Negative Caching of DNS Queries (DNS NCACHE)

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 . "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 ) 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 , and is expected when reading this document.

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:

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 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.

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.

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.

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.

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 the SOA and appropriate NXT and SIG records MUST be added.

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 1035 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 [RFC 2065 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.

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.

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.

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.

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>.

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.

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.

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.

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.

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 ... . ...

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 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.

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.