draft-ietf-dhc-ddns-resolution-04.txt   draft-ietf-dhc-ddns-resolution-05.txt 
DHC Working Group M. Stapp DHC Working Group M. Stapp
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Expires: December 20, 2002 June 21, 2002 Expires: May 2, 2003 November 1, 2002
Resolution of DNS Name Conflicts Among DHCP Clients Resolution of DNS Name Conflicts Among DHCP Clients
<draft-ietf-dhc-ddns-resolution-04.txt> <draft-ietf-dhc-ddns-resolution-05.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as other groups may also distribute working documents as
Internet-Drafts. Internet-Drafts.
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months and may be updated, replaced, or obsoleted by other documents months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference at any time. It is inappropriate to use Internet-Drafts as reference
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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on December 20, 2002. This Internet-Draft will expire on May 2, 2003.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract Abstract
DHCP provides a powerful mechanism for IP host configuration. DHCP provides a powerful mechanism for IP host configuration.
However, the configuration capability provided by DHCP does not However, the configuration capability provided by DHCP does not
include updating DNS(RFC1034[1], RFC1035[2]), and specifically include updating DNS(RFC1034[1], RFC1035[2]), and specifically
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through which DHCP clients and servers can exchange information through which DHCP clients and servers can exchange information
about client FQDNs. This document describes techniques for the about client FQDNs. This document describes techniques for the
resolution of DNS name conflicts among DHCP clients. resolution of DNS name conflicts among DHCP clients.
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 Mis-Configuration . . . . . . . . . . . . . . . . . . . 4 3.1 Client Mis-Configuration . . . . . . . . . . . . . . . . . . . 4
3.2 Multiple DHCP Servers . . . . . . . . . . . . . . . . . . . . 4 3.2 Multiple DHCP Servers . . . . . . . . . . . . . . . . . . . . 5
4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . . 5 4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . . 5
4.1 Format of the DHCID RRDATA . . . . . . . . . . . . . . . . . . 6 5. DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Procedures for performing DNS updates . . . . . . . . . . . . 6
6. Procedures for performing DNS updates . . . . . . . . . . . . 8 6.1 Adding A RRs to DNS . . . . . . . . . . . . . . . . . . . . . 6
6.1 Adding A RRs to DNS . . . . . . . . . . . . . . . . . . . . . 8 6.2 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . . . 7
6.2 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . . . 9 6.3 Removing Entries from DNS . . . . . . . . . . . . . . . . . . 7
6.3 Removing Entries from DNS . . . . . . . . . . . . . . . . . . 9 6.4 Updating Other RRs . . . . . . . . . . . . . . . . . . . . . . 8
6.4 Updating Other RRs . . . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13
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[4]. document are to be interpreted as described in RFC 2119[4].
2. Introduction 2. Introduction
"The Client FQDN Option"[3] includes a description of the operation "The Client FQDN Option"[3] includes a description of the operation
of DHCP[5] clients and servers that use the client FQDN option. of DHCP[5] clients and servers that use the client FQDN option.
Through the use of the client FQDN option, DHCP clients and servers Through the use of the client FQDN option, DHCP clients and servers
can negotiate the client's FQDN and the allocation of responsibility can negotiate the client's FQDN and the allocation of responsibility
for updating the DHCP client's A RR. This document identifies for updating the DHCP client's A RR. This document identifies
situations in which conflicts in the use of FQDNs may arise among situations in which conflicts in the use of FQDNs may arise among
DHCP clients, and describes a strategy for the use of the DHCID DNS DHCP clients, and describes a strategy for the use of the DHCID DNS
resource record[6] in resolving those conflicts. resource record[6] in resolving those conflicts.
In any case, whether a site permits all, some, or no DHCP servers In any case, whether a site permits all, some, or no DHCP servers
and clients to perform DNS updates into the zones which it controls and clients to perform DNS updates into the zones that it controls
is entirely a matter of local administrative policy. This document is entirely a matter of local administrative policy. This document
does not require any specific administrative policy, and does not does not require any specific administrative policy, and does not
propose one. The range of possible policies is very broad, from propose one. The range of possible policies is very broad, from
sites where only the DHCP servers have been given credentials that sites where only the DHCP servers have been given credentials that
the DNS servers will accept, to sites where each individual DHCP the DNS servers will accept, to sites where each individual DHCP
client has been configured with credentials which allow the client client has been configured with credentials that allow the client to
to modify its own domain name. Compliant implementations MAY support modify its own domain name. Compliant implementations MAY support
some or all of these possibilities. Furthermore, this specification some or all of these possibilities. Furthermore, this specification
applies only to DHCP client and server processes: it does not apply applies only to DHCP client and server processes: it does not apply
to other processes which initiate DNS updates. to other processes that initiate 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 anomolous situations. If DNS updaters can that allows them to detect anomolous situations. If DNS updaters can
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DHCP client wishes to be associated with a single FQDN. This 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 Mis-Configuration 3.1 Client Mis-Configuration
At many (though not all) sites, administrators wish to maintain a At many (though not all) sites, administrators wish to maintain a
one-to-one relationship between active DHCP clients and domain one-to-one relationship between active DHCP clients and domain
names, and to maintain consistency between a host's A and PTR RRs. names, and to maintain consistency between a host's A and PTR RRs.
Hosts which are not represented in the DNS, or hosts which Hosts that are not represented in the DNS, or hosts which
inadvertently share an FQDN with another host may encounter inadvertently share an FQDN with another host may encounter
inconsistent behavior or may not be able to obtain access to network inconsistent behavior or may not be able to obtain access to network
resources. Whether each DHCP client is configured with a domain name resources. Whether each DHCP client is configured with a domain name
by its administrator or whether the DHCP server is configured to by its administrator or whether the DHCP server is configured to
distribute the clients' names, the consistency of the DNS data is distribute the clients' names, the consistency of the DNS data is
entirely dependent on the accuracy of the configuration procedure. entirely dependent on the accuracy of the configuration procedure.
Sites which use Secure DNS[9] may configure credentials for each Sites that deploy Secure DNS[9] may configure credentials for each
host and its assigned name in a way that is more error-resistant, host and its assigned name in a way that is more error-resistant,
but this level of pre-configuration is still rare in DHCP but this level of pre-configuration is still rare in DHCP
environments. environments.
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 name "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 IP address lease.
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' DNS names. If the policy is that
each client that boots should replace any existing A RR that matches each client that boots should replace any existing A RR that matches
its name, Client B can proceed, though Client A may encounter its name, Client B can proceed, though Client A may encounter
problems. If Client B replaces the A RR associated with its name, problems. In this example, Client B replaces the A RR associated
Client A must have some way to recognize that when its lease is with "foo.org.nil". Client A must have some way to recognize that
about to expire, so that it can avoid removing an RR that reflects the RR associated with "foo.org.nil" now contains information for
another client's DHCP lease. Client B, so that it can avoid modifying the RR. When Client A's
lease expires, for example, it should not remove an RR that reflects
Client B's DHCP lease.
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 name should
be the only client associated with that name, Client B needs to be be the only client associated with that name, Client B needs to be
able to determine that it is not the client associated with able to determine that 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 name. 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 lease. It must either
retain persistent state about the last lease it held (in addition to retain persistent state about the last lease it held (in addition to
its current lease) or it must have some other way to detect that it its current lease) or it must have some other way to detect that it
was the last updater of "foo.org.nil" in order to implement the was the last updater of "foo.org.nil" in order to implement the
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DHCP servers to perform A RR updates on behalf of their clients. If DHCP servers to perform A RR updates on behalf of their clients. If
a single DHCP server managed all of the DHCP clients at a site, it a single DHCP server managed all of the DHCP clients at a site, it
could maintain some database of the DNS names that it was managing, could maintain some database of the DNS names that it was managing,
and check that database before initiating a DNS update for a client. and check that database before initiating a DNS update for a client.
Such a database is necessarily proprietary, however, and that Such a database is necessarily proprietary, however, and that
approach does not work once more than one DHCP 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 Consider an example in which DHCP Client A boots, obtains a DHCP
lease from Server S1, presenting the hostname "foo" in a Client FQDN lease from Server S1, presenting the hostname "foo" in a Client FQDN
option[3] in its DHCPREQUEST message. Server S1 updates its domain option[3] in its DHCPREQUEST message. Server S1 updates its domain
name, "foo.org.nil", adding an A RR which matches Client A's lease. name, "foo.org.nil", adding an A RR that matches Client A's lease.
The client then moves to another subnet, served by Server S2. When The client then moves to another subnet, served by Server S2. When
Client A boots on the new subnet, Server S2 will issue it a new Client A boots on the new subnet, Server S2 will issue it a new
lease, and will attempt to add an A RR matching the new lease to lease, and will attempt to add an A RR matching the new lease to
"foo.org.nil". At this point, without some proprietary communication "foo.org.nil". At this point, without some communication mechanism
mechanism which S2 can use to ask S1 (and every other DHCP server which S2 can use to ask S1 (and every other DHCP server that updates
which updates the zone) about the client, S2 has no way to know the zone) about the client, S2 has no way to know whether Client A
whether Client A is currently associated with the domain name, or is currently associated with the domain name, or whether A is a
whether A is a different client configured with the same hostname. different client configured with the same hostname. If the servers
If the servers cannot distinguish between these situations, they cannot distinguish between these situations, they cannot enforce the
cannot enforce the site's naming policies. 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 been associated with a DNS name, in order to offer administrators
the opportunity to configure updater behavior. the opportunity to configure updater behavior.
For this purpose, a DHCID RR, described in [6], is used to associate For this purpose, a DHCID RR, specified in [6], is used to associate
client identification information with a DNS name and the A or PTR client identification information with a DNS name and the A or PTR
RR associated with that name. When either a client or server adds an RR associated with that name. When either a client or server adds an
A or PTR RR for a client, it also adds a DHCID RR which specifies a A or PTR RR for a client, it also adds a DHCID RR that specifies a
unique client identity (based on a "client specifier" created from unique client identity, based on data from the client's DHCPREQUEST
the data in the client's DHCPREQUEST message). In this model, only message. In this model, only one A RR is associated with a given DNS
one A RR is associated with a given DNS name at a time. name at a time.
By associating this ownership information with each DNS name, By associating this ownership information with each DNS name,
cooperating DNS updaters may determine whether their client is cooperating DNS updaters may determine whether their client is
currently associated with a particular DNS name and implement the currently associated with a particular DNS name and implement the
appropriately configured administrative policy. In addition, DHCP appropriately configured administrative policy. In addition, DHCP
clients which currently have domain names may move from one DHCP clients which currently have domain names may move from one DHCP
server to another without losing their DNS names. server to another without losing their DNS names.
The specific algorithms utilizing the DHCID RR to signal client The specific algorithms utilizing the DHCID RR to signal client
ownership are explained below. The algorithms only work in the case ownership are explained below. The algorithms only work 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.
4.1 Format of the DHCID RRDATA
The DHCID RR used to hold the DHCP client's identity is formatted as
follows:
The name of the DHCID RR is the name of the A or PTR RR which refers
to the DHCP client.
The RDATA section of a DHCID RR in transmission contains RDLENGTH
bytes of binary data. From the perspective of DHCP clients and
servers, the DHCID resource record consists of a 16-bit identifier
type, followed by one or more bytes representing the actual
identifier. There are two possible forms for a DHCID RR - one that
is used when the client's link-layer address is being used to
identify it, and one that is used when some DHCP option that the
DHCP client has sent is being used to identify it.
The data following the identifier type code (for type codes other
than 0xFFFF) is derived by digesting a buffer containing identifying
information using the MD5[11] hash algorithm. The identifying
information includes some data from the DHCP client's DHCPREQUEST
message, and the FQDN which is the target of the update. The domain
name is included in the computation in order to ensure that the
DHCID RDATA will vary if a single client is associated over time
with more than one name. This makes it difficult to 'track' a client
as it is associated with various domain names. The domain name is
represented in the buffer in dns wire-format as described in
RFC1035[2], section 3.1. The domain name MUST NOT be compressed as
described in RFC1035[2], section 4.1.4. Any uppercase alphabetic
ASCII character in a label MUST be converted to lowercase before
being in the hash computation.
The MD5 hash algorithm has been shown to be weaker than the SHA-1
algorithm; it could therefore be argued that SHA-1 is a better
choice. However, SHA-1 is significantly slower than MD5. A
successful attack of MD5's weakness does not reveal the original
data that was used to generate the signature, but rather provides a
new set of input data that will produce the same signature. Because
we are using the MD5 hash to conceal the original data, the fact
that an attacker could produce a different plaintext resulting in
the same MD5 output is not a significant concern.
When the updater is using the client's link-layer address, the first
two bytes of the DHCID RRDATA MUST be zero. To generate the rest of
the resource record, the updater MUST compute a one-way hash using
the MD5 algorithm across a buffer containing the client's network
hardware type, link-layer address, and the domain name.
Specifically, the first byte of the buffer contains the network
hardware type as it appears in the DHCP htype field of the client's
DHCPREQUEST message. All of the significant bytes of the chaddr
field in the client's DHCPREQUEST message follow, in the same order
in which the bytes appear in the DHCPREQUEST message. The number of
significant bytes in the chaddr field is specified in the hlen field
of the DHCPREQUEST message. The fully-qualified domain name, as
specified above, follows.
When the updater is using a DHCP option sent by the client in its
DHCPREQUEST message, the first two bytes of the DHCID RR MUST be the
option code of that option, in network byte order. For example, if
the DHCP client identifier option is being used, the first byte of
the DHCID RR should be zero, and the second byte should be 61
decimal. The rest of the DHCID RR MUST contain the results of
computing a one-way hash across the payload of the option being used
and the FQDN (as specified above), using the MD5 algorithm. The
payload of a DHCP option consists of the bytes of the option
following the option code and length.
The two byte identifier code 0xffff is reserved for future
assignment.
In order for independent DHCP implementations to be able to use the
DHCID RR as a prerequisite in dynamic DNS updates, each updater must
be able to reliably choose the same identifier that any other would
choose. To make this possible, we specify a prioritization which
will ensure that for any given DHCP client request, any updater will
select the same client-identity data. All updaters MUST use this
order of prioritization by default, but all implementations SHOULD
be configurable to use a different prioritization if so desired by
the site administrators. Because of the possibility of future
changes in the DHCP protocol, implementors SHOULD check for updated
versions of this specification when implementing new DHCP clients
and servers which can perform DNS updates, and also when releasing
new versions of existing clients and servers.
DHCP clients and servers should use the following forms of client
identification, starting with the most preferable, and finishing
with the least preferable. If the client does not send any of these
forms of identification, the DHCP/DNS interaction is not defined by
this specification. The most preferable form of identification is
the DHCP Client Identifier option. Last is the client's link-layer
address, as conveyed in its DHCPREQUEST message. Implementors
should note that the link-layer address cannot be used if there are
no significant bytes in the chaddr field of the DHCP client's
request, because this does not constitute a unique identifier.
5. DNS RR TTLs 5. DNS RR TTLs
RRs associated with DHCP clients may be more volatile than RRs associated with DHCP clients may be more volatile than
statically configured RRs. DHCP clients and servers which perform statically configured RRs. DHCP clients and servers that perform
dynamic updates should attempt to specify resource record TTLs which dynamic updates should attempt to specify resource record TTLs which
reflect this volatility, in order to minimize the possibility that reflect this volatility, in order to minimize the possibility that
there will be stale records in resolvers' caches. A reasonable basis there will be stale records in resolvers' caches.
for RR TTLs is the lease duration itself: TTLs of 1/2 or 1/3 the
expected lease duration might be reasonable defaults. Because A reasonable basis for RR TTLs is the lease duration itself. The RR
configured DHCP lease times vary widely from site to site, it may TTL on a DNS record added for with a DHCP lease SHOULD NOT exceed
also be desirable to establish a fixed TTL ceiling. DHCP clients and 1/3 of the lease time, and SHOULD be at least 10 minutes. We
servers MAY allow administrators to configure the TTLs they will recognize that individual administrators will have varying
supply, possibly as a fraction of the actual lease time, or as a requirements: DHCP servers and clients SHOULD allow administrators
fixed value. In general, the TTLs of RRs added as a result of DHCP to configure TTLs, either as an absolute time interval or as a
lease activity SHOULD be less than the initial lease time. percentage of the lease time. In general, the TTLs or RRs added as a
result of DHCP lease activity SHOULD be less than the initial lease
time.
6. Procedures for performing DNS updates 6. Procedures for performing DNS updates
6.1 Adding A RRs to DNS 6.1 Adding A RRs to DNS
When a DHCP client or server intends to update an A RR, it first When a DHCP client or server intends to update an A RR, it first
prepares a DNS UPDATE query which includes as a prerequisite the prepares a DNS UPDATE query that includes as a prerequisite the
assertion that the name does not exist. The update section of the assertion that the name does not exist. The update section of the
query attempts to add the new name and its IP address mapping (an A query attempts to add the new name and its IP address mapping (an A
RR), and the DHCID RR with its unique client-identity. RR), and the DHCID RR with its unique client-identity.
If this update operation succeeds, the updater can conclude that it If this update operation succeeds, the updater can conclude that it
has added a new name whose only RRs are the A and DHCID RR records. has added a new name whose only RRs are the A and DHCID RR records.
The A RR update is now complete (and a client updater is finished, The A RR update is now complete (and a client updater is finished,
while a server might proceed to perform a PTR RR update). while a server might proceed to perform a PTR RR update).
If the first update operation fails with YXDOMAIN, the updater can If the first update operation fails with YXDOMAIN, the updater can
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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 domain name to remain unchanged, and to perform
disambiguation on the name of the current client in order to disambiguation on the name 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 name for the current
client. In this case, once another candidate name has been client. In this case, once another candidate name has been
generated, the updater should restart the process of adding an A generated, the updater should restart the process of adding an A
RR as specified in this section. RR as specified in this section.
6.2 Adding PTR RR Entries to DNS 6.2 Adding PTR RR Entries to DNS
The DHCP server submits a DNS query which 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 associated with the lease IP address, and adds a PTR RR whose data
is the client's (possibly disambiguated) host name. The server also is the client's (possibly disambiguated) host name. The server MAY
adds a DHCID RR as specified in Section 4. also add a DHCID RR as specified in Section 4.
6.3 Removing Entries from DNS 6.3 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 that an entity removing a DNS entry is only removing an entry that
it added, or for which an administrator has explicitly assigned it it added, or for which an administrator has explicitly assigned it
responsibility. responsibility.
When a lease expires or a DHCP client issues a DHCPRELEASE request, When a lease expires or a DHCP client issues a DHCPRELEASE request,
the DHCP server SHOULD delete the PTR RR that matches the DHCP the DHCP server SHOULD delete the PTR RR that matches the DHCP
binding, if one was successfully added. The server's update query binding, if one was successfully added. The server's update query
SHOULD assert that the name in the PTR record matches the name of SHOULD assert that the name in the PTR record matches the name of
the client whose lease has expired or been released. the client whose lease has expired or been released.
The entity chosen to handle the A record for this client (either the The entity chosen to handle the A record for this client (either the
client or the server) SHOULD delete the A record that was added when client or the server) SHOULD delete the A record that was added when
the lease was made to the client. the lease was made to the client.
In order to perform this delete, the updater prepares an UPDATE In order to perform this delete, the updater prepares an UPDATE
query which contains two prerequisites. The first prerequisite query that contains two prerequisites. The first prerequisite
asserts that the DHCID RR exists whose data is the client identity asserts that the DHCID RR exists whose data is the client identity
described in Section 4. The second prerequisite asserts that the described in Section 4. The second prerequisite asserts that the
data in the A RR contains the IP address of the lease that has data in the A RR contains the IP address of the lease that has
expired or been released. expired or been released.
If the query fails, the updater MUST NOT delete the DNS name. It If the query fails, the updater MUST NOT delete the DNS name. It
may be that the client whose lease on has expired has moved to may be that the client whose lease on has expired has moved to
another network and obtained a lease from a different server, which another network and obtained a lease from a different server, which
has caused the client's A RR to be replaced. It may also be that has caused the client's A RR to be replaced. It may also be that
some other client has been configured with a name that matches the some other client has been configured with a name that matches the
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The procedures described in this document only cover updates to the The procedures described in this document only cover updates to the
A and PTR RRs. Updating other types of RRs is outside the scope of A and PTR RRs. Updating other types of RRs is outside the scope of
this document. this document.
7. Security Considerations 7. Security Considerations
Unauthenticated updates to the DNS can lead to tremendous confusion, Unauthenticated updates to the DNS can lead to tremendous confusion,
through malicious attack or through inadvertent misconfiguration. 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 which are exposed to the global Internet. Both DHCP clients zones that are exposed to the global Internet. Both DHCP clients and
and servers SHOULD use some form of update request authentication servers SHOULD use some form of update request authentication (e.g.,
(e.g., TSIG[12]) when performing DNS updates. TSIG[12]) 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 alternatives may be based on the security model that is used with
the Dynamic DNS Update protocol (e.g., only a client may have the Dynamic DNS Update protocol (e.g., only a client may have
sufficient credentials to perform updates to the FQDN to IP address sufficient credentials to perform updates to the FQDN to IP address
mapping for its FQDN). mapping for 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
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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, ways for a DHCP server to develop a DNS name to use for a client,
but only in certain relatively rare circumstances will the DHCP but only in certain relatively rare circumstances will the DHCP
server know for certain the identity of the client. If DHCP server know for certain the identity of the client. If DHCP
Authentication[13] becomes widely deployed this may become more Authentication[13] becomes widely deployed this may become more
customary. customary.
One example of a situation which offers some extra assurances is one One example of a situation that offers some extra assurances is one
where the DHCP client is connected to a network through an MCNS where the DHCP client is connected to a network through an MCNS
cable modem, and the CMTS (head-end) of the cable modem ensures that cable modem, and the CMTS (head-end) of the cable modem ensures that
MAC address spoofing simply does not occur. Another example of a MAC address spoofing simply does not occur. Another example of a
configuration that might be trusted is one where clients obtain configuration that might be trusted is one where clients obtain
network access via a network access server using PPP. The NAS itself network access via a network access server using PPP. The NAS itself
might be obtaining IP addresses via DHCP, encoding a client might be obtaining IP addresses via DHCP, encoding a client
identification into the DHCP client-id option. In this case, the identification into the DHCP client-id option. In this case, the
network access server as well as the DHCP server might be operating network access server as well as the DHCP server might be operating
within a trusted environment, in which case the DHCP server could be within a trusted environment, in which case the DHCP server could be
configured to trust that the user authentication and authorization configured to trust that the user authentication and authorization
processing of the remote access server was sufficient, and would processing of the remote access server 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, Edie Gunter, Andreas Gustafsson, R. Barr Hibbs, Kim Kinnear, Ford, Olafur Gudmundsson, Edie Gunter, Andreas Gustafsson, R. Barr
Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Josh Littlefield, Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis,
Michael Patton, and Glenn Stump for their review and comments. Josh Littlefield, Michael Patton, Glenn Stump, and Bernie Volz for
their review and comments.
References References
[1] Mockapetris, P., "Domain names - Concepts and Facilities", RFC [1] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
1034, Nov 1987. 1034, Nov 1987.
[2] Mockapetris, P., "Domain names - Implementation and [2] Mockapetris, P., "Domain names - Implementation and
Specification", RFC 1035, Nov 1987. Specification", RFC 1035, Nov 1987.
[3] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option [3] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option
skipping to change at page 12, line 18 skipping to change at page 10, line 29
[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] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, [11] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
April 1992. April 1992.
[12] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, [12] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC "Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000. 2845, May 2000.
[13] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages [13] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
(draft-ietf-dhc-authentication-*)", January 2001. RFC 3118, June 2001.
Author's Address Author's Address
Mark Stapp Mark Stapp
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Dr. 250 Apollo Dr.
Chelmsford, MA 01824 Chelmsford, MA 01824
USA USA
Phone: 978.244.8498 Phone: 978.244.8498
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