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DHC Working Group M. Stapp
Internet-Draft Y. Rekhter
Expires: September 2000 Cisco Systems, Inc.
March 10, 2000
Interaction between DHCP and DNS
<draft-ietf-dhc-dhcp-dns-12.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
To view the entire list of Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 2000.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
DHCP provides a powerful mechanism for IP host configuration.
However, the configuration capability provided by DHCP does not
include updating DNS, and specifically updating the name to address
and address to name mappings maintained in the DNS.
This document specifies how DHCP clients and servers should use the
Dynamic DNS Updates mechanism in RFC2136[5] to update the DNS name
to address and address to name mappings so that the mappings for
DHCP clients will be consistent with the IP addresses that the
clients acquire via DHCP.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Models of Operation . . . . . . . . . . . . . . . . . . . . 3
4. Issues with DDNS in DHCP Environments . . . . . . . . . . . 4
4.1 Name Collisions . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Multiple DHCP servers . . . . . . . . . . . . . . . . . . . 6
4.3 Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . 6
4.3.1 Format of the DHCID RRDATA . . . . . . . . . . . . . . . . . 6
4.4 DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Client FQDN Option . . . . . . . . . . . . . . . . . . . . . 8
5.1 The Flags Field . . . . . . . . . . . . . . . . . . . . . . 9
5.2 The RCODE Fields . . . . . . . . . . . . . . . . . . . . . . 10
5.3 The Domain Name Field . . . . . . . . . . . . . . . . . . . 10
6. DHCP Client behavior . . . . . . . . . . . . . . . . . . . . 10
7. DHCP Server behavior . . . . . . . . . . . . . . . . . . . . 12
8. Procedures for performing DNS updates . . . . . . . . . . . 14
8.1 Adding A RRs to DNS . . . . . . . . . . . . . . . . . . . . 14
8.2 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . . 15
8.3 Removing Entries from DNS . . . . . . . . . . . . . . . . . 15
8.4 Updating other RRs . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . 16
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
References . . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 18
Full Copyright Statement . . . . . . . . . . . . . . . . . . 19
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1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119[6].
2. Introduction
DNS (RFC1034[1], RFC1035[2]) maintains (among other things) the
information about mapping between hosts' Fully Qualified Domain
Names (FQDNs) RFC1594[4] and IP addresses assigned to the hosts. The
information is maintained in two types of Resource Records (RRs): A
and PTR. The A RR contains mapping from a FQDN to an IP address; the
PTR RR contains mapping from an IP address to a FQDN. The Dynamic
DNS Updates specification (RFC2136[5]) describes a mechanism that
enables DNS information to be updated over a network.
DHCP RFC2131[3] provides a mechanism by which a host (a DHCP client)
can acquire certain configuration information, along with its IP
address(es). However, DHCP does not provide any mechanisms to update
the DNS RRs that contain the information about mapping between the
host's FQDN and its IP address(es) (A and PTR RRs). Thus the
information maintained by DNS for a DHCP client may be incorrect - a
host (the client) could acquire its address by using DHCP, but the A
RR for the host's FQDN wouldn't reflect the address that the host
acquired, and the PTR RR for the acquired address wouldn't reflect
the host's FQDN.
The Dynamic DNS Update protocol can be used to maintain consistency
between the information stored in the A and PTR RRs and the actual
address assignment done via DHCP. When a host with a particular FQDN
acquires its IP address via DHCP, the A RR associated with the
host's FQDN would be updated (by using the Dynamic DNS Updates
protocol) to reflect the new address. Likewise, when an IP address
is assigned to a host with a particular FQDN, the PTR RR associated
with this address would be updated (using the Dynamic DNS Updates
protocol) to reflect the new FQDN.
Although this document refers to the A and PTR DNS record types and
to DHCP assignment of IPv4 addresses, the same procedures and
requirements apply for updates to the analogous RR types that are
used when clients are assigned IPv6 addresses via DHCPv6.
3. Models of Operation
When a DHCP client acquires a new address, a site's administrator
may desire that one or both of the A RR for the client's FQDN and
the PTR RR for the acquired address be updated. Therefore, two
separate Dynamic DNS Update transactions occur. Acquiring an address
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via DHCP involves two entities: a DHCP client and a DHCP server. In
principle each of these entities could perform none, one, or both of
the transactions. However, in practice not all permutations make
sense. This document covers these possible design permutations:
1. DHCP client updates the A RR, DHCP server updates the PTR RR
2. DHCP server updates both the A and the PTR RRs
The only difference between these two cases is whether the FQDN to
IP address mapping is updated by a DHCP client or by a DHCP server.
The IP address to FQDN mapping is updated by a DHCP server in both
cases.
The reason these two are important, while others are unlikely, has
to do with authority over the respective DNS domain names. A DHCP
client may be given authority over mapping its own A RRs, or that
authority may be restricted to a server to prevent the client from
listing arbitrary addresses or associating its address with
arbitrary domain names. In all cases, the only reasonable place for
the authority over the PTR RRs associated with the address is in the
DHCP server that allocates the address.
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
is entirely a matter of local administrative policy. This document
does not require any specific administrative policy, and does not
propose one. The range of possible policies is very broad, from
sites where only the DHCP servers have been given credentials that
the DNS servers will accept, to sites where each individual DHCP
client has been configured with credentials which allow the client
to modify its own domain name. Compliant implementations MAY support
some or all of these possibilities. Furthermore, this specification
applies only to DHCP client and server processes: it does not apply
to other processes which initiate dynamic DNS updates.
This document describes a new DHCP option which a client can use to
convey all or part of its domain name to a DHCP server.
Site-specific policy determines whether DHCP servers use the names
that clients offer or not, and what DHCP servers may do in cases
where clients do not supply domain names.
4. Issues with DDNS in DHCP Environments
There are two DNS update situations that require special
consideration in DHCP environments: cases where more than one DHCP
client has been configured with the same FQDN, and cases where more
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
modified in inconsistent ways unless the updaters have a mechanism
that allows them to detect anomolous situations. If DNS updaters can
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detect these situations, site administrators can configure the
updaters' behavior so that the site's policies can be enforced. We
use the term "Name Collisions" to refer to cases where more than one
DHCP client has been associated with a single FQDN. This
specification describes a mechanism designed to allow updaters to
detect these situations, and requires that DHCP implementations use
this mechanism by default.
4.1 Name Collisions
How can the entity updating an A RR (either the DHCP client or DHCP
server) detect that a domain name has an A RR which is already in
use by a different DHCP client? Similarly, should a DHCP client or
server update a domain name which has an A RR that has been
configured by an administrator? In either of these cases, the
domain name in question would either have an additional A RR, or
would have its original A RR replaced by the new record. Either of
these effects may be considered undesirable by some sites. Different
authority and credential models have different levels of exposure to
name collisions.
1. Client updates A RR, uses Secure DNS Update with credentials
that are associated with the client's FQDN, and exclusive to the
client. Name collisions in this scenario are unlikely (though
not impossible), since the client has received credentials
specific to the name it desires to use. This implies that the
name has already been allocated (through some implementation- or
organization-specific procedure) to that client.
2. Client updates A RR, uses Secure DNS Update with credentials
that are valid for any name in the zone. Name collisions in this
scenario are possible, since the credentials necessary for the
client to update DNS are not necessarily name-specific. Thus,
for the client to be attempting to update a unique name requires
the existence of some administrative procedure to ensure client
configuration with unique names.
3. Server updates the A RR, uses a name for the client which is
known to the server. Name collisions in this scenario are likely
unless prevented by the server's name configuration procedures.
See Section 9 for security issues with this form of deployment.
4. Server updates the A RR, uses a name supplied by the client.
Name collisions in this scenario are highly likely, even with
administrative procedures designed to prevent them. (This
scenario is a popular one in real-world deployments in many
types of organizations.) See Section 9 for security issues with
this type of deployment.
Scenarios 2, 3, and 4 rely on administrative procedures to ensure
name uniqueness for DNS updates, and these procedures may break
down. Experience has shown that, in fact, these procedures will
break down at least occasionally. The question is what to do when
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these procedures break down or, for example in scenario #4, may not
even exist.
In all cases of name collisions, the desire is to offer two modes of
operation to the administrator of the combined DHCP-DNS capability:
first-update-wins (i.e., the first updating entity gets the name) or
most-recent-update-wins (i.e., the last updating entity for a name
gets the name).
4.2 Multiple DHCP servers
If multiple DHCP servers are able to update the same DNS zones, or
if DHCP servers are performing A RR updates on behalf of DHCP
clients, and more than one DHCP server may be able to serve
addresses to the same DHCP clients, the DHCP servers should be able
to provide reasonable and consistent DNS name update behavior for
DHCP clients.
4.3 Use of the DHCID RR
A solution to both of these problems is for the updating entities
(both DHCP clients and DHCP servers) to be able to detect that
another entity has been associated with a DNS name, and to offer
administrators the opportunity to configure update behavior.
Specifically, a DHCID RR, described in DHCID RR[12] is used to
associate client identification information with a DNS name and the
A RR associated with that name. When either a client or server adds
an A RR for a client, it also adds a DHCID RR which specifies a
unique client identity (based on a "client specifier" created from
the client's client-id or MAC address). In this model, only one A
RR is associated with a given DNS name at a time.
By associating this ownership information with each A RR,
cooperating DNS updating entities may determine whether their client
is the first or last updater of the name (and implement the
appropriately configured administrative policy), and DHCP clients
which currently have a host name may move from one DHCP server to
another without losing their DNS name.
The specific algorithms utilizing the DHCID RR to signal client
ownership are explained below. The algorithms only work in the case
where the updating entities all cooperate -- this approach is
advisory only and is not substitute for DNS security, nor is it
replaced by DNS security.
4.3.1 Format of the DHCID RRDATA
The DHCID RR used to hold the DHCP client's identity is formatted as
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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 DHC 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.
DISCUSSION:
Implementors should note that the actual identifying data is
never placed into the DNS directly. Instead, the client-identity
data is used as the input into a one-way hash algorithm, and the
output of that hash is then used as DNS RRDATA. This has been
specified in order to avoid placing data about DHCP clients that
some sites might consider sensitive into the DNS.
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[13] algorithm across a buffer containing the client's
network hardware type and link-layer address. 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.
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, using the MD5 algorithm. The payload of a DHCP option consists
of the bytes of the option following the option code and length.
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
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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 draft when implementing new DHCP clients and
servers which can perform DDNS 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/DDNS interaction is not defined by
this specification. The most preferable form of identification is
the Globally Unique Identifier Option [TBD]. Next 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.
4.4 DNS RR TTLs
RRs associated with DHCP clients may be more volatile than
statically configured RRs. DHCP clients and servers which perform
dynamic updates should attempt to specify resource record TTLs which
reflect this volatility, in order to minimize the possibility that
there will be stale records in resolvers' caches. A reasonable basis
for RR TTLs is the lease duration itself: TTLs of 1/2 or 1/3 the
expected lease duration might be reasonable defaults. Because
configured DHCP lease times vary widely from site to site, it may
also be desirable to establish a fixed TTL ceiling. DHCP clients and
servers MAY allow administrators to configure the TTLs they will
supply, possibly as a fraction of the actual lease time, or as a
fixed value.
5. Client FQDN Option
To update the IP address to FQDN mapping a DHCP server needs to know
the FQDN of the client to which the server leases the address. To
allow the client to convey its FQDN to the server this document
defines a new DHCP option, called "Client FQDN". The FQDN Option
also contains Flags and RCode fields which DHCP servers can use to
convey information about DNS updates to clients.
Clients MAY send the FQDN option, setting appropriate Flags values,
in both their DISCOVER and REQUEST messages. If a client sends the
FQDN option in its DISCOVER message, it MUST send the option in
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subsequent REQUEST messages.
The code for this option is 81. Its minimum length is 4.
Code Len Flags RCODE1 RCODE2 Domain Name
+------+------+------+------+------+------+--
| 81 | n | | | | ...
+------+------+------+------+------+------+--
5.1 The Flags Field
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |E|O|S|
+-+-+-+-+-+-+-+-+
When a DHCP client sends the FQDN option in its DHCPDISCOVER and/or
DHCPREQUEST messages, it sets the right-most bit (labelled "S") to
indicate that it will not perform any Dynamic DNS updates, and that
it expects the DHCP server to perform any FQDN-to-IP (the A RR) DNS
update on its behalf. If this bit is clear, the client indicates
that it intends to maintain its own FQDN-to-IP mapping update.
If a DHCP server intends to take responsibility for the A RR update
whether or not the client sending the FQDN option has set the "S"
bit, it sets both the "O" bit and the "S" bit, and sends the FQDN
option in its DHCPOFFER and/or DHCPACK messages.
The data in the Domain Name field may appear in one of two formats:
ASCII, or DNS-style binary encoding (without compression, of
course), as described in RFC1035[2]. A client which sends the FQDN
option MUST set the "E" bit to indicate that the data in the Domain
Name field is DNS binary encoded. If a server receives an FQDN
option from a client, and intends to include an FQDN option in its
reply, it MUST use the same encoding that the client used. The DNS
encoding is recommended. The use of ASCII-encoded domain-names is
fragile, and the use of ASCII encoding in this option should be
considered deprecated.
The remaining bits in the Flags field are reserved for future
assignment. DHCP clients and servers which send the FQDN option MUST
set the MBZ bits to 0, and they MUST ignore values in the part of
the field labelled "MBZ".
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5.2 The RCODE Fields
The RCODE1 and RCODE2 fields are used by a DHCP server to indicate
to a DHCP client the Response Code from any A or PTR RR Dynamic DNS
Updates it has performed. The server may also use these fields to
indicate whether it has attempted such an update before sending the
DHCPACK message. Each of these fields is one byte long.
Implementors should note that EDNS0 describes a mechanism for
extending the length of a DNS RCODE to 12 bits. EDNS0 is specified
in RFC2671[8]. Only the least-significant 8 bits of the RCODE from a
Dynamic DNS Update will be carried in the Client FQDN DHCP Option.
This provides enough number space to accomodate the RCODEs defined
in the Dynamic DNS Update specification.
5.3 The Domain Name Field
The Domain Name part of the option carries all or part of the FQDN
of a DHCP client. A client may be configured with a fully-qualified
domain name, or with a partial name that is not fully-qualified. If
a client knows only part of its name, it MAY send a single label,
indicating that it knows part of the name but does not necessarily
know the zone in which the name is to be embedded. The data in the
Domain Name field may appear in one of two formats: ASCII (with no
terminating NULL), or DNS encoding as specified in RFC1035[2]. If
the DHCP client wishes to use DNS encoding, it MUST set the
third-from-rightmost bit in the Flags field (the "E" bit); if it
uses ASCII encoding, it MUST clear the "E" bit.
A DHCP client that can only send a single label using ASCII encoding
includes a series of ASCII characters in the Domain Name field,
excluding the "." (dot) character. The client SHOULD follow the
character-set recommendations of RFC1034[1] and RFC1035[2]. A client
using DNS binary encoding which wants to suggest part of its FQDN
MAY send a non-terminal sequence of labels in the Domain Name part
of the option.
6. DHCP Client behavior
The following describes the behavior of a DHCP client that
implements the Client FQDN option.
If a client that owns/maintains its own FQDN wants to be responsible
for updating the FQDN to IP address mapping for the FQDN and
address(es) used by the client, then the client MUST include the
Client FQDN option in the DHCPREQUEST message originated by the
client. A DHCP client MAY choose to include the Client FQDN option
in its DISCOVER messages as well as its REQUEST messages. The
rightmost ("S") bit in the Flags field in the option MUST be set to
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0. Once the client's DHCP configuration is completed (the client
receives a DHCPACK message, and successfully completes a final check
on the parameters passed in the message), the client MAY originate
an update for the A RR (associated with the client's FQDN). The
update MUST be originated following the procedures described in
RFC2136[5] and Section 8. If the DHCP server from which the client
is requesting a lease includes the FQDN option in its ACK message,
and if the server sets both the "S" and the "O" bits (the two
rightmost bits) in the option's flags field, the DHCP client MUST
NOT initiate an update for the name in the Domain Name field.
A client can choose to delegate the responsibility for updating the
FQDN to IP address mapping for the FQDN and address(es) used by the
client to the server. In order to inform the server of this choice,
the client SHOULD include the Client FQDN option in its DHCPREQUEST
message. The rightmost (or "S") bit in the Flags field in the option
MUST be set to 1. A client which delegates this responsibility MUST
NOT attempt to perform a Dynamic DNS update for the name in the
Domain Name field of the FQDN option. The client MAY supply an FQDN
in the Client FQDN option, or it MAY supply a single label (the
most-specific label), or it MAY leave that field empty as a signal
to the server to generate an FQDN for the client in any manner the
server chooses.
Since there is a possibility that the DHCP server may be configured
to complete or replace a domain name that the client was configured
to send, the client might find it useful to send the FQDN option in
its DISCOVER messages. If the DHCP server returns different Domain
Name data in its OFFER message, the client could use that data in
performing its own eventual A RR update, or in forming the FQDN
option that it sends in its REQUEST message. There is no requirement
that the client send identical FQDN option data in its DISCOVER and
REQUEST messages. In particular, if a client has sent the FQDN
option to its server, and the configuration of the client changes so
that its notion of its domain name changes, it MAY send the new name
data in an FQDN option when it communicates with the server again.
This may allow the DHCP server to update the name associated with
the PTR record, and, if the server updated the A record representing
the client, to delete that record and attempt an update for the
client's current domain name.
A client that delegates the responsibility for updating the FQDN to
IP address mapping to a server might not receive any indication
(either positive or negative) from the server whether the server was
able to perform the update. In this case the client MAY use a DNS
query to check whether the mapping is updated.
A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option.
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If a client releases its lease prior to the lease expiration time
and the client is responsible for updating its A RR, the client
SHOULD delete the A RR (following the procedures described in
Section 8) associated with the leased address before sending a DHCP
RELEASE message. Similarly, if a client was responsible for updating
its A RR, but is unable to renew its lease, the client SHOULD
attempt to delete the A RR before its lease expires. A DHCP client
which has not been able to delete an A RR which it added (because it
has lost the use of its DHCP IP address) should attempt to notify
its administrator.
7. DHCP Server behavior
When a server receives a DHCPREQUEST message from a client, if the
message contains the Client FQDN option, and the server replies to
the message with a DHCPACK message, the server may be configured to
originate an update for the PTR RR (associated with the address
leased to the client). Any such update MUST be originated following
the procedures described in Section 8. The server MAY complete the
update before the server sends the DHCPACK message to the client. In
this case the RCODE from the update MUST be carried to the client in
the RCODE1 field of the Client FQDN option in the DHCPACK message.
Alternatively, the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the
RCODE1 field of the Client FQDN option in the DHCPACK message MUST
be set to 255. The choice between the two alternatives is entirely
determined by the configuration of the DHCP server. Servers SHOULD
support both configuration options.
When a server receives a DHCPREQUEST message containing the Client
FQDN option, the server MUST ignore the values carried in the RCODE1
and RCODE2 fields of the option.
In addition, if the Client FQDN option carried in the DHCPREQUEST
message has the "S" bit in its Flags field set, then the server MAY
originate an update for the A RR (associated with the FQDN carried
in the option) if it is configured to do so by the site's
administrator, and if it has the necessary credentials. The server
MAY be configured to use the name supplied in the client's FQDN
option, or it MAY be configured to modify the supplied name, or
substitute a different name.
Any such update MUST be originated following the procedures
described in Section 8. The server MAY originate the update before
the server sends the DHCPACK message to the client. In this case the
RCODE from the update [RFC2136] MUST be carried to the client in the
RCODE2 field of the Client FQDN option in the DHCPACK message.
Alternatively the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the
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RCODE2 field of the Client FQDN option in the DHCPACK message MUST
be set to 255. The choice between the two alternatives is entirely
up to the DHCP server. In either case, if the server intends to
perform the DNS update and the client's REQUEST message included the
FQDN option, the server SHOULD include the FQDN option in its ACK
message, and MUST set the "S" bit in the option's Flags field.
Even if the Client FQDN option carried in the DHCPREQUEST message
has the "S" bit in its Flags field clear (indicating that the client
wants to update the A RR), the server MAY be configured by the local
administrator to update the A RR on the client's behalf. A server
which is configured to override the client's preference SHOULD
include an FQDN option in its ACK message, and MUST set both the "O"
and "S" bits in the FQDN option's Flags field. The update MUST be
originated following the procedures described in Section 8. The
server MAY originate the update before the server sends the DHCPACK
message to the client. In this case the RCODE from the update
[RFC2136] MUST be carried to the client in the RCODE2 field of the
Client FQDN option in the DHCPACK message. Alternatively, the server
MAY send the DHCPACK message to the client without waiting for the
update to be completed. In this case the RCODE2 field of the Client
FQDN option in the DHCPACK message MUST be set to 255. Whether the
DNS update occurs before or after the DHCPACK is sent is entirely up
to the DHCP server's configuration.
When a DHCP server sends the Client FQDN option to a client in the
DHCPACK message, the DHCP server SHOULD send its notion of the
complete FQDN for the client in the Domain Name field. The server
MAY simply copy the Domain Name field from the Client FQDN option
that the client sent to the server in the DHCPREQUEST message. The
DHCP server MAY be configured to complete or modify the domain name
which a client sent, or it MAY be configured to substitute a
different name. If the server initiates a DDNS update which is not
complete until after the server has replied to the DHCP client, the
server's The server MUST use the same encoding format (ASCII or DNS
binary encoding) that the client used in the FQDN option in its
DHCPREQUEST, and MUST set the "E" bit in the option's Flags field
accordingly.
If a client's DHCPREQUEST message doesn't carry the Client FQDN
option (e.g., the client doesn't implement the Client FQDN option),
the server MAY be configured to update either or both of the A and
PTR RRs. The updates MUST be originated following the procedures
described in Section 8.
If a server detects that a lease on an address that the server
leases to a client has expired, the server SHOULD delete any PTR RR
which it added via dynamic update. In addition, if the server added
an A RR on the client's behalf, the server SHOULD also delete the A
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RR. The deletion MUST follow the procedures described in Section 8.
If a server terminates a lease on an address prior to the lease's
expiration time, for instance by sending a DHCPNAK to a client, the
server SHOULD delete any PTR RR which it associated with the address
via DNS Dynamic Update. In addition, if the server took
responsibility for an A RR, the server SHOULD also delete that A RR.
The deletion MUST follow the procedures described in Section 8.
8. Procedures for performing DNS updates
8.1 Adding A RRs to DNS
When a DHCP client or server intends to update an A RR, it first
prepares a DNS UPDATE query which includes as a prerequisite the
assertion that the name does not exist. The update section of the
query attempts to add the new name and its IP address mapping (an A
RR), and the DHCID RR with its unique client-identity.
If this update operation succeeds, the updater can conclude that it
has added a new name whose only RRs are the A and DHCID RR records.
The A RR update is now complete (and a client updater is finished,
while a server might proceed to perform a PTR RR update).
If the first update operation fails with YXDOMAIN, the updater can
conclude that the intended name is in use. The updater then
attempts to confirm that the DNS name is not being used by some
other host. The updater prepares a second UPDATE query in which the
prerequisite is that the desired name has attached to it a DHCID RR
whose contents match the client identity. The update section of
this query deletes the existing A records on the name, and adds the
A record that matches the DHCP binding and the DHCID RR with the
client identity.
If this query succeeds, the updater can conclude that the current
client was the last client associated with the domain name, and that
the name now contains the updated A RR. The A RR update is now
complete (and a client updater is finished, while a server would
then proceed to perform a PTR RR update).
If the second query fails with NXRRSET, the updater must conclude
that the client's desired name is in use by another host. At this
juncture, the updater can decide (based on some administrative
configuration outside of the scope of this document) whether to let
the existing owner of the name keep that name, and to (possibly)
perform some name disambiguation operation on behalf of the current
client, or to replace the RRs on the name with RRs that represent
the current client. If the configured policy allows replacement of
existing records, the updater submits a query that deletes the
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existing A RR and the existing DHCID RR, adding A and DHCID RRs that
represent the IP address and client-identity of the new client.
DISCUSSION:
The updating entity may be configured to allow the existing DNS
records on the domain name to remain unchanged, and to perform
disambiguation on the name of the current client in order to
attempt to generate a similar but unique name for the current
client. In this case, once another candidate name has been
generated, the updater should restart the process of adding an A
RR as specified in this section.
8.2 Adding PTR RR Entries to DNS
The DHCP server submits a DNS query which deletes all of the PTR RRs
associated with the lease IP address, and adds a PTR RR whose data
is the client's (possibly disambiguated) host name. The server also
adds a DHCID RR specified in Section 4.3.
8.3 Removing Entries from DNS
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 added, or for which an administrator has explicitly assigned it
responsibility.
When a lease expires or a DHCP client issues a DHCPRELEASE request,
the DHCP server SHOULD delete the PTR RR that matches the DHCP
binding, if one was successfully added. The server's update query
SHOULD assert that the name in the PTR record matches the name of
the client whose lease has expired or been released.
The entity chosen to handle the A record for this client (either the
client or the server) SHOULD delete the A record that was added when
the lease was made to the client.
In order to perform this delete, the updater prepares an UPDATE
query which contains two prerequisites. The first prerequisite
asserts that the DHCID RR exists whose data is the client identity
described in Section 4.3. The second prerequisite asserts that the
data in the A RR contains the IP address of the lease that has
expired or been released.
If the query fails, the updater MUST NOT delete the DNS name. It
may be that the host whose lease on the server has expired has moved
to another network and obtained a lease from a different server,
which has caused the client's A RR to be replaced. It may also be
that some other client has been configured with a name that matches
the name of the DHCP client, and the policy was that the last client
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to specify the name would get the name. In this case, the DHCID RR
will no longer match the updater's notion of the client-identity of
the host pointed to by the DNS name.
8.4 Updating other RRs
The procedures described in this document only cover updates to the
A and PTR RRs. Updating other types of RRs is outside the scope of
this document.
9. 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
zones which are exposed to the global Internet. Both DHCP clients
and servers SHOULD use some form of update request origin
authentication procedure (e.g., Simple Secure DNS Update[11]) when
performing DNS updates.
Whether a DHCP client may be responsible for updating an FQDN to IP
address mapping, or whether this is the responsibility of the DHCP
server is a site-local matter. The choice between the two
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 credentials to perform updates to the FQDN to IP address
mapping for its FQDN).
Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping),
regardless of the wishes of an individual DHCP client, is also a
site-local matter. The choice between the two alternatives may be
based on the security model that is being used with dynamic DNS
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 use for the client, and of the identity of the client.
Currently, it is difficult for DHCP servers to develop much
confidence in the identities of its clients, given the absence of
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 only in certain relatively unusual circumstances will the DHCP
server know for certain the identity of the client. If DHCP
Authentication[10] becomes widely deployed this may become more
customary.
One example of a situation which offers some extra assurances is one
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
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MAC address spoofing simply does not occur. Another example of a
configuration that might be trusted is one where clients obtain
network access via a network access server using PPP. The NAS itself
might be obtaining IP addresses via DHCP, encoding a client
identification into the DHCP client-id option. In this case, the
network access server as well as the DHCP server might be operating
within a trusted environment, in which case the DHCP server could be
configured to trust that the user authentication and authorization
procedure of the remote access server was sufficient, and would
therefore trust the client identification encoded within the DHCP
client-id.
10. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter
Ford, Edie Gunter, Andreas Gustafsson, R. Barr Hibbs, Kim Kinnear,
Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Josh Littlefield,
Michael Patton, and Glenn Stump for their review and comments.
References
[1] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
1034, Nov 1987.
[2] Mockapetris, P., "Domain names - Implementation and
Specification", RFC 1035, Nov 1987.
[3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[4] Marine, A., Reynolds, J. and G. Malkin, "FYI on Questions and
Answers to Commonly asked ``New Internet User'' Questions", RFC
1594, March 1994.
[5] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System", RFC 2136, April 1997.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[7] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[8] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[9] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)
(draft-ietf-dnsext-tsig-*)", July 1999.
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[10] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages
(draft-ietf-dhc-authentication-*)", June 1999.
[11] Wellington, B., "Simple Secure DNS Dynamic Updates
(draft-ietf-dnsext-simple-secure-update-*)", June 1999.
[12] Gustafsson, A., "A DNS RR for encoding DHCP client identity
(draft-ietf-dnsext-dhcid-rr-*)", October 1999.
[13] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
April 1992.
Authors' Addresses
Mark Stapp
Cisco Systems, Inc.
250 Apollo Dr.
Chelmsford, MA 01824
US
Phone: 978.244.8498
EMail: mjs@cisco.com
Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
US
Phone: 914.235.2128
EMail: yakov@cisco.com
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Full Copyright Statement
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Acknowledgement
Funding for the RFC editor function is currently provided by the
Internet Society.
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