draft-ietf-dhc-fqdn-option-01.txt   draft-ietf-dhc-fqdn-option-02.txt 
DHC Working Group M. Stapp DHC Working Group M. Stapp
Internet-Draft Y. Rekhter Internet-Draft Cisco Systems, Inc.
Expires: August 31, 2001 Cisco Systems, Inc. Expires: January 17, 2002 Y. Rekhter
March 2, 2001 Juniper Networks
July 19, 2001
The DHCP Client FQDN Option The DHCP Client FQDN Option
<draft-ietf-dhc-fqdn-option-01.txt> <draft-ietf-dhc-fqdn-option-02.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as other groups may also distribute working documents as
Internet-Drafts. Internet-Drafts.
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months and may be updated, replaced, or obsoleted by other documents months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference at any time. It is inappropriate to use Internet-Drafts as reference
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
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 31, 2001. This Internet-Draft will expire on January 17, 2002.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2001). 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
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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 . . . . . . . . . . . . . . . . . . . . 4 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 . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13 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[1].
2. Introduction 2. Introduction
DNS (RFC1034[1], RFC1035[2]) maintains (among other things) the DNS (RFC1034[2], RFC1035[3]) 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)[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 DNS PTR RR contains mapping from an IP address to a FQDN. The DNS
update specification (RFC2136[5]) describes a mechanism that enables update specification (RFC2136[5]) describes a mechanism that 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[6] provides a mechanism by which a host (a DHCP client) can
can acquire certain configuration information, along with its IP 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 DNS host's FQDN and its IP address(es) (A and PTR RRs). Thus DNS
information for a DHCP client may not exist or 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 DNS Update protocol can be used to maintain consistency between The DNS Update protocol can be used to maintain consistency between
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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 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. Another document, where clients do not supply domain names. Another document,
"Resolving Name Conflicts"[13], defines a protocol for arbitrating "Resolving Name Conflicts"[7], defines a protocol for establishing
conflicts when collisions occur in the use of FQDNs by DHCP clients. policy and arbitrating conflicts when collisions occur in the use of
FQDNs by 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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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[3]. 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 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 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 we mentioned in Section 3, we believe that in general the DHCP
server will be maintaining DNS PTR records on behalf of clients. server will be maintaining DNS PTR records on behalf of clients.
However, there may be deployments in which clients are configured to However, there may be deployments in which clients are configured to
perform all necessary DNS updates. The server MAY be configured to perform all desired DNS updates. The server MAY be configured to
honor this configuration. If the server has been configured to honor 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 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 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 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 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 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 "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 server was not configured to honor the client's suggestion, and that
the server may attempt to perform DNS updates on its behalf. the server may attempt to perform DNS updates on its behalf.
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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[3]. If
the DHCP client wishes to use DNS encoding, it MUST set the third the DHCP client wishes to use DNS encoding, it MUST set the third
bit in the Flags field (the "E" bit); if it uses ASCII encoding, it bit in the Flags field (the "E" bit); if 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[2] and RFC1035[3]. 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. Clients and servers should assume that the the name of the option. Clients and servers should assume that the the name
field contains a fully-qualified name unless one of these field contains a fully-qualified name unless one of these
partial-name conditions exists. 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.
Other DHCP options may carry data that is related to the Domain-Name
part of the FQDN option. The Host-Name option, for example, contains
an ASCII string representation of the client's host-name. In
general, a client should not need to send redundant data, and
therefore clients which send the FQDN option in their messages
SHOULD NOT also send the Host-Name option. Clients which receive
both the Host-Name option and the FQDN option from a server SHOULD
prefer FQDN option data. Servers will be asked in Section 6 to
ignore the Host-Name option in client messages which include the
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
least-significant ("S") bit in the Flags field in the option MUST be least-significant ("S") bit in the Flags field in the option MUST be
set to 0. Once the client's DHCP configuration is completed (the set to 0. Once the client's DHCP configuration is completed (the
client receives a DHCPACK message, and successfully completes a client receives a DHCPACK message, and successfully completes a
final check on the parameters passed in the message), the client MAY final check on the parameters passed in the message), the client MAY
originate an update for the A RR (associated with the client's originate an update for the A RR (associated with the client's
FQDN). The update MUST be originated following the procedures FQDN). The update SHOULD be originated following the procedures
described in RFC2136[5] and "Resolving Name Conflicts"[13]. If the described in RFC2136[5] and "Resolving Name Conflicts"[7]. If the
DHCP server from which the client is requesting a lease includes 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" FQDN option in its ACK message, and if the server sets both the "S"
and the "O" bits (the two least-significant bits) in the option's and the "O" bits (the two least-significant bits) in the option's
flags field, the DHCP client MUST NOT initiate an update for the flags field, the DHCP client MUST NOT initiate an update for the
name in the Domain Name 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
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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"[13]) associated with the leased address "Resolving Name Conflicts"[7]) 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, perhaps by emitting a
log message.
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 SHOULD be originated
the procedures described in "Resolving Name Conflicts"[13]. The following the procedures described in "Resolving Name Conflicts"[7].
server MAY complete the update before the server sends the DHCPACK The server MAY complete the update before the server sends the
message to the client. In this case the RCODE from the update MUST DHCPACK message to the client. In this case the RCODE from the
be carried to the client in the RCODE1 field of the Client FQDN update MUST be carried to the client in the RCODE1 field of the
option in the DHCPACK message. Alternatively, the server MAY send Client FQDN option in the DHCPACK message. Alternatively, the server
the DHCPACK message to the client without waiting for the update to MAY send the DHCPACK message to the client without waiting for the
be completed. In this case the RCODE1 field of the Client FQDN update to be completed. In this case the RCODE1 field of the Client
option in the DHCPACK message MUST be set to 255. The choice FQDN 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.
When a server receives a DHCPREQUEST message containing the Client When a server receives a DHCPREQUEST message containing the Client
FQDN option, the server MUST ignore the values carried in the RCODE1 FQDN option, the server MUST ignore the values carried in the RCODE1
and RCODE2 fields of the option. and RCODE2 fields of the option.
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 SHOULD be originated following the procedures
described in "Resolving Name Conflicts"[13]. The server MAY described in "Resolving Name Conflicts"[7]. The server MAY originate
originate the update before the server sends the DHCPACK message to the update before the server sends the DHCPACK message to the
the client. In this case the RCODE from the update [RFC2136] MUST be 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 a matter of the DHCP server's
if the server intends to perform the DNS update and the client's configuration. In either case, if the server intends to perform the
REQUEST message included the FQDN option, the server SHOULD include DNS update and the client's REQUEST message included the FQDN
the FQDN option in its ACK message, and MUST set the "S" bit in the option, the server SHOULD include the FQDN option in its ACK
option's Flags field. message. If the server includes the FQDN option, it MUST set the "S"
bit in the option's Flags field and MUST clear the "O" bit.
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 SHOULD be
originated following the procedures described in "Resolving Name originated following the procedures described in "Resolving Name
Conflicts"[13]. The server MAY originate the update before the Conflicts"[7]. The server MAY originate the update before the server
server sends the DHCPACK message to the client. In this case the sends the DHCPACK message to the client. In this case the RCODE from
RCODE from the update [RFC2136] MUST be carried to the client in the the update [RFC2136] MUST be carried to the client in the RCODE2
RCODE2 field of the Client FQDN option in the DHCPACK message. 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
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the dhcp client may change between two dhcp exchanges. the dhcp client may change between two dhcp exchanges.
The server MUST use the same encoding format (ASCII or DNS binary The server MUST use the same encoding format (ASCII or DNS binary
encoding) that the client used in the FQDN option in its 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 SHOULD be originated following the procedures
described in "Resolving Name Conflicts"[13]. described in "Resolving Name Conflicts"[7].
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 DNS update. In addition, if the server added an A which it added via DNS update. In addition, if the server added an A
RR on the client's behalf, the server SHOULD also delete the A RR. RR on the client's behalf, the server SHOULD also delete the A RR.
The deletion MUST follow the procedures described in "Resolving Name The deletion SHOULD follow the procedures described in "Resolving
Conflicts"[13]. Name Conflicts"[7].
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 Update. In addition, if the server took responsibility for via DNS Update. In addition, if the server took responsibility for
an A RR, the server SHOULD also delete that A RR. The deletion MUST an A RR, the server SHOULD also delete that A RR. The deletion
follow the procedures described in "Resolving Name Conflicts"[13]. SHOULD follow the procedures described in "Resolving Name
Conflicts"[7].
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., Secure DNS Dynamic Update[11]) when authentication procedure (e.g., Secure DNS Dynamic Update[10]) 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 DNS update protocol (e.g., only a client may have sufficient the DNS update protocol (e.g., only a client may have sufficient
credentials to perform updates to the FQDN to IP address mapping for credentials to perform updates to the FQDN to IP address mapping for
its FQDN). its FQDN).
skipping to change at page 11, line 38 skipping to change at page 11, line 49
cases where a DHCP server is performing DNS updates on behalf of a cases where a DHCP server is performing DNS updates on behalf of a
client, the DHCP server should be sure of the DNS name to use for client, the DHCP server should be sure of the DNS name 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[11] becomes widely deployed this may become more
customary. customary.
One example of a situation which offers some extra assurances is one One example of a situation which offers some extra assurances is one
where the DHCP client is connected to a network through an MCNS where the DHCP client is connected to a network through an MCNS
cable modem, and the CMTS (head-end) ensures that MAC address cable modem, and the CMTS (head-end) ensures that MAC address
spoofing simply does not occur. Another example of a configuration spoofing simply does not occur. Another example of a configuration
that might be trusted is one where clients obtain network access via that might be trusted is one where clients obtain network access via
a network access server using PPP. The NAS itself might be obtaining a network access server using PPP. The NAS itself might be obtaining
IP addresses via DHCP, encoding a client identification into the IP addresses via DHCP, encoding a client identification into the
DHCP client-id option. In this case, the network access server as DHCP client-id option. In this case, the network access server as
skipping to change at page 12, line 14 skipping to change at page 12, line 26
8. Acknowledgements 8. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter
Ford, Edie Gunter, Andreas Gustafsson, R. Barr Hibbs, Kim Kinnear, Ford, Edie Gunter, Andreas Gustafsson, R. Barr Hibbs, Kim Kinnear,
Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Josh Littlefield, Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Josh Littlefield,
Michael Patton, and Glenn Stump for their review and comments. Michael Patton, and Glenn Stump for their review and comments.
References References
[1] Mockapetris, P., "Domain names - Concepts and Facilities", RFC [1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[2] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
1034, Nov 1987. 1034, Nov 1987.
[2] Mockapetris, P., "Domain names - Implementation and [3] Mockapetris, P., "Domain names - Implementation and
Specification", RFC 1035, Nov 1987. 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 [4] Marine, A., Reynolds, J. and G. Malkin, "FYI on Questions and
Answers to Commonly asked ``New Internet User'' Questions", Answers to Commonly asked ``New Internet User'' Questions",
RFC 1594, March 1994. RFC 1594, March 1994.
[5] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic [5] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System", RFC 2136, April 1997. Updates in the Domain Name System", RFC 2136, April 1997.
[6] Bradner, S., "Key words for use in RFCs to Indicate [6] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
Requirement Levels", RFC 2119, March 1997. March 1997.
[7] Eastlake, D., "Domain Name System Security Extensions", RFC [7] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP
2535, March 1999. Clients (draft-ietf-dhc-ddns-resolution-*.txt)", July 2000.
[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)", RFC "Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000. 2845, May 2000.
[10] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages [10] Wellington, B., "Secure DNS Dynamic Update", RFC 3007,
(draft-ietf-dhc-authentication-*)", June 1999.
[11] Wellington, B., "Secure DNS Dynamic Update", RFC 3007,
November 2000. November 2000.
[12] Stapp, M., Gustafsson, A. and T. Lemon, "A DNS RR for encoding [11] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages
DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", July 2000. (draft-ietf-dhc-authentication-*)", June 1999.
[13] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP
Clients (draft-ietf-dhc-ddns-resolution-*.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
USA USA
Phone: 978.244.8498 Phone: 978.244.8498
EMail: mjs@cisco.com EMail: mjs@cisco.com
Yakov Rekhter Yakov Rekhter
Cisco Systems, Inc. Juniper Networks
170 Tasman Dr. 1194 North Mathilda Avenue
San Jose, CA 95134 Sunnyvale, CA 94089
USA USA
Phone: 914.235.2128 Phone: 408.745.2000
EMail: yakov@cisco.com EMail: yakov@juniper.net
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2001). 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
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