Internet DRAFT - draft-ietf-dhc-dns-pd
draft-ietf-dhc-dns-pd
Network Working Group T. Lemon
Internet-Draft Nominum
Intended status: Standards Track October 21, 2013
Expires: April 24, 2014
Populating the DNS Reverse Tree for DHCP Delegated Prefixes
draft-ietf-dhc-dns-pd-01.txt
Abstract
This document describes three alternatives for populating the DNS
reverse tree for prefixes delegated using DHCP, and provides
mechanisms for implementing each alternative.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 24, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Methods for populating the reverse tree . . . . . . . . . . . 3
3.1. Site-managed reverse tree . . . . . . . . . . . . . . . . 3
3.2. Provider-managed reverse tree . . . . . . . . . . . . . . 3
3.3. Provider-managed spoofed reverse tree . . . . . . . . . . 3
3.4. Other solutions not documented here . . . . . . . . . . . 3
4. Negotiating the reverse tree population method . . . . . . . 4
5. Configuring a site-managed reverse tree . . . . . . . . . . . 6
5.1. Requesting Router Behavior . . . . . . . . . . . . . . . 6
5.2. Delegating Router Behavior . . . . . . . . . . . . . . . 8
6. Configuring a provider-managed reverse tree . . . . . . . . . 9
6.1. Requesting Router Behavior . . . . . . . . . . . . . . . 9
6.2. Delegating Router Behavior . . . . . . . . . . . . . . . 9
7. Configuring a spoofed reverse tree . . . . . . . . . . . . . 10
8. Configuring no reverse tree . . . . . . . . . . . . . . . . . 10
9. Encoding of options . . . . . . . . . . . . . . . . . . . . . 10
9.1. Prefix Delegation Method Types . . . . . . . . . . . . . 10
9.2. Prefix Delegation Zone Preference Option . . . . . . . . 11
9.3. Prefix Delegation Zone Method Option . . . . . . . . . . 11
9.4. Prefix Delegation Zone Server Option . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 12
12.2. Informative References . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
When a site is numbered using DHCP prefix delegation [RFC3633], there
are three ways of populating the Domain Name System [RFC1035] reverse
tree. Which mechanism is chosen depends on the capabilities of the
site's DNS infrastructure, if any, on the capabilities and policies
of the service provider, and on the preferences of the site
administration.
This document does not take a position on which mechanism, if any, is
best for populating the reverse tree, but simply documents each of
the possible mechanisms for doing so, and provides a means whereby
site administrators and service providers can negotiate the mechanism
whereby the reverse tree for a particular site will be populated.
2. Terminology
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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 [RFC2119].
3. Methods for populating the reverse tree
There are three common methods of populating the reverse tree for a
delegated prefix: delegation, dynamic dns, and zone spoofing. In
addition, of course, it is possible to leave the reverse tree
unpopulated.
3.1. Site-managed reverse tree
To populate the reverse tree by delegation, the site administrator
must provide a DNS authoritative name server for the delegated zone.
The site administrator must communicate the IP address of the
authoritative name server to the service provider. The service
provider must then add a delegation for that zone using the IP
address or addresses of the DNS authoritative servers provided by the
site administrator.
3.2. Provider-managed reverse tree
To populate the reverse tree using DNS updates, the service provider
must provide an authoritative name server for the zone. The site
administrator must provide a key to the service provider that can be
used to authenticate DNS updates. The site administrator must then
provide a mechanism whereby DNS updates will automatically be
generated, using the provided key, whenever IP addresses are
allocated within the delegated prefix.
3.3. Provider-managed spoofed reverse tree
In some cases the site administrator may not be willing or able to
populate a reverse tree. However, the service provider may wish to
provide meaningful answers to reverse zone queries for the delegated
zone. It's not possible to populate the delegated zone: a fully
populated zone for a /64 would require 1.8x10^19 names. However, the
names in such a zone would never change; consequently it is possible
for a name server to spoof the zone contents, constructing answers
for queries against any name within the zone on the fly. Because the
contents of the zone never change, the zone can have a consistent
authority record.
3.4. Other solutions not documented here
It's worth noting that there are several other ways that the zone for
a delegated prefix could be populated, but we are not covering these
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mechanisms because they seem more difficult to implement and deploy.
For instance, nodes configured with addresses within a delegated
prefix could issue their own DNS updates to an authoritative server
operated by the service provider. The problem of key management in
this case becomes intractable, however.
It would also be possible for the site to have its own key management
infrastructure, and for some agent on the requesting router to act as
an intermediary in updating a zone maintained by the service
provider. However, this is substantially more complicated than
either of the proposed solutions.
Another option is to simply not populate the reverse tree. This is
an attractive option in the IETF in particular because the reverse
tree is frequently used for purposes to which it is not suited, and
some IETF participants believe that in order to discourage these
applications, it's better simply to not populate the reverse tree.
This document takes no position on this question, but does offer a
means whereby the site administrator can indicate that the reverse
tree should not be populated.
4. Negotiating the reverse tree population method
The prefix delegation process is initiated by a requesting router.
If a delegating router chooses to delegate a prefix to the requesting
router, it replies with a prefix. The requesting router may receive
responses from more than one delegating router, and may choose one or
more such delegated prefixes. For delegating routers whose offer is
accepted, the requesting router sends a request for the offered
address; at this point the delegating router commits the delegation
to stable storage and sends a confirmation to the requesting router.
The messages used to complete this transaction are the DHCP Discover,
DHCP Advertise, DHCP Request and DHCP Reply messages, respectively.
The negotiation as to how the reverse tree will be populated
piggybacks on this four-message process.
In the DHCP Discover message, the requesting router indicates the
site administrator's preference for how the reverse tree for the
delegated prefix will be populated. It does this by including, in
each IA_PD option it sends, a Prefix Delegation Zone Preference
option (PDZP) containing one or more preference codes. These codes
are listed in order of preference with the most preferred mechanism
first. A requesting router that includes a PDZP option MUST send an
Option Request option (ORO) that requests the Prefix Delegation Zone
Method (PDZM) option.
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If the delegating router chooses not to delegate a prefix to the
requesting router, no special action need be taken in response to the
PDZP option. The remainder of this section describes what happens if
the delegating router chooses to delegate a prefix to the requesting
router.
Delegating routers that implement this specification can be
configured with a list of supported reverse tree population methods.
When a requesting router receives an IA_PD option that includes a
PDZP option, if it has been configured with a reverse tree population
method list, it iterates across the list of methods in the PDZP
option. For each entry in the PDZP option, the requesting router
tests to see if that method has been configured by the site
administrator as being supported. If the method is on the list, the
iteration stops at this point.
Upon completion of this iteration, if a method was found in the PDZA
that is supported by the delegating router, that is the method that
will be used to populate the reverse tree for the delegated zone.
The delegating router constructs a PDZM option indicating that this
method will be used and includes this in the DHCP Advertise message.
If no supported method was found, this means that the service
provider will not cooperate with the site administrator in populating
the reverse tree. The delegating router indicates that this is the
case by not including a PDZM option in the DHCP Advertise message..
The requesting router may receive one or more DHCP Advertise messages
containing delegated prefixes. The requesting router MUST silently
discard any DHCP Advertise message containing a PDZM option that
indicates a method that was not listed in the PDZP option sent in the
DHCP Discover message.
The requesting router may then choose to respond to one or more of
the remaining DHCP Advertise messages, if any. The lack of a PDZM
option indicates either that the delegating router does not implement
DNS for delegated prefixes, or that it is not configured to support
DNS for delegated prefixs. The requesting router MAY prefer DHCP
Advertise messages containing PDZM options over DHCP Advertise
messages that do not contain PDZM options.
When responding to any DHCP Advertise messages containing PDZM
options, the requesting router MUST include a PDZM option containing
the same method indicated in the received PDZM option.
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Each delegating router that receives a DHCP Request message
containing a PDZM option MUST check the method indicated in the PDZM
option is supported; if not, the delegating router MUST silently
discard the DHCP Request option.
The requesting and delegating routers should follow the same
procedure specified for the DHCP Request/DHCP Reply sequence whenever
a DHCP Renew or DHCP Rebind is sent and a DHCP reply sent in
response, if that response renews the delegated prefix. In the case
that the response does not renew the prefix, the delegating router
MUST NOT send a PDZM in the IA_PD option.
5. Configuring a site-managed reverse tree
If the PDZM option returned by the delegating router in the DHCP
Advertise message specifies the Site Managed method, the requesting
router must arrange to set up one or more authoritative name servers
that will provide service for the zone or zones that correspond to
the delegated prefix. It must also communicate to the delegating
router the IP address or addresses of these servers.
5.1. Requesting Router Behavior
The requesting router MUST include a Prefix Delegation Zone Server
(PDZS) option in each IA_PD in the DHCP Request message, which
includes zero or more IP addresses of authoritative name servers for
the delegated zone. IPv4 addresses MUST be represented as
IPv4-Embedded IPv6 addresses using the Well-Known prefix [RFC6052].
Authoritative name service for these zones may be provided by any or
all of the following three types of authoritative name servers:
o An authoritative name server running on a node that has an IP
address known to the requesting router that is not obtained from
the prefix being delegated.
o An authoritative name server running on the requesting router.
o An authoritative name server running on a node that will obtain
its only IP address from the prefix being delegated.
In the first case, it is possible that the reverse zone for the
delegated prefix is already configured on the authoritative name
server. In this case, the requesting router SHOULD include the IP
address of the authoritative name servers for the delegated zone in
the PDZS option.
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However, if the prefix is being delegated for the first time, the
delegating router will not have had an opportunity to configure it
prior to sending the DHCP Request message. In this case, the
delegating router SHOULD NOT include the IP Address of this name
server in the PDZS option that's send in the DHCP Request message;
instead, it should send a DHCP Renew once the authoritative server
has been configured, and list the server's IP address in the PDZS
option in the DHCP Renew message.
In the second case, the requesting router may already have an IP
address, and may be able to configure the authoritative server for
the delegated zone before sending the DHCP Request. In this case,
the requesing router SHOULD include its own IP address in the PDZS
option in the DHCP Request message.
If the requesting router does not have an IP address at this time, it
SHOULD send a DHCP Renew message containing a PDZS option that lists
all the authoritative servers for the reverse zone or zones for the
delegated prefix after it has an IP address and has configured the
authoritative servers.
If the authoritative name server is running on a node that will
configure its IP address from the delegated prefix, this name server
cannot even be configured until it has an IP address. The process of
configuring this name server is beyond the scope of the document;
however, once the name server has been configured, the requesting
router SHOULD send a DHCP Renew message for the delegated prefix with
an IA_PD containing a PDZS option that lists the IP address of this
name server.
In general, if there are any globally-reachable name servers that are
authoritative for the zone or zones that provide the reverse tree for
the delegated prefix at the time that the DHCP Request message is
sent, the requesting router should list the IP addresses of these
name servers in the PDZS option in the associated IA_PD option in the
DHCP Request message.
If new globally-reachable name servers that are authoritative for the
reverse zone or zones become available after the DHCP Request has
been sent and the DHCP Reply received, the requesting router SHOULD
send a DHCP Renew message containing an IA_PD for the delegated
prefix and a PDZS option listing the name servers for that prefix
that have come online. The requesting router SHOULD be aware of all
outstanding name server configuration processes and minimize the
number of DHCP Renew message sent.
When a requesting router sends a DHCP Renew or DHCP Rebind message to
renew a delegated prefix, if a site-managed reverse tree was
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successfully configured, the requesting router MUST send a PDZM
option containing the same method sent in the original DHCP Request
message. The requesting router MUST also send a PDZS option that
contains one or more IP addresses for authoritative servers for the
reverse tree for the delegated prefix.
5.2. Delegating Router Behavior
When a delegating router receives a valid DHCP Request message
containing an IA_PD that contains both a PDZM option indicating the
Site Managed method and a PDZS option containing at least one IP
address, it compares the IP addresses in the PDZM option to any
previous record it may have for that delegation. If the contents of
the PDZM option differ from the previous record, or if there is no
previous record, the delegating router MUST issue a DNS Update to add
a delegation to the parent zone of the reverse tree zone for the
delegated prefix.
In the event that the PDZS option contains zero IP addresses, the
delegating router does not update the zone.
If the delegated prefix must be represented as more than one zone,
the delegating router adds delegations to the parent zone for each
such zone.
When a delegating router receives a DHCP Renew or DHCP Rebind message
for a prefix it delegated and elects to renew the prefix, it MUST
check its record for that prefix to see if a delegation exists. If
the contents of the PDZS differ from the recorded list of
authoritative name servers for that prefix, the delegating router
MUST update the parent zone with the new delegations.
When a delegating router receives a DHCP Renew or DHCP Rebind message
for a prefix it delegated, and elects not to renew the delegation,
the delegating router MUST check to see if it has a site-managed
reverse tree configuration for that pprefix. If it does, it must
update the parent zone to remove any delegations that were added, and
update its record for the delegated prefix to indicate that no site-
managed reverse tree configuration for that prefix is present.
When a delegated prefix expires without being renewed by the
requesting router, the same procedure should be followed to update
the parent zone.
In all cases where the delegating router updates the delegation for
the zone, it must first query the name server or servers listed in
the PDZS opton for an SOA record for each delegated zone. If the
name server does not respond within the standard timeout period, or
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does not provide an authoritative answer, the delegating router MUST
NOT add a delegation for that name server.
6. Configuring a provider-managed reverse tree
If the PDZM returned by a delegating router in the DHCP Advertise
message specifies the Provider Managed method, the delegating router
must arrange to set up a reverse zone for the delegated prefix. The
requesting router must communicate a key to the delegating router
that can be used to secure updates to the reverse zone.
6.1. Requesting Router Behavior
In order to update the provider-managed reverse zone, the requesting
router must provide a key to the delegating router. Because DHCP
does not provide confidentiality, this key must be the public half of
a private key.
Typically sites that wish to populate their reverse tree with
meaningful information maintain a site-specific or company-wide DNS
zone. In order to update the reverse zone, the site administrator
must publish a SIG(0) key in this zone. The requesting router MUST
include a Prefix Delegation SIG(0) Key FQDN (PDSKF) option in the
DHCP Request message and any subsequent DHCP Renew messages. It must
use the private half of the SIG(0) key in any DNS updates to the
reverse zone.
6.2. Delegating Router Behavior
There are two cases that the delegating router needs to handle: the
case where the prefix being delegated was previously delegated to the
same requesting router, and the case where it was not.
In the case where the prefix was previously delegated to the same
requesting router, the delegating router need take no action to
populate the zone, because it should already be populated.
In the case where the prefix was previously delegated to a different
requesting router, the delegating router MUST remove the old zone
information from the master authoritative name server for the zone.
In this case, and in the case where no previous delegation had been
done, the delegating router must then configure a new reverse zone on
the master server.
In any case, the delegating router must configure the reverse zone so
that it can be updated using the SIG(0) key stored on the name
provided by the requesting router in the PDSKF option.
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7. Configuring a spoofed reverse tree
A spoofed reverse tree can be configured either unilaterally by the
service provider or upon request of the site administrator. The site
administrator would list this as an option to indicate a preference
for a spoofed reverse tree over no reverse tree; the choice doesn't
make any sense otherwise.
Generally speaking, the service provider has the option of either
setting up spoofed zones on demand, or setting them up when
requested. If the service provider only offers spoofed zones, it
makes some sense to set them up in advance; otherwise they should be
set up whenever a prefix is delegated to a particular requesting
router for the first time.
In some cases the site administration may request a spoofed zone
because they do not wish to populate the reverse tree, but wish for
it to appear populated. A service provider may support this option
in addition to the site-managed option, the provider-managed option,
and the no zone option. In this case, when a prefix is delegated to
a new router for the first time, there may be an old zone configured
differently. In this case, the delegated router MUST remove the old
zone configuration before setting up the spoofed zone.
8. Configuring no reverse tree
A service provider may choose to simply not populate reverse trees
for delegated prefixes. This is a desirable option in the sense that
it minimizes the work required to support the reverse DNS tree, and
avoids creating spoofed nonsense records. The service provider may
also simply offer it as an option for sites that prefer not to have a
populated reverse tree.
In this case, if the non-populated reverse tree is an option, and the
prefix had previously been delegated to a different router, the
delegating router must remove any previously-existing zone for the
delegated prefix.
9. Encoding of options
9.1. Prefix Delegation Method Types
Prefix delegation methods are encoded as numbers. Currently three
prefix delegation methods are defined:
0 Site-Managed
1 Provider-managed
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2 Provider-managed spoofed reverse tree
9.2. Prefix Delegation Zone Preference Option
The Prefix Delegation Zone Preference option consists of an option
code, OPT_PDZP, followed by a length, followed by one or more Prefix
Delegation method type codes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPT_PDZP | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type 1 | ... | Type N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.3. Prefix Delegation Zone Method Option
The Prefix Delegation Zone Method option consists of an option code,
OPT_PDZM, followed by a length, followed by one Prefix Delegation
method type code.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPT_PDZM | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+
9.4. Prefix Delegation Zone Server Option
The Prefix Delegation Zone Server option consists of an option code,
OPT_PDZS, followed by a length, followed by zero or more IPv6
addresses.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPT_PDZS | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DNS Server IP Address 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DNS Server IP Address 1 (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DNS Server IP Address 1 (cont'd) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DNS Server IP Address 1 (cont'd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
10. Security Considerations
Some ISPs may have concerns about allowing site-managed DNS
subdelegations for the reverse zone, but this concern is a policy
issue, not a security issue. In the presence of properly agreed-to
terms of service, population of a reverse tree by the end-user is
simply a value-added service the ISP may or may not choose to
provide. Even in the absence of a legally binding ToS agreement, the
worst an end-user could do would be to publish nasty words or bogus
PTR records, neither of which is a security concern.
If an implementation were to fail to follow the advice on validating
authoritative name servers supplied by the requesting router, it
would probably be possible for a coordinated set of requesting
routers to perform a DDoS attack on a target by arranging for various
entities on the network to query the reverse tree for one or more of
the IP addresses in the delegated prefix. However, this would
require, first, that the implementation not follow the specification,
and second, a fairly complicated setup. In practice, there are
easier ways to get a DDoS amplification.
11. IANA Considerations
We request that IANA assign three new option codes from the DHCP
Option Codes table of the Dynamic Host Configuration Protocol for
IPv6 (DHCPv6) parameters registry maintained in http://www.iana.org/
assignments/dhcpv6-parameters/dhcpv6-parameters.xml These option
codes will be assigned to the Prefix Delegation Zone Preference
(OPT_PDZP), Prefix Delegation Zone Method (OPT_PDZM) and Prefix
Delegation Zone Servers (OPT_PDZS) options.
We also request that the IANA add a new table, the Prefix Delegation
Zone Method Types table, to the same registry. The first three
entries in the table will contain the values specified in the section
above titled "Prefix Delegation Zone Method Types." New entries to
the table may be added according to the "Specification Required" IANA
policy [RFC5226].
12. References
12.1. Normative References
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
12.2. Informative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
Author's Address
Ted Lemon
Nominum
2000 Seaport Blvd
Redwood City, CA 94063
USA
Phone: +1 650 381 6000
Email: mellon@nominum.com
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