Network Working Group M. Andrews
Request for Comments: 2308 CSIRO
Updates: 1034, 1035 March 1998
Category: Standards Track

Negative Caching of DNS Queries (DNS NCACHE)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

[RFC1034] provided a description of how to cache negative responses.
It however had a fundamental flaw in that it did not allow a name
server to hand out those cached responses to other resolvers, thereby
greatly redUCing the effect of the caching. This document addresses
issues raise in the light of eXPerience and replaces [RFC1034 Section
4.3.4].

Negative caching was an optional part of the DNS specification and
deals with the caching of the non-existence of an RRset [RFC2181] or
domain name.

Negative caching is useful as it reduces the response time for
negative answers. It also reduces the number of messages that have
to be sent between resolvers and name servers hence overall network
traffic. A large proportion of DNS traffic on the Internet could be
eliminated if all resolvers implemented negative caching. With this
in mind negative caching should no longer be seen as an optional part
of a DNS resolver.

1 - Terminology

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].

"Negative caching" - the storage of knowledge that something does not
exist. We can store the knowledge that a record has a particular
value. We can also do the reverse, that is, to store the knowledge
that a record does not exist. It is the storage of knowledge that
something does not exist, cannot or does not give an answer that we
call negative caching.

"QNAME" - the name in the query section of an answer, or where this
resolves to a CNAME, or CNAME chain, the data field of the last
CNAME. The last CNAME in this sense is that which contains a value
which does not resolve to another CNAME. Implementations should note
that including CNAME records in responses in order, so that the first
has the label from the query section, and then each in sequence has
the label from the data section of the previous (where more than one
CNAME is needed) allows the sequence to be processed in one pass, and
considerably eases the task of the receiver. Other relevant records
(such as SIG RRs [RFC2065]) can be interspersed amongst the CNAMEs.

"NXDOMAIN" - an alternate expression for the "Name Error" RCODE as
described in [RFC1035 Section 4.1.1] and the two terms are used
interchangeably in this document.

"NODATA" - a pseudo RCODE which indicates that the name is valid, for
the given class, but are no records of the given type. A NODATA
response has to be inferred from the answer.

"FORWARDER" - a nameserver used to resolve queries instead of
directly using the authoritative nameserver chain. The forwarder
typically either has better Access to the internet, or maintains a
bigger cache which may be shared amongst many resolvers. How a
server is identified as a FORWARDER, or knows it is a FORWARDER is
outside the scope of this document. However if you are being used as
a forwarder the query will have the recursion desired flag set.

An understanding of [RFC1034], [RFC1035] and [RFC2065] is expected
when reading this document.

2 - Negative Responses

The most common negative responses indicate that a particular RRset
does not exist in the DNS. The first sections of this document deal
with this case. Other negative responses can indicate failures of a
nameserver, those are dealt with in section 7 (Other Negative
Responses).

A negative response is indicated by one of the following conditions:

2.1 - Name Error

Name errors (NXDOMAIN) are indicated by the presence of "Name Error"
in the RCODE field. In this case the domain referred to by the QNAME
does not exist. Note: the answer section may have SIG and CNAME RRs
and the authority section may have SOA, NXT [RFC2065] and SIG RRsets.

It is possible to distinguish between a referral and a NXDOMAIN
response by the presense of NXDOMAIN in the RCODE regardless of the
presence of NS or SOA records in the authority section.

NXDOMAIN responses can be categorised into four types by the contents
of the authority section. These are shown below along with a
referral for comparison. Fields not mentioned are not important in
terms of the examples.

NXDOMAIN RESPONSE: TYPE 1.

Header:
RDCODE=NXDOMAIN
Query:
AN.EXAMPLE. A
Answer:
AN.EXAMPLE. CNAME TRIPPLE.XX.
Authority:
XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
XX. NS NS1.XX.
XX. NS NS2.XX.
Additional:
NS1.XX. A 127.0.0.2
NS2.XX. A 127.0.0.3

NXDOMAIN RESPONSE: TYPE 2.

Header:
RDCODE=NXDOMAIN
Query:
AN.EXAMPLE. A

Answer:
AN.EXAMPLE. CNAME TRIPPLE.XX.
Authority:
XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
Additional:
<empty>

NXDOMAIN RESPONSE: TYPE 3.

Header:
RDCODE=NXDOMAIN
Query:
AN.EXAMPLE. A
Answer:
AN.EXAMPLE. CNAME TRIPPLE.XX.
Authority:
<empty>
Additional:
<empty>

NXDOMAIN RESPONSE: TYPE 4

Header:
RDCODE=NXDOMAIN
Query:
AN.EXAMPLE. A
Answer:
AN.EXAMPLE. CNAME TRIPPLE.XX.
Authority:
XX. NS NS1.XX.
XX. NS NS2.XX.
Additional:
NS1.XX. A 127.0.0.2
NS2.XX. A 127.0.0.3

REFERRAL RESPONSE.

Header:
RDCODE=NOERROR
Query:
AN.EXAMPLE. A
Answer:
AN.EXAMPLE. CNAME TRIPPLE.XX.
Authority:
XX. NS NS1.XX.
XX. NS NS2.XX.
Additional:
NS1.XX. A 127.0.0.2

NS2.XX. A 127.0.0.3

Note, in the four examples of NXDOMAIN responses, it is known that
the name "AN.EXAMPLE." exists, and has as its value a CNAME record.
The NXDOMAIN refers to "TRIPPLE.XX", which is then known not to
exist. On the other hand, in the referral example, it is shown that
"AN.EXAMPLE" exists, and has a CNAME RR as its value, but nothing is
known one way or the other about the existence of "TRIPPLE.XX", other
than that "NS1.XX" or "NS2.XX" can be consulted as the next step in
oBTaining information about it.

Where no CNAME records appear, the NXDOMAIN response refers to the
name in the label of the RR in the question section.

2.1.1 Special Handling of Name Error

This section deals with errors encountered when implementing negative
caching of NXDOMAIN responses.

There are a large number of resolvers currently in existence that
fail to correctly detect and process all forms of NXDOMAIN response.
Some resolvers treat a TYPE 1 NXDOMAIN response as a referral. To
alleviate this problem it is recommended that servers that are
authoritative for the NXDOMAIN response only send TYPE 2 NXDOMAIN
responses, that is the authority section contains a SOA record and no
NS records. If a non- authoritative server sends a type 1 NXDOMAIN
response to one of these old resolvers, the result will be an
unnecessary query to an authoritative server. This is undesirable,
but not fatal except when the server is being used a FORWARDER. If
however the resolver is using the server as a FORWARDER to such a
resolver it will be necessary to disable the sending of TYPE 1
NXDOMAIN response to it, use TYPE 2 NXDOMAIN instead.

Some resolvers incorrectly continue processing if the authoritative
answer flag is not set, looping until the query retry threshold is
exceeded and then returning SERVFAIL. This is a problem when your
nameserver is listed as a FORWARDER for such resolvers. If the
nameserver is used as a FORWARDER by such resolver, the authority
flag will have to be forced on for NXDOMAIN responses to these
resolvers. In practice this causes no problems even if turned on
always, and has been the default behaviour in BIND from 4.9.3
onwards.

2.2 - No Data

NODATA is indicated by an answer with the RCODE set to NOERROR and no
relevant answers in the answer section. The authority section will
contain an SOA record, or there will be no NS records there.

NODATA responses have to be algorithmically determined from the
response's contents as there is no RCODE value to indicate NODATA.
In some cases to determine with certainty that NODATA is the correct
response it can be necessary to send another query.

The authority section may contain NXT and SIG RRsets in addition to
NS and SOA records. CNAME and SIG records may exist in the answer
section.

It is possible to distinguish between a NODATA and a referral
response by the presence of a SOA record in the authority section or
the absence of NS records in the authority section.

NODATA responses can be categorised into three types by the contents
of the authority section. These are shown below along with a
referral for comparison. Fields not mentioned are not important in
terms of the examples.

NODATA RESPONSE: TYPE 1.

Header:
RDCODE=NOERROR
Query:
ANOTHER.EXAMPLE. A
Answer:
<empty>
Authority:
EXAMPLE. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
EXAMPLE. NS NS1.XX.
EXAMPLE. NS NS2.XX.
Additional:
NS1.XX. A 127.0.0.2
NS2.XX. A 127.0.0.3

NO DATA RESPONSE: TYPE 2.

Header:
RDCODE=NOERROR
Query:
ANOTHER.EXAMPLE. A
Answer:
<empty>
Authority:
EXAMPLE. SOA NS1.XX. HOSTMASTER.NS1.XX. ....
Additional:
<empty>

NO DATA RESPONSE: TYPE 3.

Header:
RDCODE=NOERROR
Query:
ANOTHER.EXAMPLE. A
Answer:
<empty>
Authority:
<empty>
Additional:
<empty>

REFERRAL RESPONSE.

Header:
RDCODE=NOERROR
Query:
ANOTHER.EXAMPLE. A
Answer:
<empty>
Authority:
EXAMPLE. NS NS1.XX.
EXAMPLE. NS NS2.XX.
Additional:
NS1.XX. A 127.0.0.2
NS2.XX. A 127.0.0.3

These examples, unlike the NXDOMAIN examples above, have no CNAME
records, however they could, in just the same way that the NXDOMAIN
examples did, in which case it would be the value of the last CNAME
(the QNAME) for which NODATA would be concluded.

2.2.1 - Special Handling of No Data

There are a large number of resolvers currently in existence that
fail to correctly detect and process all forms of NODATA response.
Some resolvers treat a TYPE 1 NODATA response as a referral. To
alleviate this problem it is recommended that servers that are
authoritative for the NODATA response only send TYPE 2 NODATA
responses, that is the authority section contains a SOA record and no
NS records. Sending a TYPE 1 NODATA response from a non-
authoritative server to one of these resolvers will only result in an
unnecessary query. If a server is listed as a FORWARDER for another
resolver it may also be necessary to disable the sending of TYPE 1
NODATA response for non-authoritative NODATA responses.

Some name servers fail to set the RCODE to NXDOMAIN in the presence
of CNAMEs in the answer section. If a definitive NXDOMAIN / NODATA
answer is required in this case the resolver must query again using
the QNAME as the query label.

3 - Negative Answers from Authoritative Servers

Name servers authoritative for a zone MUST include the SOA record of
the zone in the authority section of the response when reporting an
NXDOMAIN or indicating that no data of the requested type exists.
This is required so that the response may be cached. The TTL of this
record is set from the minimum of the MINIMUM field of the SOA record
and the TTL of the SOA itself, and indicates how long a resolver may
cache the negative answer. The TTL SIG record associated with the
SOA record should also be trimmed in line with the SOA's TTL.

If the containing zone is signed [RFC2065] the SOA and appropriate
NXT and SIG records MUST be added.

4 - SOA Minimum Field

The SOA minimum field has been overloaded in the past to have three
different meanings, the minimum TTL value of all RRs in a zone, the
default TTL of RRs which did not contain a TTL value and the TTL of
negative responses.

Despite being the original defined meaning, the first of these, the
minimum TTL value of all RRs in a zone, has never in practice been
used and is hereby deprecated.

The second, the default TTL of RRs which contain no explicit TTL in
the master zone file, is relevant only at the primary server. After
a zone transfer all RRs have explicit TTLs and it is impossible to
determine whether the TTL for a record was explicitly set or derived
from the default after a zone transfer. Where a server does not
require RRs to include the TTL value explicitly, it should provide a
mechanism, not being the value of the MINIMUM field of the SOA
record, from which the missing TTL values are obtained. How this is
done is implementation dependent.

The Master File format [RFC1035 Section 5] is extended to include
the following directive:

$TTL <TTL> [comment]

All resource records appearing after the directive, and which do not
explicitly include a TTL value, have their TTL set to the TTL given
in the $TTL directive. SIG records without a explicit TTL get their
TTL from the "original TTL" of the SIG record [RFC2065 Section 4.5].

The remaining of the current meanings, of being the TTL to be used
for negative responses, is the new defined meaning of the SOA minimum
field.

5 - Caching Negative Answers

Like normal answers negative answers have a time to live (TTL). As
there is no record in the answer section to which this TTL can be
applied, the TTL must be carried by another method. This is done by
including the SOA record from the zone in the authority section of
the reply. When the authoritative server creates this record its TTL
is taken from the minimum of the SOA.MINIMUM field and SOA's TTL.
This TTL decrements in a similar manner to a normal cached answer and
upon reaching zero (0) indicates the cached negative answer MUST NOT
be used again.

A negative answer that resulted from a name error (NXDOMAIN) should
be cached such that it can be retrieved and returned in response to
another query for the same <QNAME, QCLASS> that resulted in the
cached negative response.

A negative answer that resulted from a no data error (NODATA) should
be cached such that it can be retrieved and returned in response to
another query for the same <QNAME, QTYPE, QCLASS> that resulted in
the cached negative response.

The NXT record, if it exists in the authority section of a negative
answer received, MUST be stored such that it can be be located and
returned with SOA record in the authority section, as should any SIG
records in the authority section. For NXDOMAIN answers there is no
"necessary" obvious relationship between the NXT records and the
QNAME. The NXT record MUST have the same owner name as the query
name for NODATA responses.

Negative responses without SOA records SHOULD NOT be cached as there
is no way to prevent the negative responses looping forever between a
pair of servers even with a short TTL.

Despite the DNS forming a tree of servers, with various mis-
configurations it is possible to form a loop in the query graph, e.g.
two servers listing each other as forwarders, various lame server
configurations. Without a TTL count down a cache negative response

when received by the next server would have its TTL reset. This
negative indication could then live forever circulating between the
servers involved.

As with caching positive responses it is sensible for a resolver to
limit for how long it will cache a negative response as the protocol
supports caching for up to 68 years. Such a limit should not be
greater than that applied to positive answers and preferably be
tunable. Values of one to three hours have been found to work well
and would make sensible a default. Values exceeding one day have
been found to be problematic.

6 - Negative answers from the cache

When a server, in answering a query, encounters a cached negative
response it MUST add the cached SOA record to the authority section
of the response with the TTL decremented by the amount of time it was
stored in the cache. This allows the NXDOMAIN / NODATA response to
time out correctly.

If a NXT record was cached along with SOA record it MUST be added to
the authority section. If a SIG record was cached along with a NXT
record it SHOULD be added to the authority section.

As with all answers coming from the cache, negative answers SHOULD
have an implicit referral built into the answer. This enables the
resolver to locate an authoritative source. An implicit referral is
characterised by NS records in the authority section referring the
resolver towards a authoritative source. NXDOMAIN types 1 and 4
responses contain implicit referrals as does NODATA type 1 response.

7 - Other Negative Responses

Caching of other negative responses is not covered by any existing
RFC. There is no way to indicate a desired TTL in these responses.
Care needs to be taken to ensure that there are not forwarding loops.

7.1 Server Failure (OPTIONAL)

Server failures fall into two major classes. The first is where a
server can determine that it has been misconfigured for a zone. This
may be where it has been listed as a server, but not configured to be
a server for the zone, or where it has been configured to be a server
for the zone, but cannot obtain the zone data for some reason. This
can occur either because the zone file does not exist or contains
errors, or because another server from which the zone should have
been available either did not respond or was unable or unwilling to
supply the zone.

The second class is where the server needs to obtain an answer from
elsewhere, but is unable to do so, due to network failures, other
servers that don't reply, or return server failure errors, or
similar.

In either case a resolver MAY cache a server failure response. If it
does so it MUST NOT cache it for longer than five (5) minutes, and it
MUST be cached against the specific query tuple <query name, type,
class, server IP address>.

7.2 Dead / Unreachable Server (OPTIONAL)

Dead / Unreachable servers are servers that fail to respond in any
way to a query or where the transport layer has provided an
indication that the server does not exist or is unreachable. A
server may be deemed to be dead or unreachable if it has not
responded to an outstanding query within 120 seconds.

Examples of transport layer indications are:

ICMP error messages indicating host, net or port unreachable.
TCP resets
IP stack error messages providing similar indications to those above.

A server MAY cache a dead server indication. If it does so it MUST
NOT be deemed dead for longer than five (5) minutes. The indication
MUST be stored against query tuple <query name, type, class, server
IP address> unless there was a transport layer indication that the
server does not exist, in which case it applies to all queries to
that specific IP address.

8 - Changes from RFC1034

Negative caching in resolvers is no-longer optional, if a resolver
caches anything it must also cache negative answers.

Non-authoritative negative answers MAY be cached.

The SOA record from the authority section MUST be cached. Name error
indications must be cached against the tuple <query name, QCLASS>.
No data indications must be cached against <query name, QTYPE,
QCLASS> tuple.

A cached SOA record must be added to the response. This was
explicitly not allowed because previously the distinction between a
normal cached SOA record, and the SOA cached as a result of a
negative response was not made, and simply extracting a normal cached
SOA and adding that to a cached negative response causes problems.

The $TTL TTL directive was added to the master file format.

9 - History of Negative Caching

This section presents a potted history of negative caching in the DNS
and forms no part of the technical specification of negative caching.

It is interesting to note that the same concepts were re-invented in
both the CHIVES and BIND servers.

The history of the early CHIVES work (Section 9.1) was supplied by
Rob Austein <sra@epilogue.com> and is reproduced here in the form in
which he supplied it [MPA].

Sometime around the spring of 1985, I mentioned to Paul Mockapetris
that our experience with his JEEVES DNS resolver had pointed out the
need for some kind of negative caching scheme. Paul suggested that
we simply cache authoritative errors, using the SOA MINIMUM value for
the zone that would have contained the target RRs. I'm pretty sure
that this conversation took place before RFC-973 was written, but it
was never clear to me whether this idea was something that Paul came
up with on the spot in response to my question or something he'd
already been planning to put into the document that became RFC-973.
In any case, neither of us was entirely sure that the SOA MINIMUM
value was really the right metric to use, but it was available and
was under the control of the administrator of the target zone, both
of which seemed to us at the time to be important feature.

Late in 1987, I released the initial beta-test version of CHIVES, the
DNS resolver I'd written to replace Paul's JEEVES resolver. CHIVES
included a search path mechanism that was used pretty heavily at
several sites (including my own), so CHIVES also included a negative
caching mechanism based on SOA MINIMUM values. The basic strategy
was to cache authoritative error codes keyed by the exact query
parameters (QNAME, QCLASS, and QTYPE), with a cache TTL equal to the
SOA MINIMUM value. CHIVES did not attempt to track down SOA RRs if
they weren't supplied in the authoritative response, so it never
managed to completely eliminate the gratuitous DNS error message
traffic, but it did help considerably. Keep in mind that this was
happening at about the same time as the near-collapse of the ARPANET
due to congestion caused by exponential growth and the the "old"
(pre-VJ) TCP retransmission algorithm, so negative caching resulted
in drasticly better DNS response time for our users, mailer daemons,
etcetera.

As far as I know, CHIVES was the first resolver to implement negative
caching. CHIVES was developed during the twilight years of TOPS-20,
so it never ran on very many machines, but the few machines that it
did run on were the ones that were too critical to shut down quickly
no matter how much it cost to keep them running. So what few users
we did have tended to drive CHIVES pretty hard. Several interesting
bits of DNS technology resulted from that, but the one that's
relevant here is the MAXTTL configuration parameter.

Experience with JEEVES had already shown that RRs often showed up
with ridiculously long TTLs (99999999 was particularly popular for
many years, due to bugs in the code and documentation of several
early versions of BIND), and that robust software that blindly
believed such TTLs could create so many strange failures that it was
often necessary to reboot the resolver frequently just to clear this
garbage out of the cache. So CHIVES had a configuration parameter
"MAXTTL", which specified the maximum "reasonable" TTL in a received
RR. RRs with TTLs greater than MAXTTL would either have their TTLs
reduced to MAXTTL or would be discarded entirely, depending on the
setting of another configuration parameter.

When we started getting field experience with CHIVES's negative
caching code, it became clear that the SOA MINIMUM value was often
large enough to cause the same kinds of problems for negative caching
as the huge TTLs in RRs had for normal caching (again, this was in
part due to a bug in several early versions of BIND, where a
secondary server would authoritatively deny all knowledge of its
zones if it couldn't contact the primaries on reboot). So we started
running the negative cache TTLs through the MAXTTL check too, and
continued to experiment.

The configuration that seemed to work best on WSMR-SIMTEL20.ARMY.MIL
(last of the major Internet TOPS-20 machines to be shut down, thus
the last major user of CHIVES, thus the place where we had the
longest experimental baseline) was to set MAXTTL to about three days.
Most of the traffic initiated by SIMTEL20 in its last years was
mail-related, and the mail queue timeout was set to one week, so this
gave a "stuck" message several tries at complete DNS resolution,
without bogging down the system with a lot of useless queries. Since
(for reasons that now escape me) we only had the single MAXTTL
parameter rather than separate ones for positive and negative
caching, it's not clear how much effect this setting of MAXTTL had on
the negative caching code.

CHIVES also included a second, somewhat controversial mechanism which
took the place of negative caching in some cases. The CHIVES
resolver daemon could be configured to load DNS master files, giving
it the ability to act as what today would be called a "stealth

secondary". That is, when configured in this way, the resolver had
direct access to authoritative information for heavily-used zones.
The search path mechanisms in CHIVES reflected this: there were
actually two separate search paths, one of which only searched local
authoritative zone data, and one which could generate normal
iterative queries. This cut down on the need for negative caching in
cases where usage was predictably heavy (e.g., the resolver on
XX.LCS.MIT.EDU always loaded the zone files for both LCS.MIT.EDU and
AI.MIT.EDU and put both of these suffixes into the "local" search
path, since between them the hosts in these two zones accounted for
the bulk of the DNS traffic). Not all sites running CHIVES chose to
use this feature; C.CS.CMU.EDU, for example, chose to use the
"remote" search path for everything because there were too many
different sub-zones at CMU for zone shadowing to be practical for
them, so they relied pretty heavily on negative caching even for
local traffic.

Overall, I still think the basic design we used for negative caching
was pretty reasonable: the zone administrator specified how long to
cache negative answers, and the resolver configuration chose the
actual cache time from the range between zero and the period
specified by the zone administrator. There are a lot of details I'd
do differently now (like using a new SOA field instead of overloading
the MINIMUM field), but after more than a decade, I'd be more worried
if we couldn't think of at least a few improvements.

9.2 BIND

While not the first attempt to get negative caching into BIND, in
July 1993, BIND 4.9.2 ALPHA, Anant Kumar of ISI supplied code that
implemented, validation and negative caching (NCACHE). This code had
a 10 minute TTL for negative caching and only cached the indication
that there was a negative response, NXDOMAIN or NOERROR_NODATA. This
is the origin of the NODATA pseudo response code mentioned above.

Mark Andrews of CSIRO added code (RETURNSOA) that stored the SOA
record such that it could be retrieved by a similar query. UUnet
complained that they were getting old answers after loading a new
zone, and the option was turned off, BIND 4.9.3-alpha5, April 1994.
In reality this indicated that the named needed to purge the space
the zone would occupy. Functionality to do this was added in BIND
4.9.3 BETA11 patch2, December 1994.

RETURNSOA was re-enabled by default, BIND 4.9.5-T1A, August 1996.

10 Example

The following example is based on a signed zone that is empty apart
from the nameservers. We will query for WWW.XX.EXAMPLE showing
initial response and again 10 minutes later. Note 1: during the
intervening 10 minutes the NS records for XX.EXAMPLE have expired.
Note 2: the TTL of the SIG records are not explicitly set in the zone
file and are hence the TTL of the RRset they are the signature for.

Zone File:

$TTL 86400
$ORIGIN XX.EXAMPLE.
@ IN SOA NS1.XX.EXAMPLE. HOSTMATER.XX.EXAMPLE. (
1997102000 ; serial
1800 ; refresh (30 mins)
900 ; retry (15 mins)
604800 ; expire (7 days)
1200 ) ; minimum (20 mins)
IN SIG SOA ...
1200 IN NXT NS1.XX.EXAMPLE. A NXT SIG SOA NS KEY
IN SIG NXT ... XX.EXAMPLE. ...
300 IN NS NS1.XX.EXAMPLE.
300 IN NS NS2.XX.EXAMPLE.
IN SIG NS ... XX.EXAMPLE. ...
IN KEY 0x4100 1 1 ...
IN SIG KEY ... XX.EXAMPLE. ...
IN SIG KEY ... EXAMPLE. ...
NS1 IN A 10.0.0.1
IN SIG A ... XX.EXAMPLE. ...
1200 IN NXT NS2.XX.EXAMPLE. A NXT SIG
IN SIG NXT ...
NS2 IN A 10.0.0.2
IN SIG A ... XX.EXAMPLE. ...
1200 IN NXT XX.EXAMPLE. A NXT SIG
IN SIG NXT ... XX.EXAMPLE. ...

Initial Response:

Header:
RDCODE=NXDOMAIN, AA=1, QR=1, TC=0
Query:
WWW.XX.EXAMPLE. IN A
Answer:
<empty>
Authority:
XX.EXAMPLE. 1200 IN SOA NS1.XX.EXAMPLE. ...
XX.EXAMPLE. 1200 IN SIG SOA ... XX.EXAMPLE. ...

NS2.XX.EXAMPLE. 1200 IN NXT XX.EXAMPLE. NXT A NXT SIG
NS2.XX.EXAMPLE. 1200 IN SIG NXT ... XX.EXAMPLE. ...
XX.EXAMPLE. 86400 IN NS NS1.XX.EXAMPLE.
XX.EXAMPLE. 86400 IN NS NS2.XX.EXAMPLE.
XX.EXAMPLE. 86400 IN SIG NS ... XX.EXAMPLE. ...
Additional
XX.EXAMPLE. 86400 IN KEY 0x4100 1 1 ...
XX.EXAMPLE. 86400 IN SIG KEY ... EXAMPLE. ...
NS1.XX.EXAMPLE. 86400 IN A 10.0.0.1
NS1.XX.EXAMPLE. 86400 IN SIG A ... XX.EXAMPLE. ...
NS2.XX.EXAMPLE. 86400 IN A 10.0.0.2
NS3.XX.EXAMPLE. 86400 IN SIG A ... XX.EXAMPLE. ...

After 10 Minutes:

Header:
RDCODE=NXDOMAIN, AA=0, QR=1, TC=0
Query:
WWW.XX.EXAMPLE. IN A
Answer:
<empty>
Authority:
XX.EXAMPLE. 600 IN SOA NS1.XX.EXAMPLE. ...
XX.EXAMPLE. 600 IN SIG SOA ... XX.EXAMPLE. ...
NS2.XX.EXAMPLE. 600 IN NXT XX.EXAMPLE. NXT A NXT SIG
NS2.XX.EXAMPLE. 600 IN SIG NXT ... XX.EXAMPLE. ...
EXAMPLE. 65799 IN NS NS1.YY.EXAMPLE.
EXAMPLE. 65799 IN NS NS2.YY.EXAMPLE.
EXAMPLE. 65799 IN SIG NS ... XX.EXAMPLE. ...
Additional
XX.EXAMPLE. 65800 IN KEY 0x4100 1 1 ...
XX.EXAMPLE. 65800 IN SIG KEY ... EXAMPLE. ...
NS1.YY.EXAMPLE. 65799 IN A 10.100.0.1
NS1.YY.EXAMPLE. 65799 IN SIG A ... EXAMPLE. ...
NS2.YY.EXAMPLE. 65799 IN A 10.100.0.2
NS3.YY.EXAMPLE. 65799 IN SIG A ... EXAMPLE. ...
EXAMPLE. 65799 IN KEY 0x4100 1 1 ...
EXAMPLE. 65799 IN SIG KEY ... . ...

11 Security Considerations

It is believed that this document does not introduce any significant
additional security threats other that those that already exist when
using data from the DNS.

With negative caching it might be possible to propagate a denial of
service attack by spreading a NXDOMAIN message with a very high TTL.
Without negative caching that would be much harder. A similar effect
could be achieved previously by spreading a bad A record, so that the
server could not be reached - which is almost the same. It has the
same effect as far as what the end user is able to do, but with a
different psychological effect. With the bad A, I feel "damn the
network is broken again" and try again tomorrow. With the "NXDOMAIN"
I feel "Oh, they've turned off the server and it doesn't exist any
more" and probably never bother trying this server again.

A practical example of this is a SMTP server where this behaviour is
encoded. With a NXDOMAIN attack the mail message would bounce
immediately, where as with a bad A attack the mail would be queued
and could potentially get through after the attack was suspended.

For such an attack to be successful, the NXDOMAIN indiction must be
injected into a parent server (or a busy caching resolver). One way
this might be done by the use of a CNAME which results in the parent
server querying an attackers server. Resolvers that wish to prevent
such attacks can query again the final QNAME ignoring any NS data in
the query responses it has received for this query.

Implementing TTL sanity checking will reduce the effectiveness of
such an attack, because a successful attack would require re-
injection of the bogus data at more frequent intervals.

DNS Security [RFC2065] provides a mechanism to verify whether a
negative response is valid or not, through the use of NXT and SIG
records. This document supports the use of that mechanism by
promoting the transmission of the relevant security records even in a
non security aware server.

Acknowledgments

I would like to thank Rob Austein for his history of the CHIVES
nameserver. The DNSIND working group, in particular Robert Elz for
his valuable technical and editorial contributions to this document.

References

[RFC1034]
Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES,"
STD 13, RFC1034, November 1987.

[RFC1035]
Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION," STD 13, RFC1035, November 1987.

[RFC2065]
Eastlake, D., and C. Kaufman, "Domain Name System Security
Extensions," RFC2065, January 1997.

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC2119, March 1997.

[RFC2181]
Elz, R., and R. Bush, "Clarifications to the DNS
Specification," RFC2181, July 1997.

Author's Address

Mark Andrews
CSIRO - Mathematical and Information Sciences
Locked Bag 17
North Ryde NSW 2113
AUSTRALIA

Phone: +61 2 9325 3148
EMail: Mark.Andrews@cmis.csiro.au

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