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Network Working Group                                          M. AndrewsRequest for Comments: 2308                                          CSIROUpdates:1034,1035                                            March 1998Category: Standards TrackNegative 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 [RFC1034Section4.3.4].   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.Andrews                     Standards Track                     [Page 1]

RFC 2308                       DNS NCACHE                     March 19981 - 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 [RFC1035Section 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.Andrews                     Standards Track                     [Page 2]

RFC 2308                       DNS NCACHE                     March 19982 - 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 insection 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. AAndrews                     Standards Track                     [Page 3]

RFC 2308                       DNS NCACHE                     March 1998           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 127.0.0.2Andrews                     Standards Track                     [Page 4]

RFC 2308                       DNS NCACHE                     March 1998               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.Andrews                     Standards Track                     [Page 5]

RFC 2308                       DNS NCACHE                     March 1998   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>Andrews                     Standards Track                     [Page 6]

RFC 2308                       DNS NCACHE                     March 1998           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.Andrews                     Standards Track                     [Page 7]

RFC 2308                       DNS NCACHE                     March 1998   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 [RFC 1035Section 5] is extended to include   the following directive:                           $TTL <TTL> [comment]Andrews                     Standards Track                     [Page 8]

RFC 2308                       DNS NCACHE                     March 1998   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 [RFC 2065Section 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 responseAndrews                     Standards Track                     [Page 9]

RFC 2308                       DNS NCACHE                     March 1998   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.Andrews                     Standards Track                    [Page 10]

RFC 2308                       DNS NCACHE                     March 1998   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 fromRFC 1034   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.Andrews                     Standards Track                    [Page 11]

RFC 2308                       DNS NCACHE                     March 1998   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 beforeRFC-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 becameRFC-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.Andrews                     Standards Track                    [Page 12]

RFC 2308                       DNS NCACHE                     March 1998   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 "stealthAndrews                     Standards Track                    [Page 13]

RFC 2308                       DNS NCACHE                     March 1998   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.Andrews                     Standards Track                    [Page 14]

RFC 2308                       DNS NCACHE                     March 199810 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. ...Andrews                     Standards Track                    [Page 15]

RFC 2308                       DNS NCACHE                     March 1998            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.Andrews                     Standards Track                    [Page 16]

RFC 2308                       DNS NCACHE                     March 1998   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.Andrews                     Standards Track                    [Page 17]

RFC 2308                       DNS NCACHE                     March 1998References   [RFC1034]           Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES,"           STD 13,RFC 1034, November 1987.   [RFC1035]           Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND           SPECIFICATION," STD 13,RFC 1035, November 1987.   [RFC2065]           Eastlake, D., and C. Kaufman, "Domain Name System Security           Extensions,"RFC 2065, January 1997.   [RFC2119]           Bradner, S., "Key words for use in RFCs to Indicate           Requirement Levels,"BCP 14,RFC 2119, March 1997.   [RFC2181]           Elz, R., and R. Bush, "Clarifications to the DNS           Specification,"RFC 2181, 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.auAndrews                     Standards Track                    [Page 18]

RFC 2308                       DNS NCACHE                     March 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Andrews                     Standards Track                    [Page 19]