A Common Operational Problem in DNS Servers - Failure To Respond.
draft-ietf-dnsop-no-response-issue-10
Network Working Group M. Andrews
Internet-Draft R. Bellis
Intended status: Best Current Practice ISC
Expires: January 21, 2019 July 20, 2018
A Common Operational Problem in DNS Servers - Failure To Respond.
draft-ietf-dnsop-no-response-issue-10
Abstract
The DNS is a query / response protocol. Failing to respond to
queries, or responding incorrectly, causes both immediate operational
problems and long term problems with protocol development.
This document identifies a number of common kinds of queries to which
some servers either fail to respond or else respond incorrectly.
This document also suggests procedures for TLD and other 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://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 January 21, 2019.
Copyright Notice
Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Consequences . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Common queries kinds that result in non responses. . . . . . 5
3.1. Basic DNS Queries . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Zone Existence . . . . . . . . . . . . . . . . . . . 5
3.1.2. Unknown / Unsupported Type Queries . . . . . . . . . 5
3.1.3. DNS Flags . . . . . . . . . . . . . . . . . . . . . . 6
3.1.4. Unknown DNS opcodes . . . . . . . . . . . . . . . . . 6
3.1.5. Recursive Queries . . . . . . . . . . . . . . . . . . 6
3.1.6. TCP Queries . . . . . . . . . . . . . . . . . . . . . 6
3.2. EDNS Queries . . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. EDNS Queries - Version Independent . . . . . . . . . 7
3.2.2. EDNS Queries - Version Specific . . . . . . . . . . . 7
3.2.3. EDNS Options . . . . . . . . . . . . . . . . . . . . 7
3.2.4. EDNS Flags . . . . . . . . . . . . . . . . . . . . . 7
3.2.5. Truncated EDNS Responses . . . . . . . . . . . . . . 8
3.2.6. DO Bit Handling . . . . . . . . . . . . . . . . . . . 8
3.2.7. EDNS over TCP . . . . . . . . . . . . . . . . . . . . 8
4. Firewalls and Load Balancers . . . . . . . . . . . . . . . . 8
5. Scrubbing Services . . . . . . . . . . . . . . . . . . . . . 9
6. Whole Answer Caches . . . . . . . . . . . . . . . . . . . . . 10
7. Response Code Selection . . . . . . . . . . . . . . . . . . . 10
8. Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Testing - Basic DNS . . . . . . . . . . . . . . . . . . . 11
8.1.1. Is The Server Configured For The Zone? . . . . . . . 11
8.1.2. Testing Unknown Types . . . . . . . . . . . . . . . . 11
8.1.3. Testing Header Bits . . . . . . . . . . . . . . . . . 12
8.1.4. Testing Unknown Opcodes . . . . . . . . . . . . . . . 13
8.1.5. Testing Recursive Queries . . . . . . . . . . . . . . 14
8.1.6. Testing TCP . . . . . . . . . . . . . . . . . . . . . 14
8.2. Testing - Extended DNS . . . . . . . . . . . . . . . . . 15
8.2.1. Testing Minimal EDNS . . . . . . . . . . . . . . . . 15
8.2.2. Testing EDNS Version Negotiation . . . . . . . . . . 16
8.2.3. Testing Unknown EDNS Options . . . . . . . . . . . . 16
8.2.4. Testing Unknown EDNS Flags . . . . . . . . . . . . . 17
8.2.5. Testing EDNS Version Negotiation With Unknown EDNS
Flags . . . . . . . . . . . . . . . . . . . . . . . . 18
8.2.6. Testing EDNS Version Negotiation With Unknown EDNS
Options . . . . . . . . . . . . . . . . . . . . . . . 18
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8.2.7. Testing Truncated Responses . . . . . . . . . . . . . 19
8.2.8. Testing DNSSEC Queries . . . . . . . . . . . . . . . 19
8.2.9. Testing EDNS Version Negotiation With DNSSEC . . . . 20
8.2.10. Testing With Multiple Defined EDNS Options . . . . . 21
8.3. When EDNS Is Not Supported . . . . . . . . . . . . . . . 21
9. Remediation . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
12.1. Normative References . . . . . . . . . . . . . . . . . . 23
12.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
The DNS [RFC1034], [RFC1035] is a query / response protocol. Failing
to respond to queries, or responding incorrectly, causes both
immediate operational problems and long term problems with protocol
development.
Failure to respond to a query is indistinguishable from packet loss
without doing an analysis of query-response patterns. Additionally
failure to respond results in unnecessary queries being made by DNS
clients, and introduces delays to the resolution process.
Due to the inability to distinguish between packet loss and
nameservers dropping EDNS [RFC6891] queries, packet loss is sometimes
misclassified as lack of EDNS support which can lead to DNSSEC
validation failures.
The existance of servers which fail to respond to queries results in
developers being hesitant to deploy new standards. Such servers need
to be identified and remediated.
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. There should be no need to drop queries because a
nameserver does not understand them.
Unless a nameserver is under attack, it should respond to all queries
directed to it. 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 accepted or not. While
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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. Misclassification of server behaviour is
unavoidable when rate limiting is used until the population of
servers which fail to respond to well formed queries drops to near
zero.
A nameserver should not assume that there isn't a delegation to the
server even if it is not configured to serve the zone. Misconfigured
nameservers are a common occurrence in the DNS and receiving queries
for zones that the server is not configured for is not necessarily an
indication that the server is under attack. Parent zone operators
are advised 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]. Doing this regularly should reduce the
instances of broken delegations.
2. Consequences
Failure to follow the relevant DNS RFCs has multiple adverse
consequences. Some are caused directly from the non-compliant
behaviour and others as a result of work-arounds forced on recursive
servers. Addressing known issues now will reduce future
interoperability issues as the DNS protocol continues to evolve and
clients make use of newly-introduced DNS features.
Some examples of known consequences include:
o The AD flag bit in a response cannot be trusted to mean anything
as some servers incorrectly copy the flag bit from the request to
the response [RFC1035], [RFC4035].
o Widespread non-response to EDNS queries has lead to recursive
servers having to assume that EDNS is not supported and that
fallback to plain DNS is required, potentially causing DNSSEC
validation failures.
o Widespread non-response to EDNS options, requires recursive
servers to have to decide whether to probe to see if it is the
EDNS option or just EDNS that is causing the non response. In the
limited amount of time required to resolve a query before the
client times out this is not possible.
o Incorrectly returning FORMERR to a EDNS option being present,
leads to the recursive server not being able to determine if the
server is just broken in the handling of the EDNS option or
doesn't support EDNS at all.
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o Mishandling of unknown query types has contributed to the
abandoning of the transition of the SPF type.
o Mishandling of unknown query types has slowed up the development
of DANE and resulted in additional rules being specified to reduce
the probability of interacting with a broken server when making
TLSA queries.
The consequences of servers not following the RFCs will only grow if
measures are not put in place to remove non compliant servers from
the ecosystem. Working around issues due to non-compliance with RFCs
is not sustainable.
Most (if not all) of these consequences could have been avoided if
action had been taken to remove non-compliant servers as soon as
people were aware of them, i.e. to actively seek out broken
implementations and servers and inform their developers and operators
that they need to fix their servers.
3. Common queries kinds that result in non responses.
There are a number common query kinds that fail to respond today.
They are: EDNS queries with and without extensions; queries for
unknown (unallocated) or unsupported types; and filtering of TCP
queries.
3.1. Basic DNS Queries
3.1.1. Zone Existence
Initially, to test existence of the zone, an SOA query should be
made. If the SOA record is not returned but some other response is
returned, this is an indication of a bad delegation.
3.1.2. 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, then 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.
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3.1.3. 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.
3.1.4. 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 or unimplemented opcode.
Note: while new opcodes will most probably use the current layout
structure for the rest of the message there is no requirement that
anything other than the DNS header match.
3.1.5. Recursive Queries
A non-recursive server is supposed to respond to recursive queries as
if the RD bit is not set [RFC1034].
3.1.6. TCP Queries
All DNS servers are supposed to respond to queries over TCP
[RFC7766]. While firewalls should not block TCP connection attempts
if they do they should cleanly terminate the connection by sending
TCP RESET or sending ICMP/ICMPv6 Administratively Prohibited
messages. Dropping TCP connections introduces 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.
3.2. EDNS Queries
EDNS queries are specified in [RFC6891].
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3.2.1. EDNS Queries - Version Independent
Identifying servers that fail to respond to EDNS queries can be done
by first confirming that the server responds to regular DNS queries,
followed by a series of otherwise identical queries 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 mis-
configured.
3.2.2. 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.
3.2.3. 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], the original EDNS specifion left this behaviour undefined
[RFC2671].
3.2.4. EDNS Flags
Some servers fail to respond to EDNS queries with EDNS flags set.
Server should ignore EDNS flags they do not understand and should not
add them to the response [RFC6891].
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3.2.5. Truncated EDNS Responses
Some EDNS aware servers fail to include an OPT record when a
truncated response is sent. An OPT record is supposed to be included
in a truncated response [RFC6891].
Some EDNS aware server fail to honour the advertised EDNS buffer size
and send over-sized responses.
3.2.6. DO Bit Handling
Some nameservers incorrectly only return a EDNS response when the DO
bit is present in the query. Additionally some nameservers fail to
copy the DO bit to the response despite clearly supporting DNSSEC by
returning RRSIG records to EDNS queries with the DO bit set.
3.2.7. EDNS over TCP
Some EDNS aware servers incorrectly limit the TCP response sizes to
the advertised UDP response size.
4. Firewalls and Load Balancers
Firewalls and load balancers can affect the externally visible
behaviour of a nameserver. Tests for conformance should to be done
from outside of any firewall so that the system is tested as a whole.
Firewalls and load balancers should not drop DNS packets that they
don't understand. They should either pass the packets or generate an
appropriate error response.
Requests for unknown query types are normal client behaviour and
should not be construed as an attack. Nameservers have always been
expected to be able to handle such queries.
Requests for unknown query classes are normal client behaviour and
should not be construed as an attack. Nameservers have always been
expected to be able to handle such queries.
Requests with unknown opcodes are normal client behaviour and should
not be construed as an attack. Nameservers have always been expected
to be able to handle such queries.
Requests with unassigned flags set (DNS or EDNS) are expected client
behaviour and should not be construed as an attack. The behaviour
for unassigned flags is to ignore them in the request and to not set
them in the response. Dropping DNS / EDNS packets with unassigned
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flags makes it difficult to deploy extensions that make use of them
due to the need to reconfigure and update firewalls.
Requests with unknown EDNS options are expected client behaviour and
should not be construed as an attack. The correct behaviour for
unknown EDNS options is to ignore their presence when constructing a
reply.
Requests with unknown EDNS versions are expected client behaviour and
should not be construed as an attack. The correct behaviour for
unknown EDNS versions is to return BADVERS along with the highest
EDNS version the server supports. Dropping EDNS packets breaks EDNS
version negotiation.
Firewalls should not assume that there will only be a single response
message to a request. 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.
DNS, and EDNS in particular, are designed to allow clients to be able
to use new features against older servers without having to validate
every option. Indiscriminate blocking of messages breaks that
design.
However, there may be times when a nameserver mishandles messages
with a particular flag, EDNS option, EDNS version field, opcode, type
or class field or combination thereof to the point where the
integrity of the nameserver is compromised. Firewalls should offer
the ability to selectively reject messages using an appropriately
constructed response based on all these fields while awaiting a fix
from the nameserver vendor.
5. Scrubbing Services
Scrubbing services can affect the externally visible behaviour of a
nameserver in a similar way to firewalls. If a operator uses a
scrubbing service, they 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.
Ideally, Operators should run these tests against a scrubbing service
to ensure that these tests are not seen as attack vectors.
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6. Whole Answer Caches
Whole answer caches take a previously constructed answer and return
it to a subsequent query for the same question. However, they can
return the wrong response if they do not take all of the relevant
attributes of the query into account.
In addition to the standard tuple of <qname,qtype,qclass> a non-
exhaustive set of attributes that must be considered include: RD, AD,
CD, OPT record, DO, EDNS buffer size, EDNS version, EDNS options, and
transport.
7. Response Code Selection
Choosing the correct response code when responding to DNS queries is
important. Response codes should be chosen considering how clients
will handle them.
For unimplemented opcodes NOTIMP is the expected response code. For
example, a new opcode could change the message format by extending
the header or changing the structure of the records etc.
For unimplemented type codes, and in the absence of other errors, the
only valid response is NoError if the qname exists, and NameError
(NXDOMAIN) otherwise. For Meta-RRs NOTIMP may be returned
instead.

If a zone cannot be loaded because it contains unimplemented type
codes that are not encoded as unknown record types according to
[RFC3597] then the expected response is SERVFAIL.
If the server supports EDNS and receives a query with an unsupported
EDNS version, the correct response is BADVERS [RFC6891].
If the server does not support EDNS at all, FORMERR and NOTIMP are
the expected error codes. That said a minimal EDNS server
implementation 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 EDNS options are expected to be
ignored [RFC6891].
8. Testing
Testing is divided into two sections. "Basic DNS", which all servers
should meet, and "Extended DNS", which should be met by all servers
that support EDNS (a server is deemed to support EDNS if it gives a
valid EDNS response to any EDNS query). If a server does not support
EDNS it should still respond to all the tests.
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It is advisable to run all of the tests below in parallel so as to
minimise the delays due to multiple timeouts when the servers do not
respond. There are 16 queries directed to each nameserver (assuming
no packet loss) testing different aspects of Basic DNS and Extended
DNS.
The tests below use dig from BIND 9.11.0.
8.1. Testing - Basic DNS
This first set of tests cover basic DNS server behaviour and all
servers should pass these tests.
8.1.1. Is The Server Configured For The Zone?
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set and without
EDNS.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, RA may also be set [RFC1034]. We do not expect
an OPT record to be returned [RFC6891].
Verify the server is configured for the zone:
dig +noedns +noad +norec soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: flag: aa to be present
expect: flag: rd to NOT be present
expect: flag: ad to NOT be present
expect: the OPT record to NOT be present
8.1.2. Testing Unknown Types
Ask for the TYPE1000 record at the zone's name. This query is made
with no DNS flag bits set and without EDNS. TYPE1000 has been chosen
for this purpose as IANA is unlikely to allocate this type in the
near future and it is not in a range reserved for private use
[RFC6895].
We expect no records to be returned in the answer section with the
rcode set to NOERROR and the AA and QR bits to be set in the
response. RA may also be set [RFC1034]. We do not expect an OPT
record to be returned [RFC6891].
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Check that queries for an unknown type work:
dig +noedns +noad +norec type1000 $zone @$server
expect: status: NOERROR
expect: an empty answer section.
expect: flag: aa to be present
expect: flag: rd to NOT be present
expect: flag: ad to NOT be present
expect: the OPT record to NOT be present
8.1.3. Testing Header Bits
8.1.3.1. Testing CD=1 Queries
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with only the CD DNS flag bit set and
all other DNS bits clear and without EDNS.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response. We do not expect an OPT record to be returned.
If the server supports DNSSEC, CD should be set in the response
[RFC4035] otherwise CD should be clear [RFC1034].
Check that queries with CD=1 work:
dig +noedns +noad +norec +cd soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: flag: aa to be present
expect: flag: rd to NOT be present
expect: flag: ad to NOT be present
expect: the OPT record to NOT be present
8.1.3.2. Testing AD=1 Queries
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with only the AD DNS flag bit set and
all other DNS bits clear and without EDNS.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response. We do not expect an OPT record to be returned.
The purpose of this query is to detect blocking of queries with the
AD bit present, not the specific value of AD in the response.
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Check that queries with AD=1 work:
dig +noedns +norec +ad soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: flag: aa to be present
expect: flag: rd to NOT be present
expect: the OPT record to NOT be present
AD use in queries is defined in [RFC6840].
8.1.3.3. Testing Reserved Bit
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with only the final reserved DNS flag
bit set and all other DNS bits clear and without EDNS.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, RA may be set. The final reserved bit must not
be set [RFC1034]. We do not expect an OPT record to be returned
[RFC6891].
Check that queries with the last unassigned DNS header flag work and
that the flag bit is not copied to the response:
dig +noedns +noad +norec +zflag soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: MBZ to NOT be in the response (see below)
expect: flag: aa to be present
expect: flag: rd to NOT be present
expect: flag: ad to NOT be present
expect: the OPT record to NOT be present
MBZ (Must Be Zero) is a dig-specific indication that 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."
8.1.4. Testing Unknown Opcodes
Construct a DNS message that consists of only a DNS header with
opcode set to 15 (currently not allocated), no DNS header bits set
and empty question, answer, authority and additional sections.
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Check that new opcodes are handled:
dig +noedns +noad +opcode=15 +norec +header-only @$server
expect: status: NOTIMP
expect: opcode: 15
expect: all sections to be empty
expect: flag: aa to NOT be present
expect: flag: rd to NOT be present
expect: flag: ad to NOT be present
expect: the OPT record to NOT be present
8.1.5. Testing Recursive Queries
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with only the RD DNS flag bit set and
without EDNS.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA, QR and RD bits to
be set in the response, RA may also be set [RFC1034]. We do not
expect an OPT record to be returned [RFC6891].
Check that recursive queries work:
dig +noedns +noad +rec soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: flag: aa to be present
expect: flag: rd to be present
expect: flag: ad to NOT be present
expect: the OPT record to NOT be present
8.1.6. Testing TCP
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set and without
EDNS. This query is to be sent using TCP.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, RA may also be set [RFC1034]. We do not expect
an OPT record to be returned [RFC6891].
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Check that TCP queries work:
dig +noedns +noad +norec +tcp soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: flag: aa to be present
expect: flag: rd to NOT be present
expect: flag: ad to NOT be present
expect: the OPT record to NOT be present
The requirement that TCP be supported is defined in [RFC7766].
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.
8.2.1. Testing Minimal EDNS
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 0 is used without any EDNS options or EDNS flags set.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, RA may also be set [RFC1034]. We expect an OPT
record to be returned. There should be no EDNS flags present in the
response. The EDNS version field should be zero and there should be
no EDNS options present [RFC6891].
Check that plain EDNS queries work:
dig +nocookie +edns=0 +noad +norec soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: an OPT record to be present in the additional section
expect: EDNS Version 0 in response
expect: flag: aa to be present
expect: flag: ad to NOT be present
+nocookie disables sending a EDNS COOKIE option in which is on by
default in BIND 9.11.0.
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8.2.2. Testing EDNS Version Negotiation
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 1 is used without any EDNS options or EDNS flags set.
We expect the SOA record for the zone to NOT be returned in the
answer section with the extended rcode set to BADVERS and the QR bit
to be set in the response, RA may also be set [RFC1034]. We expect
an OPT record to be returned. There should be no EDNS flags present
in the response. The EDNS version field should be zero as EDNS
versions other than 0 are yet to be specified and there should be no
EDNS options present [RFC6891].
Check that EDNS version 1 queries work (EDNS supported):
dig +nocookie +edns=1 +noednsneg +noad +norec soa $zone @$server
expect: status: BADVERS
expect: the SOA record to NOT be present in the answer section
expect: an OPT record to be present in the additional section
expect: EDNS Version 0 in response
expect: flag: aa to NOT be present
expect: flag: ad 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].
8.2.3. Testing Unknown EDNS Options
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 0 is used without any EDNS flags. A EDNS option is present
with a value from the yet to be assigned range. The unassigned value
chosen is 100 and will need to be adjusted when IANA assigns this
value formally.
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, RA may also be set [RFC1034]. We expect an OPT
record to be returned. There should be no EDNS flags present in the
response. The EDNS version field should be zero as EDNS versions
other than 0 are yet to be specified and there should be no EDNS
options present as unknown EDNS options are supposed to be ignored by
the server [RFC6891].
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Check that EDNS queries with an unknown option work (EDNS supported):
dig +nocookie +edns=0 +noad +norec +ednsopt=100 soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: an OPT record to be present in the additional section
expect: OPT=100 to NOT be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
expect: flag: ad to NOT be present
Unknown EDNS options are supposed to be ignored, Section 6.1.2,
[RFC6891].
8.2.4. Testing Unknown EDNS Flags
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 0 is used without any EDNS options. A unassigned EDNS flag
bit is set (0x40 in this case).
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, RA may also be set [RFC1034]. We expect an OPT
record to be returned. There should be no EDNS flags present in the
response as unknown EDNS flags are supposed to be ignored. The EDNS
version field should be zero and there should be no EDNS options
present [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: the SOA record to be present in the answer section
expect: an OPT record to be present in the additional section
expect: MBZ not to be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
expect: flag: ad to NOT be present
MBZ (Must Be Zero) presence indicates the flag bit has been
incorrectly copied as per Section 6.1.4, [RFC6891].
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8.2.5. Testing EDNS Version Negotiation With Unknown EDNS Flags
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 1 is used without any EDNS options. A unassigned EDNS flag
bit is set (0x40 in this case).
We expect the SOA record for the zone to NOT be returned in the
answer section with the extended rcode set to BADVERS and the QR bit
to be set in the response, RA may also be set [RFC1034]. We expect
an OPT record to be returned. There should be no EDNS flags present
in the response as unknown EDNS flags are supposed to be ignored.
The EDNS version field should be zero as EDNS versions other than 0
are yet to be specified and there should be no EDNS options present
[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: an OPT record to be present in the additional section
expect: MBZ not to be present
expect: EDNS Version 0 in response
expect: flag: aa to NOT be present
expect: flag: ad to NOT be present
+noednsneg disables EDNS version negotiation in DiG; MBZ (Must Be
Zero) presence indicates the flag bit has been incorrectly copied.
8.2.6. Testing EDNS Version Negotiation With Unknown EDNS Options
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 1 is used. A unknown EDNS option is present (option code 100
has been chosen).
We expect the SOA record for the zone to NOT be returned in the
answer section with the extended rcode set to BADVERS and the QR bit
to be set in the response, RA may also be set [RFC1034]. We expect
an OPT record to be returned. There should be no EDNS flags present
in the response. The EDNS version field should be zero as EDNS
versions other than 0 are yet to be specified and there should be no
EDNS options present [RFC6891].
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Check that EDNS version 1 queries with unknown options work (EDNS
supported):
dig +nocookie +edns=1 +noednsneg +noad +norec +ednsopt=100 soa \
$zone @$server
expect: status: BADVERS
expect: SOA record to NOT be present
expect: an OPT record to be present in the additional section
expect: OPT=100 to NOT be present
expect: EDNS Version 0 in response
expect: flag: aa to be present
expect: flag: ad to NOT be present
+noednsneg disables EDNS version negotiation in DiG.
8.2.7. Testing Truncated Responses
Ask for the DNSKEY records of the zone the server is nominally
configured to serve. This query is made with no DNS flag bits set.
EDNS version 0 is used without any EDNS options. The only EDNS flag
set is DO. The EDNS UDP buffer size is set to 512. The intention of
this query is elicit a truncated response from the server. Most
signed DNSKEY responses are bigger than 512 bytes.
We expect a response with the rcode set to NOERROR and the AA and QR
bits to be set, AD may be set in the response if the server supports
DNSSEC otherwise it should be clear. TC and RA may also be set
[RFC1034]. We expect an OPT record to be present in the response.
There should be no EDNS flags other than DO present in the response.
The EDNS version field should be zero and there should be no EDNS
options present [RFC6891].
If TC is not set it is not possible to confirm that the server
correctly adds the OPT record to the truncated responses or not.
dig +norec +dnssec +bufsize=512 +ignore dnskey $zone @$server
expect: NOERROR
expect: OPT record with version set to 0
8.2.8. Testing DNSSEC Queries
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 0 is used without any EDNS options. The only EDNS flag set
is DO.
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We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, AD may be set in the response if the server
supports DNSSEC otherwise it should be clear. RA may also be set
[RFC1034]. We expect an OPT record to be returned. There should be
no EDNS flags other than DO present in the response which should be
present if the server supports DNSSEC. The EDNS version field should
be zero and there should be no EDNS options present [RFC6891].
Check that a DNSSEC queries work (EDNS supported):
dig +nocookie +edns=0 +noad +norec +dnssec soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: an OPT record to be present in the additional section
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].
8.2.9. Testing EDNS Version Negotiation With DNSSEC
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 1 is used without any EDNS options. The only EDNS flag set
is DO.
We expect the SOA record for the zone to NOT be returned in the
answer section with the rcode set to BADVERS and the only the QR bit
and possibly the RA bit to be set [RFC1034]. We expect an OPT record
to be returned. There should be no EDNS flags other than DO present
in the response which should be present if the server supports
DNSSEC. The EDNS version field should be zero and there should be no
EDNS options present [RFC6891].
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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: an OPT record to be present in the additional section
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.
8.2.10. Testing With Multiple Defined EDNS Options
Ask for the SOA record of the zone the server is nominally configured
to serve. This query is made with no DNS flag bits set. EDNS
version 0 is used. A number of defined EDNS options are present
(NSID [RFC5001], DNS COOKIE [RFC7873], EDNS Client Subnet [RFC7871]
and EDNS Expire [RFC7314]).
We expect the SOA record for the zone to be returned in the answer
section with the rcode set to NOERROR and the AA and QR bits to be
set in the response, RA may also be set [RFC1034]. We expect an OPT
record to be returned. There should be no EDNS flags present in the
response. The EDNS version field should be zero. Any of the
requested EDNS options supported by the server and permitted server
configuration may be returned [RFC6891].
Check that EDNS queries with multiple defined EDNS options work:
dig +edns=0 +noad +norec +cookie +nsid +expire +subnet=0.0.0.0/0 \
soa $zone @$server
expect: status: NOERROR
expect: the SOA record to be present in the answer section
expect: an OPT record to be present in the additional section
expect: EDNS Version 0 in response
expect: flag: aa to be present
expect: flag: ad to NOT be present
8.3. When EDNS Is Not Supported
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.
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Some nameservers only return a EDNS response when a particular EDNS
option or flag (e.g. DO=1) is present in the request. This
behaviour is not compliant behaviour and may hide other incorrect
behaviour from the above tests. Re-testing with the triggering
option / flag present will expose this misbehaviour.
9. Remediation
Name server operators are generally expected to test their own
infrastructure for compliance to standards. The above tests should
be run when new systems are brought online, and should be repeated
periodically to ensure continued interoperability.
Domain registrants who do not maintain their own DNS infrastructure
are entitled to a DNS service that conforms to standards and
interoperates well. Registrants who become aware that their DNS
operator does not have a well maintained or compliant infrastructure
should insist that their service provider correct issues, and switch
providers if they do not.
In the event that an operator experiences problems due to the
behaviour of name servers outside their control, the above tests will
help in narrowing down the precise issue(s) which can then be
reported to the relevant party.
If contact information for the operator of a misbehaving name server
is not already known, the following methods of communication could be
considered:
o the RNAME of the zone authoritative for the name of the
misbehaving server
o the RNAME of zones for which the offending server is authoritative
o administrative or technical contacts listed in the registration
information for the parent domain of the name of the misbehaving
server, or for zones for which the name server is authoritative
o the registrar or registry for such zones
o DNS-specific operational fora (e.g. mailing lists)
Operators of parent zones may wish to regularly test the
authoritative name servers of their child zones. However, parent
operators can have widely varying capabilities in terms of
notification or remediation depending on whether they have a direct
relationship with the child operator. Many TLD registries, for
example, cannot directly contact their registrants and may instead
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need to communicate through the relevant registrar. In such cases
it may be most efficient for registrars to take on the responsibility
for testing the name servers of their registrants, since they have a
direct relationship.
When notification is not effective at correcting problems with a
misbehaving name server, parent operators can choose to remove NS
record sets (and glue records below) that refer to the faulty server.
This should only be done as a last resort and with due consideration,
as removal of a delegation can have unanticipated side effects. For
example, other parts of the DNS tree may depend on names below the
removed zone cut, and the parent operator may find themselves
responsible for causing new DNS failures to occur.
10. 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.
When removing delegations for non-compliant servers there can be a
knock on effect on other zones that require these zones to be
operational for the nameservers addresses to be resolved.
11. IANA Considerations
There are no actions for IANA.
12. References
12.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
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[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, DOI 10.17487/RFC3225, December 2001,
<https://www.rfc-editor.org/info/rfc3225>.
[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,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
DOI 10.17487/RFC6840, February 2013,
<https://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,
<https://www.rfc-editor.org/info/rfc6891>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.
12.2. Informative References
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, DOI 10.17487/RFC2671, August 1999,
<https://www.rfc-editor.org/info/rfc2671>.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://www.rfc-editor.org/info/rfc3597>.
[RFC5001] Austein, R., "DNS Name Server Identifier (NSID) Option",
RFC 5001, DOI 10.17487/RFC5001, August 2007,
<https://www.rfc-editor.org/info/rfc5001>.
[RFC7314] Andrews, M., "Extension Mechanisms for DNS (EDNS) EXPIRE
Option", RFC 7314, DOI 10.17487/RFC7314, July 2014,
<https://www.rfc-editor.org/info/rfc7314>.
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[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/info/rfc7871>.
[RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
<https://www.rfc-editor.org/info/rfc7873>.
Authors' Addresses
M. Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Email: marka@isc.org
Ray Bellis
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Email: ray@isc.org
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