DNSOP W. Hardaker
Internet-Draft Parsons
Intended status: Best Current Practice O. Gudmundsson
Expires: February 12, 2017 CloudFlare
S. Krishnaswamy
Parsons
August 11, 2016
DNSSEC Roadblock Avoidance
draft-ietf-dnsop-dnssec-roadblock-avoidance-05.txt
Abstract
This document describes problems that a Validating DNS resolver,
stub-resolver or application might run into within a non-compliant
infrastructure. It outlines potential detection and mitigation
techniques. The scope of the document is to create a shared approach
to detect and overcome network issues that a DNSSEC software/system
may face.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 12, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Implementation experiences . . . . . . . . . . . . . . . 4
1.3.1. Test Zone Implementation . . . . . . . . . . . . . . 4
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Detecting DNSSEC Non-Compliance . . . . . . . . . . . . . . . 5
3.1. Determining DNSSEC support in recursive resolvers . . . . 6
3.1.1. Supports UDP answers . . . . . . . . . . . . . . . . 6
3.1.2. Supports TCP answers . . . . . . . . . . . . . . . . 6
3.1.3. Supports EDNS0 . . . . . . . . . . . . . . . . . . . 7
3.1.4. Supports the DO bit . . . . . . . . . . . . . . . . . 7
3.1.5. Supports the AD bit DNSKEY algorithm 5 and 8 . . . . 7
3.1.6. Returns RRsig for signed answer . . . . . . . . . . . 8
3.1.7. Supports querying for DNSKEY records . . . . . . . . 8
3.1.8. Supports querying for DS records . . . . . . . . . . 8
3.1.9. Supports negative answers with NSEC records . . . . . 9
3.1.10. Supports negative answers with NSEC3 records . . . . 9
3.1.11. Supports queries where DNAME records lead to an
answer . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.12. Permissive DNSSEC . . . . . . . . . . . . . . . . . . 10
3.1.13. Supports Unknown RRtypes . . . . . . . . . . . . . . 10
3.2. Direct Network Queries . . . . . . . . . . . . . . . . . 10
3.2.1. Support for Remote UDP Over Port 53 . . . . . . . . . 11
3.2.2. Support for Remote UDP With Fragmentation . . . . . . 11
3.2.3. Support for Outbound TCP Over Port 53 . . . . . . . . 11
3.3. Support for DNSKEY and DS combinations . . . . . . . . . 12
4. Aggregating The Results . . . . . . . . . . . . . . . . . . . 12
4.1. Resolver capability description . . . . . . . . . . . . . 12
5. Roadblock Avoidance . . . . . . . . . . . . . . . . . . . . . 13
5.1. Partial Resolver Usage . . . . . . . . . . . . . . . . . 16
5.1.1. Known Insecure Lookups . . . . . . . . . . . . . . . 16
5.1.2. Partial NSEC/NSEC3 Support . . . . . . . . . . . . . 16
6. Start-Up and Network Connectivity Issues . . . . . . . . . . 16
6.1. What To Do . . . . . . . . . . . . . . . . . . . . . . . 17
7. Quick Test . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Test negative answers Algorithm 5 . . . . . . . . . . . . 18
7.2. Test Algorithm 8 . . . . . . . . . . . . . . . . . . . . 18
7.3. Test Algorithm 13 . . . . . . . . . . . . . . . . . . . . 18
7.4. Fails when DNSSEC does not validate . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
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9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
11. Normative References . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
This document describes problems observable during DNSSEC ([RFC4034],
[RFC4035]) deployment that derive from non-compliant infrastructure.
It poses potential detection and mitigation techniques.
1.1. Notation
In this document a "Host Validator" can either be a validating stub-
resolver, such as library that an application has linked in, or a
validating resolver daemon running on the same machine. It may or
may not be trying to use upstream caching resolvers during its own
resolution process; both cases are covered by the tests defined in
this document.
The sub-variant of this is a "Validating Forwarding Resolver", which
is a resolver that is configured to use upstream Resolvers when
possible. A Validating Forward Resolver also needs to perform the
tests outlined in this document before using an upstream recursive
resolver.
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].
1.2. Background
Deployment of DNSSEC validation is hampered by network components
that make it difficult or sometimes impossible for validating
resolvers to effectively obtain the DNSSEC data they need. This can
occur for many different reasons including, but not limited to:
o Because recursive resolvers and DNS proxies [RFC5625] are not
fully DNSSEC compliant
o Because resolvers are not DNSSEC aware
o Because "middle-boxes" actively block, modify and/or restrict
outbound traffic to the DNS port (53) either UDP and/or TCP .
o In-path network components do not allow UDP fragments
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This document talks about ways that a Host Validator can detect the
state of the network it is attached to, and ways to hopefully
circumvent the problems associated with the network defects it
discovers. The tests described in this document may be performed on
any validating resolver to detect and prevent problems. While these
recommendations are mainly aimed at Host Validators it it prudent to
perform these tests from regular Validating Resolvers before enabling
just to make sure things work.
There are situations where a host can not talk directly to a
Resolver; the tests below can not address how to overcome that, and
inconsistent results can be seen in such cases. This can happen, for
instance, when there are DNS proxies/forwarders between the user and
the actual resolvers.
1.3. Implementation experiences
Multiple lessons learned from multiple implementations led to the
development of this document, including (in alphabetical order)
DNSSEC-Tools' DNSSEC-Check, DNSSEC_Resolver_Check, dnssec-trigger,
FCC_Grade.
Detecting lack of support for specified DNSKEY algorithms and DS
digest algorithms is outside the scope of this document but the
document provides information on how to do that, see sample test
tool: https://github.com/ogud/DNSSEC_ALG_Check
This document does describe compliance tests for algorithms 5, 7 and
13 with DS digest algorithms 1 and 2.
1.3.1. Test Zone Implementation
In this document, the "test.example.com" domain is used to refer to
DNS records which contain test records that have known DNSSEC
properties associated with them. For example, the "badsign-
a.test.example.com" domain is used below to refer to a DNS A record
where the signatures published for it are invalid (i.e., they are
"bad signatures" that should cause a validation failure).
At the time of this publication, the "test.dnssec-tools.org" domain
implements all of these test records. Thus, it may be possible to
replace "test.example.com" in this document with "test.dnssec-
tools.org" when performing real-world tests.
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2. Goals
This document is intended to show how a Host Validator can detect the
capabilities of a recursive resolver, and work around any problems
that could potentially affect DNSSEC resolution. This enables the
Host Validator to make use of the caching functionality of the
recursive resolver, which is desirable in that it decreases network
traffic and improves response times.
A Host Validator has two choices: it can wait to determine that it
has problems with a recursive resolver based on the results that it
is getting from real-world queries issued to it, or it can
proactively test for problems (Section 3) to build a work around list
ahead of time (Section 5). There are pros and cons to both of these
paths that are application specific, and this document does not
attempt to provide guidance about whether proactive tests should or
should not be used. Either way, DNSSEC roadblock avoidance
techniques ought to be used when needed and if possible.
Note: Besides being useful for Host Validators, the same tests can be
used for a recursive resolver to check if its upstream connections
hinder DNSSEC validation.
3. Detecting DNSSEC Non-Compliance
A Host Validator may choose to determine early-on what roadblocks
exist that may hamper its ability to perform DNSSEC look-ups. This
section outlines tests that can be done to test certain features of
the surrounding network.
These tests should be performed when a resolver determines its
network infrastructure has changed. Certainly a resolver should
perform these tests when first starting, but MAY also perform these
tests when they've detected network changes (e.g. address changes, or
network reattachment, etc).
NOTE: when performing these tests against an address, we make the
following assumption about that address: It is a uni-cast address or
an any-cast [RFC4786] cluster where all servers have identical
configuration and connectivity.
NOTE: when performing these tests we also assume that the path is
clear of "DNS interfering" middle-boxes, like firewalls, proxies,
forwarders. Presence of such infrastructure can easily make a
recursive resolver appear to be improperly performing. It is beyond
the scope of the document as how to work around such interference,
although the tests defined in this document may indicate when such
misbehaving middle-ware is causing interference.
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NOTE: This document specifies two sets of tests to perform: a
comprehensive one and a fast one. The fast one will detect most
common problems, thus if the fast one passes then the comprehensive
MAY be considered passed as well.
3.1. Determining DNSSEC support in recursive resolvers
Ideally, a Host Validator can make use of the caching present in
recursive resolvers. This section discusses the tests that a
recursive resolver MUST pass in order to be fully usable as a DNS
cache.
Unless stated otherwise, all of the following tests SHOULD have the
Recursion Desired (RD) flag set when sending out a query and SHOULD
be sent over UDP. Unless otherwise stated, the tests MUST NOT have
the DO bit set or utilize any of the other DNSSEC related
requirements, like EDNS0, unless otherwise specified. The tests are
designed to check for support of one feature at a time.
3.1.1. Supports UDP answers
Purpose: This tests basic DNS over UDP functionality to a resolver.
Test: A DNS request is sent to the resolver under test for an A
record for a known existing domain, such as good-a.test.example.com.
SUCCESS: A DNS response was received that contains an A record in the
answer section. (The data itself does not need to be checked.)
Note: an implementation MAY chose to not perform the rest of the
tests if this test fails, as it is highly unlikely that the resolver
under test will pass any of the remaining tests.
3.1.2. Supports TCP answers
Purpose: This tests basic TCP functionality to a resolver.
Test: A DNS request is sent over TCP to the resolver under test for
an A record for a known existing domain, such as good-
a.test.example.com.
SUCCESS: A DNS response was received that contains an A record in the
answer section. (The data itself does not need to be checked.)
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3.1.3. Supports EDNS0
Purpose: Test whether a resolver properly supports the EDNS0
extension option.
Pre-requisite: "Supports UDP or TCP".
Test: Send a request to the resolver under test for an A record for a
known existing domain, such as good-a.test.example.com, with an EDNS0
OPT record in the additional section.
SUCCESS: A DNS response was received that contains an EDNS0 option
with version number 0.
3.1.4. Supports the DO bit
Purpose: This tests whether a resolver has minimal support of the DO
bit.
Pre-requisite: "Supports EDNS0".
Test: Send a request to the resolver under test for an A record for a
known existing domain such as good-a.test.example.com. Set the DO
bit in the outgoing query.
SUCCESS: A DNS response was received that contains the DO bit set.
Note: this only tests that the resolver sets the DO bit in the
response. Later tests will determine if the DO bit was actually made
use of. Some resolvers successfully pass this test because they
simply copy the unknown flags into the response. These resolvers
will fail the later tests.
3.1.5. Supports the AD bit DNSKEY algorithm 5 and 8
Purpose: This tests whether the resolver is a validating resolver.
Pre-requisite: "Supports the DO bit".
Test: Send requests to the resolver under test for an A record for a
known existing domain in a DNSSEC signed zone which is verifiable to
a configured trust anchor, such as good-a.test.example.com using the
root's published DNSKEY or DS record as a trust anchor. Set the DO
bit in the outgoing query. This test should be done twice, once for
a zone that contains algorithm 5 (RSASHA1) and another for algorithm
8 (RSASHA256).
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SUCCESS: A DNS response was received that contains the AD bit set for
algorithm 5 (RSASHA1).
BONUS: The AD bit is set for a resolver that supports Algorithm 8
RSASHA256
3.1.6. Returns RRsig for signed answer
Purpose: This tests whether a resolver will properly return RRSIG
records when the DO bit is set.
Pre-requisite: "Supports the DO bit".
Test: Send a request to the resolver under test for an A record for a
known existing domain in a DNSSEC signed zone, such as good-
a.test.example.com. Set the DO bit in the outgoing query.
SUCCESS: A DNS response was received that contains at least one RRSIG
record.
3.1.7. Supports querying for DNSKEY records
Purpose: This tests whether a resolver can query for and receive a
DNSKEY record from a signed zone.
Pre-requisite: "Supports the DO bit."
Test: Send a request to the resolver under test for an DNSKEY record
which is known to exist in a signed zone, such as test.example.com/
DNSKEY. Set the DO bit in the outgoing query.
SUCCESS: A DNS response was received that contains a DNSKEY record in
the answer section.
Note: Some DNSKEY RRset's are large and if the network path has
problems with large answers this query may result in either false
positive or false negative. In general the DNSKEY queried for should
be small enough to fit into a 1220 byte answer, to avoid false
negative result when TCP is disabled. However, querying many zones
will result in answers greater than 1220 bytes so DNS over TCP MUST
be available for DNSSEC use in general.
3.1.8. Supports querying for DS records
Purpose: This tests whether a resolver can query for and receive a DS
record from a signed zone.
Pre-requisite: "Supports the DO bit."
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Test: Send a request to the resolver under test for an DS record
which is known to exist in a signed zone, such as test.example.com/
DS. Set the DO bit in the outgoing query.
SUCCESS: A DNS response was received that contains a DS record in the
answer section.
3.1.9. Supports negative answers with NSEC records
Purpose: This tests whether a resolver properly returns NSEC records
for a non-existing domain in a DNSSEC signed zone.
Pre-requisite: "Supports the DO bit."
Test: Send a request to the resolver under test for an A record which
is known to not exist in an NSEC signed zone, such as non-
existent.test.example.com. Set the DO bit in the outgoing query.
SUCCESS: A DNS response was received that contains an NSEC record.
Note: The query issued in this test MUST be sent to a NSEC signed
zone. Getting back appropriate NSEC3 records does not indicate a
failure, but a bad test.
3.1.10. Supports negative answers with NSEC3 records
Purpose: This tests whether a resolver properly returns NSEC3 records
([RFC5155]) for a non-existing domain in a DNSSEC signed zone.
Pre-requisite: "Supports the DO bit."
Test: Send a request to the resolver under test for an A record which
is known to be non-existent in a zone signed using NSEC3, such as
non-existent.nsec3-ns.test.example.com. Set the DO bit in the
outgoing query.
SUCCESS: A DNS response was received that contains an NSEC3 record.
Bonus: If the AD bit is set, this validator supports algorithm 7
RSASHA1-NSEC3-SHA1
Note: The query issued in this test MUST be sent to a NSEC3 signed
zone. Getting back appropriate NSEC records does not indicate a
failure, but a bad test.
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3.1.11. Supports queries where DNAME records lead to an answer
Purpose: This tests whether a resolver can query for an A record in a
zone with a known DNAME referral for the record's parent.
Test: Send a request to the resolver under test for an A record which
is known to exist in a signed zone within a DNAME referral child
zone, such as good-a.dname-good-ns.test.example.com.
SUCCESS: A DNS response was received that contains a DNAME in the
answer section. An RRSIG MUST also be received in the answer section
that covers the DNAME record.
3.1.12. Permissive DNSSEC
Purpose: To see if a validating resolver is ignoring DNSSEC
validation failures.
Pre-requisite: Supports the AD bit.
Test: ask for data from a broken DNSSEC delegation such as badsign-
a.test.example.com.
SUCCESS: A reply was received with the Rcode set to SERVFAIL
3.1.13. Supports Unknown RRtypes
Purpose: Some DNS Resolvers/gateways only support some RRtypes. This
causes problems for applications that need recently defined types.
Pre-requisite: "Supports UDP or TCP".
Test: Send a request for recently defined type or unknown type in the
20000-22000 range, that resolves to a server that will return an
answer for all types, such as alltypes.example.com (a server today
that supports this: alltypes.res.dnssecready.org)
SUCCESS: A DNS response was retrieved that contains the type
requested in the answer section.
3.2. Direct Network Queries
If need be, a Host Validator may need to make direct queries to
authoritative servers or known Open Recursive Resolvers in order to
collect data. To do that, a number of key network features MUST be
functional.
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3.2.1. Support for Remote UDP Over Port 53
Purpose: This tests basic UDP functionality to outside the local
network.
Test: A DNS request is sent to a known distant authoritative server
for a record known to be within that server's authoritative data.
Example: send a query to the address of ns1.test.example.com for the
good-a.test.example.com/A record.
SUCCESS: A DNS response was received that contains an A record in the
answer section.
Note: an implementation can use the local resolvers for determining
the address of the name server that is authoritative for the given
zone. The recursive bit MAY be set for this request, but does not
need to be.
3.2.2. Support for Remote UDP With Fragmentation
Purpose: This tests if the local network can receive fragmented UDP
answers
Pre-requisite: Local UDP traffic > 1500 in size is possible
Test: A DNS request is sent over UDP to a known distant DNS address
asking for a record that has answer larger than 2000 bytes. For
example, send a query for the test.example.com/DNSKEY record with the
DO bit set in the outgoing query.
Success: A DNS response was received that contains the large answer.
Note: A failure in getting large answers over UDP is not a serious
problem if TCP is working.
3.2.3. Support for Outbound TCP Over Port 53
Purpose: This tests basic TCP functionality to outside the local
network.
Test: A DNS request is sent over TCP to a known distant authoritative
server for a record known to be within that server's authoritative
data. Example: send a query to the address of ns1.test.example.com
for the good-a.test.example.com/A record.
SUCCESS: A DNS response was received that contains an A record in the
answer section.
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Note: an implementation can use the local resolvers for determining
the address of the name server that is authoritative for the given
zone. The recursive bit MAY be set for this request, but does not
need to be.
3.3. Support for DNSKEY and DS combinations
Purpose: These tests can check what algorithm combinations are
supported.
Pre-requisite: At least one of above tests has returned the AD bit
set proving that the upstream is validating
Test: A DNS request is sent over UDP to the resolver under test for a
known combination of the DS algorithm number (N) and DNSKEY algorithm
number (M) of the example form ds-N.alg-M-nsec.test.example.com.
Examples:
ds-2.alg-13-nsec.test.example.com TXT
or
ds-4.alg-13-nsec3.test.example.com TXT.
SUCCESS: a DNS response is received with the AD bit set and with a
matching record type in the answer section.
Note: for algorithms 6 and 7, NSEC is not defined thus query for alg-
M-nsec3 is required. Similarly NSEC3 is not defined for algorithms
1, 3 and 5. Furthermore algorithms 2, 4, 9, 11 do not currently have
definitions for signed zones.
4. Aggregating The Results
Some conclusions can be drawn from the results of the above tests in
an "aggregated" form. This section defines some labels to assign to
a resolver under test given the results of the tests run against
them.
4.1. Resolver capability description
This section will group and label certain common results
Resolvers are classified into following broad behaviors:
Validator: The resolver passes all DNSSEC tests and had the AD bit
appropriately set.
DNSSEC Aware: The resolver passes all DNSSEC tests, but does not
appropriately set the AD bit on answers, indicating it is not
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validating. A Host Validator will function fine using this
resolver as a forwarder.
Non-DNSSEC capable: The resolver is not DNSSEC aware and will make
it hard for a Host Validator to operate behind it. It MAY be
usable for querying for data that is in known insecure sections of
the DNS tree.
Not a DNS Resolver: This is a improperly behaving resolver and not
should not be used at all.
While it would be great if all resolvers fell cleanly into one of the
broad categories above, that is not the case. For that reason it is
necessary to augment the classification with more descriptive result,
this is done by adding the word "Partial" in front of Validator/
DNSSEC Aware classifications, followed by sub-descriptors of what is
not working.
Unknown: Failed the Unknown test
DNAME: Failed the DNAME test
NSEC3: Failed the NSEC3 test
TCP: TCP not available
SlowBig: UDP is size limited but TCP fallback works
NoBig: TCP not available and UDP is size limited
Permissive: Passes data known to fail validation
5. Roadblock Avoidance
The goal of this document is to tie the above tests and aggregations
to avoidance practices; however the document does not specify exactly
how to do that.
Once we have determined what level of support is available in the
network, we can determine what must be done in order to effectively
act as a validating resolver. This section discusses some of the
options available given the results from the previous sections.
The general fallback approach can be described by the following
sequence:
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If the resolver is labeled as "Validator" or "DNSSEC aware":
Send queries through this resolver and perform local
validation on the results.
If validation fails, try the next resolver
Else if the resolver is labeled "Not a DNS Resolver" or
"Non-DNSSEC capable":
Mark it as unusable and try next resolver
Else if no more resolvers are configured and if direct queries
are supported:
1. Try iterating from the Root
2. If the answer is SECURE/BOGUS:
Return the result of the iteration
3. If the answer is INSECURE:
Re-query "Non-DNSSEC capable" servers and return
answers from them w/o the AD bit set to the client.
This will increase the likelihood that split-view unsigned
answers are found.
Else:
Return an error code and log a failure
While attempting resolution through a particular recursive name
server with a particular transport method that worked, any transport-
specific parameters MUST be remembered in order to avoid any
unnecessary fallback attempts.
Transport-specific parameters MUST also be remembered for each
authoritative name server that is queried while performing an
iterative mode lookup.
Any transport settings that are remembered for a particular name
server MUST be periodically refreshed; they should also be refreshed
when an error is encountered as described below.
For a stub resolver, problems with the name server can manifest
themselves under the following types of error conditions:
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o No Response, error response or missing DNSSEC meta-data
o Illegal Response: An illegal response is received, which prevents
the validator from fetching all necessary records required for
constructing an authentication chain. This could result when
referral loops are encountered, when any of the antecedent zone
delegations are lame, when aliases are erroneously followed for
certain RRtypes (such as SOA, DNSKEYs or DS records), or when
resource records for certain types (e.g. DS) are returned from a
zone that is not authoritative for such records.
o Bogus Response: A Bogus Response is received, when the
cryptographic assertions in the authentication chain do not
validate properly.
For each of the above error conditions a validator MAY adopt the
following dynamic fallback technique, preferring a particular
approach if it is known to work for a given name server or zone from
previous attempts.
o No response, error response, or missing DNSSEC meta-data:
* Re-try with different EDNS0 sizes (4096, 1492, None)
* Re-try with TCP only
* Perform an iterative query starting from the Root if the
previous error was returned from a lookup that had recursion
enabled.
* Re-try using an alternative transport method, if this
alternative method is known (configured) to be supported by the
nameserver in question.
o Illegal Response
* Perform an iterative query starting from the Root if the
previous error was returned from a lookup that had recursion
enabled.
* Check if any of the antecedent zones up to the closest
configured trust anchor are provably insecure.
o Bogus Response
* Perform an iterative query starting from the Root if the
previous error was returned from a lookup that had recursion
enabled.
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For each fallback technique, attempts to multiple potential name
servers should be skewed such that the next name server is tried when
the previous one encounters an error, a timeout is reached, or
whichever is earlier.
The validator SHOULD remember, in its zone-specific fallback cache,
any broken behavior identified for a particular zone for a duration
of that zone's SOA negative TTL.
The validator MAY place name servers that exhibit broken behavior
into a blacklist, and bypass these name servers for all zones that
they are authoritative for. The validator MUST time out entries in
this name server blacklist periodically, where this interval could be
set to be the same as the DNSSEC BAD cache default TTL.
5.1. Partial Resolver Usage
It may be possible to use Non-DNSSEC Capable caching resolvers in
careful ways if maximum optimization is desired. This section
describes some of the advanced techniques that could be used to use a
resolver in at least a minimal way. Most of the time this would be
unnecessary, except in the case where none of the resolvers are fully
compliant and thus the choices would be to use them at least
minimally or not at all (and no caching benefits would be available).
5.1.1. Known Insecure Lookups
If a resolver is Non-DNSSEC Capable but a section of the DNS tree has
been determined to be Provably Insecure [RFC4035], then queries to
this section of the tree MAY be sent through Non-DNSSEC Capable
caching resolver.
5.1.2. Partial NSEC/NSEC3 Support
Resolvers that understand DNSSEC generally, and understand NSEC but
not NSEC3 are partially usable. These resolvers generally also lack
support for Unknown types, rendering them mostly useless and to be
avoided.
6. Start-Up and Network Connectivity Issues
A number of scenarios will produce either short-term or long-term
connectivity issues with respect to DNSSEC validation. Consider the
following cases:
Time Synchronization: Time synchronization problems can occur when
a device which has been off for a period of time and the clock is
no longer in close synchronization with "real time" or when a
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device always has clock set to the same time during start-up.
This will cause problems when the device needs to resolve their
source of time synchronization, such as "ntp.example.com".
Changing Network Properties: A newly established network
connection may change state shortly after a HTTP-based pay-wall
authentication system has been used. This especially common in
hotel, airport and coffee-shop style networks, where DNSSEC,
validation and even DNS are not functional until the user proceeds
through a series of forced web pages used to enable their network.
The tests in Section 3 will produce very different results before
and after the network authorization has succeeded. APIs exist on
many operating systems to detect initial network device status
changes, such as right after DHCP has finished, but few (none?)
exist to detect that authentication through a pay-wall has
succeeded.
There are only two choices when situations like this happen:
Continue to perform DNSSEC processing, which will likely result in
all DNS requests failing. This is the most secure route, but
causes the most operational grief for users.
Turn off DNSSEC support until the network proves to be usable.
This allows the user to continue using the network, at the
sacrifice of security. It also allows for a denial of security-
service attack if a man-in-the-middle can convince a device that
DNSSEC is impossible.
6.1. What To Do
If the Host Validator detects that DNSSEC resolution is not possible
it SHOULD log the event and/or SHOULD report an error to the user.
In the case there is no user, then no reporting can be performed and
thus the device MAY have a policy of action, like continue or fail.
Until middle boxes allow DNSSEC protected information to traverse
them consistently, software implementations may need to offer this
choice to let users pick the security level they require. Note that
continuing without DNSSEC protection in the absence of a notification
or report could lead to situations where users assume a level of
security that does not exist.
7. Quick Test
The quick tests defined below make the assumption that the questions
to be asked are of a real resolver and the only real question is:
"how complete is the DNSSEC support?". This quick test as been
implemented in few programs developed at IETF hackthons at IETF-91
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and IETF-92. The programs use a common grading method. For each
question that returns expected answer the resolver gets a point. If
the AD bit is set as expected the resolver gets a second point.
7.1. Test negative answers Algorithm 5
Query: realy-doesnotexist.test.example.com. A
Answer: RCODE= NXDOMAIN, Empty Answer, Authority: NSEC proof
7.2. Test Algorithm 8
Query: alg-8-nsec3.test.example.com. SOA
Answer: RCODE= 0, Answer: SOA record
7.3. Test Algorithm 13
Query: alg-13-nsec.test.example.com. SOA
Answer: RCODE= 0, Answer: SOA record
7.4. Fails when DNSSEC does not validate
Query: dnssec-failed.test.example.com. SOA
Answer: RCODE= SERVFAIL, empty answer, and authority, AD=0
8. Security Considerations
This document discusses problems that may occur while deploying the
DNSSEC protocol. It describes what may be possible to help detect
and mitigate these problems. Following the outlined suggestions will
result in a more secure DNSSEC operational environment than if DNSSEC
was simply disabled.
9. IANA Considerations
No IANA actions are required.
10. Acknowledgments
We thank the IESG and DNSOP working group members for their extensive
comments and suggestions.
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11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <http://www.rfc-editor.org/info/rfc4786>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
<http://www.rfc-editor.org/info/rfc5625>.
Authors' Addresses
Wes Hardaker
Parsons
P.O. Box 382
Davis, CA 95617
US
Email: ietf@hardakers.net
Olafur Gudmundsson
CloudFlare
San Francisco, CA 94107
USA
Email: olafur+ietf@cloudflare.com
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Suresh Krishnaswamy
Parsons
7110 Samuel Morse Dr
Columbia, MD 21046
US
Email: suresh@tislabs.com
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