DNS Operations S. Krishnaswamy
Internet-Draft SPARTA Inc.
Expires: March 11, 2006 September 7, 2005
Split-View DNSSEC Operational Practices
draft-krishnaswamy-dnsop-dnssec-split-view-01.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 . . . . . . . . . . . . . . . . . . . . . . 8
3.1. No Internal Validation . . . . . . . . . . . . . . . . . . 8
3.2. Same Key Signing . . . . . . . . . . . . . . . . . . . . . 9
3.3. Partial Decoupling of Chains-Of-Trust . . . . . . . . . . 10
3.4. Complete Decoupling of Chains-Of-Trust . . . . . . . . . . 11
3.5. Multiple DS Records . . . . . . . . . . . . . . . . . . . 11
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 multiple views of DNS
information for a domain based on where and by whom the query is
sent. 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 view of information hidden from the outside 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. Attack
vectors in recent 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 chain-of-trust from the signed record to some trusted key
configured at the end resolver. In the case of split-view DNS every
chain-of-trust 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 a chain-of-trust from a trusted key
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 an important
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 made 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.
Single name server approaches are susceptible to leakage of DNS
information if the host on which they operate is compromised.
Confidentiality of the namespace is directly tied to how resilient
the name server is against such attacks. 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.
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They have the drawback that they do not allow for transparency during
name resolution; queries have to be made for specific names in
specific views.
This document describes a generic configuration for split-view DNS
using multiple nameservers without relying on any special 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, this configuration is not recommended for
hiding sensitive names because of the ease with which names can be
leaked out due to trivial configuration errors. Again, if name
hiding is the main objective for providing split-views, 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
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resides in the boundary network.
The two-level recursive scheme controls where 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 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
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can be relaxed while forwarding queries to name servers that are
simultaneously authoritative for the child as well as the parent
zone.
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.
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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
external chains-of-trust validate properly. This concern is
addressed by making an appropriate choice of trusted 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.
3.1. No Internal Validation
(No trusted key) parent zone (trusted)
^
|
|
split zone split zone
(internal) (external)
^
|
|
sub split zone sub split zone
(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
trusted key configured and does not perform any validation.
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3.2. Same Key Signing
-----------> parent zone (trusted)
/ ^ ^
/ | |
/ (same key)-->|
/ |
split zone split zone
(internal) (external)
^ ^
| |
| |
sub split zone sub split zone
(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
the delegation point of the split zone all correspond to the external
view data. Validation proceeds by constructing two separate segments
of the chain-of-trust. In the first segment, data at the level of
the split and below is validated by constructing a chain-of-trust
contained entirely within the internal view. If the trusted key is
configured at or below the level of the split, validation stops at
this point and queries are never sent to the outer view. If not, a
second validation chain segment is constructed from the DS record
covering the split to the trusted key. In forming the second
validation segment all queries (including the query for the DS record
of the split zone) are sent to the outer zone. Since the key
referenced in the DS record is present in the apex key-set of both
views, the chain-of-trust can be completed.
This approach allows flexibility in choosing the level at which the
trusted key is configured, with the possibility of using the same
trusted key 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. Partial Decoupling of Chains-Of-Trust
parent zone
(trusted) split zone split zone (trusted)
(internal) (external)
^ ^
| |
| |
sub split zone sub split zone
(internal) (external)
Figure 4
With the DS record in the parent always pointing to a key in the
outer view, the construction of the chain-of-trust becomes
problematic when the keys used to sign data in the two views of the
split are different. The trusted key cannot be configured above the
level of the split since there would be no way of linking the DS
record in the outer zone to the apex DNSKEY set in the internal view
of the split zone.
A simple solution is to configure the trusted key at the level of the
split such that the chains-of-trust for the internal and external
zones share no common records that might 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. Cache pollution can be detected because such cases would lead to
validation failures.
This configuration however involves more configuration overhead since
trusted keys 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 trusted
keys.
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3.4. Complete Decoupling of Chains-Of-Trust
(trusted) parent zone parent zone (trusted)
(internal) (external)
^ ^
| |
| |
split zone split zone
(internal) (external)
^ ^
| |
| |
sub split zone sub split zone
(internal) (external)
Figure 5
One problem with Section 3.3 is that trusted keys need to be
configured for every zone that is split under the parent. An option
to circumvent this while still retaining the advantages of the
earlier setup is to split the parent also, and configure the trusted
key at the level of the parent. An internal name server is
configured as the authoritative server for the internal view of this
split and the internal recursive forwarder is modified to forward all
internal queries for the parent zone to it.
Although this option reduces the number of trusted keys at the end
resolver, the trusted key still needs to change when moving between
the two views. Since splitting the parent essentially creates two
new zones, records in the parent that were previously common in both
views would now need to be duplicated in the two split 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.
3.5. Multiple DS Records
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-----------> parent zone (trusted)
/ (DS1) ^
/ |
/ | (DS2)
/ |
split zone split zone
(internal) (external)
^ ^
| |
| |
sub split zone sub split zone
(internal) (external)
Figure 6
In this approach, the parent is not split; however, DS records
corresponding to each of the two different views are published at the
delegation point. The glue and NS records at the delegation point
still corresponds to the external zone but this information is never
used by the internal zone. As in option 4, the trusted key is
configured at the level of the parent zone. The chain-of-trust from
this trusted key to either zone is formed by using one of the two DS
records, which ever is applicable at that view.
Since some coordination between the split zone and the parent 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 (e.g. for example.com, the split is made at a level
lower than example.com). This approach lends itself to using
different keys in different views while still allowing for minimal
configuration at the end resolvers; trusted keys 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 chain-of-trust.
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 the objective of the split-view setup this should not
really be a problem in most cases. An attendent problem with
multiple DS records is that since the validation algorithm
iteratively looks for a DS record in the parent while completing the
chain-of-trust there is some 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
include all DS records in the parent, especially if the child is a
part of a different organization. Lastly, the internal view is still
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susceptible to an insider "attack", where data from the outside view
injected in response to internal queries can corrupt the cache. This
attack is common to all scenarios that use a common key for
validating internal and external zone contents. Any cache pollution
introduced due to administrator errors can also escape detection for
the same reason.
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 trusted keys. It should be
possible to configure more than one trusted key.
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.
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
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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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
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.
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
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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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
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
Note that in this configuration, queries from all recursive name
servers in the boundary network for any external view information may
need to transit outward through the second-level packet filter and
then back again into the boundary network. If the existing packet
filter policy prevents such traffic patterns, all such recursive name
servers would need additional forwarding statements to forward these
queries directly to their respective authoritative name servers
without going through the packet filter.
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,
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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
trusted keys in end resolvers which may change between views. Proper
care must be taken to ensure that correct split-view behavior is
consistently maintained.
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 either result in cache pollution and cause
incorrect results to be returned, or impede the ability for end
resolvers to validate data returned in response to queries. An
improperly configured packet filter that allows errant DNS traffic
through or denies legitimate responses can lead to aggressive
retransmission of queries.
Each of the validation options outlined in Section 3 also introduce
their own security considerations. 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.
All approaches that use a common key for validating internal and
external data are also susceptible to an insider attack where data
from the outside view injected in response to internal queries can
corrupt the cache. On the other hand, using a multitude of keys at
end resolvers only increases the operator overhead and thus the
chances for configuration errors.
8. Acknowledgements
The contributions, suggestions and remarks of the following persons
to this draft are particularly acknowledged: Wesley Griffin, John
Kelley, Russ Mundy and Sam Weiler. 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@tislabs.com
URI: http://www.sparta.com
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