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+Network Working Group J. Rosenberg, Ed.
+Request for Comments: 4367 IAB
+Category: Informational February 2006
+
+
+ What's in a Name: False Assumptions about DNS Names
+
+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 (2006).
+
+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, Uniform Resource
+ Identifiers (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. There is a danger in this trend; the humans and
+ automata that consume and use such names will associate specific
+ semantics with some names and thereby make assumptions about the
+ services that are, or should be, provided by the hosts associated
+ with the names. Those assumptions can often be false, resulting in a
+ variety of failure conditions. This document discusses this problem
+ in more detail and makes recommendations on how it can be avoided.
+
+
+
+
+
+
+
+
+
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+
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+
+
+
+
+
+
+Rosenberg Informational [Page 1]
+
+RFC 4367 Name Assumptions February 2006
+
+
+Table of Contents
+
+ 1. Introduction ....................................................2
+ 2. Target Audience .................................................4
+ 3. Modeling Usage of the DNS .......................................4
+ 4. Possible Assumptions ............................................5
+ 4.1. By the User ................................................5
+ 4.2. By the Client ..............................................6
+ 4.3. By the Server ..............................................7
+ 5. Consequences of False Assumptions ...............................8
+ 6. Reasons Why the Assumptions Can Be False ........................9
+ 6.1. Evolution ..................................................9
+ 6.2. Leakage ...................................................10
+ 6.3. Sub-Delegation ............................................10
+ 6.4. Mobility ..................................................12
+ 6.5. Human Error ...............................................12
+ 7. Recommendations ................................................12
+ 8. A Note on RFC 2219 and RFC 2782 ................................13
+ 9. Security Considerations ........................................14
+ 10. Acknowledgements ..............................................14
+ 11. IAB Members ...................................................14
+ 12. Informative References ........................................15
+
+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], Real Time Streaming Protocol (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, based on
+ their expectations and understanding of what the name implies. This,
+ in turn, triggers attempts by organizations to register domain names
+ based on that presumed user expectation. Examples of this are the
+
+
+
+Rosenberg Informational [Page 2]
+
+RFC 4367 Name Assumptions February 2006
+
+
+ various proposals for a Top-Level Domain (TLD) that could be
+ associated with adult content [8], the requests for creation of TLDs
+ associated with mobile devices and services, and even phishing
+ attacks.
+
+ When these assumptions are codified into the behavior of an
+ automaton, such as an application client or server, as a result of
+ implementor choice, management directive, or domain owner policy, the
+ overall system can fail in various ways. This document describes a
+ number of typical ways in which these assumptions can be codified,
+ how they can be wrong, the consequences of those mistakes, and the
+ recommended ways in which they can be avoided.
+
+ Section 4 describes some of the possible assumptions that clients,
+ servers, and people can make about a domain 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 codified in protocol specifications. Frequently, these
+ assumptions involve ignoring parts of a specification based on an
+ assumption that the client or server is deployed in an environment
+ that is more rigid than the specification allows. Section 5
+ overviews some of the consequences of these false assumptions.
+ Generally speaking, these consequences can include a variety of
+ different interoperability failures, user experience failures, and
+ system failures. Section 6 discusses why these assumptions can be
+ false from the very beginning or become false at some point in the
+ future. Most commonly, they become false because the environment
+ changes in unexpected ways over time, and what was a valid assumption
+ before, no longer is. Other times, the assumptions prove wrong
+ because they were based on the belief that a specific community of
+ clients and servers was participating, and an element outside of that
+ community began participating.
+
+ Section 7 then provides some recommendations. These recommendations
+ encapsulate some of the engineering mantras that have been at the
+ root of Internet protocol design for decades. These include:
+
+ Follow the specifications.
+
+ Use the capability negotiation techniques provided in the
+ protocols.
+
+ Be liberal in what you accept, and conservative in what you send.
+ [18]
+
+ Overall, automata should not change their behavior within a protocol
+ based on the domain name, or some component of the domain name, of
+ the host they are communicating with.
+
+
+
+Rosenberg Informational [Page 3]
+
+RFC 4367 Name Assumptions February 2006
+
+
+2. Target Audience
+
+ This document has several audiences. Firstly, it is aimed at
+ implementors who ultimately develop the software that make the false
+ assumptions that are the subject of this document. The
+ recommendations described here are meant to reinforce the engineering
+ guidelines that are often understood by implementors, but frequently
+ forgotten as deadlines near and pressures mount.
+
+ The document is also aimed at technology managers, who often develop
+ the requirements that lead to these false assumptions. For them,
+ this document serves as a vehicle for emphasizing the importance of
+ not taking shortcuts in the scope of applicability of a project.
+
+ Finally, this document is aimed at domain name policy makers and
+ administrators. For them, it points out the perils in establishing
+ domain policies that get codified into the operation of applications
+ running within that domain.
+
+3. Modeling Usage of the DNS
+
+
+ +--------+
+ | |
+ | |
+ | DNS |
+ |Service |
+ | |
+ +--------+
+ ^ |
+ | |
+ | |
+ | |
+ /--\ | |
+ | | | V
+ | | +--------+ +--------+
+ \--/ | | | |
+ | | | | |
+ ---+--- | Client |-------------------->| Server |
+ | | | | |
+ | | | | |
+ /\ +--------+ +--------+
+ / \
+ / \
+
+ User
+ Figure 1
+
+
+
+
+Rosenberg Informational [Page 4]
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+RFC 4367 Name Assumptions February 2006
+
+
+ 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 its 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
+ 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. Furthermore,
+ applications can potentially contain a multiplicity of humans,
+ clients, and servers, all of which can independently make these false
+ assumptions.
+
+4. Possible Assumptions
+
+ For each of the three elements, there are many types of false
+ assumptions that can be made.
+
+4.1. By the User
+
+ The set of possible assumptions here is nearly boundless. Users
+ 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 are not the focus of this
+ document.
+
+
+
+
+
+
+
+
+
+Rosenberg Informational [Page 5]
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+RFC 4367 Name Assumptions February 2006
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+4.2. 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
+ when a standardized protocol negotiation technique defined by the
+ protocol is ignored, and instead, some kind of rule is coded into the
+ software that comes to its own conclusion about what the negotiation
+ would have determined. 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 is apparently used to identify mobile
+ devices (for example, www.example.cellular) might assume that the
+ server supports the optional Authentication and Key Agreement
+ (AKA) digest technique [10], just because of the domain name that
+ was used to access the server. As another example, a web client
+ might assume that a server with the name https.example.com
+ supports HTTP over Transport Layer Security (TLS) [16].
+
+ 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 in order to determine the actual content type.
+
+ Protocol Extensions: A client may attempt an operation on the server
+ that requires the server to support an optional protocol
+ extension. However, rather than implementing the necessary
+ fallback logic, the client may falsely assume that the extension
+ is supported. As an example, a SIP client that requires reliable
+ provisional responses to its request (RFC 3262 [17]) might assume
+ that this extension is supported on servers in the domain
+
+
+
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+RFC 4367 Name Assumptions February 2006
+
+
+ sip.example.telecom. Furthermore, the client would not implement
+ the fallback behavior defined in RFC 3262, since it would assume
+ that all servers it will communicate with are in this domain and
+ that all therefore support this extension. However, if the
+ assumptions prove wrong, the client is unable to make any phone
+ calls.
+
+ Languages: A client may support 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 might assume 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 a server
+ within the .de country code TLD (ccTLD) is in German, and attempt
+ a translation to Finnish. This would fail dramatically if the
+ text was actually in French. Unfortunately, this client behavior
+ is sometimes exhibited because the server has not properly labeled
+ the language of the content in the first place, often because the
+ server assumed such a labeling was not needed. This is an example
+ of how these false assumptions can create vicious cycles.
+
+4.3. By the Server
+
+ The server, like the client, is an automaton. Let us consider one
+ 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.
+
+ Language: The server can serve content in a particular language,
+ based on an assumption that clients accessing the domain speak a
+ particular language, or based on an assumption that clients coming
+ from a particular IP address speak a certain language.
+
+ 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.
+
+
+
+
+
+
+
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+ 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 the
+ client 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.
+
+5. 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, and this assumption 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 misinterpreted a
+ protocol operation, and this misinterpretation 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 decompression prior
+ to storage, the codec might crash when it unexpectedly receives an
+ image compressed in a different format. Of course, it might crash
+ even if the Content-Type was correct, but the compressed bitstream
+ was invalid. False assumptions merely introduce additional
+ failure cases.
+
+
+
+
+
+
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+RFC 4367 Name Assumptions February 2006
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+
+ 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), the user 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.
+
+ Degraded Security: In these cases, a weaker security mechanism is
+ used than the one that ought to have been used. As an example, a
+ server in a domain might assume that it is only contacted by
+ clients with a limited set of authentication algorithms, even
+ though the clients have been recently upgraded to support a
+ stronger set.
+
+6. 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 holder of the domain
+ name assumes something about those clients, and can assume that only
+ those clients use the domain name, then 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.
+
+6.1. Evolution
+
+ The Internet and the devices that 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
+
+
+
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+
+RFC 4367 Name Assumptions February 2006
+
+
+ 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
+ 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 that, if
+ they had been in place and used to begin with, would have avoided the
+ problem altogether.
+
+6.2. Leakage
+
+ Servers also make assumptions because of the belief that they will
+ only be accessed by specific clients, and in particular, those that
+ 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.
+
+ Indeed, this leakage is a strength of the Internet architecture, not
+ a weakness. It enables global access to services from any client
+ with a connection to the Internet. That, in turn, allows for rapid
+ growth in the number of customers for any particular service.
+
+6.3. 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
+
+
+
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+RFC 4367 Name Assumptions February 2006
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+
+ 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 the domain administrators begin
+ 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.
+
+ Similarly, the usage of domain names with human semantic connotation
+ tends to lead to a registration of multiple domains in which a
+ particular service is to run. As an example, a service provider with
+ the name "example" might register and set up its services in
+ "example.com", "example.net", and generally example.foo for each foo
+ that is a valid TLD. This, like sub-delegation, results in a growth
+ in the number of domains over which it is difficult to maintain
+ centralized control.
+
+ Not that it is not possible, since there are many examples of
+ successful administration of policies across sub-domains many levels
+ deep. However, it takes an increasing amount of effort to ensure
+ this result, as it requires human intervention and the creation of
+ process and procedure. Automated validation of adherence to policies
+ is very difficult to do, as there is no way to automatically verify
+ many policies that might be put into place.
+
+ A less costly process for providing centralized management of
+ policies is to just hope that any centralized policies are being
+ followed, and then wait for complaints or perform random audits.
+ Those approaches have many problems.
+
+ The invalidation of assumptions due to sub-delegation is discussed in
+ further detail in Section 4.1.3 of [8] and in Section 3.3 of [20].
+
+ 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 increasingly 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, unless the
+ holder of ".wifi" is working diligently, the properties that the
+ holder 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.
+
+
+
+
+
+
+
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+6.4. Mobility
+
+ One of the primary value propositions of a hostname as an identifier
+ is its persistence. A client can change IP addresses, yet still
+ retain a persistent identifier used by other hosts to reach it.
+ Because their value derives from their persistence, hostnames tend to
+ move with a host not just as it changes IP addresses, but as it
+ changes access network providers and technologies. For this reason,
+ assumptions made about a host based on the presumed access network
+ corresponding to that hostname tend to be wrong over time. As an
+ example, a PC might normally be connected to its broadband provider,
+ and through dynamic DNS have a hostname within the domain of that
+ provider. However, one cannot assume that any host within that
+ network has access over a broadband link; the user could connect
+ their PC over a low-bandwidth wireless access network and still
+ retain its domain name.
+
+6.5. Human Error
+
+ Of course, human error can be the source of errors in any system, and
+ the same is true here. There are many examples relevant to the
+ problem under discussion.
+
+ A client implementation may make the assumption that, just because a
+ DNS SRV record exists for a particular protocol in a particular
+ domain, indicating that the service is available on some port, that
+ the service is, in fact, running there. This assumption could be
+ wrong because the SRV records haven't been updated by the system
+ administrators to reflect the services currently running. As another
+ example, a client might assume that a particular domain policy
+ applies to all sub-domains. However, a system administrator might
+ have omitted to apply the policy to servers running in one of those
+ sub-domains.
+
+7. 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
+
+
+
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+RFC 4367 Name Assumptions February 2006
+
+
+ most of the time. Put more simply, the behavior of the protocol
+ machinery should never change based on the domain name of the
+ host.
+
+ 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.
+ A corollary is that protocol designers should include such
+ mechanisms when evolution is expected in the usage of the
+ protocol.
+
+ "Be liberal in what you accept, and conservative in what you send"
+ [18]: This axiom of Internet protocol design is applicable here
+ as well. Implementations should be prepared for the full breadth
+ of what a protocol allows another entity to send, rather than be
+ limiting in what it is willing to receive.
+
+ 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.
+
+8. A Note on RFC 2219 and RFC 2782
+
+ Based on the definition of an assumption given here, the behavior
+ hinted at by records in the DNS also represents an assumption. RFC
+ 2219 [19] defines well-known aliases that can be used to construct
+ domain names for reaching various well-known services in a domain.
+ This approach was later followed by the definition of a new resource
+ record, the SRV record [2], which specifies that a particular service
+ is running on a server in a domain. Although both of these
+ mechanisms are useful as a hint that a particular service is running
+ in a domain, both of them represent assumptions that may be false.
+ However, they differ in the set of reasons why those assumptions
+ might be false.
+
+ A client that assumes that "ftp.example.com" is an FTP server may be
+ wrong because the presumed naming convention in RFC 2219 was not
+ known by, or not followed by, the owner of domain.com. With RFC
+ 2782, an SRV record for a particular service would be present only by
+ explicit choice of the domain administrator, and thus a client that
+
+
+
+
+Rosenberg Informational [Page 13]
+
+RFC 4367 Name Assumptions February 2006
+
+
+ assumes that the corresponding host provides this service would be
+ wrong only because of human error in configuration. In this case,
+ the assumption is less likely to be wrong, but it certainly can be.
+
+ The only way to determine with certainty that a service is running on
+ a host is to initiate a connection to the port for that service, and
+ check. Implementations need to be careful not to codify any
+ behaviors that cause failures should the information provided in the
+ record actually be false. This borders on common sense for robust
+ implementations, but it is valuable to raise this point explicitly.
+
+9. 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. These kinds of assumptions are
+ fundamentally unsound even if the records themselves are secured with
+ DNSSEC.
+
+10. Acknowledgements
+
+ The IAB would like to thank John Klensin, Keith Moore and Peter Koch
+ for their comments.
+
+11. IAB Members
+
+ Internet Architecture Board members at the time of writing of this
+ document are:
+
+ Bernard Aboba
+
+ Loa Andersson
+
+ Brian Carpenter
+
+ Leslie Daigle
+
+ Patrik Faltstrom
+
+
+
+
+Rosenberg Informational [Page 14]
+
+RFC 4367 Name Assumptions February 2006
+
+
+ Bob Hinden
+
+ Kurtis Lindqvist
+
+ David Meyer
+
+ Pekka Nikander
+
+ Eric Rescorla
+
+ Pete Resnick
+
+ Jonathan Rosenberg
+
+12. 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.
+
+ [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.
+
+
+
+
+Rosenberg Informational [Page 15]
+
+RFC 4367 Name Assumptions February 2006
+
+
+ [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] Braden, R., "Requirements for Internet Hosts - Communication
+ Layers", STD 3, RFC 1122, October 1989.
+
+ [19] Hamilton, M. and R. Wright, "Use of DNS Aliases for Network
+ Services", BCP 17, RFC 2219, October 1997.
+
+ [20] Faltstrom, P., "Design Choices When Expanding DNS", Work in
+ Progress, June 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
+
+
+
+
+
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+
+RFC 4367 Name Assumptions February 2006
+
+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (2006).
+
+ 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.
+
+ 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
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+ 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.
+
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+
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+
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+
+Acknowledgement
+
+ Funding for the RFC Editor function is provided by the IETF
+ Administrative Support Activity (IASA).
+
+
+
+
+
+
+
+Rosenberg Informational [Page 17]
+
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