DHC M. Stapp
Internet-Draft B. Volz
Expires: January 14, 2005 Cisco Systems, Inc.
Y. Rekhter
Juniper Networks
Jul 16, 2004
The DHCP Client FQDN Option
<draft-ietf-dhc-fqdn-option-07.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
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RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies a DHCP for IPv4, DHCPv4, option which can be
used to exchange information about a DHCPv4 client's fully-qualified
domain name and about responsibility for updating the DNS RR related
to the client's address assignment.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Models of Operation . . . . . . . . . . . . . . . . . . . . . 3
4. The Client FQDN Option . . . . . . . . . . . . . . . . . . . . 4
4.1 The Flags Field . . . . . . . . . . . . . . . . . . . . . 5
4.2 The RCODE Fields . . . . . . . . . . . . . . . . . . . . . 6
4.3 The Domain Name Field . . . . . . . . . . . . . . . . . . 6
4.3.1 Deprecated ASCII Encoding . . . . . . . . . . . . . . 7
5. DHCP Client Behavior . . . . . . . . . . . . . . . . . . . . . 7
6. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . . 9
7. DNS Update Conflicts . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
10.2 Informative References . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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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 [1].
2. Introduction
DNS ([2], [3]) maintains (among other things) the information about
mapping between hosts' Fully Qualified Domain Names (FQDNs) [6] and
IP addresses assigned to the hosts. The information is maintained in
two types of Resource Records (RRs): A and PTR. The DNS update
specification ([4]) describes a mechanism that enables DNS
information to be updated over a network.
The Dynamic Host Configuration Protocol for IPv4 (DHCPv4 or just DHCP
in this document) [5] provides a mechanism by which a host (a DHCP
client) can acquire certain configuration information, along with its
address. This document specifies a DHCP option, the Client FQDN
option, which can be used by DHCP clients and servers to exchange
information about the client's fully-qualified domain name for an
address and who has the responsibility for updating the DNS with the
associated A and PTR RRs.
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 DNS
update transactions may 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, in practice not all permutations make sense.
The DHCP Client FQDN option is intended to operate in the following
two cases:
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
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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 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. The 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 Client 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 Client FQDN option, setting appropriate Flags
values, in both their DISCOVER and REQUEST messages. If a client
sends the Client 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.
The Format of the Client FQDN Option:
Code Len Flags RCODE1 RCODE2 Domain Name
+------+------+------+------+------+------+--
| 81 | n | | | | ...
+------+------+------+------+------+------+--
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4.1 The Flags Field
The Format of the Flags Field:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |N|E|O|S|
+-+-+-+-+-+-+-+-+
When a DHCP client sends the Client FQDN option in its DHCPDISCOVER
and/or DHCPREQUEST messages, it sets the "S" bit 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 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 Client FQDN option has set the
"S" bit, it sets both the "O" bit and the "S" bit, and sends the
Client FQDN option in its DHCPOFFER and/or DHCPACK messages.
The data in the Domain Name field SHOULD appear in DNS-style binary
encoding (without compression, of course), as described in RFC 1035
[3]. A client which sends the Client FQDN option SHOULD use this
encoding. The client MUST set the "E" bit when the data in the
Domain Name field is in DNS binary encoding. If a server receives a
Client FQDN option from a client, and intends to include a Client
FQDN option in its reply, it MUST use the same encoding that the
client used, and MUST set the "E" bit accordingly.
Server implementers should note that earlier draft versions of this
specification permitted an ASCII encoding of the domain name.
Clients which implemented this encoding were deployed before this
specification was completed. Server implementers which need to
support these clients should note the section on the deprecated ASCII
encoding (Section 4.3.1).
A client MAY set the "N" bit 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 desired 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 Client 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
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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
assignment. DHCP clients and servers which send the Client FQDN
option MUST set the MBZ bits to 0, and they MUST ignore these bits.
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 DNS updates it
has performed. The server may also use these fields to indicate
whether it has attempted such an update before sending the DHCPACK
message. Each of these fields is one byte long.
Implementers should note that EDNS0 describes a mechanism for
extending the length of a DNS RCODE to 12 bits. EDNS0 is specified
in RFC 2671 [7]. Only the least-significant 8 bits of the RCODE from
a DNS update will be carried in the Client FQDN option. This
provides enough number space to accommodate the RCODEs defined in the
DNS update specification.
4.3 The Domain Name Field
The Domain Name part of the option carries all or part of the FQDN of
a DHCP client. The data in the Domain Name field SHOULD appear in
uncompressed DNS encoding as specified in RFC 1035 [3]. If the DHCP
client uses DNS encoding, it MUST set the third bit in the Flags
field (the "E" bit). In order to determine whether a name has
changed between message exchanges, an unambiguous canonical form is
necessary. Eventually, the IETF IDN Working Group is expected to
produce a standard canonicalization specification, and this
specification may be updated to include its standard. Until that
time, servers and clients should be sensitive to canonicalization
when comparing names in the Domain Name field and the name
canonicalization defined in RFC 2535 [10] MAY be used.
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 name that is not
fully-qualified, indicating that it knows part of the name but does
not necessarily know the zone in which the name is to be embedded. A
client which wants to convey part of its FQDN sends a non-terminal
sequence of labels in the Domain Name part of the option. Clients
and servers should assume that the name field contains a
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fully-qualified name unless this partial-name format exists.
4.3.1 Deprecated ASCII Encoding
The DNS encoding specified above MUST be supported by DHCP servers.
However, a substantial population of clients implemented an earlier
version of this specification, which permitted an ASCII encoding of
the Domain Name field. Server implementations should be aware that
clients which send the Client FQDN option with the "E" bit clear are
using an ASCII version of the Domain Name field. Servers MAY be
prepared to return an ASCII encoded version of the Domain Name field
to such clients. The use of ASCII encoding in this option should be
considered deprecated.
A DHCP client which used ASCII encoding was permitted to suggest a
single label if it was not configured with a fully-qualified name.
Such clients send a single label as a series of ASCII characters in
the Domain Name field, excluding the "." (dot) character. Such
clients SHOULD follow the character-set recommendations of RFC 1034
[2] and RFC 1035 [3].
Server implementers should also be aware that some client software
may attempt to use UTF-8 [9] character encoding. This information is
included for informational purposes only: this specification does not
require any support for UTF-8.
5. DHCP Client Behavior
The following describes the behavior of a DHCP client that implements
the Client FQDN option.
Other DHCP options may carry data that is related to the Domain-Name
part of the Client 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 Client FQDN option in their messages
MUST NOT also send the Host-Name option. Clients which receive both
the Host-Name option and the Client FQDN option from a server SHOULD
prefer Client FQDN option data. Servers will be asked in Section 6
to ignore the Host-Name option in client messages which include 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 "S"
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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). If the DHCP server
from which the client is requesting a lease includes the Client 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 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 "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 DNS update for the name in the Domain Name field of the
Client 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 Client 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 Client FQDN option that it sends in its REQUEST message. There
is no requirement that the client send identical Client FQDN option
data in its DISCOVER and REQUEST messages. In particular, if a
client has sent the Client 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 Client FQDN option
when it communicates with the server again. This may allow the DHCP
server to update the name associated with the PTR record, and, if the
server updated the A record representing the client, to delete that
record and attempt an update for the client's current domain name.
A client that delegates the responsibility for updating the FQDN to
IP address mapping to a server might not receive any indication
(either positive or negative) from the server whether the server was
able to perform the update. In this case the client MAY use a DNS
query to check whether the mapping is updated.
A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option.
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If a client releases its lease prior to the lease expiration time and
the client is responsible for updating its A RR, the client SHOULD
delete the A RR associated with the leased address before sending a
DHCP RELEASE message. Similarly, if a client was responsible for
updating its A RR, but is unable to renew its lease, the client
SHOULD attempt to delete the A RR before its lease expires. A DHCP
client which has not been able to delete an A RR which it added
(because it has lost the use of its DHCP IP address) should attempt
to notify its administrator, perhaps by emitting a log message.
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). 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 Client FQDN
option, or it MAY be configured to modify the supplied name, or
substitute a different name.
The server MAY originate the update before the server sends the
DHCPACK message to the client. In this case the RCODE from the
update RFC 2136 [4] 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 a matter of the DHCP
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server's configuration. In either case, if the server intends to
perform the DNS update and the client's REQUEST message included the
Client FQDN option, the server SHOULD include the Client FQDN option
in its ACK message. If the server includes the Client 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 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 a Client FQDN option in its ACK message, and MUST set both
the "O" and "S" bits in the Client FQDN option's Flags field. The
server MAY originate the update before the server sends the DHCPACK
message to the client. In this case the RCODE from the update RFC
2136 [4] 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 DNS update that is not complete until 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 Client 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),
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the server MAY be configured to update either or both of the A and
PTR RRs.
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 DNS update. In addition, if the server added an A RR on
the client's behalf, the server SHOULD also delete the A RR.
If a server terminates a lease on an address prior to the lease's
expiration time, for instance by sending a DHCPNAK to a client, the
server SHOULD delete any PTR RR which it associated with the address
via DNS Update. In addition, if the server took responsibility for
an A RR, the server SHOULD also delete that A RR.
7. DNS Update Conflicts
This document does not resolve how a DHCP client or server prevent
name conflicts. This document addresses only how a DHCP client and
server negotiate who will perform the DNS updates and the fully
qualified domain name requested or used.
Implementers of this work will need to consider how name conflicts
will be prevented. It may be that the DNS updater must hold a
security token in order to successfully perform DNS updates on a
specific name, in which case name conflicts can only occur if
multiple clients are given a security token for that name. Or, the
fully qualified domains may be based on the specific address bound to
a client and in this case conflicts should not occur. However,
without this level of security in the DNS system or use of
non-conflicting names, other techniques need to be developed. This
is an area for future work (see "Resolving Name Conflicts" [12]).
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 permitting unsecured DNS updates to
zones which are exposed to the global Internet. Both DHCP clients
and servers SHOULD use some form of update request origin
authentication procedure (e.g., Secure DNS Dynamic Update [11]) when
performing DNS updates.
Whether a DHCP client may be responsible for updating an FQDN to IP
address mapping or whether this is the responsibility of the DHCP
server is a site-local matter. The choice between the two
alternatives may be based on the security model that is used with the
DNS update protocol (e.g., only a client may have sufficient
credentials to perform updates to the FQDN to IP address mapping for
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its FQDN).
Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping),
regardless of the wishes of an individual DHCP client, is also a
site-local matter. The choice between the two alternatives may be
based on the security model that is being used with DNS updates. In
cases where a DHCP server is performing DNS updates on behalf of a
client, the DHCP server should be sure of the DNS name to use for the
client, and of the identity of the client.
Currently, it is difficult for DHCP servers to develop much
confidence in the identities of its clients, given the absence of
entity authentication from the DHCP protocol itself. There are many
ways for a DHCP server to develop a DNS name to use for a client, but
only in certain relatively unusual circumstances will the DHCP server
know for certain the identity of the client. If DHCP Authentication
[13] 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) 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 be configured to trust that the user
authentication and authorization procedure of the remote access
server was sufficient, and would therefore trust the client
identification encoded within the DHCP client-id.
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.
10. References
10.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Mockapetris, P., "Domain names - concepts and facilities", STD
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13, RFC 1034, November 1987.
[3] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[4] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April
1997.
[5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
10.2 Informative References
[6] Marine, A., Reynolds, J. and G. Malkin, "FYI on Questions and
Answers - Answers to Commonly asked "New Internet User"
Questions", RFC 1594, March 1994.
[7] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[8] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[9] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[10] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[11] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[12] Stapp, M. and B. Volz, "Resolution of DNS Name Conflicts Among
DHCP Clients (draft-ietf-dhc-ddns-resolution-*.txt)", July
2004.
[13] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
Stapp, et al. Expires January 14, 2005 [Page 13]
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Authors' Addresses
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.1535
EMail: mjs@cisco.com
Bernie Volz
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.0382
EMail: volz@cisco.com
Yakov Rekhter
Juniper Networks
1194 North Mathilda Avenue
Sunnyvale, CA 94089
USA
Phone: 408.745.2000
EMail: yakov@juniper.net
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Internet-Draft The DHCP Client FQDN Option Jul 2004
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Stapp, et al. Expires January 14, 2005 [Page 15]
