A delete of the existing AAAA RRs on the FQDN if the updater does
not desire AAAA records on the FQDN or this update is adding an not desire AAAA records on the FQDN or this update is adding an
AAAA and the updater only desires a single IP address on the AAAA and the updater only desires a single IP address on the
FQDN. FQDN.
3. An add of the A RR that matches the DHCP binding if this is an A
update. 3. An add (or adds) of the A RR that matches the DHCP binding if
this is an A update.
4. Adds of the AAAA RRs that match the DHCP bindings if this is an 4. Adds of the AAAA RRs that match the DHCP bindings if this is an
AAAA update. AAAA update.
Whether A or AAAA RRs are deleted depends on the updater or updater's
policy. For example, if the updater is the client or configured as
the only DHCP server for the link on which the client is located, the
updater may find it beneficial to delete all A and/or AAAA RRs and
then add the current set of A and/or AAAA RRs, if any, for the
client.
If the update succeeds, the updater can conclude that the current If the update succeeds, the updater can conclude that the current
client was the last client associated with the FQDN, and that the client was the last client associated with the FQDN, and that the
FQDN now contains the updated A and/or AAAA RRs. The update is now FQDN now contains the updated A and/or AAAA RRs. The update is now
complete (and a client updater is finished, while a server would then complete (and a client updater is finished, while a server would then
proceed to perform a PTR RR update). proceed to perform a PTR RR update).
If the update returns NXDOMAIN, the FQDN is no longer in use and the If the update returns NXDOMAIN, the FQDN is no longer in use and the
updater proceeds back to Section 6.3.1. updater proceeds back to Section 5.3.1.
If the update returns NXRRSET, there are two possibilities - there If the update returns NXRRSET, there are two possibilities - there
are no DHCID RRs for the FQDN or the DHCID RR does not match. In are no DHCID RRs for the FQDN or the DHCID RR does not match. In
either case, the updater proceeds to Section 6.3.3. either case, the updater proceeds to Section 5.3.3.
6.3.3. FQDN in Use by another Client 5.3.3. FQDN in Use by another Client
As the FQDN appears to be in use by another client or is not As the FQDN appears to be in use by another client or is not
associated with any client, the updater can decide (based on some associated with any client, the updater SHOULD either choose another
administrative configuration outside of the scope of this document) FQDN and restart the update process with this new FQDN or terminate
whether to let the existing owner of the FQDN keep that FQDN, and to the update with a failure.
(possibly) perform some FQDN disambiguation operation on behalf of
the current client, or to replace the RRs on the FQDN with RRs that
represent the current client. If the configured policy allows
replacement of existing records either if another DHCID RR is present
or no DHCID RR is present, the updater submits a query that deletes
all RRs for the FQDN (with a prerequisite that a DHCID RR exists or
does not exist) and adds the A and/or AAAA and DHCID RRs that
represent the IP address and client identity of the new client.
Techniques that may be considered to disambiguate FQDNs include Techniques that may be considered to disambiguate FQDNs include
adding some suffix or prefix to the hostname portion of the FQDN or adding some suffix or prefix to the hostname portion of the FQDN or
randomly generating a hostname. randomly generating a hostname.
DISCUSSION: 5.4. Adding PTR RR Entries to DNS
The updating entity may be configured to allow the existing DNS
records on the FQDN to remain unchanged, and to perform
disambiguation on the FQDN of the current client in order to
attempt to generate a similar but unique FQDN for the current
client. In this case, once another candidate FQDN has been
generated, the updater should restart the process of adding A
and/or AAAA RRs as specified in this section.
6.4. Adding PTR RR Entries to DNS
The DHCP server submits a DNS query that deletes all of the PTR RRs The DHCP server submits a DNS query that deletes all of the PTR RRs
associated with the client's assigned IP address, and adds a PTR RR associated with the client's assigned IP address, and adds a PTR RR
whose data is the client's (possibly disambiguated) FQDN. The server whose data is the client's (possibly disambiguated) FQDN. The server
MAY also add a DHCID RR as specified in Section 4, in which case it MAY also add a DHCID RR as specified in Section 4, in which case it
would include a delete of all of the DHCID RRs associated with the would include a delete of all of the DHCID RRs associated with the
client's assigned IP address, and adds a DHCID RR for the client. client's assigned IP address, and adds a DHCID RR for the client.
There is no need to validate the DHCID RR for PTR updates as the DHCP There is no need to validate the DHCID RR for PTR updates as the DHCP
server (or servers) only assigns an address to a single client at a server (or servers) only assigns an address to a single client at a
time. time.
6.5. Removing Entries from DNS 5.5. Removing Entries from DNS
The most important consideration in removing DNS entries is be sure The most important consideration in removing DNS entries is to be
that an entity removing a DNS entry is only removing an entry that it sure that an entity removing a DNS entry is only removing an entry
added, or for which an administrator has explicitly assigned it that it added, or for which an administrator has explicitly assigned
responsibility. it responsibility.
When an address' lease time or valid lifetime expires or a DHCP When an address' lease time or valid lifetime expires or a DHCP
client issues a DHCPRELEASE [5] or Release [6] request, the DHCP client issues a DHCPRELEASE [4] or Release [5] request, the DHCP
server SHOULD delete the PTR RR that matches the DHCP binding, if one server SHOULD delete the PTR RR that matches the DHCP binding, if one
was successfully added. The server's update query SHOULD assert that was successfully added. The server's update query SHOULD assert that
the domain name (PTRDNAME field) in the PTR record matches the FQDN the domain name (PTRDNAME field) in the PTR record matches the FQDN
of the client whose address has expired or been released and should of the client whose address has expired or been released and should
delete all RRs for the FQDN. delete all RRs for the FQDN.
The entity chosen to handle the A or AAAA records for this client The entity chosen to handle the A or AAAA records for this client
(either the client or the server) SHOULD delete the A or AAAA records (either the client or the server) SHOULD delete the A or AAAA records
that was added when the address was assigned to the client. However, that were added when the address was assigned to the client.
the updater should only remove the DHCID RR if there are no A or AAAA However, the updater should only remove the DHCID RR if there are no
RRs remaining for the client. A or AAAA RRs remaining for the client.
In order to perform this A or AAAA RR delete, the updater prepares an In order to perform this A or AAAA RR delete, the updater prepares an
UPDATE query that contains a prerequisite that asserts that the DHCID UPDATE query that contains a prerequisite that asserts that the DHCID
RR exists whose data is the client identity described in Section 4 RR exists whose data is the client identity described in Section 4
and contains an update section that deletes the client's specific A and contains an update section that deletes the client's specific A
or AAAA RR. or AAAA RR.
If the query succeeds, the updater prepares a second UPDATE query If the query succeeds, the updater prepares a second UPDATE query
that contains three prerequisites and contains an update section that that contains three prerequisites and contains an update section that
deletes all RRs for the FQDN. The first prerequisite asserts that deletes all RRs for the FQDN. The first prerequisite asserts that
the DHCID RR exists whose data is the client identity described in the DHCID RR exists whose data is the client identity described in
Section 4. The second prerequisite asserts that there are no A RRs. Section 4. The second prerequisite asserts that there are no A RRs.
The third prerequisite asserts that there are no AAAA RRs. The third prerequisite asserts that there are no AAAA RRs.
If either query fails, the updater MUST NOT delete the FQDN. It may If either query fails, the updater MUST NOT delete the FQDN. It may
be that the client whose address has expired has moved to another be that the client whose address has expired has moved to another
network and obtained an address from a different server, which has network and obtained an address from a different server, which has
caused the client's A or AAAA RR to be replaced. It may also be that caused the client's A or AAAA RR to be replaced. Or, the DNS data
some other client has been configured with a FQDN that matches the may have been removed or altered by an administrator.
FQDN of the DHCP client, and the policy was that the last client to
specify the FQDN would get the FQDN. In these cases, the DHCID RR
will no longer match the updater's notion of the client identity of
the client pointed to by the FQDN.
6.6. Updating Other RRs 5.6. Updating Other RRs
The procedures described in this document only cover updates to the The procedures described in this document only cover updates to the
A, AAAA, PTR, and DHCID RRs. Updating other types of RRs is outside A, AAAA, PTR, and DHCID RRs. Updating other types of RRs is outside
the scope of this document. the scope of this document.
7. Security Considerations 6. Security Considerations
Administrators should be wary of permitting unsecured DNS updates to Administrators should be wary of permitting unsecured DNS updates to
zones, where or not they are exposed to the global Internet. Both zones, whether or not they are exposed to the global Internet. Both
DHCP clients and servers SHOULD use some form of update request DHCP clients and servers SHOULD use some form of update request
authentication (e.g., TSIG [16]) when performing DNS updates. authentication (e.g., TSIG [14]) when performing DNS updates.
Whether a DHCP client may be responsible for updating an FQDN to IP Whether a DHCP client may be responsible for updating an FQDN to IP
address mapping, or whether this is the responsibility of the DHCP address mapping, or whether this is the responsibility of the DHCP
server is a site-local matter. The choice between the two server is a site-local matter. The choice between the two
alternatives may be based on the security model that is used with the alternatives may be based on the security model that is used with the
Dynamic DNS Update protocol (e.g., only a client may have sufficient Dynamic DNS Update protocol (e.g., only a client may have sufficient
credentials to perform updates to the FQDN to IP address mapping for credentials to perform updates to the FQDN to IP address mapping for
its FQDN). its FQDN).
Whether a DHCP server is always responsible for updating the FQDN to Whether a DHCP server is always responsible for updating the FQDN to
skipping to change at page 11, line 35 skipping to change at page 10, line 35
updates. In cases where a DHCP server is performing DNS updates on updates. In cases where a DHCP server is performing DNS updates on
behalf of a client, the DHCP server should be sure of the FQDN to use behalf of a client, the DHCP server should be sure of the FQDN to use
for the client, and of the identity of the client. for the client, and of the identity of the client.
Currently, it is difficult for DHCP servers to develop much Currently, it is difficult for DHCP servers to develop much
confidence in the identities of their clients, given the absence of confidence in the identities of their clients, given the absence of
entity authentication from the DHCP protocol itself. There are many entity authentication from the DHCP protocol itself. There are many
ways for a DHCP server to develop a FQDN to use for a client, but ways for a DHCP server to develop a FQDN to use for a client, but
only in certain relatively rare circumstances will the DHCP server only in certain relatively rare circumstances will the DHCP server
know for certain the identity of the client. If DHCP Authentication know for certain the identity of the client. If DHCP Authentication
[17] becomes widely deployed this may become more customary. [15] becomes widely deployed this may become more customary.
One example of a situation that offers some extra assurances is when One example of a situation that offers some extra assurances is when
the DHCP client is connected to a network through a DOCSIS cable the DHCP client is connected to a network through a DOCSIS cable
modem, and the Cable Modem Termination System (head-end) of the cable modem, and the Cable Modem Termination System (head-end) of the cable
modem ensures that MAC address spoofing simply does not occur. modem ensures that MAC address spoofing simply does not occur.
Another example of a configuration that might be trusted is when Another example of a configuration that might be trusted is when
clients obtain network access via a network access server using PPP. clients obtain network access via a network access server using PPP.
The Network Access Server (NAS) itself might be obtaining IP The Network Access Server (NAS) itself might be obtaining IP
addresses via DHCP, encoding client identification into the DHCP addresses via DHCP, encoding client identification into the DHCP
client-id option. In this case, the NAS as well as the DHCP server client-id option. In this case, the NAS as well as the DHCP server
might be operating within a trusted environment, in which case the might be operating within a trusted environment, in which case the
DHCP server could be configured to trust that the user authentication DHCP server could be configured to trust that the user authentication
and authorization processing of the NAS was sufficient, and would and authorization processing of the NAS was sufficient, and would
therefore trust the client identification encoded within the DHCP therefore trust the client identification encoded within the DHCP
client-id. client-id.
8. Acknowledgements 7. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter
Ford, Olafur Gudmundsson, Edie Gunter, Andreas Gustafsson, David W. Ford, Olafur Gudmundsson, Edie Gunter, Andreas Gustafsson, David W.
Hankins, R. Barr Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Ed Hankins, R. Barr Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Ed
Lewis, Michael Lewis, Josh Littlefield, Michael Patton, and Glenn Lewis, Michael Lewis, Josh Littlefield, Michael Patton, Pekka Savola,
Stump for their review and comments. and Glenn Stump for their review and comments.
9. References 8. References
9.1. Normative References 8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Stapp, M., Gustafsson, A., and T. Lemon, "A DNS RR for Encoding [2] Stapp, M., Gustafsson, A., and T. Lemon, "A DNS RR for Encoding
DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", February 2005. DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", February 2006.
[3] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic [3] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136, Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
April 1997. April 1997.
9.2. Informative References [4] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[4] Mockapetris, P., "Domain names - implementation and [5] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
specification", STD 13, RFC 1035, November 1987. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 8.2. Informative References
March 1997.
[6] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. [6] Mockapetris, P., "Domain names - implementation and
Carney, "Dynamic Host Configuration Protocol for IPv6 specification", STD 13, RFC 1035, November 1987.
(DHCPv6)", RFC 3315, July 2003.
[7] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996. [7] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.
[8] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option [8] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option
(draft-ietf-dhc-fqdn-option-*.txt)", February 2005. (draft-ietf-dhc-fqdn-option-*.txt)", February 2006.
[9] Volz, B., "The DHCPv6 Client FQDN Option [9] Volz, B., "The DHCPv6 Client FQDN Option
(draft-ietf-dhc-dhcpv6-fqdn-*.txt)", February 2005. (draft-ietf-dhc-dhcpv6-fqdn-*.txt)", February 2006.
[10] Wellington, B., "Secure Domain Name System (DNS) Dynamic [10] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000. Update", RFC 3007, November 2000.
[11] Eastlake, D., "Domain Name System Security Extensions", [11] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999. RFC 2535, March 1999.
[12] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, [12] Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers
August 1996. for Dynamic Host Configuration Protocol Version Four (DHCPv4)",
RFC 4361, February 2006.
[13] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
(DNS NOTIFY)", RFC 1996, August 1996.
[14] Lemon, T. and B. Sommerfeld, "Node-Specific Client Identifiers
for DHCPv4 (draft-ietf-dhc-3315id-for-v4-*txt)", June 2005.
[15] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor [13] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997. Extensions", RFC 2132, March 1997.
[16] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington, [14] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", "Secret Key Transaction Authentication for DNS (TSIG)",
RFC 2845, May 2000. RFC 2845, May 2000.
[17] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", [15] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001. RFC 3118, June 2001.
Authors' Addresses Authors' Addresses
Mark Stapp Mark Stapp
Cisco Systems, Inc. Cisco Systems, Inc.
1414 Massachusetts Ave. 1414 Massachusetts Ave.
Boxborough, MA 01719 Boxborough, MA 01719
USA USA
skipping to change at page 15, line 41 skipping to change at page 14, line 41
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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