NetworkDHC Working Group Yakov Rekhter
INTERNET-DRAFT Mark M. Stapp
Internet-Draft Y. Rekhter
Expires: April 2000 Cisco Systems
December Systems, Inc.
Interaction between DHCP and DNS
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire April, 2000.
Copyright (C) The Internet Society (1999). All Rights Reserved.
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] RFC2136 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.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Models of Operation . . . . . . . . . . . . . . . . . . . . 3
4. Client FQDN Option . . . . . . . . . . . . . . . . . . . . . 4
4.1 The Flags Field . . . . . . . . . . . . . . . . . . . . . . 5
4.2 The RCODE Fields . . . . . . . . . . . . . . . . . . . . . . 6
4.3 The Domain Name Field . . . . . . . . . . . . . . . . . . . 6
5. DHCP Client behavior . . . . . . . . . . . . . . . . . . . . 6
6. DHCP Server behavior . . . . . . . . . . . . . . . . . . . . 8
7. Procedures for performing DNS updates . . . . . . . . . . . 10
7.1 Name Collisions . . . . . . . . . . . . . . . . . . . . . . 10
7.2 Multiple DHCP servers . . . . . . . . . . . . . . . . . . . 11
7.3 Use of the KEY RR . . . . . . . . . . . . . . . . . . . . . 11
7.3.1 Format of the KEY RR . . . . . . . . . . . . . . . . . . . . 12
7.4 DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . 12
7.5 Adding A RRs to DNS . . . . . . . . . . . . . . . . . . . . 13
7.6 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . . 14
7.7 Removing Entries from DNS . . . . . . . . . . . . . . . . . 14
7.8 Updating other RRs . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
References . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
Full Copyright Statement . . . . . . . . . . . . . . . . . . 18
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 [RFC2119]. 2119.
2. Interaction between DHCP and DNS Introduction
DNS [RFC1034, RFC1035] RFC1034, RFC1035 maintains (among other things) the
information about mapping between hosts' Fully Qualified Domain
[RFC1594] RFC1594 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 a mapping from an a FQDN to an IP address; the
PTR RR con-
tains a contains mapping from an IP address to a FQDN. The Dynamic
DNS Updates specification [RFC2136] RFC2136 describes a mechanism that
enables DNS information to be updated over a network.
DHCP [RFC2131] RFC2131 provides a mechanism by which a host (a DHCP client)
can acquire certain configuration information, and specifically 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 gets
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 should apply for updates to the analogous RR types that are
used when clients are assigned IPv6 addresses via DHCPv6.
3. Models of operation Operation
When a DHCP client acquires a new address, a site's administrator
may desire that one or both of the A RR (for for the client's FQDN) FQDN and
the PTR RR (for for the acquired address) have to address be updated. Therefore, two
separate Dynamic DNS Update transactions occur. Acquiring an address
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, upon reflec-
tion one could realize that in practice not all permutations make
sense. This document covers the 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
One could observe that the
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
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
trary 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 them. 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 will 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 should may do in cases
where clients do not supply domain names.
4. 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 sub-
subsequent REQUEST messages.
The code for this option is 81. Its minimum length is 4.
Code Len Flags RCODE1 RCODE2 Domain Name
| 81 | n | | | | ...
4.1 The Flags Field
This figure presents the format of the Flags field, which is one byte
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 perform 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 corresponding 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).
course), as described in RFC1035. A client which sends the FQDN
option sets MUST set the "E" bit to indicate that the data in the Domain
Name field is DNS-encoded, as described DNS-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 [RFC1035]. this option should be considered
The remaining bits in the Flags field are reserved for future assign-
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".
4.2 The RCODE Fields
The RCODE1 and RCODE2 fields are used by a DHCP server to indicate
to a DHCP client the Response Code from the an any A or PTR RR Dynamic DNS Update
performed on the client's behalf.
Updates it has performed. The server also uses these fields to
indicate whether it has attempted such an update before sending the
DHCPACK message. Each of these fields is one byte long.
4.3 The Domain Name Field
The Domain Name part of the option carries 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]. RFC1035. 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
that Flags 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] RFC1034 and [RFC1035]. RFC1035. A client
using DNS encoding sends a single label as which wants to suggest part of its FQDN MAY send
a single byte count, fol-
lowed by that number non-terminal sequence of bytes labels in the Domain Name part of data, without a terminal reference
to the root.
5. 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
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 SHOULD MAY originate
an update for the A RR (associated with the client's FQDN). The
update MUST be originated following the procedures described in section 3.4.
RFC2136, and Section 7. 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" (the two rightmost)
bits in the option's Flags 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
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 should 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 which that delegates the responsibility for updating the FQDN to
IP address mapping to a server might not receive any indication
(either positive or negative) about from the status of server whether the update from server was
able to perform the update. In this case the
server. 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.
If a client releases its lease prior to the lease expiration time
and the client is responsible for updating its A RR(s), RR, the client
SHOULD delete the A RR (following the procedures described in [RFC2136])
Section 7) associated with the leased address before sending a DHCPRELEASE mes-
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 add an entry should attempt to its logfile and/or notify
6. 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 SHOULD may be configured to
originate an update for the PTR RR associated (associated with the address
leased to the
client if the server is configured to perform DNS updates. The client). Any such update MUST be originated following
the procedures described in Section 3.4. 7. The server MAY complete 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 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 configura-
tion 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 by in the client, 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 3.4. 7. 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. Alterna-
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. 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, MAY be configured byt 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 3.4. 7. 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 server receives a DHCPREQUEST message from a client, and the
message contains the Client FQDN option, the server MUST ignore the
values carried in the RCODE1 and RCODE2 fields of the option.
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 MUST
MAY simply copy the Domain Name field from the Client FQDN option
that the client sent to the server in the DHCPRE-
QUEST DHCPREQUEST message. If, however, the client sent only a single label, or
if the The
DHCP server has been MAY be configured to assign complete or modify the client a domain name
different from the one the
which a client has sent, or it MAY be configured to substitute a
different name. If the server SHOULD send
its notion of the initiates a DDNS update which is not
complete FQDN for until after the client. server has replied to the DHCP client, the
server's The server MUST use the same encoding format (ASCII or
DNS-encoding) that the client used in the FQDN option in its
DHCPREQUEST, and MUST set the "E" bit in the option's Flags field
If the a client's DHCPREQUEST message received by a DHCP server from a DHCP
client doesn't carry the Client FQDN
option (e.g., the client doesn't implement the Client FQDN option), and the DHCP client acquires its
FQDN from a DHCP server (as part of a normal DHCP transaction), then
the server MAY be configured to update either or both of the A and
PTR RRs. Any The updates MUST be originated following the procedures
described in Sec-
tion 3.4. In this case, the server MAY NOT wish to return the FQDN
option to a client which may not be able to understand it. If it can,
the DHCP server MAY (optionally) return the host part of the domain
name that it will use for the client in the host-name DHCP option
(defined in [RFC2132]). Note that it may not be possible to con-
sistently encode some domain name data in the format specified by the
host-name option. Section 7.
If a server detects that a lease on an address that the server
leases to a client has expired or has been released by the client, expired, the server SHOULD delete the any PTR RR
which it associated with the address added via DNS Dynamic Update. dynamic update. In addition, if the server added
an A RR on
behalf of the client, client's behalf, the server SHOULD also delete the A
RR. The deletion MUST follow the procedures described in Section 3.4. 7.
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 the any PTR RR which it associated with the address
via DNS Dynamic Update. In addition, if the server took responsibil-
responsibility for the client's an A RR , RR, the server SHOULD also delete that A RR.
The deletion MUST follow the procedures described in Section 3.4.
7. Procedures for performing DNS updates
There are two principal issues that need to be addressed concerning
A RR DNS updates:
7.1 Name Collisions
If the entity updating the A RR (either the DHCP client or DHCP
server) attempts to perform a DNS update to a domain name that has
an A RR which is already in use by a different DHCP client, what
should be done? 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 in some sites. This specification describes
behavior designed to prevent these undesirable effects, and requires
that DHCP implementations make this behavior the default. In some scenarios these name collisions
are unlikely, in some scenarios they are very likely:
1. Client updates A RR, uses DNSSEC: DNSSEC. Name collisions in this
scenario are unlikely (though not impossible), since for the
client to use DNSSEC, it has already 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, and presumably uniquely) to
2. Client updates A RR, uses some form of TSIG: TSIG. Name colli-
sions 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 adminis-
trative 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: server. Name collisions in this scenario are likely
unless prevented by the server's name configuration procedures.
See Section 5 8 for security issues with this form of deployment.
4. Server updates the A RR, uses a name supplied by the
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 5 8 for security issues with
this type of deployment.
Scenarios 3 and 4 are much more attractive given some form of DHCP
Authentication, but the difficulties remain.
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 pro-
cedures procedures will
break down at least occasionally. The question is what to do when
these procedures break down or, for example in scenario #4, may not
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 updat-
ing updating entity for a name
gets the name).
7.2 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 population of DHCP clients, the DHCP servers should be able
to provide reasonable and consistent DNS name update behavior for
7.3 Use of the KEY RR
A solution to both of these problems is for the updating entities
(both DHCP clients or and DHCP servers) to be able to cooperate when
updating DNS A RRs.
Specifically, a KEY RR, described in [RFC2535] RFC2535 is used to associate
client ownership 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 KEY RR which specifies a unique client iden-
identity (based on a "client specifier" created from the client's
client-id or MAC address: see Appendix A). 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
ured 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.
See Appendix A for the details of the use of the KEY RR for this pur-
The specific algorithms utilizing the KEY RR to signal client owner-
ownership are explained below. The algorithms only work in the case
where the updating entities all cooperate -- this approach is
advisory only and does not substitute for DNS security, nor is it
replaced by DNS security.
3.4.1. Adding A RRs
7.3.1 Format of the KEY RR
The KEY RR used to DNS
When a hold the DHCP client or server intends to update an A RR, it first
prepares a DNS UPDATE query which includes client's identity is formatted as a prerequisite
The name of the
assertion that KEY RR is the name does not of the A or PTR RR which refers
to the client.
The flags field is set to 0x4200 - that is, the 1 bit and the 6 bit
The protocol field is set to [TBD].
The algorithm field is set to [TBD].
0 15 31
| Version | Identity-length |
/ Client-identity... /
The Version field indicates the version of the data used in this RR.
The Version field is a 2-byte integer in network byte-order. Its
value MUST be 1.
The remainder of the Key field contains the length of the
client-identity, followed by that number of bytes of client-identity
data. The data length is represented as a 2-byte integer in network
byte order. If a DHCP client sent the client-id option in its
request message, the client-identity MUST be identical to the data
in the client-id option. If a client did not send the client-id
option, the client-identity is constructed from the htype byte, the
hlen byte, and hlen bytes of the client's chaddr from its request
7.4 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
7.5 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 KEY 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 KEY RR records.
The A RR update is now complete (and a client updater is finished,
while a server would then 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 KEY RR
whose contents match the client identity (see Appendix A). 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 KEY RR with the
If this query succeeds, the updater can conclude that the current
client was the last user of this 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 confi-
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 'take-over' replace the name RRs on behalf of 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
existing A RR and the existing KEY RR, adding A and KEY RRs that
represent the IP address and client-identity of the new client.
The updating entity may be configured to allow the existing owner
to keep DNS
records on the name, 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 such a
similar another candidate name has been
generated, the updating entity updater should res-
tart restart the process of adding an A
RR as specified in this section.
7.6 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 KEY RR whose data is the client's client-identity as described specified in
3.4.3. Section 7.3.
7.7 Removing Entries from DNS
The first rule in removing DNS entries is be sure that an entity
removing a DNS entry is only removing an entry for which that it is
When a lease expires or a DHCP client issues a DHCPRELEASE request,
the DHCP server SHOULD delete the PTR RR that matches the DHCP bind-
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 and KEY records record that were 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 KEY RR exists whose data is the client identity
Appendix A. Section 7.3. 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's prerequisites fail, 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 dif-
ferent 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 administrative pol-
icy at the site policy was that the last client
to specify the name would get the name. In this case, the KEY RR
will no longer match the updater's notion of the client-identity of
the host pointed to by the DNS name.
7.8 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
8. Security Considerations
Unauthenticated updates to the DNS can lead to tremendous confusion,
through malicious attack or through inadvertent misconfiguration.
Administrators should be wary of unsecured DNS updates to zones
which are exposed to the global Internet.
Whether the client may be responsible for updating the FQDN to IP
address mapping, or whether the this responsibility lies with the
DHCP server is a site-local matter. The choice between the two alter-
alternatives may be based on a particular security model that is
used with the Dynamic DNS Update protocol (e.g., only a client may
cient sufficient credentials to perform updates to the FQDN to IP
ping 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 a DHCP client, is also a site-local matter.
The choice between the two alternatives may be based on a particular
The client Both DHCP clients and servers SHOULD use some form
of data update request origin authentication pro-
cedures (e.g. [TSIG], [DNSSEC]) procedure (e.g., TSIG,
Simple Secure DNS Update) when performing DNS updates.
While the DHCP client MAY be the one to update the DNS A record, in
certain specialized cases configurations a DHCP server MAY do so instead. In this
case, the DHCP server MUST be sure of both the name to use for the
client, as well as the identity of the client.
In the general case, both of these conditions are not satisfied --
one needs difficult to be mindful of
satisfy, given the possibility absence of MAC address spoofing in
a security from the DHCP packet. It is not difficult protocol
itself. There are many ways for a DHCP server to know unambi-
guously the develop a DNS name
to use for a client, but only in certain cir-
cumstances relatively unusual
circumstances will the DHCP server know for sure certain the identity of
the client. If DHCP authentication [DHCPAUTH] authentication becomes widely deployed this
may become more customary. An
One example of a situation which offers some extra assurances is one
where the DHCP client is con-
nected connected to a network through an MCNS
cable modem, and the CMTS (head-
end) (head-end) of the cable modem ensures that
MAC address spoofing simply does not occur. Another example where the DHCP server would know the identity of the
client would be in a case where it was interacting with a remote
access server which encoded a client identification into the DHCP
client-id option. In this case, the remote access server as well as
the DHCP server would
configuration that might be operating within a trusted environment, and
the DHCP server could trust that the user authentication and authori-
zation procedure of the remote is one where clients obtain
network access server was sufficient, and
would therefore trust the client identification encoded within the
In either of these cases, a DHCP server would be able to correctly
enter the DNS A record on behalf of a client, since it would know the
name associated with a client (through some administrative procedure
outside the scope of this protocol), and it would also know the
client's identity in via a secure manner.
6. Appendix A - Use of the KEY RR
The KEY RR used to hold the DHCP client's identity is formatted as
The name of the KEY RR is the name of the A or PTR RR which refers to
The flags field is set to 0x42 - that is, the 1 bit and the 6 bit are
The protocol field is set to [TBD].
The algorithm field is set to [TBD].
The first byte in the key field contains the length of the client-
identity, and is followed by that number of bytes of client-identity
data. If network access server using PPP. The NAS itself
might be obtaining IP addresses via DHCP, encoding a DHCP client sent
identification into the DHCP client-id option in its request mes-
sage, option. In this case, the client-identity MUST be identical to
network access server as well as the data DHCP server might be operating
within a trusted environment, in which case the
client-id option. If a client did not send DHCP server could
trust that the client-id option, user authentication and authorization procedure of
client-identity is constructed from remote access server was sufficient, and would therefore trust
the htype byte, client identification encoded within the hlen byte, DHCP client-id.
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 hlen bytes of the client's chaddr from its request message.
7. Glenn Stump for their review and comments.
 Mockapetris, P., "Domain names - concepts Concepts and facilities",
[RFC1035] P. Facilities", RFC
1034, Nov 1987.
 Mockapetris, P., "Domain names - implementation Implementation and specif-
ication", RFC1035, 11/01/1987
Specification", RFC 1035, Nov 1987.
 Droms, R., "Dynamic Host Configuration Protocol", RFC2131,
[RFC2132] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC2132, RFC 2131,
 Marine, J. A., Reynolds, J. and G. Malkin, "FYI on Questions and
Answers to Commonly asked ``New Internet User'' Questions", RFC1594, 03/11/1994
[RFC2136] P. RFC
1594, March 1994.
 Vixie, S. P., Thomson, Y. S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC2136, System", RFC 2136, April 1997
 Bradner, S. S., "Key words for use in RFCs to Indicate Requirement
[DNSSEC] D. RFC 2119, March 1997.
 Eastlake, D., "Domain Name System Security Extensions",
2535, March 1999.
 Vixie, O. P., Gudmundsson, D. O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Signatures for DNS", draft-ietf-
dnsind-tsig-*, Work in Progress.
[DHCPAUTH] R. DNS (TSIG)
(draft-ietf-dnsind-tsig-*)", July 1999.
 Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
draft-ietf-dhc-authentication-*, Work in Progress.
Many thanks to Messages
(draft-ietf-dhc-authentication-*)", June 1999.
 Wellington, B., "Simple Secure DNS Dynamic Updates
(draft-ietf-dnsind-simple-secure-update-*)", June 1999.
Mark Beyer, Jim Bound, Ralph Droms, Robert Elz,
Peter Ford, Edie Gunter, R. Barr Hibbs, Kim Kinnear, Stuart Kwan,
Ted Lemon, Michael Lewis, Michael Patton, and Glenn Stump for
their review and comments.
9. Author information Stapp
Cisco Systems, Inc.
250 Apollo Dr.
Chelmsford, MA 01824
Cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 235-2128
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8498
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