draft-ietf-dhc-ddns-resolution-08.txt   draft-ietf-dhc-ddns-resolution-09.txt 
Dynamic Host Configuration M. Stapp Dynamic Host Configuration M. Stapp
Internet-Draft B. Volz Internet-Draft B. Volz
Expires: March 22, 2005 Cisco Systems, Inc. Expires: December 30, 2005 Cisco Systems, Inc.
September 21, 2004 June 28, 2005
Resolution of DNS Name Conflicts among DHCP Clients Resolution of FQDN Conflicts among DHCP Clients
<draft-ietf-dhc-ddns-resolution-08.txt> <draft-ietf-dhc-ddns-resolution-09.txt>
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). Copyright (C) The Internet Society (2005).
Abstract Abstract
DHCP provides a powerful mechanism for IP host configuration. DHCP provides a mechanism for host configuration that includes
However, the configuration capability provided by DHCP does not dynamic assignment of IP addresses and fully qualified domain names.
include updating DNS, and specifically updating the name to address To maintain accurate name to IP address and IP address to name
and address to name mappings maintained in the DNS. This document mappings in the DNS, these dynamically assigned addresses and fully
describes techniques for the resolution of DNS name conflicts among qualified domain names require updates to the DNS. This document
DHCP clients. identifies situations in which conflicts in the use of fully
qualified domain names may arise among DHCP clients and servers, and
describes a strategy for the use of the DHCID DNS resource record in
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. DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Procedures for performing DNS updates . . . . . . . . . . . . 6 6. Procedures for performing DNS updates . . . . . . . . . . . . 7
6.1 Error Return Codes . . . . . . . . . . . . . . . . . . . . 6 6.1 Error Return Codes . . . . . . . . . . . . . . . . . . . . 7
6.2 Dual IPv4/IPv6 Client Considerations . . . . . . . . . . . 7 6.2 Dual IPv4/IPv6 Client Considerations . . . . . . . . . . . 7
6.3 Adding A or AAAA RRs to DNS . . . . . . . . . . . . . . . 7 6.3 Adding A and/or AAAA RRs to DNS . . . . . . . . . . . . . 7
6.3.1 Initial DHCID RR Query . . . . . . . . . . . . . . . . 7 6.3.1 Initial DHCID RR Query . . . . . . . . . . . . . . . . 8
6.3.2 DNS UPDATE When Name Not in Use . . . . . . . . . . . 8 6.3.2 DNS UPDATE When FQDN Not in Use . . . . . . . . . . . 8
6.3.3 DNS UPDATE When Name in Use . . . . . . . . . . . . . 8 6.3.3 DNS UPDATE When FQDN in Use . . . . . . . . . . . . . 8
6.3.4 Name in Use by another Client . . . . . . . . . . . . 8 6.3.4 FQDN in Use by another Client . . . . . . . . . . . . 9
6.4 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . 9 6.4 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . 10
6.5 Removing Entries from DNS . . . . . . . . . . . . . . . . 9 6.5 Removing Entries from DNS . . . . . . . . . . . . . . . . 10
6.6 Updating Other RRs . . . . . . . . . . . . . . . . . . . . 10 6.6 Updating Other RRs . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 11 9.1 Normative References . . . . . . . . . . . . . . . . . . . 12
9.2 Informative References . . . . . . . . . . . . . . . . . . . 12 9.2 Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
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].
FQDN, or Fully Qualified Domain Name, is the full name of a system,
rather than just its hostname. For example, "venera" is a hostname
and "venera.isi.edu" is an FQDN. See [7].
DOCSIS, or Data-Over-Cable Service Interface Specifications, is
defined by CableLabs (www.cablelabs.com).
Additional terms used in this document are likely defined in [7].
2. Introduction 2. Introduction
"The Client FQDN Option" [4] includes a description of the operation "The Client FQDN Option" [4] includes a description of the operation
of DHCPv4 [7] clients and servers that use the DHCPv4 client FQDN of DHCPv4 [8] clients and servers that use the DHCPv4 client FQDN
option. And, "The DHCPv6 Client FQDN Option" [5] includes a option. And, "The DHCPv6 Client FQDN Option" [5] includes a
description of the operation of DHCPv6 [9] clients and servers that description of the operation of DHCPv6 [10] 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
or AAAA RR. This document identifies situations in which conflicts and/or AAAA RRs. This document identifies situations in which
in the use of FQDNs may arise among DHCP clients, and describes a conflicts in the use of FQDNs may arise among DHCP clients and
strategy for the use of the DHCID DNS resource record [2] in servers, and describes a strategy for the use of the DHCID DNS
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 [11]) into the clients to perform DNS updates (RFC 2136 [3], RFC 3007 [12]) into the
zones that it controls is entirely a matter of local administrative zones that it controls is entirely a matter of local administrative
policy. This document does not require any specific administrative policy. This document does not require any specific administrative
policy, and does not propose one. The range of possible policies is policy, and does not propose one. The range of possible policies is
very broad, from sites where only the DHCP servers have been given very broad, from sites where only the DHCP servers have been given
credentials that the DNS servers will accept, to sites where each credentials that the DNS servers will accept, to sites where each
individual DHCP client has been configured with credentials that individual DHCP client has been configured with credentials that
allow the client to modify its own domain name. Compliant allow the client to modify its own FQDN. Compliant implementations
implementations MAY support some or all of these possibilities. MAY support some or all of these possibilities. Furthermore, this
Furthermore, this specification applies only to DHCP client and specification applies only to DHCP client and server processes; it
server processes: it does not apply to other processes that initiate does not apply to other processes that initiate DNS updates.
DNS updates.
3. Issues with DNS Update in DHCP Environments 3. Issues with DNS Update in DHCP Environments
There are two DNS update situations that require special There are two DNS update situations that require special
consideration in DHCP environments: cases where more than one DHCP consideration in DHCP environments: cases where more than one DHCP
client has been configured with the same FQDN, and cases where more client has been configured with the same FQDN and cases where more
than one DHCP server has been given authority to perform DNS updates than one DHCP server has been given authority to perform DNS updates
in a zone. In these cases, it is possible for DNS records to be in a zone. In these cases, it is possible for DNS records to be
modified in inconsistent ways unless the updaters have a mechanism modified in inconsistent ways unless the updaters have a mechanism
that allows them to detect anomalous situations. If DNS updaters can that allows them to detect anomalous situations. If DNS updaters can
detect these situations, site administrators can configure the detect these situations, site administrators can configure the
updaters' behavior so that the site's policies can be enforced. We updaters' behavior so that the site's policies can be enforced. This
use the term "Name Conflict" to refer to cases where more than one
DHCP client wishes to be associated with a single FQDN. This
specification describes a mechanism designed to allow updaters to specification describes a mechanism designed to allow updaters to
detect these situations, and suggests that DHCP implementations use detect these situations, and suggests that DHCP implementations use
this mechanism by default. this mechanism by default.
3.1 Client Misconfiguration 3.1 Client Misconfiguration
At many (though not all) sites, administrators wish to maintain a Administrators may wish to maintain a one-to-one relationship between
one-to-one relationship between active DHCP clients and domain names, active DHCP clients and FQDNs, and to maintain consistency between a
and to maintain consistency between a host's A and PTR RRs. Hosts client's A, AAAA, and PTR RRs. Clients that are not represented in
that are not represented in the DNS, or hosts which inadvertently the DNS, or clients that inadvertently share an FQDN with another
share an FQDN with another host may encounter inconsistent behavior client may encounter inconsistent behavior or may not be able to
or may not be able to obtain access to network resources. Whether obtain access to network resources. Whether each DHCP client is
each DHCP client is configured with a domain name by its configured with a FQDN by its administrator or whether the DHCP
administrator or whether the DHCP server is configured to distribute server is configured to distribute the clients' FQDN, the consistency
the clients' names, the consistency of the DNS data is entirely of the DNS data is entirely dependent on the accuracy of the
dependent on the accuracy of the configuration procedure. Sites that configuration procedure. Sites that deploy Secure DNS [11] may
deploy Secure DNS [10] may configure credentials for each host and configure credentials for each client and its assigned FQDN in a way
its assigned name in a way that is more error-resistant, but this that is more error-resistant, as both the FQDN and credentials must
level of pre-configuration is still rare in DHCP environments. match.
Consider an example in which two DHCP clients in the "org.nil" Consider an example in which two DHCP clients in the "org.nil"
network are both configured with the name "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.org.nil", and its
DHCP server adds a PTR RR corresponding to its IP address lease. DHCP server adds a PTR RR corresponding to its assigned IP address.
When the second client, client B, boots, it is also configured via When the second client, client B, boots, it is also configured via
DHCP, and it also begins to update "foo.org.nil". DHCP, and it also begins to update "foo.org.nil".
At this point, the "org.nil" administrators may wish to establish At this point, the "org.nil" administrators may wish to establish
some policy about DHCP clients' DNS names. If the policy is that some policy about DHCP clients' FQDNs. If the policy is that each
each client that boots should replace any existing A RR that matches client that boots should replace any existing A RR that matches its
its name, Client B can proceed, though Client A may encounter FQDN, Client B can proceed, though Client A may encounter problems.
problems. In this example, Client B replaces the A RR associated In this example, Client B replaces the A RR associated with
with "foo.org.nil". Client A must have some way to recognize that "foo.org.nil". Client A must have some way to recognize that the RR
the RR associated with "foo.org.nil" now contains information for associated with "foo.org.nil" now contains information for Client B,
Client B, so that it can avoid modifying the RR. When Client A's so that it can avoid modifying the RR. When Client A's assigned IP
lease expires, for example, it should not remove an RR that reflects address expires, for example, it should not remove a RR that reflects
Client B's DHCP lease. Client B's DHCP assigned IP address.
If the policy is that the first DHCP client with a given name should If the policy is that the first DHCP client with a given FQDN should
be the only client associated with that name, Client B needs to be be the only client associated with that FQDN, Client B needs to be
able to determine that 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.org.nil". It could be that Client A booted first, and that
Client B should choose another name. 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 lease. It must either booted on a new subnet, and received a new IP address assignment, in
retain persistent state about the last lease it held (in addition to which case B should update the DNS with its new IP address. It must
its current lease) or it must have some other way to detect that it either retain persistent state about the last IP address it was
was the last updater of "foo.org.nil" in order to implement the assigned (in addition to its current IP address) or it must have some
site's policy. other way to detect that it was the last updater of "foo.org.nil" in
order to implement the site's policy.
3.2 Multiple DHCP Servers 3.2 Multiple DHCP Servers
At many sites, the difficulties with distributing DNS update It is possible to arrange for DHCP servers to perform A and/or AAAA
credentials to all of the DHCP clients lead to the desire for the RR updates on behalf of their clients. If a single DHCP server
DHCP servers to perform A RR updates on behalf of their clients. If manages all of the DHCP clients at a site, it can maintain a database
a single DHCP server managed all of the DHCP clients at a site, it of the FQDNs in use, and can check that database before assigning a
could maintain some database of the DNS names that it was managing, FQDN to a client. Such a database is necessarily proprietary,
and check that database before initiating a DNS update for a client. however, and the approach does not work once more than one DHCP
Such a database is necessarily proprietary, however, and that server is deployed.
approach does not work once more than one DHCP server is deployed.
Consider an example in which DHCP Client A boots, obtains a DHCP When multiple DHCP servers are deployed, the servers require a way to
lease from Server S1, presenting the hostname "foo" in a Client FQDN coordinate the identities of DHCP clients. Consider an example in
option [4] in its DHCPREQUEST message. Server S1 updates its domain which DHCP Client A boots, obtains an IP address from Server S1,
name, "foo.org.nil", adding an A RR that matches Client A's lease. presenting the hostname "foo" in a Client FQDN option [4] in its
The client then moves to another subnet, served by Server S2. When DHCPREQUEST message. Server S1 updates the FQDN "foo.org.nil",
Client A boots on the new subnet, Server S2 will issue it a new adding an A RR containing the IP address assigned to A. The client
lease, and will attempt to add an A RR matching the new lease to then moves to another subnet, served by Server S2. When Client A
"foo.org.nil". At this point, without some communication mechanism boots on the new subnet, Server S2 will assign it a new IP address,
which S2 can use to ask S1 (and every other DHCP server that updates and will attempt to add an A RR containing the newly assigned IP
the zone) about the client, S2 has no way to know whether Client A is address to the FQDN "foo.org.nil". At this point, without some
currently associated with the domain name, or whether A is a communication mechanism which S2 can use to ask S1 (and every other
different client configured with the same hostname. If the servers DHCP server that updates the zone) about the client, S2 has no way to
cannot distinguish between these situations, they cannot enforce the know whether Client A is currently associated with the FQDN, or
site's naming policies. whether A is a different client configured with the same FQDN. If
the servers cannot distinguish between these situations, they cannot
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 DNS name, 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 DNS name and the A or PTR RR client identification information with a FQDN and the A, AAAA, and
associated with that name. When either a client or server adds an A PTR RRs associated with that FQDN. When either a client or server
or PTR RR for a client, it also adds a DHCID RR that specifies a adds A, AAAA, or PTR RRs for a client, it also adds a DHCID RR that
unique client identity, based on data from the client's DHCPREQUEST specifies a unique client identity, based on data from the client's
message. In this model, only one A RR is associated with a given DNS DHCPREQUEST message. In this model, only one client is associated
name at a time. with a given FQDN at a time.
By associating this ownership information with each DNS name, By associating this ownership information with each FQDN, cooperating
cooperating DNS updaters may determine whether their client is DNS updaters may determine whether their client is currently
currently associated with a particular DNS name and implement the associated with a particular FQDN and implement the appropriately
appropriately configured administrative policy. In addition, DHCP configured administrative policy. In addition, DHCP clients which
clients which currently have domain names may move from one DHCP currently have FQDNs may move from one DHCP server to another without
server to another without losing their DNS names. losing their FQDNs.
The specific algorithms utilizing the DHCID RR to signal client The specific algorithm utilizing the DHCID RR to signal client
ownership are explained below. The algorithms only work 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. DNS RR TTLs
RRs associated with DHCP clients may be more volatile than statically RRs associated with DHCP clients may be more volatile than statically
configured RRs. DHCP clients and servers that perform dynamic configured RRs. DHCP clients and servers that perform dynamic
updates should attempt to specify resource record TTLs which reflect updates should attempt to specify resource record TTLs which reflect
this volatility, in order to minimize the possibility that answers to this volatility, in order to minimize the possibility that answers to
DNS queries will return records that refer to DHCP lease bindings DNS queries will return records that refer to DHCP IP address
that have expired. assignments that have expired or been released.
The coupling among primary, secondary, and caching DNS servers is The coupling among primary, secondary, and caching DNS servers is
'loose'; that is a fundamental part of the design of the DNS. This 'loose'; that is a fundamental part of the design of the DNS. This
looseness makes it impossible to prevent all possible situations in looseness makes it impossible to prevent all possible situations in
which a resolver may return a record reflecting a DHCP lease binding which a resolver may return a record reflecting a DHCP assigned IP
that has expired. In deployment, this rarely if ever represents a address that has expired or been released. In deployment, this
significant problem. Most DHCP-managed hosts are rarely looked-up by rarely, if ever, represents a significant problem. Most DHCP-managed
name in the DNS, and the deployment of IXFR (RFC 1995 [14]) and clients are infrequently looked-up by name in the DNS, and the
NOTIFY (RFC 1996 [15]) can reduce the latency between updates and deployment of IXFR (RFC 1995 [15]) and NOTIFY (RFC 1996 [16]) can
their visibility at secondary servers. reduce the latency between updates and their visibility at secondary
servers.
We suggest these basic guidelines for implementers. In general, the We suggest these basic guidelines for implementers. In general, the
TTLs for RRs added as a result of DHCP lease activity SHOULD be less TTLs for RRs added as a result of DHCP IP address assignment activity
than the initial lease time. The RR TTL on a DNS record added for a SHOULD be less than the initial lease time or lifetime. The RR TTL
DHCP lease SHOULD NOT exceed 1/3 of the lease time, and SHOULD be at on a DNS record added SHOULD NOT exceed 1/3 of the lease time or
least 10 minutes. We recognize that individual administrators will lifetime, and SHOULD be at least 10 minutes. We recognize that
have varying requirements: DHCP servers and clients SHOULD allow individual administrators will have varying requirements: DHCP
administrators to configure TTLs, either as an absolute time interval servers and clients SHOULD allow administrators to configure TTLs and
or as a percentage of the lease time. 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. Procedures for performing DNS updates
6.1 Error Return Codes 6.1 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 6.2 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 will be represented by have dual-stack clients present. IPv6 clients require updates of
AAAA RRs; IPv4 clients by A RRs. The administrators of mixed AAAA RRs; IPv4 client require updates of A RRs. The administrators
deployments will likely wish to permit a single name to contain A and of mixed deployments will likely wish to permit a single FQDN to
AAAA RRs from the same client. contain A and AAAA RRs from the same client.
Sites that wish to permit a single name 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 for DHCPv4 client identifiers, see the DHCP Unique Identifier (DUID) for DHCPv4 client identifiers such
Node-Specific Client Identifiers for DHCPv4 [6]. Otherwise, a that this DUID is used in computing the RDATA of the DHCID RR by both
dual-stack client that uses older-style DHCPv4 client identifiers DHCPv4 and DHCPv6 for the client, see Node-Specific Client
(see [7] and [8]) will only be able to have either its A or AAAA Identifiers for DHCPv4 [6]. Otherwise, a dual-stack client that uses
record in DNS under a name because of the DHCID RR conflicts that older-style DHCPv4 client identifiers (see [8] and [9]) will only be
result. able to have either its A or AAAA records in DNS under a single FQDN
because of the DHCID RR conflicts that result.
6.3 Adding A or AAAA RRs to DNS 6.3 Adding A and/or AAAA RRs to DNS
When a DHCP client or server intends to update A and/or AAAA RRs, it
has two choices as to where to start the update sequence. The choice
of whether to start with the lookup query in Section 6.3.1 or to
start directly with the update in Section 6.3.2 is left to the
implementer.
Implementers MAY use other algorithms, provided that the algorithm
assures that the DNS updates are done with the proper prerequisites
to prevent incorrect or incomplete updates should multiple updaters
be updating the same FQDN at once.
As the update sequence below can result in loops, implementers SHOULD
limit the total number of attempts for a single transaction.
6.3.1 Initial DHCID RR Query 6.3.1 Initial DHCID RR Query
When a DHCP client or server intends to update an A or AAAA RR, it When a DHCP client or server intends to update A and/or AAAA RRs, it
performs a DNS query with QNAME of the target name and with QTYPE of performs a DNS query with QNAME of the target FQDN and with QTYPE of
DHCID. DHCID.
If the query returns NXDOMAIN, the updater can conclude that the name If the query returns NXDOMAIN, the updater can conclude that the FQDN
is not in use and proceeds to Section 6.3.2. is not in use and proceeds to Section 6.3.2.
If the query returns NOERROR but without an answer, the updater can If the query returns NOERROR but without an answer, the updater can
conclude that the target name is in use, but that no DHCID RR is conclude that the target FQDN is in use, but that no DHCID RR is
present. This indicates that some records have been configured by an present. This indicates that some records have been configured by an
administrator. Whether the updater proceeds with an update is a administrator. Whether the updater proceeds with the update is a
matter of local administrative policy. matter of local administrative policy. Or, the updater may proceed
to Section 6.3.4.
If the DHCID rrset is returned, the updater uses the hash calculation If the query returns NOERROR with a DHCID rrset, the updater uses the
defined in the DHCID RR specification [4] to determine whether the hash calculation defined in the DHCID RR specification [2] to
client associated with the name matches the current client's determine whether the client associated with the FQDN matches the
identity. If so, the updater proceeds to Section 6.3.3. Otherwise current client's identity. If so, the updater proceeds to
the updater must conclude that the client's desired name is in use by Section 6.3.3. Otherwise the updater must conclude that the client's
another host and proceeds to Section 6.3.4. desired FQDN is in use by another client and proceeds to
Section 6.3.4.
If any other status is returned, the updater MUST NOT attempt an If any other status is returned, the updater SHOULD NOT attempt an
update. update.
6.3.2 DNS UPDATE When Name Not in Use 6.3.2 DNS UPDATE When FQDN Not in Use
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 name 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 name and its IP address section of the query attempts to add the new FQDN and its IP address
mapping (an A or AAAA RR), and the DHCID RR with its unique mapping (A and/or AAAA RRs) and the DHCID RR with its unique client
client-identity. identity.
If the update operation succeeds, the A 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 name is in use and proceeds to Section 6.3.3. intended FQDN is in use and proceeds to Section 6.3.3.
6.3.3 DNS UPDATE When Name in Use 6.3.3 DNS UPDATE When FQDN in Use
The updater next attempts to confirm that the DNS name is not being The updater next attempts to confirm that the FQDN is not being used
used by some other host. The updater prepares a UPDATE query in by some other client by preparing an UPDATE query in which there are
which the prerequisite is that the desired name has attached to it a two prerequisites. The first prerequisite is that the FQDN exists.
DHCID RR whose contents match the client identity. The update The second is that the desired FQDN has attached to it a DHCID RR
section of the UPDATE query contains: whose contents match the client identity. The update section of the
1. A delete of any existing A RRs on the name if this is an A update UPDATE query contains:
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 name. the FQDN.
2. A delete of the existing AAAA RRs on the name 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 name 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 address on the name. AAAA and the updater only desires a single IP address on the
FQDN.
3. An add of the A RR that matches the DHCP binding if this is an A 3. An add of the A RR that matches the DHCP binding if this is an A
update. update.
4. An add of the AAAA RR that matches the DHCP binding if this is an 4. Adds of the AAAA RRs that match the DHCP bindings if this is an
AAAA update. AAAA update.
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 domain name, and that client was the last client associated with the FQDN, and that the
the name now contains the updated A or AAAA RR. 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 NXRRSET, the updater must conclude that the If the update returns NXDOMAIN, the FQDN is no longer in use and the
client's desired name is in use by another host and proceeds to updater proceeds to Section 6.3.2.
Section 6.3.4.
6.3.4 Name in Use by another Client 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
either case, the updater proceeds to Section 6.3.4.
6.3.4 FQDN in Use by another Client
At this juncture, the updater can decide (based on some At this juncture, the updater can decide (based on some
administrative configuration outside of the scope of this document) administrative configuration outside of the scope of this document)
whether to let the existing owner of the name keep that name, and to whether to let the existing owner of the FQDN keep that FQDN, and to
(possibly) perform some name disambiguation operation on behalf of (possibly) perform some FQDN disambiguation operation on behalf of
the current client, or to replace the RRs on the name with RRs that the current client, or to replace the RRs on the FQDN with RRs that
represent the current client. If the configured policy allows represent the current client. If the configured policy allows
replacement of existing records, the updater submits a query that replacement of existing records, the updater submits a query that
deletes all RRs for the name and adds the A or AAAA and DHCID RRs deletes all RRs for the FQDN and adds the A and/or AAAA and DHCID RRs
that represent the address and client-identity of the new client. that represent the IP address and client identity of the new client.
Techniques that may be considered to disambiguate FQDNs include
adding some suffix or prefix to the hostname portion of the FQDN or
randomly generating a hostname.
DISCUSSION: DISCUSSION:
The updating entity may be configured to allow the existing DNS The updating entity may be configured to allow the existing DNS
records on the domain name to remain unchanged, and to perform records on the FQDN to remain unchanged, and to perform
disambiguation on the name of the current client in order to disambiguation on the FQDN of the current client in order to
attempt to generate a similar but unique name for the current attempt to generate a similar but unique FQDN for the current
client. In this case, once another candidate name has been client. In this case, once another candidate FQDN has been
generated, the updater should restart the process of adding an A generated, the updater should restart the process of adding A
RR as specified in this section. and/or AAAA RRs as specified in this section.
6.4 Adding PTR RR Entries to DNS 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 lease IP address, and adds a PTR RR whose data is associated with the client's assigned IP address, and adds a PTR RR
the client's (possibly disambiguated) host name. The server MAY also whose data is the client's (possibly disambiguated) FQDN. The server
add a DHCID RR as specified in Section 4, in which case it would MAY also add a DHCID RR as specified in Section 4, in which case it
include a delete of all of the DHCID RRs associated with the lease IP would include a delete of all of the DHCID RRs associated with the
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
server (or servers) only assigns an address to a single client at a
time.
6.5 Removing Entries from DNS 6.5 Removing Entries from DNS
The most important consideration in removing DNS entries is be sure The most important consideration in removing DNS entries is be sure
that an entity removing a DNS entry is only removing an entry that it that an entity removing a DNS entry is only removing an entry that it
added, or for which an administrator has explicitly assigned it added, or for which an administrator has explicitly assigned it
responsibility. responsibility.
When a lease expires or a DHCP client issues a DHCPRELEASE [7] or When an address' lease time or valid lifetime expires or a DHCP
Release [9] request, the DHCP server SHOULD delete the PTR RR that client issues a DHCPRELEASE [8] or Release [10] request, the DHCP
matches the DHCP binding, if one was successfully added. The server SHOULD delete the PTR RR that matches the DHCP binding, if one
server's update query SHOULD assert that the name in the PTR record was successfully added. The server's update query SHOULD assert that
matches the name of the client whose lease has expired or been the domain name (PTRDNAME field) in the PTR record matches the FQDN
released and should delete all RRs for the name. of the client whose address has expired or been released and should
delete all RRs for the FQDN.
The entity chosen to handle the A or AAAA record 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 record (either the client or the server) SHOULD delete the A or AAAA records
that was added when the lease was made to the client. However, the that was added when the address was assigned to the client. However,
updater should only remove the DHCID RR if there are no A or AAAA RRs the updater should only remove the DHCID RR if there are no A or AAAA
for the client. 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 deletes all RRs for the name. that contains three prerequisites and contains an update section that
The first prerequisite asserts that the DHCID RR exists whose data is deletes all RRs for the FQDN. The first prerequisite asserts that
the client identity described in Section 4. The second prerequisite the DHCID RR exists whose data is the client identity described in
asserts that there are no A RRs. The third prerequisite asserts that Section 4. The second prerequisite asserts that there are no A RRs.
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 DNS name. It If either query fails, the updater MUST NOT delete the FQDN. It may
may be that the client whose lease has expired has moved to another be that the client whose address has expired has moved to another
network and obtained a lease 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. It may also be that
some other client has been configured with a name that matches the some other client has been configured with a FQDN that matches the
name of the DHCP client, and the policy was that the last client to FQDN of the DHCP client, and the policy was that the last client to
specify the name would get the name. In these cases, the DHCID RR 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 will no longer match the updater's notion of the client identity of
the host pointed to by the DNS name. the client pointed to by the FQDN.
6.6 Updating Other RRs 6.6 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 7. Security Considerations
Unauthenticated updates to the DNS can lead to tremendous confusion,
through malicious attack or through inadvertent misconfiguration.
Administrators should be wary of permitting unsecured DNS updates to Administrators should be wary of permitting unsecured DNS updates to
zones that are exposed to the global Internet. Both DHCP clients and zones, where or not they are exposed to the global Internet. Both
servers SHOULD use some form of update request authentication (e.g., DHCP clients and servers SHOULD use some form of update request
TSIG [12]) when performing DNS updates. authentication (e.g., TSIG [13]) 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
IP address mapping (in addition to updating the IP to FQDN mapping), IP address mapping (in addition to updating the IP to FQDN mapping),
regardless of the wishes of an individual DHCP client, is also a regardless of the wishes of an individual DHCP client, is also a
site-local matter. The choice between the two alternatives may be site-local matter. The choice between the two alternatives may be
based on the security model that is being used with dynamic DNS based on the security model that is being used with dynamic DNS
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 DNS name to behalf of a client, the DHCP server should be sure of the FQDN to use
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 DNS name 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
[13] becomes widely deployed this may become more customary. [14] becomes widely deployed this may become more customary.
One example of a situation that offers some extra assurances is one One example of a situation that offers some extra assurances is when
where the DHCP client is connected to a network through an MCNS cable the DHCP client is connected to a network through a DOCSIS cable
modem, and the CMTS (head-end) of the cable modem ensures that MAC modem, and the Cable Modem Termination System (head-end) of the cable
address spoofing simply does not occur. Another example of a modem ensures that MAC address spoofing simply does not occur.
configuration that might be trusted is one where clients obtain Another example of a configuration that might be trusted is when
network access via a network access server using PPP. The NAS itself clients obtain network access via a network access server using PPP.
might be obtaining IP addresses via DHCP, encoding a client The Network Access Server (NAS) itself might be obtaining IP
identification into the DHCP client-id option. In this case, the addresses via DHCP, encoding client identification into the DHCP
network access server as well as the DHCP server might be operating client-id option. In this case, the NAS as well as the DHCP server
within a trusted environment, in which case the DHCP server could be might be operating within a trusted environment, in which case the
configured to trust that the user authentication and authorization DHCP server could be configured to trust that the user authentication
processing of the remote access server 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 8. 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, R. Barr Ford, Olafur Gudmundsson, Edie Gunter, Andreas Gustafsson, R. Barr
Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis,
Josh Littlefield, Michael Patton, and Glenn Stump for their review Josh Littlefield, Michael Patton, and Glenn Stump for their review
and comments. and comments.
9. References 9. References
9.1 Normative References 9.1 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-*)", July 2004. DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", February 2005.
[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, April Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
1997. April 1997.
9.2 Informative References 9.2 Informative References
[4] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option [4] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option
(draft-ietf-dhc-fqdn-option-*.txt)", July 2004. (draft-ietf-dhc-fqdn-option-*.txt)", February 2005.
[5] Volz, B., "The DHCPv6 Client FQDN Option [5] Volz, B., "The DHCPv6 Client FQDN Option
(draft-ietf-dhc-dhcpv6-fqdn-*.txt)", September 2004. (draft-ietf-dhc-dhcpv6-fqdn-*.txt)", February 2005.
[6] Lemon, T. and B. Sommerfeld, "Node-Specific Client Identifiers [6] Lemon, T. and B. Sommerfeld, "Node-Specific Client Identifiers
for DHCPv4 (draft-ietf-dhc-3315id-for-v4-*txt)", July 2004. for DHCPv4 (draft-ietf-dhc-3315id-for-v4-*txt)", June 2005.
[7] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [7] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.
[8] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997. March 1997.
[8] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor [9] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997. Extensions", RFC 2132, March 1997.
[9] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. [10] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003. (DHCPv6)", RFC 3315, July 2003.
[10] Eastlake, D., "Domain Name System Security Extensions", RFC [11] Eastlake, D., "Domain Name System Security Extensions",
2535, March 1999. RFC 2535, March 1999.
[11] Wellington, B., "Secure Domain Name System (DNS) Dynamic [12] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000. Update", RFC 3007, November 2000.
[12] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, [13] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC "Secret Key Transaction Authentication for DNS (TSIG)",
2845, May 2000. RFC 2845, May 2000.
[13] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", [14] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001. RFC 3118, June 2001.
[14] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, August [15] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
1996. August 1996.
[15] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes [16] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
(DNS NOTIFY)", RFC 1996, August 1996. (DNS NOTIFY)", RFC 1996, August 1996.
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
Phone: 978.936.1535 Phone: 978.936.1535
EMail: mjs@cisco.com Email: mjs@cisco.com
Bernie Volz Bernie Volz
Cisco Systems, Inc. Cisco Systems, Inc.
1414 Massachusetts Ave. 1414 Massachusetts Ave.
Boxborough, MA 01719 Boxborough, MA 01719
USA USA
Phone: 978.936.0382 Phone: 978.936.0382
EMail: volz@cisco.com Email: volz@cisco.com
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
skipping to change at page 14, line 41 skipping to change at page 15, 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 (2004). This document is subject Copyright (C) The Internet Society (2005). 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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