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Network Working Group                                          D. AtkinsRequest for Comments: 3833                              IHTFP ConsultingCategory: Informational                                       R. Austein                                                                     ISC                                                             August 2004Threat Analysis of the Domain Name System (DNS)Status of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2004).Abstract   Although the DNS Security Extensions (DNSSEC) have been under   development for most of the last decade, the IETF has never written   down the specific set of threats against which DNSSEC is designed to   protect.  Among other drawbacks, this cart-before-the-horse situation   has made it difficult to determine whether DNSSEC meets its design   goals, since its design goals are not well specified.  This note   attempts to document some of the known threats to the DNS, and, in   doing so, attempts to measure to what extent (if any) DNSSEC is a   useful tool in defending against these threats.1. Introduction   The earliest organized work on DNSSEC within the IETF was an open   design team meeting organized by members of the DNS working group in   November 1993 at the 28th IETF meeting in Houston.  The broad   outlines of DNSSEC as we know it today are already clear in Jim   Galvin's summary of the results of that meeting [Galvin93]:   - While some participants in the meeting were interested in     protecting against disclosure of DNS data to unauthorized parties,     the design team made an explicit decision that "DNS data is     `public'", and ruled all threats of data disclosure explicitly out     of scope for DNSSEC.   - While some participants in the meeting were interested in     authentication of DNS clients and servers as a basis for access     control, this work was also ruled out of scope for DNSSEC per se.Atkins & Austein             Informational                      [Page 1]

RFC 3833                  DNS Threat Analysis                August 2004   - Backwards compatibility and co-existence with "insecure DNS" was     listed as an explicit requirement.   - The resulting list of desired security services was     1) data integrity, and     2) data origin authentication.   - The design team noted that a digital signature mechanism would     support the desired services.   While a number of detail decisions were yet to be made (and in some   cases remade after implementation experience) over the subsequent   decade, the basic model and design goals have remained fixed.   Nowhere, however, does any of the DNSSEC work attempt to specify in   any detail the sorts of attacks against which DNSSEC is intended to   protect, or the reasons behind the list of desired security services   that came out of the Houston meeting.  For that, we have to go back   to a paper originally written by Steve Bellovin in 1990 but not   published until 1995, for reasons that Bellovin explained in the   paper's epilogue [Bellovin95].   While it may seem a bit strange to publish the threat analysis a   decade after starting work on the protocol designed to defend against   it, that is, nevertheless, what this note attempts to do.  Better   late than never.   This note assumes that the reader is familiar with both the DNS and   with DNSSEC, and does not attempt to provide a tutorial on either.   The DNS documents most relevant to the subject of this note are:   [RFC1034], [RFC1035],section 6.1 of [RFC1123], [RFC2181], [RFC2308],   [RFC2671], [RFC2845], [RFC2930], [RFC3007], and [RFC2535].   For purposes of discussion, this note uses the term "DNSSEC" to refer   to the core hierarchical public key and signature mechanism specified   in the DNSSEC documents, and refers to TKEY and TSIG as separate   mechanisms, even though channel security mechanisms such as TKEY and   TSIG are also part of the larger problem of "securing DNS" and thus   are often considered part of the overall set of "DNS security   extensions".  This is an arbitrary distinction that in part reflects   the way in which the protocol has evolved (introduction of a   putatively simpler channel security model for certain operations such   as zone transfers and dynamic update requests), and perhaps should be   changed in a future revision of this note.Atkins & Austein             Informational                      [Page 2]

RFC 3833                  DNS Threat Analysis                August 20042.  Known Threats   There are several distinct classes of threats to the DNS, most of   which are DNS-related instances of more general problems, but a few   of which are specific to peculiarities of the DNS protocol.2.1.  Packet Interception   Some of the simplest threats against DNS are various forms of packet   interception: monkey-in-the-middle attacks, eavesdropping on requests   combined with spoofed responses that beat the real response back to   the resolver, and so forth.  In any of these scenarios, the attacker   can simply tell either party (usually the resolver) whatever it wants   that party to believe.  While packet interception attacks are far   from unique to DNS, DNS's usual behavior of sending an entire query   or response in a single unsigned, unencrypted UDP packet makes these   attacks particularly easy for any bad guy with the ability to   intercept packets on a shared or transit network.   To further complicate things, the DNS query the attacker intercepts   may just be a means to an end for the attacker: the attacker might   even choose to return the correct result in the answer section of a   reply message while using other parts of the message to set the stage   for something more complicated, for example, a name chaining attack   (seesection 2.3).   While it certainly would be possible to sign DNS messages using a   channel security mechanism such as TSIG or IPsec, or even to encrypt   them using IPsec, this would not be a very good solution for   interception attacks.  First, this approach would impose a fairly   high processing cost per DNS message, as well as a very high cost   associated with establishing and maintaining bilateral trust   relationships between all the parties that might be involved in   resolving any particular query.  For heavily used name servers (such   as the servers for the root zone), this cost would almost certainly   be prohibitively high.  Even more important, however, is that the   underlying trust model in such a design would be wrong, since at best   it would only provide a hop-by-hop integrity check on DNS messages   and would not provide any sort of end-to-end integrity check between   the producer of DNS data (the zone administrator) and the consumer of   DNS data (the application that triggered the query).   By contrast, DNSSEC (when used properly) does provide an end-to-end   data integrity check, and is thus a much better solution for this   class of problems during basic DNS lookup operations.Atkins & Austein             Informational                      [Page 3]

RFC 3833                  DNS Threat Analysis                August 2004   TSIG does have its place in corners of the DNS protocol where there's   a specific trust relationship between a particular client and a   particular server, such as zone transfer, dynamic update, or a   resolver (stub or otherwise) that is not going to check all the   DNSSEC signatures itself.   Note that DNSSEC does not provide any protection against modification   of the DNS message header, so any properly paranoid resolver must:   - Perform all of the DNSSEC signature checking on its own,   - Use TSIG (or some equivalent mechanism) to ensure the integrity of     its communication with whatever name servers it chooses to trust,     or   - Resign itself to the possibility of being attacked via packet     interception (and via other techniques discussed below).2.2.  ID Guessing and Query Prediction   Since DNS is for the most part used over UDP/IP, it is relatively   easy for an attacker to generate packets which will match the   transport protocol parameters.  The ID field in the DNS header is   only a 16-bit field and the server UDP port associated with DNS is a   well-known value, so there are only 2**32 possible combinations of ID   and client UDP port for a given client and server.  This is not a   particularly large range, and is not sufficient to protect against a   brute force search; furthermore, in practice both the client UDP port   and the ID can often be predicted from previous traffic, and it is   not uncommon for the client port to be a known fixed value as well   (due to firewalls or other restrictions), thus frequently reducing   the search space to a range smaller than 2**16.   By itself, ID guessing is not enough to allow an attacker to inject   bogus data, but combined with knowledge (or guesses) about QNAMEs and   QTYPEs for which a resolver might be querying, this leaves the   resolver only weakly defended against injection of bogus responses.   Since this attack relies on predicting a resolver's behavior, it's   most likely to be successful when the victim is in a known state,   whether because the victim rebooted recently, or because the victim's   behavior has been influenced by some other action by the attacker, or   because the victim is responding (in a predictable way) to some third   party action known to the attacker.Atkins & Austein             Informational                      [Page 4]

RFC 3833                  DNS Threat Analysis                August 2004   This attack is both more and less difficult for the attacker than the   simple interception attack described above: more difficult, because   the attack only works when the attacker guesses correctly; less   difficult, because the attacker doesn't need to be on a transit or   shared network.   In most other respects, this attack is similar to a packet   interception attack.  A resolver that checks DNSSEC signatures will   be able to detect the forged response; resolvers that do not perform   DNSSEC signature checking themselves should use TSIG or some   equivalent mechanism to ensure the integrity of their communication   with a recursive name server that does perform DNSSEC signature   checking.2.3.  Name Chaining   Perhaps the most interesting class of DNS-specific threats are the   name chaining attacks.  These are a subset of a larger class of   name-based attacks, sometimes called "cache poisoning" attacks.  Most   name-based attacks can be partially mitigated by the long-standing   defense of checking RRs in response messages for relevance to the   original query, but such defenses do not catch name chaining attacks.   There are several variations on the basic attack, but what they all   have in common is that they all involve DNS RRs whose RDATA portion   (right hand side) includes a DNS name (or, in a few cases, something   that is not a DNS name but which directly maps to a DNS name).  Any   such RR is, at least in principle, a hook that lets an attacker feed   bad data into a victim's cache, thus potentially subverting   subsequent decisions based on DNS names.   The worst examples in this class of RRs are CNAME, NS, and DNAME RRs   because they can redirect a victim's query to a location of the   attacker's choosing.  RRs like MX and SRV are somewhat less   dangerous, but in principle they can also be used to trigger further   lookups at a location of the attacker's choosing.  Address RR types   such as A or AAAA don't have DNS names in their RDATA, but since the   IN-ADDR.ARPA and IP6.ARPA trees are indexed using a DNS encoding of   IPv4 and IPv6 addresses, these record types can also be used in a   name chaining attack.   The general form of a name chaining attack is something like this:   - Victim issues a query, perhaps at the instigation of the attacker     or some third party; in some cases the query itself may be     unrelated to the name under attack (that is, the attacker is just     using this query as a means to inject false information about some     other name).Atkins & Austein             Informational                      [Page 5]

RFC 3833                  DNS Threat Analysis                August 2004   - Attacker injects response, whether via packet interception, query     guessing, or by being a legitimate name server that's involved at     some point in the process of answering the query that the victim     issued.   - Attacker's response includes one or more RRs with DNS names in     their RDATA; depending on which particular form this attack takes,     the object may be to inject false data associated with those names     into the victim's cache via the Additional section of this     response, or may be to redirect the next stage of the query to a     server of the attacker's choosing (in order to inject more complex     lies into the victim's cache than will fit easily into a single     response, or in order to place the lies in the Authority or Answer     section of a response where they will have a better chance of     sneaking past a resolver's defenses).   Any attacker who can insert resource records into a victim's cache   can almost certainly do some kind of damage, so there are cache   poisoning attacks which are not name chaining attacks in the sense   discussed here.  However, in the case of name chaining attacks, the   cause and effect relationship between the initial attack and the   eventual result may be significantly more complex than in the other   forms of cache poisoning, so name chaining attacks merit special   attention.   The common thread in all of the name chaining attacks is that   response messages allow the attacker to introduce arbitrary DNS names   of the attacker's choosing and provide further information that the   attacker claims is associated with those names; unless the victim has   better knowledge of the data associated with those names, the victim   is going to have a hard time defending against this class of attacks.   This class of attack is particularly insidious given that it's quite   easy for an attacker to provoke a victim into querying for a   particular name of the attacker's choosing, for example, by embedding   a link to a 1x1-pixel "web bug" graphic in a piece of Text/HTML mail   to the victim.  If the victim's mail reading program attempts to   follow such a link, the result will be a DNS query for a name chosen   by the attacker.   DNSSEC should provide a good defense against most (all?) variations   on this class of attack.  By checking signatures, a resolver can   determine whether the data associated with a name really was inserted   by the delegated authority for that portion of the DNS name space.   More precisely, a resolver can determine whether the entity that   injected the data had access to an allegedly secret key whoseAtkins & Austein             Informational                      [Page 6]

RFC 3833                  DNS Threat Analysis                August 2004   corresponding public key appears at an expected location in the DNS   name space with an expected chain of parental signatures that start   with a public key of which the resolver has prior knowledge.   DNSSEC signatures do not cover glue records, so there's still a   possibility of a name chaining attack involving glue, but with DNSSEC   it is possible to detect the attack by temporarily accepting the glue   in order to fetch the signed authoritative version of the same data,   then checking the signatures on the authoritative version.2.4.  Betrayal By Trusted Server   Another variation on the packet interception attack is the trusted   server that turns out not to be so trustworthy, whether by accident   or by intent.  Many client machines are only configured with stub   resolvers, and use trusted servers to perform all of their DNS   queries on their behalf.  In many cases the trusted server is   furnished by the user's ISP and advertised to the client via DHCP or   PPP options.  Besides accidental betrayal of this trust relationship   (via server bugs, successful server break-ins, etc), the server   itself may be configured to give back answers that are not what the   user would expect, whether in an honest attempt to help the user or   to promote some other goal such as furthering a business partnership   between the ISP and some third party.   This problem is particularly acute for frequent travelers who carry   their own equipment and expect it to work in much the same way   wherever they go.  Such travelers need trustworthy DNS service   without regard to who operates the network into which their equipment   is currently plugged or what brand of middle boxes the local   infrastructure might use.   While the obvious solution to this problem would be for the client to   choose a more trustworthy server, in practice this may not be an   option for the client.  In many network environments a client machine   has only a limited set of recursive name servers from which to   choose, and none of them may be particularly trustworthy.  In extreme   cases, port filtering or other forms of packet interception may   prevent the client host from being able to run an iterative resolver   even if the owner of the client machine is willing and able to do so.   Thus, while the initial source of this problem is not a DNS protocol   attack per se, this sort of betrayal is a threat to DNS clients, and   simply switching to a different recursive name server is not an   adequate defense.   Viewed strictly from the DNS protocol standpoint, the only difference   between this sort of betrayal and a packet interception attack is   that in this case the client has voluntarily sent its request to theAtkins & Austein             Informational                      [Page 7]

RFC 3833                  DNS Threat Analysis                August 2004   attacker.  The defense against this is the same as with a packet   interception attack: the resolver must either check DNSSEC signatures   itself or use TSIG (or equivalent) to authenticate the server that it   has chosen to trust.  Note that use of TSIG does not by itself   guarantee that a name server is at all trustworthy: all TSIG can do   is help a resolver protect its communication with a name server that   it has already decided to trust for other reasons.  Protecting a   resolver's communication with a server that's giving out bogus   answers is not particularly useful.   Also note that if the stub resolver does not trust the name server   that is doing work on its behalf and wants to check the DNSSEC   signatures itself, the resolver really does need to have independent   knowledge of the DNSSEC public key(s) it needs in order to perform   the check.  Usually the public key for the root zone is enough, but   in some cases knowledge of additional keys may also be appropriate.   It is difficult to escape the conclusion that a properly paranoid   resolver must always perform its own signature checking, and that   this rule even applies to stub resolvers.2.5.  Denial of Service   As with any network service (or, indeed, almost any service of any   kind in any domain of discourse), DNS is vulnerable to denial of   service attacks.  DNSSEC does not help this, and may in fact make the   problem worse for resolvers that check signatures, since checking   signatures both increases the processing cost per DNS message and in   some cases can also increase the number of messages needed to answer   a query.  TSIG (and similar mechanisms) have equivalent problems.   DNS servers are also at risk of being used as denial of service   amplifiers, since DNS response packets tend to be significantly   longer than DNS query packets.  Unsurprisingly, DNSSEC doesn't help   here either.2.6.  Authenticated Denial of Domain Names   Much discussion has taken place over the question of authenticated   denial of domain names.  The particular question is whether there is   a requirement for authenticating the non-existence of a name.  The   issue is whether the resolver should be able to detect when an   attacker removes RRs from a response.   General paranoia aside, the existence of RR types whose absence   causes an action other than immediate failure (such as missing MX and   SRV RRs, which fail over to A RRs) constitutes a real threat.   Arguably, in some cases, even the absence of an RR might beAtkins & Austein             Informational                      [Page 8]

RFC 3833                  DNS Threat Analysis                August 2004   considered a problem.  The question remains: how serious is this   threat?  Clearly the threat does exist; general paranoia says that   some day it'll be on the front page of some major newspaper, even if   we cannot conceive of a plausible scenario involving this attack   today.  This implies that some mitigation of this risk is required.   Note that it's necessary to prove the non-existence of applicable   wildcard RRs as part of the authenticated denial mechanism, and that,   in a zone that is more than one label deep, such a proof may require   proving the non-existence of multiple discrete sets of wildcard RRs.   DNSSEC does include mechanisms which make it possible to determine   which authoritative names exist in a zone, and which authoritative   resource record types exist at those names.  The DNSSEC protections   do not cover non-authoritative data such as glue records.2.7.  Wildcards   Much discussion has taken place over whether and how to provide data   integrity and data origin authentication for "wildcard" DNS names.   Conceptually, RRs with wildcard names are patterns for synthesizing   RRs on the fly according to the matching rules described insection4.3.2 of RFC 1034.  While the rules that control the behavior of   wildcard names have a few quirks that can make them a trap for the   unwary zone administrator, it's clear that a number of sites make   heavy use of wildcard RRs, particularly wildcard MX RRs.   In order to provide the desired services for wildcard RRs, we need to   do two things:   - We need a way to attest to the existence of the wildcard RR itself     (that is, we need to show that the synthesis rule exists), and   - We need a way to attest to the non-existence of any RRs which, if     they existed, would make the wildcard RR irrelevant according to     the synthesis rules that govern the way in which wildcard RRs are     used (that is, we need to show that the synthesis rule is     applicable).   Note that this makes the wildcard mechanisms dependent upon the   authenticated denial mechanism described in the previous section.   DNSSEC includes mechanisms along the lines described above, which   make it possible for a resolver to verify that a name server applied   the wildcard expansion rules correctly when generating an answer.Atkins & Austein             Informational                      [Page 9]

RFC 3833                  DNS Threat Analysis                August 20043.  Weaknesses of DNSSEC   DNSSEC has some problems of its own:   - DNSSEC is complex to implement and includes some nasty edge cases     at the zone cuts that require very careful coding.  Testbed     experience to date suggests that trivial zone configuration errors     or expired keys can cause serious problems for a DNSSEC-aware     resolver, and that the current protocol's error reporting     capabilities may leave something to be desired.   - DNSSEC significantly increases the size of DNS response packets;     among other issues, this makes DNSSEC-aware DNS servers even more     effective as denial of service amplifiers.   - DNSSEC answer validation increases the resolver's work load, since     a DNSSEC-aware resolver will need to perform signature validation     and in some cases will also need to issue further queries.  This     increased workload will also increase the time it takes to get an     answer back to the original DNS client, which is likely to trigger     both timeouts and re-queries in some cases.  Arguably, many current     DNS clients are already too impatient even before taking the     further delays that DNSSEC will impose into account, but that topic     is beyond the scope of this note.   - Like DNS itself, DNSSEC's trust model is almost totally     hierarchical.  While DNSSEC does allow resolvers to have special     additional knowledge of public keys beyond those for the root, in     the general case the root key is the one that matters.  Thus any     compromise in any of the zones between the root and a particular     target name can damage DNSSEC's ability to protect the integrity of     data owned by that target name.  This is not a change, since     insecure DNS has the same model.   - Key rollover at the root is really hard.  Work to date has not even     come close to adequately specifying how the root key rolls over, or     even how it's configured in the first place.   - DNSSEC creates a requirement of loose time synchronization between     the validating resolver and the entity creating the DNSSEC     signatures.  Prior to DNSSEC, all time-related actions in DNS could     be performed by a machine that only knew about "elapsed" or     "relative" time.  Because the validity period of a DNSSEC signature     is based on "absolute" time, a validating resolver must have the     same concept of absolute time as the zone signer in order to     determine whether the signature is within its validity period or     has expired.  An attacker that can change a resolver's opinion of     the current absolute time can fool the resolver using expiredAtkins & Austein             Informational                     [Page 10]

RFC 3833                  DNS Threat Analysis                August 2004     signatures.  An attacker that can change the zone signer's opinion     of the current absolute time can fool the zone signer into     generating signatures whose validity period does not match what the     signer intended.   - The possible existence of wildcard RRs in a zone complicates the     authenticated denial mechanism considerably.  For most of the     decade that DNSSEC has been under development these issues were     poorly understood.  At various times there have been questions as     to whether the authenticated denial mechanism is completely     airtight and whether it would be worthwhile to optimize the     authenticated denial mechanism for the common case in which     wildcards are not present in a zone.  However, the main problem is     just the inherent complexity of the wildcard mechanism itself.     This complexity probably makes the code for generating and checking     authenticated denial attestations somewhat fragile, but since the     alternative of giving up wildcards entirely is not practical due to     widespread use, we are going to have to live with wildcards. The     question just becomes one of whether or not the proposed     optimizations would make DNSSEC's mechanisms more or less fragile.   - Even with DNSSEC, the class of attacks discussed insection 2.4 is     not easy to defeat.  In order for DNSSEC to be effective in this     case, it must be possible to configure the resolver to expect     certain categories of DNS records to be signed.  This may require     manual configuration of the resolver, especially during the initial     DNSSEC rollout period when the resolver cannot reasonably expect     the root and TLD zones to be signed.4.  Topics for Future Work   This section lists a few subjects not covered above which probably   need additional study, additional mechanisms, or both.4.1.  Interactions With Other Protocols   The above discussion has concentrated exclusively on attacks within   the boundaries of the DNS protocol itself, since those are (some of)   the problems against which DNSSEC was intended to protect.  There   are, however, other potential problems at the boundaries where DNS   interacts with other protocols.4.2.  Securing DNS Dynamic Update   DNS dynamic update opens a number of potential problems when combined   with DNSSEC.  Dynamic update of a non-secure zone can use TSIG to   authenticate the updating client to the server.  While TSIG does not   scale very well (it requires manual configuration of shared keysAtkins & Austein             Informational                     [Page 11]

RFC 3833                  DNS Threat Analysis                August 2004   between the DNS name server and each TSIG client), it works well in a   limited or closed environment such as a DHCP server updating a local   DNS name server.   Major issues arise when trying to use dynamic update on a secure   zone.  TSIG can similarly be used in a limited fashion to   authenticate the client to the server, but TSIG only protects DNS   transactions, not the actual data, and the TSIG is not inserted into   the DNS zone, so resolvers cannot use the TSIG as a way of verifying   the changes to the zone.  This means that either:   a) The updating client must have access to a zone-signing key in      order to sign the update before sending it to the server, or   b) The DNS name server must have access to an online zone-signing key      in order to sign the update.   In either case, a zone-signing key must be available to create signed   RRsets to place in the updated zone.  The fact that this key must be   online (or at least available) is a potential security risk.   Dynamic update also requires an update to the SERIAL field of the   zone's SOA RR.  In theory, this could also be handled via either of   the above options, but in practice (a) would almost certainly be   extremely fragile, so (b) is the only workable mechanism.   There are other threats in terms of describing the policy of who can   make what changes to which RRsets in the zone.  The current access   control scheme in Secure Dynamic Update is fairly limited.  There is   no way to give fine-grained access to updating DNS zone information   to multiple entities, each of whom may require different kinds of   access.  For example, Alice may need to be able to add new nodes to   the zone or change existing nodes, but not remove them; Bob may need   to be able to remove zones but not add them; Carol may need to be   able to add, remove, or modify nodes, but only A records.   Scaling properties of the key management problem here are a   particular concern that needs more study.4.3.  Securing DNS Zone Replication   As discussed in previous sections, DNSSEC per se attempts to provide   data integrity and data origin authentication services on top of the   normal DNS query protocol.  Using the terminology discussed in   [RFC3552], DNSSEC provides "object security" for the normal DNS query   protocol.  For purposes of replicating entire DNS zones, however,   DNSSEC does not provide object security, because zones include   unsigned NS RRs and glue at delegation points.  Use of TSIG toAtkins & Austein             Informational                     [Page 12]

RFC 3833                  DNS Threat Analysis                August 2004   protect zone transfer (AXFR or IXFR) operations provides "channel   security", but still does not provide object security for complete   zones. The trust relationships involved in zone transfer are still   very much a hop-by-hop matter of name server operators trusting other   name server operators rather than an end-to-end matter of name server   operators trusting zone administrators.   Zone object security was not an explicit design goal of DNSSEC, so   failure to provide this service should not be a surprise.   Nevertheless, there are some zone replication scenarios for which   this would be a very useful additional service, so this seems like a   useful area for future work.  In theory it should not be difficult to   add zone object security as a backwards compatible enhancement to the   existing DNSSEC model, but the DNSEXT WG has not yet discussed either   the desirability of or the requirements for such an enhancement.5.  Conclusion   Based on the above analysis, the DNSSEC extensions do appear to solve   a set of problems that do need to be solved, and are worth deploying.Security Considerations   This entire document is about security considerations of the DNS.   The authors believe that deploying DNSSEC will help to address some,   but not all, of the known threats to the DNS.Acknowledgments   This note is based both on previous published works by others and on   a number of discussions both public and private over a period of many   years, but particular thanks go to   Jaap Akkerhuis,   Steve Bellovin,   Dan Bernstein,   Randy Bush,   Steve Crocker,   Olafur Gudmundsson,   Russ Housley,   Rip Loomis,   Allison Mankin,   Paul Mockapetris,   Thomas Narten   Mans Nilsson,   Pekka Savola,   Paul Vixie,   Xunhua Wang,Atkins & Austein             Informational                     [Page 13]

RFC 3833                  DNS Threat Analysis                August 2004   and any other members of the DNS, DNSSEC, DNSIND, and DNSEXT working   groups whose names and contributions the authors have forgotten, none   of whom are responsible for what the authors did with their ideas.   As with any work of this nature, the authors of this note acknowledge   that we are standing on the toes of those who have gone before us.   Readers interested in this subject may also wish to read   [Bellovin95], [Schuba93], and [Vixie95].Normative References   [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.   [RFC1123]    Braden, R., "Requirements for Internet Hosts -                Application and Support", STD 3,RFC 1123, October 1989.   [RFC2181]    Elz, R. and R. Bush, "Clarifications to the DNS                Specification",RFC 2181, July 1997.   [RFC2308]    Andrews, M., "Negative Caching of DNS Queries (DNS                NCACHE)",RFC 2308, March 1998.   [RFC2671]    Vixie, P., "Extension Mechanisms for DNS (EDNS0)",RFC2671, August 1999.   [RFC2845]    Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.                Wellington, "Secret Key Transaction Authentication for                DNS (TSIG)",RFC 2845, May 2000.   [RFC2930]    Eastlake 3rd, D., "Secret Key Establishment for DNS                (TKEY RR)",RFC 2930, September 2000.   [RFC3007]    Wellington, B., "Secure Domain Name System (DNS) Dynamic                Update",RFC 3007, November 2000.   [RFC2535]    Eastlake 3rd, D., "Domain Name System Security                Extensions",RFC 2535, March 1999.Atkins & Austein             Informational                     [Page 14]

RFC 3833                  DNS Threat Analysis                August 2004Informative References   [RFC3552]    Rescorla, E. and B. Korver, "Guidelines for Writing RFC                Text on Security Considerations",BCP 72,RFC 3552, July                2003.   [Bellovin95] Bellovin, S., "Using the Domain Name System for System                Break-Ins", Proceedings of the Fifth Usenix Unix                Security Symposium, June 1995.   [Galvin93]   Design team meeting summary message posted to dns-                security@tis.com mailing list by Jim Galvin on 19                November 1993.   [Schuba93]   Schuba, C., "Addressing Weaknesses in the Domain Name                System Protocol", Master's thesis, Purdue University                Department of Computer Sciences,  August 1993.   [Vixie95]    Vixie, P, "DNS and BIND Security Issues", Proceedings of                the Fifth Usenix Unix Security Symposium, June 1995.Authors' Addresses   Derek Atkins   IHTFP Consulting, Inc.   6 Farragut Ave   Somerville, MA  02144   USA   EMail: derek@ihtfp.com   Rob Austein   Internet Systems Consortium   950 Charter Street   Redwood City, CA 94063   USA   EMail: sra@isc.orgAtkins & Austein             Informational                     [Page 15]

RFC 3833                  DNS Threat Analysis                August 2004Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78, and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Atkins & Austein             Informational                     [Page 16]