DNS Operations S. Krishnaswamy
Internet-Draft SPARTA Inc.
Expires: September 4, 2006 March 3, 2006
Split-View DNSSEC Operational Practices
draft-krishnaswamy-dnsop-dnssec-split-view-02.txt
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Abstract
The security extensions to the Domain Name System (DNSSEC) allow for
integrity protection, whereby it is possible to make a determination
of the verity of data returned from the Domain Name System in
response to a query. Current operation of the Domain Name System
also allows for the creation of multiple views of data, where the
answer returned in response to a query is dependent on the origin of
the query. Data integrity and the ability to return possibly
conflicting values as in split-views may be construed to be mutually
conflicting goals; but this apparent dichotomy is resolvable in
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practice through proper configuration. This document provides
recommendations for correctly configuring the split-view DNSSEC
environment in a typical enterprise network.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Split-View DNS . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Query Channeling . . . . . . . . . . . . . . . . . . . . . 5
2.3. Controlling Errant Queries . . . . . . . . . . . . . . . . 7
2.4. Name Server Requirements . . . . . . . . . . . . . . . . . 7
2.4.1. Internal Recursive Forwarder . . . . . . . . . . . . . 7
2.4.2. Second-Level Recursive Name Server . . . . . . . . . . 7
2.4.3. Authoritative Internal and External-View Name
Servers . . . . . . . . . . . . . . . . . . . . . . . 7
3. Split-View DNSSEC . . . . . . . . . . . . . . . . . . . . . . 7
3.1. No Internal Validation . . . . . . . . . . . . . . . . . . 8
3.2. Same Key Signing . . . . . . . . . . . . . . . . . . . . . 9
3.3. Complete Decoupling of Authentication Chains . . . . . . . 10
3.4. Partial Decoupling of Authentication Chains . . . . . . . 11
3.5. Multiple DS Records . . . . . . . . . . . . . . . . . . . 12
3.6. Name Server Requirements . . . . . . . . . . . . . . . . . 13
4. Packet Filtering Considerations . . . . . . . . . . . . . . . 13
4.1. Inner Packet Filter . . . . . . . . . . . . . . . . . . . 13
4.2. Outer Packet Filter . . . . . . . . . . . . . . . . . . . 14
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
Split-view DNS is the term used to describe the scenario where the
DNS returns different answers for the same question depending on who
asked the question and from where. Split-views help contain DNS
names to only those portions of the network that need to see these
names. Although primarily meant to be a network management
technique, the tailoring of the DNS to create an internal separation
of names is also seen by some as improving their organization's
security posture, by preventing the exposure of internal host names,
knowledge of whose existence is deemed to be sensitive.
Relying solely on split-view DNS to protect sensitive hosts from
attacks has proven to be less than adequate in the past and is not
recommended. Attack vectors in Internet exploits have been able to
successfully infect hosts with or without their IP addresses being
published in the DNS. Conversely, publishing the IP addresses of
hosts that are otherwise secured does not necessarily increase their
vulnerability to these attacks. Name hiding through split-view DNS
is primarily useful as part of a more comprehensive defense-in-depth
strategy to provide one line of defense against name-based attacks.
The security extensions to DNS [1] provide for origin authenticity
and data integrity. These properties are determined by validating
the authentication chain from some trust anchor configured at the end
resolver to the signed record. In the case of split-view DNS every
authentication chain in every view must validate properly. Some
names may be common between multiple views but contain different
data. Cache pollution is a possibility when data from the wrong view
is returned in response to a query. Building an authentication chain
from a trust anchor above the zone that has split views, to data in
the internal view of a zone can be especially problematic, caching
problems notwithstanding.
The objective of this document is to describe approaches for
configuring split-view DNSSEC environments with the additional
requirement that no server be both authoritative and recursive at the
same time. Separation of authoritative and recursive name servers
not only provides simple role separation, but is also a well
recognized security measure in DNS for protecting authoritative name
servers against compromised caches.
In cases where the different views of DNS information correspond to
different physical networks, the name servers authoritative for the
internal and external views of data are often separated by a
firewall. Among some of the frequently observed DNS resolution
misbehaviour [3] is the problem of resolvers aggressively
retransmitting queries from behind misconfigured firewalls that allow
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queries out, but drop all returned responses. This problem is
exacerbated by a handful of errant queries that are sent by only a
subset of internal resolvers, which makes problem isolation extremely
difficult. This document provides recommendations for reducing the
impact of errant queries in the split-view DNS setup and also makes
recommedations for DNS-related packet filtering rules required to
support the proper operation of the suggested configuration.
Section 2 describes the general approach for configuring split-view
DNS, which by itself, is independent of DNSSEC. Considerations for
DNSSEC appear in Section 3.
2. Split-View DNS
2.1. Background
Different views of the DNS can be created by a process of "query
channeling". Here, different servers are authoritative for the
different views of the DNS information and queries are channeled to
these name servers based upon their origination address.
It is also possible to use a single machine as the authoritative name
server for both views of data by running multiple instances of the
name server process on a machine with multiple network interfaces and
answering differently based on the query source. Some name server
implementations also directly support split-view DNS. Variants
include the view-based approach and the data tagging approach. In
the former, the name server loads multiple zone databases and makes
available answers from a particular zone based on the origin of the
query. The second approach tags the data in the database itself as
either being internally, externally or globally available.
Split-view DNS is not recommended for hiding DNS names. A much
better alternative to protect a namespace of sensitive hosts is to
have that entire namespace reside within a private delegation. By
doing so, it is possible to have the protection given to the name
server that serves these names commensurate with the protection given
to the hosts themselves. Since hosts in the private branch are
explicitly marked as such by virtue of their domain name, this method
also allows the network administrator to better classify hosts as
being public or private and lessens the opportunity for sensitive
hosts to be inadvertantly placed in public domains. Private
delegations are useful when name hiding is the only reason for
namespace separation. They do not, however, allow for transparency
during name resolution; queries have to be made for specific names in
specific views.
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This document describes a generic configuration for split-view DNS
without relying on any special capablities from any machine or name
server implementation. The architecture is modeled around a typical
enterprise structure: the two views are for the internal and external
portions of the network, with the external portion residing within
the boundary network. The two networks are separated by a packet
filtering firewall. A packet filtering firewall also separates the
boundary network from the external Internet. Name hiding is not an
objective of this split-view setup, but avoiding cache pollution is.
Although the two concepts are related, split-views is not recommended
for hiding sensitive names because of the ease with which names can
be leaked out. Again, if name hiding is the main objective, the
approach of using a private delegation for sensitive names is
strongly encouraged.
The suggested configuration uses a combination of multiple name
servers and query forwarding. One name server answers queries for
the internal view and forwards all requests for external data to a
second name server. The second name server recursively answers
queries but only if asked by the first. Other name servers are
configured in such a way so as to decouple the roles of the
authoritative and recursive name servers.
2.2. Query Channeling
Query channeling is the process of carefully controlling how the
queries are sent to different name servers so as to avoid cache
pollution.
Resolving outer data is straightforward since queries follow their
normative paths. For the internal view, a two-level recursive server
scheme is recommended. One server functions as a recursive forwarder
and is responsible for answering all internal queries. This server
forwards all queries for internal data to their respective
authoritative name servers while recursively obtaining external
answers from a second-level name server. The authoritative name
servers do not perform any recursion themselves. The second-level
name server is a simple caching name server that asks questions from
the outside, but only if asked by the recursive forwarder. The
recursive forwarder and the name servers authoritative for the
internal data reside in the internal network; the second-level
recursive name server that is used for returning external answers
resides in the boundary network.
The two-level recursive scheme controls which name servers the
queries are directed to. Since queries for internal data are sent to
authoritative name servers which are not also recursive, this scheme
also controls where data is received from. In this way internal data
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is kept totally separate from external data, thus preventing cache
pollution. Figure 1 illustrates the above setup.
^ root, TLD servers etc
| ^ (queries) EXTERNAL NAMESERVERS
[Outside] | ^ (responses)
| | |
v | |
------------[O u t e r----P a c k e t---F i l t e r]-------------------
^ | |
| | v (queries)
| | AUTHORITATIVE EXT-VIEW SERV
| v (responses)
[Boundary] RECURSIVE NAMESERVER
Net ^ (queries from the
| CLIENTS | recursive forwarder
| (responses ^ | protected by TSIG)
| for internal | |
v data) | |
------------[I n n e r----P a c k e t---F i l t e r]-------------------
^ | |
| (queries)| |
| v v (external data)
| RECURSIVE FORWARDER <---------> INTERNAL
[Internal] ^ (internal data) (query/ CLIENT
| | response)
| |
| v
| AUTHORITATIVE INT-VIEW SERV
v
Figure 1
It is useful to note that the internal recursive forwarder must not
attempt to recursively answer queries if the authoritative name
server for internal-view data fails to respond. If it did so,
external data could be returned in such circumstances and lead to
cache pollution. Since neither the server authoritative for a
forwarded zone nor the server doing the forwarding can recursively
answer queries for delegations from that zone, the internal recursive
forwarder must explicitly forward queries for every internal
delegation to its respective authoritative name server. This rule
can be relaxed while forwarding queries to name servers that are
simultaneously authoritative for the child as well as the parent
zone.
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2.3. Controlling Errant Queries
DNS queries that are sent from the internal recursive forwarder to
the outside should only be directed towards the second-level
recursive name server. Since the second-level name server has no
knowledge of internal-view data, internal resolvers must not use it
directly for resolving queries. Only properly configured internal
recursive forwarders that are approved to send queries to this name
server must do so, and that too solely for the purpose of resolving
external answers. TSIG is the recommended method for controlling
which recursive forwarders are approved for sending queries to the
second-level name server. Having these rules alternatively
configured in the packet filter is also possible, but using TSIG for
performing this authorization eases packet filter administration for
DNS.
2.4. Name Server Requirements
This section summarizes the list of requirements for the various name
servers involved in the split-view configuration.
2.4.1. Internal Recursive Forwarder
o Ability to forward queries to specific name servers.
o Ability to control forwarding behaviour such that the recursive
option is not tried, even if the name server that queries are
normally forwarded to fails to respond.
o Ability to recursively answer queries.
o Ability to protect the integrity of messages using TSIG for
selected destinations.
2.4.2. Second-Level Recursive Name Server
o Ability to recursively answer queries.
o Ability to verify TSIG protection on messages.
o Ability to filter incoming queries based on the TSIG key used to
protect the message.
2.4.3. Authoritative Internal and External-View Name Servers
o Ability to authoritatively answer queries for a zone.
o Ability to disable all recursive behaviour.
3. Split-View DNSSEC
Any data in any view that is likely to be spoofed has to be signed.
The DNSSEC concern for split-view is ensuring that the internal and
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external authentication chains validate properly. This concern is
addressed by making an appropriate choice of trust anchors and Secure
Entry Point (SEP) keys.
While validating external data is relatively straightforward, there
are multiple approaches that can be used for validating internal
data. The method of choice depends on what the threat environment
for the internal view is perceived to be, the amount of end-resolver
configuration overhead that is needed, the ease of debugging and the
ability to have administrative separation between the two split-
views. The configuration overhead at end resolvers is mainly
associated with the task of defining trust anchors at different
validating resolvers. Having fewer keys is desirable in that it
makes key management easier. It is also desirable to reduce the
amount of reconfiguration required for clients that move between the
two views of data, while still being able to tie an answer to a
particular view. Often two views of a split zone are administered
separately, so having different zone signing keys for the different
views is also desirable.
The different options for internal data validation are further
outlined below. In each of these options, the zone that is split is
assumed to be a delegation under example.com. and validation of
internal data occurs at the recursive forwarder.
3.1. No Internal Validation
(No trust anchors) example.com. (trusted)
^
|
|
split.example.com. split.example.com.
(internal) (external)
^
|
|
sub.split.example.com. sub.split.example.com.
(internal) (external)
Figure 2
This is an option if the security requirements for the internal zone
are more relaxed than the external zone. The threat environment for
the internal zone in this scenario does not include DNS compromise
and validation results returned from the internal recursive forwarder
is not important. The internal recursive forwarder does not have any
trust anchor configured and does not perform any validation.
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3.2. Same Key Signing
-----------> example.com. (trusted)
/ ^ ^
/ | |
/ (same key)-->|
/ |
split.example.com. split.example.com.
(internal) (external)
^ ^
| |
| |
sub.split.example.com. sub.split.example.com.
(internal) (external)
Figure 3
In this scenario, a single private key is used to sign both the
internal and the external zone data. The glue DS and NS records at
example.com. all correspond to the external view data. If the trust
anchor is configured at or below the level of split.example.com.,
queries sent for validating internal data are never sent to the outer
view. If not, queries for the data from the trust anchor to the DS
record covering the split in example.com. are sent to the outer zone,
while queries for others data are sent to the internal zone. Since
the key referenced in the DS record at example.com. is present in the
apex key-set of both views, the authentication chain can always be
completed.
This approach allows flexibility in choosing the level at which the
trust anchor is configured, with the possibility of using the same
trust anchor for validating answers in both views.
Although easy to setup, this approach can be difficult to
troubleshoot. There is no easy way to identify if the record
obtained for a query corresponds to the internal view or the external
view. Using the same key also makes administrative separation of the
two views of data difficult.
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3.3. Complete Decoupling of Authentication Chains
example.com. (trusted)
^
|
(trusted) split.example.com. split.example.com.
(internal) (external)
^ ^
| |
| |
sub.split.example.com. sub.split.example.com.
(internal) (external)
Figure 4
With the DS record in example.com. always pointing to a key in the
outer view, the construction of the authentication chain is a problem
when the keys used to sign data in the two views of the split are
different. The trust anchor cannot be configured above the level of
split.example.com. since there would be no way of linking the DS
record in example.com. to the apex DNSKEY set in the internal view of
the split zone.
A simple solution is to configure the trust anchor for the internal
view at the level of split.example.com. so that the authentication
chains for the internal and external zones share no common records
that may cause any ambiguity.
Having separate keys for the two views of data is useful for
troubleshooting and in determining which view a given record belongs
to. This configuration however involves more configuration overhead
since trust anchors need to be configured for every zone that is
split. This problem is more pronounced when dealing with validating
stub resolvers on mobile nodes, where moving between the internal and
external views would involve constant reconfiguration of its trust
anchors. Multiple trust anchors, one for each view, may also be
configured at the end resolver but doing so would not allow cache
pollution to be automatically detected by way of validation failures.
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3.4. Partial Decoupling of Authentication Chains
com.
^
| (common key)
+--------+
/ \
(trusted) example.com. example.com. (trusted)
(internal) (external)
^ ^
| |
| |
split.example.com. split.example.com.
(internal) (external)
^ ^
| |
| |
sub.split.example.com. sub.split.example.com.
(internal) (external)
Figure 5
One problem with the approach described in Section 3.3 is that trust
anchors need to be configured for every zone that is split under the
example.com.. An option to circumvent this while still retaining the
advantages of the earlier setup is to split example.com. also, and
configure the trust anchor at the level of example.com. or higher.
The apex keyset in example.com. is the same for both views, but the
glue and DS information for delegations is different across views.
An internal name server is configured as the authoritative server for
the internal view of the split example.com. zone and the internal
recursive forwarder is modified to forward all internal queries for
this zone to it.
This option reduces the number of trust anchors at the end resolver.
However, since a new example.com. zone is created, records that were
previously common in both views would now need to be duplicated
between the two zones. The number of such records is typically not
very large, but the overhead and complexity in maintaining duplicate
records can still be a burden in some cases. Also, since this
approach uses a common trust anchor for both views, cache pollution
cannot be automatically detected.
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3.5. Multiple DS Records
-----------> example.com. (trusted)
/ (DS1) ^
/ |
/ | (DS2)
/ |
split.example.com. split.example.com.
(internal) (external)
^ ^
| |
| |
sub.split.example.com. sub.split.example.com.
(internal) (external)
Figure 6
In this approach, example.com. is not split; however, DS records
corresponding to each of the two different views of
split.example.com. are published at the delegation point. The glue
and NS records at the delegation point still corresponds to the
external view of split.example.com. but this information is never
used by the internal zone. As in Section 3.4, the trust anchor is
configured at the level of example.com. or higher. The
authentication chain from this trust anchor to data in either zone is
constructed by using one of the two DS records, which ever is
applicable at that view.
Since some coordination between split.example.com. and its parent,
example.com, is required to publish multiple DS records, this
approach is most suitable when the split is made at a level lower
than the organization apex. This approach lends itself to using
different keys in different views while still allowing for minimal
configuration at validating stub resolvers; trust anchors need not be
changed even if the nodes are mobile across the two views. This
approach has the advantage that administrative separation of the two
views of the split can be maintained while still having a single key
configured at the end resolvers. Identifying which view a given
record belongs to can be done by tracing the keys used to form the
authentication chain; however, cache pollution cannot be
automatically detected.
While most of the internal zone contents can be kept private to the
internal view, the DS record must still be exposed. Since data
hiding is not an objective of the split-view setup this is not really
a problem. An attendent problem with multiple DS records is that
since the validation algorithm iteratively verifies parent DS records
while trying to complete the authentication chain, there is some
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added computational overhead, which increases as the number of DS
records in the delegation point grows. In some circumstances there
may not be sufficient flexibility to introduce multiple DS records in
the parent, especially if the child is a part of a different
organization.
3.6. Name Server Requirements
All name servers listed below must conform to the specifications
given in [2]. Additionally, the Internal Recursive Forwarder must
support the following:
o Ability to validate DNSSEC responses.
o Support for configurable DNSSEC trust anchors. It should be
possible to configure more than one trust anchor.
4. Packet Filtering Considerations
The following subsections define the rules that must be configured in
the two packet filters depicted in Figure 1 in order to support the
split-view configuration.
4.1. Inner Packet Filter
In order to allow the above configuration to work, any packet
filtering system between the internal network and the boundary
network must allow all of the following types of packets.
o DNS queries from any internal address to the second-level
recursive name server (finer level access control is done by
TSIG):
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP internal >1023 rec.srv 53 N/A
UDP internal 53 rec.srv 53 N/A
TCP internal >1023 rec.srv 53 Any
o Responses to the above queries from the second-level recursive
name server to any internal address:
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP rec.srv 53 internal >1023 N/A
UDP rec.srv 53 internal 53 N/A
TCP rec.srv 53 internal >1023 Set
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o Queries from clients in the boundary network to any internal name
server:
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP client >1023 internal 53 N/A
TCP client >1023 internal 53 Any
o Responses to the above queries from the (any) recursive forwarder
to clients in the boundary network:
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP internal 53 client >1023 N/A
TCP internal 53 client >1023 Set
4.2. Outer Packet Filter
Any packet filtering system configured between the boundary network
and the external network needs to allow the following.
o Queries from the recursive name server in the boundary network to
the outside network:
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP rec.srv >1023 outside 53 N/A
UDP rec.srv 53 outside 53 N/A
TCP rec.srv >1023 outside 53 Any
o Responses to the above queries from the outside to the boundary
network recursive name server:
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP outside 53 rec.srv >1023 N/A
UDP outside 53 rec.srv 53 N/A
TCP outside 53 rec.srv >1023 Set
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o Queries from outside clients to the external-view authoritative
servers:
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP outside >1023 auth.serv(ext view) 53 N/A
UDP outside 53 auth.serv(ext view) 53 N/A
TCP outside >1023 auth.serv(ext view) 53 Any
o Responses to the above queries from the external-view
authoritative server to the outside:
Proto SrcIP SrcPort DestIP DstPort AckBit
UDP auth.serv(ext view) 53 outside >1023 N/A
UDP auth.serv(ext view) 53 outside 53 N/A
TCP auth.serv(ext view) 53 outside >1023 Set
5. Summary
This document describes an approach for configuring split-view
DNSSEC. The approach uses a two-level recursive scheme where an
internal recursive forwarder resolves inside answers and marshalls
all outside queries to a second-level recursive name server. TSIG
between the internal and the second-level name servers protects
against errant queries.
The recommended configuration has been shown to be adjustable for
various needs and security consideration levels. Differences in
these approaches make trade-offs between configuration overhead and
validation overhead. Trading-off in favour of minimal operator
overhead is useful for overall maintainability of the system,
especially when split-view DNS is considered in the context of nodes
that are mobile across the two views.
Although split-view DNSSEC is possible using the recommended setup,
it still involves significant effort: for configuring the various
name servers, for setting up zone forwarding, for configuring and
distributing shared keys for TSIG, and, depending on the
configuration, for performing DS (or keyset) exchanges for every view
of a split zone. Some configurations may also require multiple trust
anchors in end resolvers which may change between views. Proper care
must be taken to ensure that correct split-view behavior is
consistently maintained.
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6. IANA Considerations
This document has no actions for IANA.
7. Security Considerations
The configuration suggested in this document tries to minimize cache
pollution, but misconfigurations are still easily possible. Any
misconfigurations in the three different types of name servers or the
two packet filters can result in cache pollution and cause incorrect
results to be returned between views. The resolver may or may not
detect this depending on the manner in which secure entry points to
split zones are defined and which trust anchors are configured in the
resolver. Leakage of information in this context is not an issue
since split-views by itself is not meant to provide confidentiality
of data.
An improperly configured packet filter that allows errant DNS traffic
through or denies legitimate responses can lead to aggressive
retransmission of queries by resolvers to name servers, leading to
increased query load at such name servers.
Each of the validation options outlined in Section 3 also introduce
their own security considerations. Not using DNSSEC in the internal
view creates the possibility of a malicious entity supplying bogus
information in response to queries, without detection. Using a
common key between both views of the split does not allow one to
differentiate between internal and external data and troubleshooting
is greatly encumbered. On the other hand, using a multitude of keys
at end resolvers only increases the operator overhead and thus the
chances for configuration errors. All approaches that use a common
key for validating internal and external data are unable to
automatically detect cache pollution so they may escape the attention
of a system administration until the time they are specifically being
troubleshooted. Approaches using a common trust anchor are also
susceptible to an insider attack where data from one view injected in
response to queries for the other can corrupt the cache.
8. Acknowledgements
The contributions, suggestions and remarks of the following persons
to this draft are particularly acknowledged: Wes Griffin, John
Kelley, Ed Lewis, Russ Mundy, Scott Rose and Mike StJohns. The two-
level name server scheme described in this document builds upon work
that was originally performed by Ed Lewis.
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9. References
9.1. Normative References
[1] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[2] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
9.2. Informative References
[3] Larson, M. and P. Barber, "Observed DNS Resolution Misbehavior",
Work in Progress ietf-dnsop-bad-dns-res, July 2005.
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Author's Address
Suresh Krishnaswamy
SPARTA Inc.
7075 Samuel Morse Dr.
Columbia, MD 21046
US
Email: suresh AT sparta DOT com
URI: http://www.sparta.com
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