Network Working Group                                          D. Atkins
draft-ietf-dnsext-dns-threats-01.txt                    IHTFP Consulting
                                                              R. Austein
                                             InterNetShare, Incorporated
                                                           February 2002

               Threat Analysis Of The Domain Name System

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   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.

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draft-ietf-dnsext-dns-threats-01.txt February 2002 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. DNS Transaction Signatures (TSIG) were eventually developed as a separate mechanism to address threats of unauthorized access to DNS's zone transfer and dynamic update mechanisms. - 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 eight years, 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 eight years after starting work on the protocol designed to defend against it, that is nevertheless what this note attempts to do. Better late than never. Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 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. 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 refer to TKEY and TSIG as separate mechanisms, even though 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 transaction model certain operations), and perhaps should be changed in a future revision of this note. 2. 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 chose 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-based attack (q.v., below). While it certainly would be possible to sign DNS messages using TSIG or IPsec, or even to encrypt them using IPsec, this would not be a very good solution. 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 Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 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. 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 all of the DNSSEC signature checking on its own, - Use TSIG (or some equivalent mechanism) to insure 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 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, 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 proof 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 Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 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. 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 themselves perform DNSSEC signature checking should use TSIG or some equivalent mechanism to insure the integrity of their communication with a recursing name server that does perform DNSSEC signature checking. 2.3. Name Games Perhaps the most interesting class of DNS-specific threats are the name-based attacks. There are several variations within this class, sometimes called "cache poisoning" or "fake authority" attacks. What all of these attacks have in common is that they all involve DNS RRs whose RDATA portion (right hand side) includes 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. The general form of a name-based attack is something like this: - Victim issues a query, perhaps at the instigation of the attacker or some third party; in some the query itself may be unrelated to the name under attack (ie, the attacker is just using this query as a means to inject false information about some other name). - 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. Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 - 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). The common thread in all of these 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" in a piece of Text/HTML mail to the victim. 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 whose 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-based attack involving glue, but it should be 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 Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 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 further 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 no matter which network it's plugged into at any given moment (and no matter what brand of middle boxes a particular hotel chain might have installed when adding network drops in every guest room...). From the 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 the 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 to perform the check (usually the public key for the root zone, 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 Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 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 immediate failure of a missing RR might be 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 the New York Times, even if we cannot conceive of a plausible scenario involving this attack today. This implies that some mitigation of this risk is required. 3. 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 will almost certainly trigger both timeouts and re-queries. (Arguably, many current DNS clients are already too impatient even before taking the further delays that DNSSEC will impose into account, but that's a separate topic for another document....) - Like DNS itself, DNSSEC's trust model is almost totally Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 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 really a change, since insecure DNS has essentially the same problem, but it's not good either. - 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 resolver and the host 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 resolver must have the same concept of absolute time 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 expired 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. 4. Other issues [Odds and ends that don't yet fit anywhere else, to be revised...] 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 the problems against (some of) which DNSSEC was intended to protect. There are, however, other potential problems at the boundaries where DNS interacts with other protocols. This topic needs further study. 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 keys 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. Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 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 find-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; Charlie may need to be able to add, remove, or modify nodes, but only A records. NOTE: Scaling properties of the key management problem here is a particular concern that needs more study. 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. We believe that deploying DNSSEC will help to address some, but not all, of the known threats to with DNS. Atkins & Austein Expires 30 August 2002 [Page
draft-ietf-dnsext-dns-threats-01.txt February 2002 IANA Considerations None known. Acknowledgments This note is based both previous published works by others and on on a number of discussions both public and private over a period of many years, but particular thanks go to Steve Bellovin, Dan Bernstein, Randy Bush, Olafur Gudmundsson, Allison Mankin, Paul Vixie, and our libel lawyers at the firm of Dewey, Chetham, & Howe, none of whom are responsible for what the authors did with their ideas. References [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- 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. [DNS-CONCEPTS] Mockapetris, P., "Domain names - concepts and facilities", RFC 1034, November 1987. [DNS-IMPLEMENTATION] Mockapetris, P., "Domain names - implementation and specification", RFC 1035, November 1987. [HOST-REQUIREMENTS] Braden, R., Editor, "Requirements for Internet Hosts - Application and Support", RFC 1123, October 1989. [DNS-CLARIFY] Elz, R., and Bush, R., "Clarifications to the DNS Specification" RFC 2181, July 1997. [NCACHE] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)" RFC 2308, March 1998. [DNSSEC] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. Atkins & Austein Expires 30 August 2002 [Page 11]
draft-ietf-dnsext-dns-threats-01.txt February 2002 [EDNS0] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999. [TSIG] Vixie, P., Gudmundsson, O., Eastlake, D., and Wellington, B., "Secret Key Transaction Authentication for DNS (TSIG)" RFC 2845, May 2000. [TKEY] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)" RFC 2930, September 2000. [SECURE-UPDATE] Wellington, B., "Secure Domain Name System (DNS) Dynamic Update" RFC 3007, November 2000. [SIGNING-AUTHORITY] Wellington, B., "Domain Name System Security (DNSSEC) Signing Authority" RFC 3008, November 2000. [DNSSEC-ZONE-STATUS] Lewis, E., "DNS Security Extension Clarification on Zone Status" RFC 3090, March 2001. Author's addresses: Derek Atkins IHTFP Consulting 6 Farragut Ave Somerville, MA 02144 USA Email: Rob Austein InterNetShare, Incorporated 325M Sharon Park Drive, Suite 308 Menlo Park, CA 94025 USA Atkins & Austein Expires 30 August 2002 [Page 12]