draft-ietf-dhc-fqdn-option-00.txt   draft-ietf-dhc-fqdn-option-01.txt 
DHC Working Group M. Stapp DHC Working Group M. Stapp
Internet-Draft Y. Rekhter Internet-Draft Y. Rekhter
Expires: January 12, 2001 Cisco Systems, Inc. Expires: August 31, 2001 Cisco Systems, Inc.
July 14, 2000 March 2, 2001
The DHCP Client FQDN Option The DHCP Client FQDN Option
<draft-ietf-dhc-fqdn-option-00.txt> <draft-ietf-dhc-fqdn-option-01.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as other groups may also distribute working documents as
Internet-Drafts. Internet-Drafts.
skipping to change at page 1, line 32 skipping to change at page 1, line 32
months and may be updated, replaced, or obsoleted by other documents months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on January 12, 2001. This Internet-Draft will expire on August 31, 2001.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract Abstract
DHCP provides a powerful mechanism for IP host configuration. DHCP provides a powerful mechanism for IP host configuration.
However, the configuration capability provided by DHCP does not However, the configuration capability provided by DHCP does not
include updating DNS, and specifically updating the name to address include updating DNS, and specifically updating the name to address
and address to name mappings maintained in the DNS. and address to name mappings maintained in the DNS.
This document specifies a DHCP option which can be used to exchange This document specifies a DHCP option which can be used to exchange
information about a DHCP client's fully-qualified domain name, or information about a DHCP client's fully-qualified domain name, and
'FQDN'. about responsibility for updating DNS RRs related to the client's
DHCP lease.
Table of Contents Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Models of Operation . . . . . . . . . . . . . . . . . . . . . 3 3. Models of Operation . . . . . . . . . . . . . . . . . . . . . 3
4. The Client FQDN Option . . . . . . . . . . . . . . . . . . . . 5 4. The Client FQDN Option . . . . . . . . . . . . . . . . . . . . 4
4.1 The Flags Field . . . . . . . . . . . . . . . . . . . . . . . 5 4.1 The Flags Field . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 The RCODE Fields . . . . . . . . . . . . . . . . . . . . . . . 6 4.2 The RCODE Fields . . . . . . . . . . . . . . . . . . . . . . . 6
4.3 The Domain Name Field . . . . . . . . . . . . . . . . . . . . 6 4.3 The Domain Name Field . . . . . . . . . . . . . . . . . . . . 6
5. DHCP Client behavior . . . . . . . . . . . . . . . . . . . . . 7 5. DHCP Client behavior . . . . . . . . . . . . . . . . . . . . . 7
6. DHCP Server behavior . . . . . . . . . . . . . . . . . . . . . 8 6. DHCP Server behavior . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14
1. Terminology 1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119[6]. document are to be interpreted as described in RFC 2119[6].
2. Introduction 2. Introduction
DNS (RFC1034[1], RFC1035[2]) maintains (among other things) the DNS (RFC1034[1], RFC1035[2]) maintains (among other things) the
information about mapping between hosts' Fully Qualified Domain information about mapping between hosts' Fully Qualified Domain
Names (FQDNs) RFC1594[4] and IP addresses assigned to the hosts. The Names (FQDNs) RFC1594[4] and IP addresses assigned to the hosts. The
information is maintained in two types of Resource Records (RRs): A 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 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 PTR RR contains mapping from an IP address to a FQDN. The DNS
DNS Updates specification (RFC2136[5]) describes a mechanism that update specification (RFC2136[5]) describes a mechanism that enables
enables DNS information to be updated over a network. DNS information to be updated over a network.
DHCP RFC2131[3] provides a mechanism by which a host (a DHCP client) DHCP RFC2131[3] provides a mechanism by which a host (a DHCP client)
can acquire certain configuration information, along with its IP can acquire certain configuration information, along with its IP
address(es). However, DHCP does not provide any mechanisms to update address(es). However, DHCP does not provide any mechanisms to update
the DNS RRs that contain the information about mapping between the 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 host's FQDN and its IP address(es) (A and PTR RRs). Thus DNS
information maintained by DNS for a DHCP client may be incorrect - a information for a DHCP client may not exist or may be incorrect - a
host (the client) could acquire its address by using DHCP, but the 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 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 acquired, and the PTR RR for the acquired address wouldn't reflect
the host's FQDN. the host's FQDN.
The Dynamic DNS Update protocol can be used to maintain consistency The DNS Update protocol can be used to maintain consistency between
between the information stored in the A and PTR RRs and the actual the information stored in the A and PTR RRs and the actual address
address assignment done via DHCP. When a host with a particular FQDN assignment done via DHCP. When a host with a particular FQDN
acquires its IP address via DHCP, the A RR associated with the acquires its IP address via DHCP, the A RR associated with the
host's FQDN would be updated (by using the Dynamic DNS Updates host's FQDN would be updated (by using the DNS Update protocol) to
protocol) to reflect the new address. Likewise, when an IP address reflect the new address. Likewise, when an IP address is assigned to
is assigned to a host with a particular FQDN, the PTR RR associated a host with a particular FQDN, the PTR RR associated with this
with this address would be updated (using the Dynamic DNS Updates address would be updated (using the DNS Update protocol) to reflect
protocol) to reflect the new FQDN. the new FQDN.
Although this document refers to the A and PTR DNS record types and Although this document refers to the A and PTR DNS record types and
to DHCP assignment of IPv4 addresses, the same procedures and to DHCP assignment of IPv4 addresses, the same procedures and
requirements apply for updates to the analogous RR types that are requirements apply for updates to the analogous RR types that are
used when clients are assigned IPv6 addresses via DHCPv6. used when clients are assigned IPv6 addresses via DHCPv6.
3. Models of Operation 3. Models of Operation
When a DHCP client acquires a new address, a site's administrator 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 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 the PTR RR for the acquired address be updated. Therefore, two
separate Dynamic DNS Update transactions occur. Acquiring an address separate DNS update transactions may occur. Acquiring an address via
via DHCP involves two entities: a DHCP client and a DHCP server. In DHCP involves two entities: a DHCP client and a DHCP server. In
principle each of these entities could perform none, one, or both of principle each of these entities could perform none, one, or both of
the transactions. However, in practice not all permutations make the transactions. However, in practice not all permutations make
sense. The DHCP client FQDN option is intended to operate in the sense. The DHCP client FQDN option is intended to operate in the
following two cases: following two cases:
1. DHCP client updates the A RR, DHCP server updates the PTR RR 1. DHCP client updates the A RR, DHCP server updates the PTR RR
2. DHCP server updates both the A and the PTR RRs 2. DHCP server updates both the A and the PTR RRs
The only difference between these two cases is whether the FQDN to 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. IP address mapping is updated by a DHCP client or by a DHCP server.
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and clients to perform DNS updates into the zones which it controls and clients to perform DNS updates into the zones which it controls
is entirely a matter of local administrative policy. This document is entirely a matter of local administrative policy. This document
does not require any specific administrative policy, and does not does not require any specific administrative policy, and does not
propose one. The range of possible policies is very broad, from propose one. The range of possible policies is very broad, from
sites where only the DHCP servers have been given credentials that sites where only the DHCP servers have been given credentials that
the DNS servers will accept, to sites where each individual DHCP the DNS servers will accept, to sites where each individual DHCP
client has been configured with credentials which allow the client client has been configured with credentials which allow the client
to modify its own domain name. Compliant implementations MAY support to modify its own domain name. Compliant implementations MAY support
some or all of these possibilities. Furthermore, this specification some or all of these possibilities. Furthermore, this specification
applies only to DHCP client and server processes: it does not apply applies only to DHCP client and server processes: it does not apply
to other processes which initiate dynamic DNS updates. to other processes which initiate DNS updates.
This document describes a new DHCP option which a client can use to 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. convey all or part of its domain name to a DHCP server.
Site-specific policy determines whether DHCP servers use the names Site-specific policy determines whether DHCP servers use the names
that clients offer or not, and what DHCP servers may do in cases that clients offer or not, and what DHCP servers may do in cases
where clients do not supply domain names. Two other documents, where clients do not supply domain names. Another document,
"Resolving Name Conflicts"[14] and "Update Procedures"[15], provide "Resolving Name Conflicts"[13], defines a protocol for arbitrating
additional guidance about the use of this new DHCP option.
"Resolving Name Conflicts"[14] defines a protocol for arbitrating
conflicts when collisions occur in the use of FQDNs by DHCP clients. conflicts when collisions occur in the use of FQDNs by DHCP clients.
"Update Procedures"[15] recommends a procedure which may be used to
update DNS RRs for DHCP clients.
4. The Client FQDN Option 4. The Client FQDN Option
To update the IP address to FQDN mapping a DHCP server needs to know 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 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 allow the client to convey its FQDN to the server this document
defines a new DHCP option, called "Client FQDN". The FQDN Option defines a new DHCP option, called "Client FQDN". The FQDN Option
also contains Flags and RCode fields which DHCP servers can use to also contains Flags and RCode fields which DHCP servers can use to
convey information about DNS updates to clients. convey information about DNS updates to clients.
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The Format of the FQDN Option: The Format of the FQDN Option:
Code Len Flags RCODE1 RCODE2 Domain Name Code Len Flags RCODE1 RCODE2 Domain Name
+------+------+------+------+------+------+-- +------+------+------+------+------+------+--
| 81 | n | | | | ... | 81 | n | | | | ...
+------+------+------+------+------+------+-- +------+------+------+------+------+------+--
4.1 The Flags Field 4.1 The Flags Field
The Format of the Flags Field:
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| MBZ |E|O|S| | MBZ |N|E|O|S|
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
When a DHCP client sends the FQDN option in its DHCPDISCOVER and/or When a DHCP client sends the FQDN option in its DHCPDISCOVER and/or
DHCPREQUEST messages, it sets the right-most bit (labelled "S") to DHCPREQUEST messages, it sets the least-significant bit (labelled
indicate that it will not perform any Dynamic DNS updates, and that "S") to indicate that it will not perform any DNS updates, and that
it expects the DHCP server to perform any FQDN-to-IP (the A RR) DNS 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 update on its behalf. If this bit is clear, the client indicates
that it intends to maintain its own FQDN-to-IP mapping update. 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 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" 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 bit, it sets both the "O" bit and the "S" bit, and sends the FQDN
option in its DHCPOFFER and/or DHCPACK messages. option in its DHCPOFFER and/or DHCPACK messages.
The data in the Domain Name field may appear in one of two formats: The data in the Domain Name field may appear in one of two formats:
ASCII, or DNS-style binary encoding (without compression, of ASCII, or DNS-style binary encoding (without compression, of
course), as described in RFC1035[2]. A client which sends the FQDN 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 option MUST set the "E" bit to indicate that the data in the Domain
Name field is DNS binary encoded. If a server receives an FQDN Name field is DNS binary encoded. If a server receives an FQDN
option from a client, and intends to include an FQDN option in its 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 reply, it MUST use the same encoding that the client used. The DNS
encoding is recommended. The use of ASCII-encoded domain-names is encoding is recommended. The use of ASCII-encoded domain-names is
fragile, and the use of ASCII encoding in this option should be fragile, and the use of ASCII encoding in this option should be
considered deprecated. considered deprecated.
A client MAY set the "N" flag in its request messages to indicate
that the server should not perform any DNS updates on its behalf. As
we mentioned in Section 3, we believe that in general the DHCP
server will be maintaining DNS PTR records on behalf of clients.
However, there may be deployments in which clients are configured to
perform all necessary DNS updates. The server MAY be configured to
honor this configuration. If the server has been configured to honor
a client's "N" indication, it SHOULD set the "N" bit in fqdn options
which it sends to the client in its OFFER or ACK messages. Clients
which have set the "N" bit in their requests SHOULD use the state of
the "N" bit in server responses to determine whether the server was
prepared to honor the client's indication. If a client has set the
"N" bit but its server does not, the client SHOULD conclude that the
server was not configured to honor the client's suggestion, and that
the server may attempt to perform DNS updates on its behalf.
The remaining bits in the Flags field are reserved for future The remaining bits in the Flags field are reserved for future
assignment. DHCP clients and servers which send the FQDN option MUST 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 set the MBZ bits to 0, and they MUST ignore values in the part of
the field labelled "MBZ". the field labelled "MBZ".
4.2 The RCODE Fields 4.2 The RCODE Fields
The RCODE1 and RCODE2 fields are used by a DHCP server to indicate 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 to a DHCP client the Response Code from any A or PTR RR DNS updates
Updates it has performed. The server may also use these fields to it has performed. The server may also use these fields to indicate
indicate whether it has attempted such an update before sending the whether it has attempted such an update before sending the DHCPACK
DHCPACK message. Each of these fields is one byte long. message. Each of these fields is one byte long.
Implementors should note that EDNS0 describes a mechanism for Implementors should note that EDNS0 describes a mechanism for
extending the length of a DNS RCODE to 12 bits. EDNS0 is specified extending the length of a DNS RCODE to 12 bits. EDNS0 is specified
in RFC2671[8]. Only the least-significant 8 bits of the RCODE from a in RFC2671[8]. Only the least-significant 8 bits of the RCODE from a
Dynamic DNS Update will be carried in the Client FQDN DHCP Option. DNS update will be carried in the Client FQDN DHCP Option. This
This provides enough number space to accomodate the RCODEs defined provides enough number space to accomodate the RCODEs defined in the
in the Dynamic DNS Update specification. DNS update specification.
4.3 The Domain Name Field 4.3 The Domain Name Field
The Domain Name part of the option carries all or part of the FQDN The Domain Name part of the option carries all or part of the FQDN
of a DHCP client. A client may be configured with a fully-qualified 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 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, 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 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 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 Domain Name field may appear in one of two formats: ASCII (with no
terminating NULL), or DNS encoding as specified in RFC1035[2]. If terminating NULL), or DNS encoding as specified in RFC1035[2]. If
the DHCP client wishes to use DNS encoding, it MUST set the the DHCP client wishes to use DNS encoding, it MUST set the third
third-from-rightmost bit in the Flags field (the "E" bit); if it bit in the Flags field (the "E" bit); if it uses ASCII encoding, it
uses ASCII encoding, it MUST clear the "E" bit. MUST clear the "E" bit.
A DHCP client that can only send a single label using ASCII encoding A DHCP client that can only send a single label using ASCII encoding
includes a series of ASCII characters in the Domain Name field, includes a series of ASCII characters in the Domain Name field,
excluding the "." (dot) character. The client SHOULD follow the excluding the "." (dot) character. The client SHOULD follow the
character-set recommendations of RFC1034[1] and RFC1035[2]. A client character-set recommendations of RFC1034[1] and RFC1035[2]. A client
using DNS binary encoding which wants to suggest part of its FQDN using DNS binary encoding which wants to suggest part of its FQDN
MAY send a non-terminal sequence of labels in the Domain Name part MAY send a non-terminal sequence of labels in the Domain Name part
of the option. of the option. Clients and servers should assume that the the name
field contains a fully-qualified name unless one of these
partial-name conditions exists.
5. DHCP Client behavior 5. DHCP Client behavior
The following describes the behavior of a DHCP client that The following describes the behavior of a DHCP client that
implements the Client FQDN option. implements the Client FQDN option.
If a client that owns/maintains its own FQDN wants to be responsible 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 for updating the FQDN to IP address mapping for the FQDN and
address(es) used by the client, then the client MUST include the address(es) used by the client, then the client MUST include the
Client FQDN option in the DHCPREQUEST message originated by the Client FQDN option in the DHCPREQUEST message originated by the
client. A DHCP client MAY choose to include the Client FQDN option client. A DHCP client MAY choose to include the Client FQDN option
in its DISCOVER messages as well as its REQUEST messages. The 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 least-significant ("S") bit in the Flags field in the option MUST be
0. Once the client's DHCP configuration is completed (the client set to 0. Once the client's DHCP configuration is completed (the
receives a DHCPACK message, and successfully completes a final check client receives a DHCPACK message, and successfully completes a
on the parameters passed in the message), the client MAY originate final check on the parameters passed in the message), the client MAY
an update for the A RR (associated with the client's FQDN). The originate an update for the A RR (associated with the client's
update MUST be originated following the procedures described in FQDN). The update MUST be originated following the procedures
RFC2136[5] and "Resolving Name Conflicts"[14]. If the DHCP server described in RFC2136[5] and "Resolving Name Conflicts"[13]. If the
from which the client is requesting a lease includes the FQDN option DHCP server from which the client is requesting a lease includes the
in its ACK message, and if the server sets both the "S" and the "O" FQDN option in its ACK message, and if the server sets both the "S"
bits (the two rightmost bits) in the option's flags field, the DHCP and the "O" bits (the two least-significant bits) in the option's
client MUST NOT initiate an update for the name in the Domain Name flags field, the DHCP client MUST NOT initiate an update for the
field. name in the Domain Name field.
A client can choose to delegate the responsibility for updating the 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 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, client to the server. In order to inform the server of this choice,
the client SHOULD include the Client FQDN option in its DHCPREQUEST the client SHOULD include the Client FQDN option in its DHCPREQUEST
message. The rightmost (or "S") bit in the Flags field in the option message. The least-significant (or "S") bit in the Flags field in
MUST be set to 1. A client which delegates this responsibility MUST the option MUST be set to 1. A client which delegates this
NOT attempt to perform a Dynamic DNS update for the name in the responsibility MUST NOT attempt to perform a DNS update for the name
Domain Name field of the FQDN option. The client MAY supply an FQDN in the Domain Name field of the FQDN option. The client MAY supply
in the Client FQDN option, or it MAY supply a single label (the an FQDN in the Client FQDN option, or it MAY supply a single label
most-specific label), or it MAY leave that field empty as a signal (the most-specific label), or it MAY leave that field empty as a
to the server to generate an FQDN for the client in any manner the signal to the server to generate an FQDN for the client in any
server chooses. manner the server chooses.
Since there is a possibility that the DHCP server may be configured 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 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 to send, the client might find it useful to send the FQDN option in
its DISCOVER messages. If the DHCP server returns different Domain its DISCOVER messages. If the DHCP server returns different Domain
Name data in its OFFER message, the client could use that data in 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 performing its own eventual A RR update, or in forming the FQDN
option that it sends in its REQUEST message. There is no requirement option that it sends in its REQUEST message. There is no requirement
that the client send identical FQDN option data in its DISCOVER and that the client send identical FQDN option data in its DISCOVER and
REQUEST messages. In particular, if a client has sent the FQDN REQUEST messages. In particular, if a client has sent the FQDN
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(either positive or negative) from the server whether the server was (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 able to perform the update. In this case the client MAY use a DNS
query to check whether the mapping is updated. query to check whether the mapping is updated.
A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option. option to 0 when sending the option.
If a client releases its lease prior to the lease expiration time If a client releases its lease prior to the lease expiration time
and the client is responsible for updating its A RR, the client and the client is responsible for updating its A RR, the client
SHOULD delete the A RR (following the procedures described in SHOULD delete the A RR (following the procedures described in
"Resolving Name Conflicts"[14]) associated with the leased address "Resolving Name Conflicts"[13]) associated with the leased address
before sending a DHCP RELEASE message. Similarly, if a client was before sending a DHCP RELEASE message. Similarly, if a client was
responsible for updating its A RR, but is unable to renew its lease, 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 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 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) which it added (because it has lost the use of its DHCP IP address)
should attempt to notify its administrator. should attempt to notify its administrator.
6. DHCP Server behavior 6. DHCP Server behavior
When a server receives a DHCPREQUEST message from a client, if the When a server receives a DHCPREQUEST message from a client, if the
message contains the Client FQDN option, and the server replies to message contains the Client FQDN option, and the server replies to
the message with a DHCPACK message, the server may be configured to the message with a DHCPACK message, the server may be configured to
originate an update for the PTR RR (associated with the address originate an update for the PTR RR (associated with the address
leased to the client). Any such update MUST be originated following leased to the client). Any such update MUST be originated following
the procedures described in "Resolving Name Conflicts"[14]. The the procedures described in "Resolving Name Conflicts"[13]. The
server MAY complete the update before the server sends the DHCPACK server MAY complete the update before the server sends the DHCPACK
message to the client. In this case the RCODE from the update MUST 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 be carried to the client in the RCODE1 field of the Client FQDN
option in the DHCPACK message. Alternatively, the server MAY send option in the DHCPACK message. Alternatively, the server MAY send
the DHCPACK message to the client without waiting for the update to the DHCPACK message to the client without waiting for the update to
be completed. In this case the RCODE1 field of the Client FQDN be completed. In this case the RCODE1 field of the Client FQDN
option in the DHCPACK message MUST be set to 255. The choice option in the DHCPACK message MUST be set to 255. The choice
between the two alternatives is entirely determined by the between the two alternatives is entirely determined by the
configuration of the DHCP server. Servers SHOULD support both configuration of the DHCP server. Servers SHOULD support both
configuration options. configuration options.
skipping to change at page 9, line 13 skipping to change at page 9, line 27
In addition, if the Client FQDN option carried in the DHCPREQUEST 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 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 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 in the option) if it is configured to do so by the site's
administrator, and if it has the necessary credentials. The server administrator, and if it has the necessary credentials. The server
MAY be configured to use the name supplied in the client's FQDN 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 option, or it MAY be configured to modify the supplied name, or
substitute a different name. substitute a different name.
Any such update MUST be originated following the procedures Any such update MUST be originated following the procedures
described in "Resolving Name Conflicts"[14]. The server MAY described in "Resolving Name Conflicts"[13]. The server MAY
originate the update before the server sends the DHCPACK message to originate the update before the server sends the DHCPACK message to
the client. In this case the RCODE from the update [RFC2136] MUST be 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 carried to the client in the RCODE2 field of the Client FQDN option
in the DHCPACK message. Alternatively the server MAY send the in the DHCPACK message. Alternatively the server MAY send the
DHCPACK message to the client without waiting for the update to be DHCPACK message to the client without waiting for the update to be
completed. In this case the RCODE2 field of the Client FQDN option 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 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, 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 if the server intends to perform the DNS update and the client's
REQUEST message included the FQDN option, the server SHOULD include REQUEST message included the FQDN option, the server SHOULD include
skipping to change at page 9, line 35 skipping to change at page 9, line 49
option's Flags field. option's Flags field.
Even if the Client FQDN option carried in the DHCPREQUEST message Even if the Client FQDN option carried in the DHCPREQUEST message
has the "S" bit in its Flags field clear (indicating that the client 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 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 administrator to update the A RR on the client's behalf. A server
which is configured to override the client's preference SHOULD which is configured to override the client's preference SHOULD
include an FQDN option in its ACK message, and MUST set both the "O" 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 and "S" bits in the FQDN option's Flags field. The update MUST be
originated following the procedures described in "Resolving Name originated following the procedures described in "Resolving Name
Conflicts"[14]. The server MAY originate the update before the Conflicts"[13]. The server MAY originate the update before the
server sends the DHCPACK message to the client. In this case 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 RCODE from the update [RFC2136] MUST be carried to the client in the
RCODE2 field of the Client FQDN option in the DHCPACK message. RCODE2 field of the Client FQDN option in the DHCPACK message.
Alternatively, the server MAY send the DHCPACK message to the client Alternatively, the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the without waiting for the update to be completed. In this case the
RCODE2 field of the Client FQDN option in the DHCPACK message MUST 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 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. 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 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 DHCPACK message, the DHCP server SHOULD send its notion of the
complete FQDN for the client in the Domain Name field. The server complete FQDN for the client in the Domain Name field. The server
MAY simply copy the Domain Name field from the Client FQDN option MAY simply copy the Domain Name field from the Client FQDN option
skipping to change at page 9, line 52 skipping to change at page 10, line 18
be set to 255. Whether the DNS update occurs before or after the 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. 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 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 DHCPACK message, the DHCP server SHOULD send its notion of the
complete FQDN for the client in the Domain Name field. The server complete FQDN for the client in the Domain Name field. The server
MAY simply copy the Domain Name field from the Client FQDN option MAY simply copy the Domain Name field from the Client FQDN option
that the client sent to the server in the DHCPREQUEST message. The that the client sent to the server in the DHCPREQUEST message. The
DHCP server MAY be configured to complete or modify the domain name DHCP server MAY be configured to complete or modify the domain name
which a client sent, or it MAY be configured to substitute a which a client sent, or it MAY be configured to substitute a
different name. If the server initiates a DDNS update which is not different name.
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 If the server initiates a DNS update which is not complete until
binary encoding) that the client used in the FQDN option in its after the server has replied to the DHCP client, the server's
interaction with the DNS server may cause the DHCP server to change
the domain name that it associates with the client. This may occur,
for example, if the server detects and resolves a domain-name
conflict. In such cases, the domain name that the server returns to
the dhcp client may change between two dhcp exchanges.
The server MUST use the same encoding format (ASCII or DNS binary
encoding) that the client used in the FQDN option in its
DHCPREQUEST, and MUST set the "E" bit in the option's Flags field DHCPREQUEST, and MUST set the "E" bit in the option's Flags field
accordingly. accordingly.
If a client's DHCPREQUEST message doesn't carry the Client FQDN If a client's DHCPREQUEST message doesn't carry the Client FQDN
option (e.g., the client doesn't implement the Client FQDN option), 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 the server MAY be configured to update either or both of the A and
PTR RRs. The updates MUST be originated following the procedures PTR RRs. The updates MUST be originated following the procedures
described in "Resolving Name Conflicts"[14]. described in "Resolving Name Conflicts"[13].
If a server detects that a lease on an address that the server 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 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 which it added via DNS update. In addition, if the server added an A
an A RR on the client's behalf, the server SHOULD also delete the A RR on the client's behalf, the server SHOULD also delete the A RR.
RR. The deletion MUST follow the procedures described in "Resolving The deletion MUST follow the procedures described in "Resolving Name
Name Conflicts"[14]. Conflicts"[13].
If a server terminates a lease on an address prior to the lease's 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 expiration time, for instance by sending a DHCPNAK to a client, the
server SHOULD delete any PTR RR which it associated with the address server SHOULD delete any PTR RR which it associated with the address
via DNS Dynamic Update. In addition, if the server took via DNS Update. In addition, if the server took responsibility for
responsibility for an A RR, the server SHOULD also delete that A RR. an A RR, the server SHOULD also delete that A RR. The deletion MUST
The deletion MUST follow the procedures described in "Resolving Name follow the procedures described in "Resolving Name Conflicts"[13].
Conflicts"[14].
7. Security Considerations 7. Security Considerations
Unauthenticated updates to the DNS can lead to tremendous confusion, Unauthenticated updates to the DNS can lead to tremendous confusion,
through malicious attack or through inadvertent misconfiguration. through malicious attack or through inadvertent misconfiguration.
Administrators should be wary of permitting unsecured DNS updates to Administrators should be wary of permitting unsecured DNS updates to
zones which are exposed to the global Internet. Both DHCP clients zones which are exposed to the global Internet. Both DHCP clients
and servers SHOULD use some form of update request origin and servers SHOULD use some form of update request origin
authentication procedure (e.g., Simple Secure DNS Update[11]) when authentication procedure (e.g., Secure DNS Dynamic Update[11]) when
performing DNS updates. performing DNS updates.
Whether a DHCP client may be responsible for updating an FQDN to IP Whether a DHCP client may be responsible for updating an FQDN to IP
address mapping or whether this is the responsibility of the DHCP address mapping or whether this is the responsibility of the DHCP
server is a site-local matter. The choice between the two server is a site-local matter. The choice between the two
alternatives may be based on the security model that is used with alternatives may be based on the security model that is used with
the Dynamic DNS Update protocol (e.g., only a client may have the DNS update protocol (e.g., only a client may have sufficient
sufficient credentials to perform updates to the FQDN to IP address credentials to perform updates to the FQDN to IP address mapping for
mapping for its FQDN). its FQDN).
Whether a DHCP server is always responsible for updating the FQDN to Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping), IP address mapping (in addition to updating the IP to FQDN mapping),
regardless of the wishes of an individual DHCP client, is also a regardless of the wishes of an individual DHCP client, is also a
site-local matter. The choice between the two alternatives may be site-local matter. The choice between the two alternatives may be
based on the security model that is being used with dynamic DNS based on the security model that is being used with DNS updates. In
updates. In cases where a DHCP server is performing DNS updates on cases where a DHCP server is performing DNS updates on behalf of a
behalf of a client, the DHCP server should be sure of the DNS name client, the DHCP server should be sure of the DNS name to use for
to use for the client, and of the identity of the client. the client, and of the identity of the client.
Currently, it is difficult for DHCP servers to develop much Currently, it is difficult for DHCP servers to develop much
confidence in the identities of its clients, given the absence of confidence in the identities of its clients, given the absence of
entity authentication from the DHCP protocol itself. There are many entity authentication from the DHCP protocol itself. There are many
ways for a DHCP server to develop a DNS name to use for a client, ways for a DHCP server to develop a DNS name to use for a client,
but only in certain relatively unusual circumstances will the DHCP but only in certain relatively unusual circumstances will the DHCP
server know for certain the identity of the client. If DHCP server know for certain the identity of the client. If DHCP
Authentication[10] becomes widely deployed this may become more Authentication[10] becomes widely deployed this may become more
customary. customary.
skipping to change at page 12, line 18 skipping to change at page 12, line 40
[6] Bradner, S., "Key words for use in RFCs to Indicate [6] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997. Requirement Levels", RFC 2119, March 1997.
[7] Eastlake, D., "Domain Name System Security Extensions", RFC [7] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999. 2535, March 1999.
[8] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, [8] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999. August 1999.
[9] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, [9] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG) "Secret Key Transaction Authentication for DNS (TSIG)", RFC
(draft-ietf-dnsext-tsig-*)", July 1999. 2845, May 2000.
[10] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages [10] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages
(draft-ietf-dhc-authentication-*)", June 1999. (draft-ietf-dhc-authentication-*)", June 1999.
[11] Wellington, B., "Simple Secure DNS Dynamic Updates [11] Wellington, B., "Secure DNS Dynamic Update", RFC 3007,
(draft-ietf-dnsext-simple-secure-update-*)", June 1999. November 2000.
[12] Stapp, M., Gustafsson, A. and T. Lemon, "A DNS RR for encoding [12] Stapp, M., Gustafsson, A. and T. Lemon, "A DNS RR for encoding
DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", July 2000. DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", July 2000.
[13] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, [13] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP
April 1992.
[14] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP
Clients (draft-ietf-dhc-ddns-resolution-*.txt)", July 2000. Clients (draft-ietf-dhc-ddns-resolution-*.txt)", July 2000.
[15] Stapp, M., "Recommended Procedures for DHCP Updates of DNS RRs
(draft-ietf-dhc-ddns-procedure-*.txt)", July 2000.
Authors' Addresses Authors' Addresses
Mark Stapp Mark Stapp
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Dr. 250 Apollo Dr.
Chelmsford, MA 01824 Chelmsford, MA 01824
US USA
Phone: 978.244.8498 Phone: 978.244.8498
EMail: mjs@cisco.com EMail: mjs@cisco.com
Yakov Rekhter Yakov Rekhter
Cisco Systems, Inc. Cisco Systems, Inc.
170 Tasman Dr. 170 Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
US USA
Phone: 914.235.2128 Phone: 914.235.2128
EMail: yakov@cisco.com EMail: yakov@cisco.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
 End of changes. 

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