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Network Working Group Yakov Rekhter
INTERNET-DRAFT Mark Stapp
Cisco Systems
June 1999
Expires
December 1999
Interaction between DHCP and DNS
<draft-ietf-dhc-dhcp-dns-10.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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society (1999). 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] 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
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acquire via DHCP.
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 [RFC2119].
2. Interaction between DHCP and DNS
DNS [RFC1034, RFC1035] maintains (among other things) the information
about mapping between hosts' Fully Qualified Domain Names (FQDNs)
[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 FQDN to an IP address; the PTR RR con-
tains a mapping from an IP address to a FQDN. The Dynamic DNS
Updates specification [RFC2136] describes a mechanism that enables
DNS information to be updated over a network.
DHCP [RFC2131] provides a mechanism by which a host (a DHCP client)
could acquire certain configuration information, and specifically 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 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.
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3. Models of operation
When a DHCP client acquires a new address, both the A RR (for the
client's FQDN) and the PTR RR (for the acquired address) have to 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 not all permutations make sense. This
document covers the 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 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 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 arbi-
trary 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.
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 support some or all
of these possibilities.
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 do in cases where clients
do not supply domain names.
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3.1. 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-
sequent REQUEST messages.
The code for this option is 81. Its minimum length is 4.
Code Len Flags RCODE1 RCODE2 Domain Name
+------+------+------+------+------+------+--
| 81 | n | | | | ...
+------+------+------+------+------+------+--
3.1.1. The Flags Field
This figure presents the format of the Flags field, which is one byte
long:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |E|O|S|
+-+-+-+-+-+-+-+-+
When a 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 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
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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).
A client which sends the FQDN option sets the "E" bit to indicate
that the data in the Domain Name field is DNS-encoded, as described
in [RFC1035].
The remaining bits in the Flags field are reserved for future assign-
ment. 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".
3.1.2. 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 A RR Dynamic DNS Update
performed on the client's behalf. 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.
3.1.3. 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]. 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 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] and [RFC1035]. A client
using DNS encoding sends a single label as a single byte count, fol-
lowed by that number of bytes of data, without a terminal reference
to the root.
3.2. DHCP Client behavior
The following describes the behavior of a DHCP client that implements
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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
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 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.
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 in the option's Flags field, the 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 should send the new 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.
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A client which delegates the responsibility for updating the FQDN to
IP address mapping to a server might not receive any indication
(either positive or negative) about the status of the update from 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), the client SHOULD
delete the A RR (following the procedures described in [RFC2136])
associated with the leased address before sending a DHCPRELEASE mes-
sage. 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 client which has not been able
to delete an A RR which it added (because it has lost its IP address)
SHOULD add an entry to its logfile and/or notify its administrator.
3.3. 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 originate an
update for the PTR RR associated with the address leased to the
client if the server is configured to perform DNS updates. The update
MUST be originated following the procedures described in Section 3.4.
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 of the DHCP server. Servers SHOULD support both configuration
options.
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 the client, or it MAY be configured to
modify the supplied name, or substitute a different name.
The update MUST be originated following the procedures described in
Section 3.4. The server MAY originate the update before the server
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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-
tively 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 its Flags field clear (indicating that the client wants
to update the A RR), the server MAY, be configured byt 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. 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 server MUST copy the Domain Name field from the
Client FQDN option that the client sent to the server in the DHCPRE-
QUEST message. If, however, the client sent only a single label, or
if the DHCP server has been configured to assign the client a name
different from the one the client has sent, the server SHOULD send
its notion of the complete FQDN for the client. 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 accordingly.
If the 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
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FQDN from a DHCP server (as part of a normal DHCP transaction), then
the server MAY be configured to update both A and PTR RRs. Any
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.
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, the
server SHOULD delete the PTR RR which it associated with the address
via DNS Dynamic Update. In addition, if the server added an A RR on
behalf of the client, the server SHOULD also delete the A RR. The
deletion MUST follow the procedures described in Section 3.4.
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 PTR RR which it associated with the address
via DNS Dynamic Update. In addition, if the server took responsibil-
ity for the client's A RR , the server SHOULD also delete that A RR.
The deletion MUST follow the procedures described in Section 3.4.
3.4. Procedures for performing DNS updates
There are two principal issues that need to be addressed concerning A
RR DNS updates:
o 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 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: Name collisions in this
scenario are unlikely (though not impossible), since for the
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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 that client.
2. Client updates A RR, uses some form of TSIG: Name colli-
sions 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 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 5 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 5 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 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 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 updat-
ing entity for a name gets the name).
o 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
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addresses to the same population of DHCP clients, the DHCP
servers should be able to provide reasonable DNS name update
behavior for DHCP clients.
The solution to both of these problems is for the updating entities
(either DHCP clients or DHCP servers) to be able to cooperate when
updating DNS A RRs.
Specifically, a KEY RR, described in [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-
tity (based on a "client specifier" created from the client's
client-id or MAC address: see Appendix A). 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 config-
ured 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-
pose.
The specific algorithms utilizing the KEY RR to signal client owner-
ship 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 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 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 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.
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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). 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 client
identity.
If this query succeeds, the updater can conclude that the current
client was the last user of this 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-
guration 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' the name on behalf of the current client.
DISCUSSION:
The updating entity may be configured to allow the existing owner
to keep the name, 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 name has been generated, the updating entity should res-
tart the process of adding an A RR as specified in this section.
3.4.2. 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 KEY RR whose data is the client's client-identity as described in
Appendix A.
3.4.3. 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 it is
responsible.
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-
ing, 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
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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 that were
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 described in
Appendix A. 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, 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 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 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.
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.
5. Security Considerations
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-
natives may be based on a particular security model that is used with
the Dynamic DNS Update protocol (e.g., only a client may have suffi-
cient credentials to perform updates to the FQDN to IP address map-
ping 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 security model.
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The client SHOULD use some form of data origin authentication pro-
cedures (e.g. [TSIG], [DNSSEC]) when performing DNS updates.
While the DHCP client MAY be the one to update the DNS A record, in
certain specialized cases 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 to be mindful of the possibility of MAC address spoofing in
a DHCP packet. It is not difficult for a DHCP server to know unambi-
guously the DNS name to use for a client, but only in certain cir-
cumstances will the DHCP server know for sure the identity of the
client. If DHCP authentication [DHCPAUTH] becomes widely deployed
this may become more customary. An example of a situation which
offers some extra assurances is one where the DHCP client is con-
nected to a network through an MCNS cable modem, and the CMTS (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 be operating within a trusted environment, and
the DHCP server could trust that the user authentication and authori-
zation procedure of the remote access server was sufficient, and
would therefore trust the client identification encoded within the
DHCP client-id.
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 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
follows:
The name of the KEY RR is the name of the A or PTR RR which refers to
the client.
The flags field is set to 0x42 - that is, the 1 bit and the 6 bit are
set.
The protocol field is set to [TBD].
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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 a DHCP client sent the client-id option in its request mes-
sage, 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 message.
7. References
[RFC1034] P. Mockapetris, "Domain names - concepts and facilities",
RFC1034, 11/01/1987
[RFC1035] P. Mockapetris, "Domain names - implementation and specif-
ication", RFC1035, 11/01/1987
[RFC2131] R. Droms, "Dynamic Host Configuration Protocol", RFC2131,
March 1997.
[RFC2132] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC2132, March 1997.
[RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and
Answer Answers to Commonly asked ``New Internet User''
Questions", RFC1594, 03/11/1994
[RFC2136] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC2136,
April 1997
[RFC2119] Bradner, S. "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119.
[DNSSEC] D. Eastlake, "Domain Name System Security Extensions",
RFC2535, March 1999.
[TSIG] P. Vixie, O. Gudmundsson, D. Eastlake, B. Wellington,
"Secret Key Transaction Signatures for DNS", draft-ietf-
dnsind-tsig-*, Work in Progress.
[DHCPAUTH] R. Droms, W. Arbaugh, "Authentication for DHCP Messages",
draft-ietf-dhc-authentication-*, Work in Progress.
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8. Acknowledgements
Many thanks to 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
Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 235-2128
email: yakov@cisco.com
Mark Stapp
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8498
email: mjs@cisco.com
10. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
explain it or assist in its implementation may be prepared,
copied, published and distributed, in whole or in part, without
restriction of any kind, provided that the above copyright notice
and this paragraph are included on all such copies and derivative
works. However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the
Internet Society or other Internet organizations, except as needed
for the purpose of developing Internet standards in which case
the procedures for copyrights defined in the Internet Standards
process must be followed, or as required to translate it into
languages other than English.
The limited permissions granted above are perpetual and will not
be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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DHC Working Group M. Stapp
Internet-Draft Y. Rekhter
Expires: April 2000 Cisco Systems, Inc.
October, 1999
Interaction between DHCP and DNS
<draft-ietf-dhc-dhcp-dns-11.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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire April, 2000.
Copyright Notice
Copyright (C) The Internet Society (1999). 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. 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
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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. 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.
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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
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
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-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
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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 any A or PTR RR Dynamic DNS
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[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 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.
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 MAY originate
an update for the A RR (associated with the client's FQDN). The
update MUST be originated following the procedures described in
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RFC2136[5], 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 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.
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 7) associated with the leased address before sending a DHCP
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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.
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 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 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 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 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. 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
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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 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 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-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 7.
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
RR. The deletion MUST follow the procedures described in Section 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
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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 7.
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.
1. Client updates A RR, uses 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
that client.
2. Client updates A RR, uses some form of TSIG. 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 8 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 8 for security issues with
this type of deployment.
Scenarios 3 and 4 are much more attractive given some form of DHCP
Authentication, but difficulties remain.
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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
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).
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 DHCP clients, the DHCP servers should be able
to provide reasonable and consistent DNS name update behavior for
DHCP clients.
7.3 Use of the KEY RR
A solution to both of these problems is for the updating entities
(both DHCP clients and DHCP servers) to be able to cooperate when
updating DNS A RRs.
Specifically, a KEY RR, described in RFC2535[7] 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
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 KEY 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 does not substitute for DNS security, nor is it
replaced by DNS security.
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7.3.1 Format of the KEY RR
The KEY RR used to hold the DHCP client's identity is formatted as
follows:
The name of the KEY RR is the name 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
are set.
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
message.
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
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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.
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 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. 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
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
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.
DISCUSSION:
The updating entity may be configured to allow the existing DNS
records on the domain name to remain unchanged, and to perform
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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.
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 specified in 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 that it added.
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 KEY RR exists whose data is the client identity
described in 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 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
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
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this document.
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
alternatives may be based on a particular 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 a DHCP client, is a site-local matter.
The choice between the two alternatives may be based on a particular
security model. Both DHCP clients and servers SHOULD use some form
of update request origin authentication procedure (e.g., TSIG[8],
Simple Secure DNS Update[10]) when performing DNS updates.
While the DHCP client MAY be the one to update the DNS A record, in
certain 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 difficult to
satisfy, given the absence of security 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[9] 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
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
trust that the user authentication and authorization procedure of
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the remote access server was sufficient, and would therefore trust
the client identification encoded within the DHCP client-id.
9. 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., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Signatures for DNS (TSIG)
(draft-ietf-dnsind-tsig-*)", July 1999.
[9] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages
(draft-ietf-dhc-authentication-*)", June 1999.
[10] Wellington, B., "Simple Secure DNS Dynamic Updates
(draft-ietf-dnsind-simple-secure-update-*)", June 1999.
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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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