Internet DRAFT - draft-andrews-dns-no-response-issue
draft-andrews-dns-no-response-issue
Network Working Group M. Andrews
Internet-Draft ISC
Intended status: Best Current Practice November 26, 2015
Expires: May 29, 2016
A Common Operational Problem in DNS Servers - Failure To Respond.
draft-andrews-dns-no-response-issue-16
Abstract
The DNS is a query / response protocol. Failure to respond or to
respond correctly to queries causes both immediate operational
problems and long term problems with protocol development.
This document identifies a number of common classes of queries to
which some servers either fail to respond or else respond
incorrectly. This document also suggests procedures for TLD and
other similar zone operators to apply to help reduce / eliminate the
problem.
The document does not look at the DNS data itself, just the structure
of the responses.
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
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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 May 29, 2016.
Copyright Notice
Copyright (c) 2015 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Common queries class that result in non responses. . . . . . 3
2.1. EDNS Queries - Version Independent . . . . . . . . . . . 4
2.2. EDNS Queries - Version Specific . . . . . . . . . . . . . 4
2.3. EDNS Options . . . . . . . . . . . . . . . . . . . . . . 4
2.4. EDNS Flags . . . . . . . . . . . . . . . . . . . . . . . 4
2.5. DNS Flags . . . . . . . . . . . . . . . . . . . . . . . . 5
2.6. Unknown / Unsupported Type Queries . . . . . . . . . . . 5
2.7. Unknown DNS opcodes . . . . . . . . . . . . . . . . . . . 5
2.8. TCP Queries . . . . . . . . . . . . . . . . . . . . . . . 5
3. Remediating . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Firewalls and Load Balancers . . . . . . . . . . . . . . . . 7
5. Scrubbing Services . . . . . . . . . . . . . . . . . . . . . 8
6. Whole Answer Caches . . . . . . . . . . . . . . . . . . . . . 9
7. Response Code Selection . . . . . . . . . . . . . . . . . . . 9
8. Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Testing - Basic DNS . . . . . . . . . . . . . . . . . . . 10
8.2. Testing - Extended DNS . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11. Normative References . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
The DNS [RFC1034], [RFC1035] is a query / response protocol. Failure
to respond to queries or to respond incorrectly causes both immediate
operational problems and long term problems with protocol
development.
Failure to respond to a query is indistinguishable from a packet loss
without doing a analysis of query response patterns and results in
unnecessary additional queries being made by DNS clients and
unnecessary delays being introduced to the resolution process.
Due to the inability to distinguish between packet loss and
nameservers dropping EDNS [RFC6891] queries, packet loss is sometimes
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misclassified as lack of EDNS support which can lead to DNSSEC
validation failures.
Allowing servers which fail to respond to queries to remain results
in developers being afraid to deploy implementations of recent
standards. Such servers need to be identified and corrected /
replaced.
The DNS has response codes that cover almost any conceivable query
response. A nameserver should be able to respond to any conceivable
query using them.
Unless a nameserver is under attack, it should respond to all queries
directed to it as a result of following delegations. Additionally
code should not assume that there isn't a delegation to the server
even if it is not configured to serve the zone. Broken delegations
are a common occurrence in the DNS and receiving queries for zones
that you are not configured for is not a necessarily a indication
that you are under attack. Parent zone operators are supposed to
regularly check that the delegating NS records are consistent with
those of the delegated zone and to correct them when they are not
[RFC1034]. If this was being done regularly, the instances of broken
delegations would be much lower.
When a nameserver is under attack it may wish to drop packets. A
common attack is to use a nameserver as a amplifier by sending
spoofed packets. This is done because response packets are bigger
than the queries and big amplification factors are available
especially if EDNS is supported. Limiting the rate of responses is
reasonable when this is occurring and the client should retry. This
however only works if legitimate clients are not being forced to
guess whether EDNS queries are accept or not. While there is still a
pool of servers that don't respond to EDNS requests, clients have no
way to know if the lack of response is due to packet loss, EDNS
packets not being supported or rate limiting due to the server being
under attack. Mis-classifications of server characteristics are
unavoidable when rate limiting is done.
2. Common queries class that result in non responses.
There are three common query classes that result in non responses
today. These are EDNS queries, queries for unknown (unallocated) or
unsupported types, and filtering of TCP queries.
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2.1. EDNS Queries - Version Independent
Identifying servers that fail to respond to EDNS queries can be done
by first identifying that the server responds to regular DNS queries,
followed by a series of otherwise identical responses using EDNS,
then making the original query again. A series of EDNS queries is
needed as at least one DNS implementation responds to the first EDNS
query with FORMERR but fails to respond to subsequent queries from
the same address for a period until a regular DNS query is made. The
EDNS query should specify a UDP buffer size of 512 bytes to avoid
false classification of not supporting EDNS due to response packet
size.
If the server responds to the first and last queries but fails to
respond to most or all of the EDNS queries, it is probably faulty.
The test should be repeated a number of times to eliminate the
likelihood of a false positive due to packet loss.
Firewalls may also block larger EDNS responses but there is no easy
way to check authoritative servers to see if the firewall is
misconfigured.
2.2. EDNS Queries - Version Specific
Some servers respond correctly to EDNS version 0 queries but fail to
respond to EDNS queries with version numbers that are higher than
zero. Servers should respond with BADVERS to EDNS queries with
version numbers that they do not support.
Some servers respond correctly to EDNS version 0 queries but fail to
set QR=1 when responding to EDNS versions they do not support. Such
answers are discarded or treated as requests.
2.3. EDNS Options
Some servers fail to respond to EDNS queries with EDNS options set.
Unknown EDNS options are supposed to be ignored by the server
[RFC6891].
2.4. EDNS Flags
Some servers fail to respond to EDNS queries with EDNS flags set.
Server should ignore EDNS flags there do not understand and should
not add them to the response [RFC6891].
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2.5. DNS Flags
Some servers fail to respond to DNS queries with various DNS flags
set, regardless of whether they are defined or still reserved. At
the time of writing there are servers that fail to respond to queries
with the AD bit set to 1 and servers that fail to respond to queries
with the last reserved flag bit set.
2.6. Unknown / Unsupported Type Queries
Identifying servers that fail to respond to unknown or unsupported
types can be done by making an initial DNS query for an A record,
making a number of queries for an unallocated type, them making a
query for an A record again. IANA maintains a registry of allocated
types.
If the server responds to the first and last queries but fails to
respond to the queries for the unallocated type, it is probably
faulty. The test should be repeated a number of times to eliminate
the likelihood of a false positive due to packet loss.
2.7. Unknown DNS opcodes
The use of previously undefined opcodes is to be expected. Since the
DNS was first defined two new opcodes have been added, UPDATE and
NOTIFY.
NOTIMP is the expected rcode to an unknown / unimplemented opcode.
Note: while new opcodes will most probably use the current layout
structure for the rest of the message there is no requirement than
anything other than the DNS header match.
2.8. TCP Queries
All DNS servers are supposed to respond to queries over TCP
[RFC5966]. Firewalls that drop TCP connection attempts rather that
resetting the connect attempt or send a ICMP/ICMPv6 administratively
prohibited message introduce excessive delays to the resolution
process.
Whether a server accepts TCP connections can be tested by first
checking that it responds to UDP queries to confirm that it is up and
operating, then attempting the same query over TCP. An additional
query should be made over UDP if the TCP connection attempt fails to
confirm that the server under test is still operating.
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3. Remediating
While the first step in remediating this problem is to get the
offending nameserver code corrected, there is a very long tail
problem with DNS servers in that it can often take over a decade
between the code being corrected and a nameserver being upgraded with
corrected code. With that in mind it is requested that TLD, and
other similar zone operators, take steps to identify and inform their
customers, directly or indirectly through registrars, that they are
running such servers and that the customers need to correct the
problem.
TLD operators are being asked to do this as they, due to the nature
of running a TLD and the hierarchical nature of the DNS, have access
to a large numbers of nameserver names as well as contact details for
the registrants of those nameservers. One can construct lists of
nameservers from other sources and that has been done to survey the
state of the Internet, but that doesn't give you the contact details
necessary to inform the operators. The SOA RNAME is often invalid
and whois data is obscured and / or not available which makes it
infeasible for others to do this.
TLD operators should construct a list of servers child zones are
delegated to along with a delegated zone name. This name shall be
the query name used to test the server as it is supposed to exist.
For each server the TLD operator shall make an SOA query of the
delegated zone name. This should result in the SOA record being
returned in the answer section. If the SOA record is not returned
but some other response is returned, this is a indication of a bad
delegation and the TLD operator should take whatever steps it
normally takes to rectify a bad delegation. If more that one zone is
delegated to the server, it should choose another zone until it finds
a zone which responds correctly or it exhausts the list of zones
delegated to the server.
If the server fails to get a response to a SOA query, the TLD
operator should make an A query as some nameservers fail to respond
to SOA queries but respond to A queries. If it gets no response to
the A query, another delegated zone should be queried for as some
nameservers fail to respond to zones they are not configured for. If
subsequent queries find a responding zone, all delegation to this
server need to be checked and rectified using the TLD's normal
procedures.
Having identified a working <server, query name> tuple the TLD
operator should now check that the server responds to EDNS, Unknown
Query Type and TCP tests as described above. If the TLD operator
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finds that server fails any of the tests, the TLD operator shall take
steps to inform the operator of the server that they are running a
faulty nameserver and that they need to take steps to correct the
matter. The TLD operator shall also record the <server, query name>
for follow-up testing.
If repeated attempts to inform and get the customer to correct /
replace the faulty server are unsuccessful the TLD operator shall
remove all delegations to said server from the zone.
It will also be necessary for TLD operators to repeat the scans
periodically. It is recommended that this be performed monthly
backing off to bi-annually once the numbers of faulty servers found
drops off to less than 1 in 100000 servers tested. Follow-up tests
for faulty servers still need to be performed monthly.
Some operators claim that they can't perform checks at registration
time. If a check is not performed at registration time, it needs to
be performed within a week of registration in order to detect faulty
servers swiftly.
Checking of delegations by TLD operators should be nothing new as
they have been required from the very beginnings of DNS to do this
[RFC1034]. Checking for compliance of nameserver operations should
just be a extension of such testing.
It is recommended that TLD operators setup a test web page which
performs the tests the TLD operator performs as part of their regular
audits to allow nameserver operators to test that they have correctly
fixed their servers. Such tests should be rate limited to avoid
these pages being a denial of service vector.
4. Firewalls and Load Balancers
Firewalls and load balancers can affect the externally visible
behaviour of a nameserver. Tests for conformance need to be done
from outside of any firewall so that the system as a whole is tested.
Firewalls and load balancers should not drop DNS packets that they
don't understand. They should either pass through the packets or
generate an appropriate error response.
Requests for unknown query types are not attacks and should not be
treated as such.
Requests with unassigned flags set (DNS or EDNS) are not attacks and
should not be treated as such. The behaviour for unassigned is to
ignore them in the request and to not set them in the response. All
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dropping DNS / EDNS packets with unassigned flags does is make it
harder to deploy extensions that make use of them due to the need to
reconfigure / update firewalls.
Requests with unknown EDNS options are not an attack and should not
be treated as such. The correct behaviour for unknown EDNS options
is to ignore them.
Requests with unknown EDNS versions are not a attack and should not
be treated as such. The correct behaviour for unknown EDNS versions
is to return BADVERS along with the highest EDNS version the server
supports. All dropping EDNS packets does is break EDNS version
negotiation.
Firewalls should not assume that there will only be a single response
message to a requests. There have been proposals to use EDNS to
signal that multiple DNS messages be returned rather than a single
UDP message that is fragmented at the IP layer.
5. Scrubbing Services
Scrubbing services, like firewalls, can affect the externally visible
behaviour of a nameserver. If you use a scrubbing service, you
should check that legitimate queries are not being blocked.
Scrubbing services, unlike firewalls, are also turned on and off in
response to denial of service attacks. One needs to take care when
choosing a scrubbing service and ask questions like:
Do they pass unknown DNS query types?
Do they pass unknown EDNS versions?
Do they pass unknown EDNS options?
Do they pass unknown EDNS flags?
Do they pass requests with unknown DNS opcodes?
Do they pass requests with the remaining reserved DNS header flag
bit set?
All of these are not attack vectors but some scrubbing services treat
them as such.
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6. Whole Answer Caches
Whole answer caches can return the wrong response to a query if they
do not take all of the query into account. This has implications
when testing and with overall protocol compliance.
e.g. There are whole answer caches that ignore the EDNS version
field which results in incorrect answers to non EDNS version 0
queries being returned if they were proceeded by a EDNS version 0
query for the same name and type.
7. Response Code Selection
Choosing the correct response code when fixing a nameserver is
important. Just because a type is not implemented does not mean that
NOTIMP is the correct response code to return. Response codes need
to be chosen considering how clients will handle them.
For unimplemented opcodes NOTIMP is the expected response code.
Additionally a new opcode could change the message format by
extending the header or changing the structure of the records etc.
This may result in FORMERR being returned though NOTIMP would be more
correct.
In general, for unimplemented type codes Name Error (NXDOMAIN) and
NOERROR (no data) are the expected response codes. A server is not
supposed to serve a zone which contains unsupported types ([RFC1034])
so the only thing left is return if the QNAME exists or not. NOTIMP
and REFUSED are not useful responses as they force the clients to try
all the authoritative servers for a zone looking for a server which
will answer the query.
Meta queries type may be the exception but these need to be thought
about on a case by case basis.
If you support EDNS and get a query with an unsupported EDNS version,
the correct response is BADVERS [RFC6891].
If you do not support EDNS at all, FORMERR and NOTIMP are the
expected error codes. That said a minimal EDNS server implementation
just requires parsing the OPT records and responding with an empty
OPT record. There is no need to interpret any EDNS options present
in the request as unsupported options are expected to be ignored
[RFC6891].
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8. Testing
Testing is broken into two sections. Basic DNS which all servers
should meet and Extended DNS which should be met by all servers that
support EDNS.
It is advisable to run all the below tests in parallel so as to
minimise the delays due to multiple timeouts when the servers do not
respond.
The below tests use dig from BIND 9.11.0 which is still in
development.
8.1. Testing - Basic DNS
This first set of tests cover basic DNS server behaviour and all
servers should pass these tests.
Verify the server is configured for the zone:
dig +noedns +noad +norec soa $zone @$server
expect: status: NOERROR
expect: SOA record
expect: flag: aa to be present
Check that TCP queries work:
dig +noedns +noad +norec +tcp soa $zone @$server
expect: status: NOERROR
expect: SOA record
expect: flag: aa to be present
The requirement that TCP be supported is defined in [RFC5966].
Check that queries for an unknown type to work:
dig +noedns +noad +norec type1000 $zone @$server
expect: status: NOERROR
expect: an empty answer section.
expect: flag: aa to be present
That new types are to be expected is specified in Section 3.6,
[RFC1035]. Servers that don't support a new type are expected to
reject a zone that contains a unsupported type as per Section 5.2,
[RFC1035]. This means that a server that does load a zone can answer
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questions for unknown types with NOERROR or NXDOMAIN as per
Section 4.3.2, [RFC1034]. [RFC5395] later reserved distinct ranges
for meta and data types which allows servers to be definitive about
whether a query should be answerable from zone content or not.
Check that queries with CD=1 work:
dig +noedns +noad +norec +cd soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: flag: aa to be present
CD use in queries is defined in [RFC4035].
Check that queries with AD=1 work:
dig +noedns +norec +ad soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: flag: aa to be present
AD use in queries is defined in [RFC6840].
Check that queries with the last unassigned DNS header flag to work
and that the flag bit is not copied to the response:
dig +noedns +noad +norec +zflag soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: MBZ to not be in the response
expect: flag: aa to be present
MBZ (Must Be Zero) presence indicates the flag bit has been
incorrectly copied. See Section 4.1.1, [RFC1035] "Z Reserved for
future use. Must be zero in all queries and responses."
Check that new opcodes are handled:
dig +noedns +noad +opcode=15 +norec +header-only @$server
expect: status: NOTIMP
expect: SOA record to not be present
expect: flag: aa to NOT be present
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As unknown opcodes have no definition, including packet format other
than there must be a DNS header present, there is only one possible
rcode that make sense to return to a request with a unknown opcode
and that is NOTIMP.
8.2. Testing - Extended DNS
The next set of test cover various aspects of EDNS behaviour. If any
of these tests succeed, then all of them should succeed. There are
servers that support EDNS but fail to handle plain EDNS queries
correctly so a plain EDNS query is not a good indicator of lack of
EDNS support.
Check that plain EDNS queries work:
dig +nocookie +edns=0 +noad +norec soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: OPT record to be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
+nocookie disables sending a EDNS COOKIE option in which is on by
default.
Check that EDNS version 1 queries work (EDNS supported):
dig +nocookie +edns=1 +noednsneg +noad +norec soa $zone @$server
expect: status: BADVERS
expect: SOA record to not be present
expect: OPT record to be present
expect: EDNS Version 0 in response
expect: flag: aa to NOT be present
Only EDNS Version 0 is currently defined so the response should
always be a 0 version. This will change when EDNS version 1 is
defined. BADVERS is the expected rcode if EDNS is supported as per
Section 6.1.3, [RFC6891].
Check that EDNS queries with an unknown option work (EDNS supported):
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dig +nocookie +edns=0 +noad +norec +ednsopt=100 soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: OPT record to be present
expect: OPT=100 to not be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
Unknown EDNS options are supposed to be ignored, Section 6.1.2,
[RFC6891].
Check that EDNS queries with unknown flags work (EDNS supported):
dig +nocookie +edns=0 +noad +norec +ednsflags=0x40 soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: OPT record to be present
expect: MBZ not to be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
MBZ (Must Be Zero) presence indicates the flag bit has been
incorrectly copied as per Section 6.1.4, [RFC6891].
Check that EDNS version 1 queries with unknown flags work (EDNS
supported):
dig +nocookie +edns=1 +noednsneg +noad +norec +ednsflags=0x40 soa \
$zone @$server
expect: status: BADVERS
expect: SOA record to NOT be present
expect: OPT record to be present
expect: MBZ not to be present
expect: EDNS Version 0 in response
expect: flag: aa to NOT be present
+noednsneg disables EDNS version negotiation in DiG; MBZ (Must Be
Zero) presence indicates the flag bit has been incorrectly copied.
Check that EDNS version 1 queries with unknown options work (EDNS
supported):
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dig +nocookie +edns=1 +noednsneg +noad +norec +ednsopt=100 soa \
$zone @$server
expect: status: BADVERS
expect: SOA record to NOT be present
expect: OPT record to be present
expect: OPT=100 to NOT be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
+noednsneg disables EDNS version negotiation in DiG.
Check that a DNSSEC queries work (EDNS supported):
dig +nocookie +edns=0 +noad +norec +dnssec soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: OPT record to be present
expect: DO=1 to be present if a RRSIG is in the response
expect: EDNS Version 0 in response
expect: flag: aa to be present
DO=1 should be present if RRSIGs are returned as they indicate that
the server supports DNSSEC. Servers that support DNSSEC are supposed
to copy the DO bit from the request to the response as per [RFC3225].
Check that EDNS version 1 DNSSEC queries work (EDNS supported):
dig +nocookie +edns=1 +noednsneg +noad +norec +dnssec soa \
$zone @$server
expect: status: BADVERS
expect: SOA record to not be present
expect: OPT record to be present
expect: DO=1 to be present if the EDNS version 0 DNSSEC query test
returned DO=1
expect: EDNS Version 0 in response
expect: flag: aa to NOT be present
+noednsneg disables EDNS version negotiation in DiG.
Check that EDNS queries with multiple defined EDNS options work.
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dig +edns=0 +noad +norec +cookie +nsid +expire +subnet=0.0.0.0/0 \
soa $zone @$server
expect: status: NOERROR
expect: SOA record to be present
expect: OPT record to be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
If EDNS is not supported by the nameserver, we expect a response to
all the above queries. That response may be a FORMERR or NOTIMP
error response or the OPT record may just be ignored.
9. Security Considerations
Testing protocol compliance can potentially result in false reports
of attempts to break services from Intrusion Detection Services and
firewalls. None of the tests listed above should break nominally
EDNS compliant servers. None of the tests above should break non
EDNS servers. All the tests above are well formed, though not
necessarily common, DNS queries.
Relaxing firewall settings to ensure EDNS compliance could
potentially expose a critical implementation flaw in the nameserver.
Nameservers should be tested for conformance before relaxing firewall
settings.
10. IANA Considerations
IANA / ICANN needs to consider what tests, if any, from above that it
should add to the zone maintenance procedures for zones under its
control including pre-delegation checks. Otherwise this document has
no actions for IANA.
11. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, DOI 10.17487/RFC3225, December 2001,
<http://www.rfc-editor.org/info/rfc3225>.
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[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
[RFC5395] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", RFC 5395, DOI 10.17487/RFC5395, November
2008, <http://www.rfc-editor.org/info/rfc5395>.
[RFC5966] Bellis, R., "DNS Transport over TCP - Implementation
Requirements", RFC 5966, DOI 10.17487/RFC5966, August
2010, <http://www.rfc-editor.org/info/rfc5966>.
[RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
DOI 10.17487/RFC6840, February 2013,
<http://www.rfc-editor.org/info/rfc6840>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
Author's Address
M. Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Email: marka@isc.org
Andrews Expires May 29, 2016 [Page 16]
