Network Working Group J. Rosenberg Internet-Draft IAB Expires: July 6, 2005 January 5, 2005 What's in a Name: False Assumptions about DNS Names draft-iab-dns-assumptions-01 Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on July 6, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract The Domain Name System (DNS) provides an essential service on the Internet, mapping structured names to a variety of data, usually IP addresses. These names appear in email addresses, URIs, and other application layer identifiers that are often rendered to human users. Because of this, there has been a strong demand to acquire names that have significance to people, through equivalence to registered trademarks, company names, types of services, and so on. A danger of this trend is that the humans and automata which consume and use Rosenberg Expires July 6, 2005 [Page 1] Internet-Draft Name Assumptions January 2005 these identifiers will make assumptions about the services that are or should be provided by the hosts associated with these identifiers. This document discusses this problem in more detail and makes recommendations on how it can be avoided. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Modeling Usage of the DNS . . . . . . . . . . . . . . . . . . 4 3. Possible Assumptions . . . . . . . . . . . . . . . . . . . . . 5 3.1 By the User . . . . . . . . . . . . . . . . . . . . . . . 5 3.2 By the Client . . . . . . . . . . . . . . . . . . . . . . 5 3.3 By the Server . . . . . . . . . . . . . . . . . . . . . . 6 4. Consequences of False Assumptions . . . . . . . . . . . . . . 7 5. Reasons why the Assumptions can be False . . . . . . . . . . . 8 5.1 Evolution . . . . . . . . . . . . . . . . . . . . . . . . 8 5.2 Leakage . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.3 Sub Delegation . . . . . . . . . . . . . . . . . . . . . . 9 6. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 9. IAB Members . . . . . . . . . . . . . . . . . . . . . . . . . 11 10. Informative References . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . . 15 Rosenberg Expires July 6, 2005 [Page 2] Internet-Draft Name Assumptions January 2005 1. Introduction The Domain Name System (DNS) [1] provides an essential service on the Internet, mapping structured names to a variety of different types of data. Most often it is used to obtain the IP address of a host associated with that name [2][1][3]. However, it can be used to obtain other information, and proposals have been made for nearly everything, including geographic information [4]. Domain names are most often used in identifiers used by application protocols. The most well known include email addresses and URIs, such as the HTTP URL [5], RTSP URL [6] and SIP URI [7]. These identifiers are ubiquitous, appearing on business cards, web pages, street signs, and so on. Because of this, there has been a strong demand to acquire domain names that have significance to people through equivalence to registered trademarks, company names, types of services, and so on. Such identifiers serve many business purposes, including extension of brand, advertising, and so on. People often make assumptions about the type of service that is or should be provided by a host associated with that name. This problem first manifested itself in the "DNS land grab" that occurred through the registration of trademarked names by people who were not owners of those trademarks. The reason people did that is that they knew that users who saw the name in an HTTP URL would assume that the URL referenced a site associated with the trademarked name. Another example are the various proposals for a TLD that could be associated with adult content [8]. Yet another example are requests for TLDs associated with mobile devices and services. The essence of the problem is that humans will frequently make assumptions about a name based on their expectations and understanding of what the name implies. When these assumptions are wrong, the user might be surprised, but the system works, and the human can do something different having realized that its assumption was false. When an automaton makes similar assumptions, the system might fail, and it might fail systematically. For this reason, this document focuses primarily on assumptions that might be made by client and server automata about the service that is or should be provided by a host associated with a DNS name. In this context, an "assumption" is defined as any behavior that is expected when accessing a service at a domain name, even though the behavior is not explicitly indicated in the DNS or otherwise codified in protocol specifications. Although the DNS can contain data besides host identifiers (i.e., IP addresses), we only consider the usage of the DNS for the purposes of translating a name to a host address. Rosenberg Expires July 6, 2005 [Page 3] Internet-Draft Name Assumptions January 2005 2. Modeling Usage of the DNS +--------+ | | | | | DNS | |Service | | | +--------+ ^ | | | | | | | /--\ | | | | | V | | +--------+ +--------+ \--/ | | | | | | | | | ---+--- | Client |-------------------->| Server | | | | | | | | | | | /\ +--------+ +--------+ / \ / \ User Figure 1 Figure 1 shows a simple conceptual model of how the DNS is used by applications. A user of the application obtains an identifier for particular content or service it wishes to obtain. This identifier is often a URL or URI that contains a domain name. The user enters this identifier into their client application (for example, by typing in the URL in a web browser window). The client is the automaton (a software and/or hardware system) that contacts a server for that application in order to provide service to the user. To do that, it contacts a DNS server to resolve the domain name in the identifier to an IP address. It then contacts the server at that IP address. This simple model applies to application protocols such as HTTP [5], SIP [7], RTSP [6], and SMTP [9]. From this model, it is clear that three entities in the system can potentially make false assumptions about the service provided by the server. The human user may form expectations relating to the content of the service based on a parsing of the host name from which the Rosenberg Expires July 6, 2005 [Page 4] Internet-Draft Name Assumptions January 2005 content originated. The server might assume that the client connecting to it supports protocols that it does not, can process content that it cannot, or has capabilities that it does not. Similarly, the client might assume that the server supports protocols, content, or capabilities that it does not. 3. Possible Assumptions For each of the three elements, there are many types of false assumptions that can be made. 3.1 By the User The set of possible assumptions here is nearly boundless. A user might assume that an HTTP URL that looks like a company name maps to a server run by that company. They might assume that an email from a email address in the .gov TLD is actually from a government employee. They might assume that the content obtained from a web server within a TLD labeled as containing adult materials (for example, .sex) actually contains adult content [8]. These assumptions are unavoidable, may all be false, and not the focus of this document. 3.2 By the Client Even though the client is an automaton, it can make some of the same assumptions that a human user might make. For example, many clients assume that any host with a hostname that begins with "www" is a web server, even though this assumption may be false. In addition, the client concerns itself with the protocols needed to communicate with the server. As a result, it might make assumptions about the operation of the protocols for communicating with the server. These assumptions manifest themselves in an implementation through the replacement of a standardized protocol negotiation technique for an ill-defined rule for determining some kind of protocol parameter. The result is often a loss of interoperability, degradation in reliability and worsening of user experience. Authentication Algorithm: Though a protocol might support a multiplicity of authentication techniques, a client might assume that a server always supports one that is only optional according to the protocol. For example, a SIP client contacting a SIP server in a domain that appeared to be used to identify mobile devices (for example, www.example.cellular) might assume it supports the optional AKA digest technique [10]. As another example, a web client might assume that a server with the name https.example.com supports HTTP over TLS [16]. Rosenberg Expires July 6, 2005 [Page 5] Internet-Draft Name Assumptions January 2005 Data Formats: Though a protocol might allow a multiplicity of data formats to be sent from the server to the client, the client might assume a specific one, rather than using the content labeling and negotiation capabilities of the underlying protocol. For example, an RTSP client might assume that all audio content delivered to it from media.example.cellular uses a low bandwidth codec. As another example, a mail client might assume that the contents of messages it retrieves from a mail server at mail.example.cellular are always text, instead of checking the MIME headers [11] in the message to determine the actual content type. Protocol Extensions: The client attempts an operation on the server which requires the server to support an optional protocol extension. However, the client merely assumes that this extension is supported, rather than implementing the fallback logic necessary if it is not. As an example, a SIP client requires reliable provisional responses to its request (RFC 3262 [17]), and it assumes that this extension is supported on servers in the domain sip.example.telecom. Furthermore, the client has not implemented the fallback behavior defined in RFC 3262, since it assumes that all servers it will communicate with are in this domain, and that all support this extension. However, this assumption proves wrong. The result is that the client is unable to make any phone calls. Languages: The client supports facilities for processing text content differently depending on the language of the text. Rather than determining the language from markers in the message from the server, the client assumes a language based on the domain name. This assumption can easily be wrong. For example, a client might assume that any text in a web page retrieved from www.example.de is in German, and attempt a translation to Finnish. This would fail dramatically if the text was actually in French. 3.3 By the Server The server, like the client, is an automaton. It is servicing a particular domain - www.company.cellular, for example. It might assume that all clients connecting to this domain support particular capabilities, rather than using the underlying protocol to make this determination. Some examples include: Authentication Algorithm: The server can assume that a client supports a particular, optional, authentication technique, and it therefore does not support the mandatory one. Rosenberg Expires July 6, 2005 [Page 6] Internet-Draft Name Assumptions January 2005 Data Formats: The server can assume that the client supports a particular set of MIME types, and is only capable of sending ones within that set. When it generates content in a protocol response, it ignores any content negotiation headers that were present in the request. For example, a web server might ignore the Accept HTTP header field and send a specific image format. Protocol Extensions: The server might assume that the client supports a particular optional protocol extension, and so it does not support the fallback behavior necessary in the case where it does not. Client Characteristics: The server might assume certain things about the physical characteristics of its clients, such as memory footprint, processing power, screen sizes, screen colors, pointing devices, and so on. Based on these assumptions, it might choose specific behaviors when processing a request. For example, a web server might always assume that clients connect through cell phones, and therefore return content that lacks images and is tuned for such devices. 4. Consequences of False Assumptions There are numerous negative outcomes that can arise from the various false assumptions that users, servers and clients can make. These include: Interoperability Failure: In these cases, the client or server assumed some kind of protocol operation which was wrong. The result is that the two are unable to communicate, and the user receives some kind of an error. This represents a total interoperability failure, manifesting itself as a lack of service to users of the system. Unfortunately, this kind of failure persists. Repeated attempts over time by the client to access the service will fail. Only a change in the server or client software can fix this problem. System Failure: In these cases, the client or server mis-interpreted a protocol operation, and this mis-interpretation was serious enough to uncover a bug in the implementation. The bug causes a system crash or some kind of outage, either transient or permanent (until user reset). If this failure occurs in a server, not only will the connecting client lose service, but other clients attempting to connect will not get service. As an example, if a web server assumes that content passed to it from a client (created, for example, by a digital camera) is of a particular content type, and it always passes image content to a codec for Rosenberg Expires July 6, 2005 [Page 7] Internet-Draft Name Assumptions January 2005 de-compression prior to storage, the codec might crash when it unexpectedly receives an image compressed in a different format. Poor User Experience: In these cases, the client and server communicate, but the user receives a diminished user experience. For example, if a client on a PC connects to a web site that provides content for mobile devices, the content may be underwhelming when viewed on the PC. Or, a client accessing a streaming media service may receive content of very low bitrate, even though the client supported better codecs. Indeed, if a user wishes to access content from both a cellular device and a PC using a shared address book (that is, an address book shared across multiple devices), they would need two entries in that address book, and would need to use the right one from the right device. This is a poor user experience. 5. Reasons why the Assumptions can be False Assumptions made by clients and servers about the operation of protocols when contacting a particular domain are brittle, and can be wrong for many reasons. On the server side, many of the assumptions are based on the notion that a domain name will only be given to, or used by, a restricted set of clients. If the owner of the domain assumes something about those clients, and can assume that only those clients use the domain name, that it can configure or program the server to operate specifically for those clients. Both parts of this assumption can be wrong, as discussed in more detail below. On the client side, the notion is similar, being based on the assumption that a server within a particular domain will provide a specific type of service. Sub-delegation and evolution, both discussed below, can make these assumptions wrong. 5.1 Evolution The Internet, and the devices which access it, are constantly evolving, often at a rapid pace. Unfortunately, there is a tendency to build for the here and now, and then worry about the future at a later time. Many of the assumptions above are predicated on characteristics of today's clients and servers. Support for specific protocols, authentication techniques or content are based on today's standards and today's devices. Even though they may, for the most part, be true, they won't always be. An excellent example is mobile devices. A server servicing a domain accessed by mobile devices might try to make assumptions about the protocols, protocol extensions, security mechanisms, screen sizes or processor power of such devices. However, all of these characteristics can and will Rosenberg Expires July 6, 2005 [Page 8] Internet-Draft Name Assumptions January 2005 change over time. When they do change, the change is usually evolutionary. The result is that the assumptions remain valid in some cases, but not in others. It is difficult to fix such systems, since it requires the server to detect what type of client is connecting, and what its capabilities are. Unless the system is built and deployed with these capability negotiation techniques built in to begin with, such detection can be extremely difficult. In fact, fixing it will often require the addition of such capability negotiation features which, if they had been in place and used to begin with, would have avoided the problem altogether. 5.2 Leakage Servers also make assumptions because of the belief that they will only be accessed by specific clients, and in particular, those which are configured or provisioned to use the domain name. In essence, there is an assumption of community - that a specific community knows and uses the domain name, while others outside of the community do not. The problem is that this notion of community is a false one. The Internet is global. The DNS is global. There is no technical barrier that separates those inside of the community from those outside. The ease with which information propagates across the Internet makes it extremely likely that such domain names will eventually find their way into clients outside of the presumed community. The ubiquitous presence of domain names in various URI formats, coupled with the ease of conveyance of URIs, makes such leakage merely a matter of time. Furthermore, since the DNS is global, and since it can only have one root [12], it becomes possible for clients outside of the community to search and find and use such "special" domain names. 5.3 Sub Delegation Clients and users make assumptions about domains because of the notion that there is some kind of centralized control that can enforce those assumptions. However, the DNS is not centralized, it is distributed. If a domain doesn't delegate its sub-domains and has its records within a single zone, it is possible to maintain a centralized policy about operation of its domain. However, once a domain gets sufficiently large that it begins to delegate sub-domains to other authorities, it becomes increasingly difficult to maintain any kind of central control on the nature of the service provided in each sub-domain. Adherance to policies could be checked by having a robot periodically sweep the records in all sub-domains; this will Rosenberg Expires July 6, 2005 [Page 9] Internet-Draft Name Assumptions January 2005 only validate policies which are explicit within the record themselves (such as TTLs), and even then, it will be slow. Worse yet, most policies (such as support for specific protocol extensions) cannot be learned from the DNS itself, and cannot be verified in an automated way. The other choice is to hope that any centralized policies are being followed, and then wait for complaints. That approach has many problems. The invalidation of assumptions due to sub-delegation is discussed in further detail by Eastlake in Section 4.1.3 of [8] and by Faltstrom in Section 3.3 of [19]. As a result of the fragility of policy continuity across sub-delegations, if a client or user assumes some kind of property associated with a TLD (such as ".wifi"), it becomes exponentially more likely with the number of sub-domains that this property will not exist in a server identified by a particular name. For example, in "store.chain.company.provider.wifi", there may be four levels of delegation from ".wifi", making it quite likely that the properties which the owner of ".wifi" wishes to enforce are not present. These properties may not be present due to human error or due to a willful decision not to adhere to them. 6. Recommendations Based on these problems, the clear conclusion is that clients, servers and users should not make assumptions on the nature of the service provided to, or by, a domain. More specifically, however, the following can be said: Follow the Specifications: When specifications define mandatory baseline procedures and formats, those should be implemented and supported, even if the expectation is that optional procedures will most often be used. For example, if a specification mandates a particular baseline authentication technique, but allows others to be negotiated and used, implementations need to implement the baseline authentication algorithm even if the other ones are used most of the time. Use Capability Negotiation: Many protocols are engineered with capability negotiation mechanisms. For example, a content negotiation framework has been defined for protocols using MIME content [13][14][15]. SIP allows for clients to negotiate the media types used in the multimedia session, as well as protocol parameters. HTTP allows for clients to negotiate the media types returned in requests for content. When such features are available in a protocol, client and servers should make use of them rather than making assumptions about supported capabilities. Rosenberg Expires July 6, 2005 [Page 10] Internet-Draft Name Assumptions January 2005 A corollary is that protocol designers should include such mechanisms when evolution is expected in the usage of the protocol. The DNS Record Type Says it All: The only assumptions one can make about the service defined by the name are encapsulated in the DNS resource record type that is retrieved. For example, if an A or AAAA record exists for the domain web.example.com, one cannot assume that web service is available at the domain, since all the A or AAAA record says is that the host with that name has a particular IP address. The only way to definitively know if a service is available at a domain is through SRV records [2]. As an example, a client can do a query for _sip._udp.example.com to learn if a server is listening for SIP requests over UDP within the example.com domain [18]. To summarize - there is never a need to make assumptions. Rather than doing so, utilize the specifications and the negotiation capabilities they provide, and the overall system will be robust and interoperable. 7. Security Considerations One of the assumptions that can be made by clients or servers is the availability and usage (or lack thereof) of certain security protocols and algorithms. For example, a client accessing a service in a particular domain might assume a specific authentication algorithm or hash function in the application protocol. It is possible that, over time, weaknesses are found in such a technique, requiring usage of a different mechanism. Similarly, a system might start with an insecure mechanism, and then decide later on to use a secure one. In either case, assumptions made on security properties can result in interoperability failures, or worse yet, providing service in an insecure way, even though the client asked for, and thought it would get, secure service. This kinds of assumptions are fundamentally unsound even if the records themselves are secured with DNSSEC. 8. Acknowledgements The IAB would like to thank Brian Carpenter for his comments. 9. IAB Members Internet Architecture Board members at the time of writing of this document are: Rosenberg Expires July 6, 2005 [Page 11] Internet-Draft Name Assumptions January 2005 Bernard Aboba Harald Alvestrand Rob Austein Leslie Daigle Patrik Faltstrom Sally Floyd Mark Handley Bob Hinden Geoff Huston Jun-ichiro Itojun Hagino Eric Rescorla Pete Resnick Jonathan Rosenberg 10 Informative References [1] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [2] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [3] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database", RFC 3403, October 2002. [4] Davis, C., Vixie, P., Goodwin, T. and I. Dickinson, "A Means for Expressing Location Information in the Domain Name System", RFC 1876, January 1996. [5] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Rosenberg Expires July 6, 2005 [Page 12] Internet-Draft Name Assumptions January 2005 [6] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. [7] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [8] Eastlake, D., ".sex Considered Dangerous", RFC 3675, February 2004. [9] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [10] Niemi, A., Arkko, J. and V. Torvinen, "Hypertext Transfer Protocol (HTTP) Digest Authentication Using Authentication and Key Agreement (AKA)", RFC 3310, September 2002. [11] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [12] Internet Architecture Board, "IAB Technical Comment on the Unique DNS Root", RFC 2826, May 2000. [13] Klyne, G., "Indicating Media Features for MIME Content", RFC 2912, September 2000. [14] Klyne, G., "A Syntax for Describing Media Feature Sets", RFC 2533, March 1999. [15] Klyne, G., "Protocol-independent Content Negotiation Framework", RFC 2703, September 1999. [16] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [17] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [18] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol (SIP): Locating SIP Servers", RFC 3263, June 2002. [19] Faltstrom, P. and R. Austein, "Design Choices When Expanding DNS", draft-iab-dns-choices-00 (work in progress), October 2004. Rosenberg Expires July 6, 2005 [Page 13] Internet-Draft Name Assumptions January 2005 Author's Address Jonathan Rosenberg, Editor IAB 600 Lanidex Plaza Parsippany, NJ 07054 US Phone: +1 973 952-5000 EMail: jdrosen@cisco.com URI: http://www.jdrosen.net Rosenberg Expires July 6, 2005 [Page 14] Internet-Draft Name Assumptions January 2005 Intellectual Property Statement 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. 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Disclaimer of Validity 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. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Rosenberg Expires July 6, 2005 [Page 15]