DHC Working Group M. Stapp
Internet-Draft Y. Rekhter
Expires: January 12, 2001 Cisco Systems, Inc.
July 14, 2000
The DHCP Client FQDN Option
<draft-ietf-dhc-fqdn-option-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on January 12, 2001.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
DHCP provides a powerful mechanism for IP host configuration.
However, the configuration capability provided by DHCP does not
include updating DNS, and specifically updating the name to address
and address to name mappings maintained in the DNS.
This document specifies a DHCP option which can be used to exchange
information about a DHCP client's fully-qualified domain name, or
'FQDN'.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Models of Operation . . . . . . . . . . . . . . . . . . . . . 3
4. The Client FQDN Option . . . . . . . . . . . . . . . . . . . . 5
4.1 The Flags Field . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 The RCODE Fields . . . . . . . . . . . . . . . . . . . . . . . 6
4.3 The Domain Name Field . . . . . . . . . . . . . . . . . . . . 6
5. DHCP Client behavior . . . . . . . . . . . . . . . . . . . . . 7
6. DHCP Server behavior . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14
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1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119[6].
2. Introduction
DNS (RFC1034[1], RFC1035[2]) maintains (among other things) the
information about mapping between hosts' Fully Qualified Domain
Names (FQDNs) RFC1594[4] and IP addresses assigned to the hosts. The
information is maintained in two types of Resource Records (RRs): A
and PTR. The A RR contains mapping from a FQDN to an IP address; the
PTR RR contains mapping from an IP address to a FQDN. The Dynamic
DNS Updates specification (RFC2136[5]) describes a mechanism that
enables DNS information to be updated over a network.
DHCP RFC2131[3] provides a mechanism by which a host (a DHCP client)
can acquire certain configuration information, along with its IP
address(es). However, DHCP does not provide any mechanisms to update
the DNS RRs that contain the information about mapping between the
host's FQDN and its IP address(es) (A and PTR RRs). Thus the
information maintained by DNS for a DHCP client may be incorrect - a
host (the client) could acquire its address by using DHCP, but the A
RR for the host's FQDN wouldn't reflect the address that the host
acquired, and the PTR RR for the acquired address wouldn't reflect
the host's FQDN.
The Dynamic DNS Update protocol can be used to maintain consistency
between the information stored in the A and PTR RRs and the actual
address assignment done via DHCP. When a host with a particular FQDN
acquires its IP address via DHCP, the A RR associated with the
host's FQDN would be updated (by using the Dynamic DNS Updates
protocol) to reflect the new address. Likewise, when an IP address
is assigned to a host with a particular FQDN, the PTR RR associated
with this address would be updated (using the Dynamic DNS Updates
protocol) to reflect the new FQDN.
Although this document refers to the A and PTR DNS record types and
to DHCP assignment of IPv4 addresses, the same procedures and
requirements apply for updates to the analogous RR types that are
used when clients are assigned IPv6 addresses via DHCPv6.
3. Models of Operation
When a DHCP client acquires a new address, a site's administrator
may desire that one or both of the A RR for the client's FQDN and
the PTR RR for the acquired address be updated. Therefore, two
separate Dynamic DNS Update transactions occur. Acquiring an address
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via DHCP involves two entities: a DHCP client and a DHCP server. In
principle each of these entities could perform none, one, or both of
the transactions. However, in practice not all permutations make
sense. 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 domain names. In all cases, the only reasonable place for
the authority over the PTR RRs associated with the address is in the
DHCP server that allocates the address.
In any case, whether a site permits all, some, or no DHCP servers
and clients to perform DNS updates into the zones which it controls
is entirely a matter of local administrative policy. This document
does not require any specific administrative policy, and does not
propose one. The range of possible policies is very broad, from
sites where only the DHCP servers have been given credentials that
the DNS servers will accept, to sites where each individual DHCP
client has been configured with credentials which allow the client
to modify its own domain name. Compliant implementations MAY support
some or all of these possibilities. Furthermore, this specification
applies only to DHCP client and server processes: it does not apply
to other processes which initiate dynamic DNS updates.
This document describes a new DHCP option which a client can use to
convey all or part of its domain name to a DHCP server.
Site-specific policy determines whether DHCP servers use the names
that clients offer or not, and what DHCP servers may do in cases
where clients do not supply domain names. Two other documents,
"Resolving Name Conflicts"[14] and "Update Procedures"[15], provide
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.
"Update Procedures"[15] recommends a procedure which may be used to
update DNS RRs for DHCP clients.
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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 FQDN Option
also contains Flags and RCode fields which DHCP servers can use to
convey information about DNS updates to clients.
Clients MAY send the FQDN option, setting appropriate Flags values,
in both their DISCOVER and REQUEST messages. If a client sends the
FQDN option in its DISCOVER message, it MUST send the option in
subsequent REQUEST messages.
The code for this option is 81. Its minimum length is 4.
The Format of the FQDN Option:
Code Len Flags RCODE1 RCODE2 Domain Name
+------+------+------+------+------+------+--
| 81 | n | | | | ...
+------+------+------+------+------+------+--
4.1 The Flags Field
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |E|O|S|
+-+-+-+-+-+-+-+-+
When a DHCP client sends the FQDN option in its DHCPDISCOVER and/or
DHCPREQUEST messages, it sets the right-most bit (labelled "S") to
indicate that it will not perform any Dynamic DNS updates, and that
it expects the DHCP server to perform any FQDN-to-IP (the A RR) DNS
update on its behalf. If this bit is clear, the client indicates
that it intends to maintain its own FQDN-to-IP mapping update.
If a DHCP server intends to take responsibility for the A RR update
whether or not the client sending the FQDN option has set the "S"
bit, it sets both the "O" bit and the "S" bit, and sends the FQDN
option in its DHCPOFFER and/or DHCPACK messages.
The data in the Domain Name field may appear in one of two formats:
ASCII, or DNS-style binary encoding (without compression, of
course), as described in RFC1035[2]. A client which sends the FQDN
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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
option from a client, and intends to include an FQDN option in its
reply, it MUST use the same encoding that the client used. The DNS
encoding is recommended. The use of ASCII-encoded domain-names is
fragile, and the use of ASCII encoding in this option should be
considered deprecated.
The remaining bits in the Flags field are reserved for future
assignment. DHCP clients and servers which send the FQDN option MUST
set the MBZ bits to 0, and they MUST ignore values in the part of
the field labelled "MBZ".
4.2 The RCODE Fields
The RCODE1 and RCODE2 fields are used by a DHCP server to indicate
to a DHCP client the Response Code from any A or PTR RR Dynamic DNS
Updates it has performed. The server 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.
Implementors should note that EDNS0 describes a mechanism for
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
Dynamic DNS Update will be carried in the Client FQDN DHCP Option.
This provides enough number space to accomodate the RCODEs defined
in the Dynamic 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. A client may be configured with a fully-qualified
domain name, or with a partial name that is not fully-qualified. If
a client knows only part of its name, it MAY send a single label,
indicating that it knows part of the name but does not necessarily
know the zone in which the name is to be embedded. The data in the
Domain Name field may appear in one of two formats: ASCII (with no
terminating NULL), or DNS encoding as specified in RFC1035[2]. If
the DHCP client wishes to use DNS encoding, it MUST set the
third-from-rightmost bit in the Flags field (the "E" bit); if it
uses ASCII encoding, it MUST clear the "E" bit.
A DHCP client that can only send a single label using ASCII encoding
includes a series of ASCII characters in the Domain Name field,
excluding the "." (dot) character. The client SHOULD follow the
character-set recommendations of RFC1034[1] and RFC1035[2]. A client
using DNS binary encoding which wants to suggest part of its FQDN
MAY send a non-terminal sequence of labels in the Domain Name part
of the option.
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5. DHCP Client behavior
The following describes the behavior of a DHCP client that
implements the Client FQDN option.
If a client that owns/maintains its own FQDN wants to be responsible
for updating the FQDN to IP address mapping for the FQDN and
address(es) used by the client, then the client MUST include the
Client FQDN option in the DHCPREQUEST message originated by the
client. A DHCP client MAY choose to include the Client FQDN option
in its DISCOVER messages as well as its REQUEST messages. The
rightmost ("S") bit in the Flags field in the option MUST be set to
0. Once the client's DHCP configuration is completed (the client
receives a DHCPACK message, and successfully completes a final check
on the parameters passed in the message), the client MAY originate
an update for the A RR (associated with the client's FQDN). The
update MUST be originated following the procedures described in
RFC2136[5] and "Resolving Name Conflicts"[14]. If the DHCP server
from which the client is requesting a lease includes the FQDN option
in its ACK message, and if the server sets both the "S" and the "O"
bits (the two rightmost bits) in the option's flags field, the DHCP
client MUST NOT initiate an update for the name in the Domain Name
field.
A client can choose to delegate the responsibility for updating the
FQDN to IP address mapping for the FQDN and address(es) used by the
client to the server. In order to inform the server of this choice,
the client SHOULD include the Client FQDN option in its DHCPREQUEST
message. The rightmost (or "S") bit in the Flags field in the option
MUST be set to 1. A client which delegates this responsibility MUST
NOT attempt to perform a Dynamic DNS update for the name in the
Domain Name field of the FQDN option. The client MAY supply an FQDN
in the Client FQDN option, or it MAY supply a single label (the
most-specific label), or it MAY leave that field empty as a signal
to the server to generate an FQDN for the client in any manner the
server chooses.
Since there is a possibility that the DHCP server may be configured
to complete or replace a domain name that the client was configured
to send, the client might find it useful to send the FQDN option in
its DISCOVER messages. If the DHCP server returns different Domain
Name data in its OFFER message, the client could use that data in
performing its own eventual A RR update, or in forming the FQDN
option that it sends in its REQUEST message. There is no requirement
that the client send identical FQDN option data in its DISCOVER and
REQUEST messages. In particular, if a client has sent the FQDN
option to its server, and the configuration of the client changes so
that its notion of its domain name changes, it MAY send the new name
data in an FQDN option when it communicates with the server again.
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This may allow the DHCP server to update the name associated with
the PTR record, and, if the server updated the A record representing
the client, to delete that record and attempt an update for the
client's current domain name.
A client that delegates the responsibility for updating the FQDN to
IP address mapping to a server might not receive any indication
(either positive or negative) from the server whether the server was
able to perform the update. In this case the client MAY use a DNS
query to check whether the mapping is updated.
A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
option to 0 when sending the option.
If a client releases its lease prior to the lease expiration time
and the client is responsible for updating its A RR, the client
SHOULD delete the A RR (following the procedures described in
"Resolving Name Conflicts"[14]) 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.
6. DHCP Server behavior
When a server receives a DHCPREQUEST message from a client, if the
message contains the Client FQDN option, and the server replies to
the message with a DHCPACK message, the server may be configured to
originate an update for the PTR RR (associated with the address
leased to the client). Any such update MUST be originated following
the procedures described in "Resolving Name Conflicts"[14]. 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
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message has the "S" bit in its Flags field set, then the server MAY
originate an update for the A RR (associated with the FQDN carried
in the option) if it is configured to do so by the site's
administrator, and if it has the necessary credentials. The server
MAY be configured to use the name supplied in the client's FQDN
option, or it MAY be configured to modify the supplied name, or
substitute a different name.
Any such update MUST be originated following the procedures
described in "Resolving Name Conflicts"[14]. The server MAY
originate the update before the server sends the DHCPACK message to
the client. In this case the RCODE from the update [RFC2136] MUST be
carried to the client in the RCODE2 field of the Client FQDN option
in the DHCPACK message. Alternatively the server MAY send the
DHCPACK message to the client without waiting for the update to be
completed. In this case the RCODE2 field of the Client FQDN option
in the DHCPACK message MUST be set to 255. The choice between the
two alternatives is entirely up to the DHCP server. In either case,
if the server intends to perform the DNS update and the client's
REQUEST message included the FQDN option, the server SHOULD include
the FQDN option in its ACK message, and MUST set the "S" bit in the
option's Flags field.
Even if the Client FQDN option carried in the DHCPREQUEST message
has the "S" bit in its Flags field clear (indicating that the client
wants to update the A RR), the server MAY be configured by the local
administrator to update the A RR on the client's behalf. A server
which is configured to override the client's preference SHOULD
include an FQDN option in its ACK message, and MUST set both the "O"
and "S" bits in the FQDN option's Flags field. The update MUST be
originated following the procedures described in "Resolving Name
Conflicts"[14]. The server MAY originate the update before the
server sends the DHCPACK message to the client. In this case the
RCODE from the update [RFC2136] MUST be carried to the client in the
RCODE2 field of the Client FQDN option in the DHCPACK message.
Alternatively, the server MAY send the DHCPACK message to the client
without waiting for the update to be completed. In this case the
RCODE2 field of the Client FQDN option in the DHCPACK message MUST
be set to 255. Whether the DNS update occurs before or after the
DHCPACK is sent is entirely up to the DHCP server's configuration.
When a DHCP server sends the Client FQDN option to a client in the
DHCPACK message, the DHCP server SHOULD send its notion of the
complete FQDN for the client in the Domain Name field. The server
MAY simply copy the Domain Name field from the Client FQDN option
that the client sent to the server in the DHCPREQUEST message. The
DHCP server MAY be configured to complete or modify the domain name
which a client sent, or it MAY be configured to substitute a
different name. If the server initiates a DDNS update which is not
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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
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
accordingly.
If a client's DHCPREQUEST message doesn't carry the Client FQDN
option (e.g., the client doesn't implement the Client FQDN option),
the server MAY be configured to update either or both of the A and
PTR RRs. The updates MUST be originated following the procedures
described in "Resolving Name Conflicts"[14].
If a server detects that a lease on an address that the server
leases to a client has expired, the server SHOULD delete any PTR RR
which it added via dynamic update. In addition, if the server added
an A RR on the client's behalf, the server SHOULD also delete the A
RR. The deletion MUST follow the procedures described in "Resolving
Name Conflicts"[14].
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 Dynamic Update. In addition, if the server took
responsibility for an A RR, the server SHOULD also delete that A RR.
The deletion MUST follow the procedures described in "Resolving Name
Conflicts"[14].
7. 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., Simple Secure DNS 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 Dynamic DNS Update protocol (e.g., only a client may have
sufficient credentials to perform updates to the FQDN to IP address
mapping for its FQDN).
Whether a DHCP server is always responsible for updating the FQDN to
IP address mapping (in addition to updating the IP to FQDN mapping),
regardless of the wishes of an individual DHCP client, is also a
site-local matter. The choice between the two alternatives may be
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based on the security model that is being used with dynamic 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[10] 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.
8. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter
Ford, Edie Gunter, Andreas Gustafsson, R. Barr Hibbs, Kim Kinnear,
Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Josh Littlefield,
Michael Patton, and Glenn Stump for their review and comments.
References
[1] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
1034, Nov 1987.
[2] Mockapetris, P., "Domain names - Implementation and
Specification", RFC 1035, Nov 1987.
[3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[4] Marine, A., Reynolds, J. and G. Malkin, "FYI on Questions and
Answers to Commonly asked ``New Internet User'' Questions",
RFC 1594, March 1994.
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[5] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System", RFC 2136, April 1997.
[6] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[7] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[8] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[9] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)
(draft-ietf-dnsext-tsig-*)", July 1999.
[10] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages
(draft-ietf-dhc-authentication-*)", June 1999.
[11] Wellington, B., "Simple Secure DNS Dynamic Updates
(draft-ietf-dnsext-simple-secure-update-*)", June 1999.
[12] Stapp, M., Gustafsson, A. and T. Lemon, "A DNS RR for encoding
DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", July 2000.
[13] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
April 1992.
[14] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP
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
Mark Stapp
Cisco Systems, Inc.
250 Apollo Dr.
Chelmsford, MA 01824
US
Phone: 978.244.8498
EMail: mjs@cisco.com
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Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
US
Phone: 914.235.2128
EMail: yakov@cisco.com
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