<pre>Network Working Group                                  J. Rosenberg, Ed.
Request for Comments: 4367                                           IAB
Category: Informational                                    February 2006


          <span class="h1">What's in a Name: False Assumptions about DNS Names</span>

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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Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
   <a href="#section-2">2</a>. Target Audience .................................................<a href="#page-4">4</a>
   <a href="#section-3">3</a>. Modeling Usage of the DNS .......................................<a href="#page-4">4</a>
   <a href="#section-4">4</a>. Possible Assumptions ............................................<a href="#page-5">5</a>
      <a href="#section-4.1">4.1</a>. By the User ................................................<a href="#page-5">5</a>
      <a href="#section-4.2">4.2</a>. By the Client ..............................................<a href="#page-6">6</a>
      <a href="#section-4.3">4.3</a>. By the Server ..............................................<a href="#page-7">7</a>
   <a href="#section-5">5</a>. Consequences of False Assumptions ...............................<a href="#page-8">8</a>
   <a href="#section-6">6</a>. Reasons Why the Assumptions Can Be False ........................<a href="#page-9">9</a>
      <a href="#section-6.1">6.1</a>. Evolution ..................................................<a href="#page-9">9</a>
      <a href="#section-6.2">6.2</a>. Leakage ...................................................<a href="#page-10">10</a>
      <a href="#section-6.3">6.3</a>. Sub-Delegation ............................................<a href="#page-10">10</a>
      <a href="#section-6.4">6.4</a>. Mobility ..................................................<a href="#page-12">12</a>
      <a href="#section-6.5">6.5</a>. Human Error ...............................................<a href="#page-12">12</a>
   <a href="#section-7">7</a>. Recommendations ................................................<a href="#page-12">12</a>
   <a href="#section-8">8</a>. A Note on <a href="./rfc2219">RFC 2219</a> and <a href="./rfc2782">RFC 2782</a> ................................<a href="#page-13">13</a>
   <a href="#section-9">9</a>. Security Considerations ........................................<a href="#page-14">14</a>
   <a href="#section-10">10</a>. Acknowledgements ..............................................<a href="#page-14">14</a>
   <a href="#section-11">11</a>. IAB Members ...................................................<a href="#page-14">14</a>
   <a href="#section-12">12</a>. Informative References ........................................<a href="#page-15">15</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   The Domain Name System (DNS) [<a href="#ref-1" title="&quot;Domain names - concepts and facilities&quot;">1</a>] 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 [<a href="#ref-2" title="&quot;A DNS RR for specifying the location of services (DNS SRV)&quot;">2</a>] [<a href="#ref-1" title="&quot;Domain names - concepts and facilities&quot;">1</a>] [<a href="#ref-3" title="&quot;Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database&quot;">3</a>].  However, it can be used to
   obtain other information, and proposals have been made for nearly
   everything, including geographic information [<a href="#ref-4" title="&quot;A Means for Expressing Location Information in the Domain Name System&quot;">4</a>].

   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 [<a href="#ref-5" title="&quot;Hypertext Transfer Protocol -- HTTP/1.1&quot;">5</a>], Real Time Streaming Protocol (RTSP) URL
   [<a href="#ref-6" title="&quot;Real Time Streaming Protocol (RTSP)&quot;">6</a>], and SIP URI [<a href="#ref-7" title="&quot;SIP: Session Initiation Protocol&quot;">7</a>].  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



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   various proposals for a Top-Level Domain (TLD) that could be
   associated with adult content [<a href="#ref-8" title="&quot;.sex Considered Dangerous&quot;">8</a>], 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.

   <a href="#section-4">Section 4</a> 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.  <a href="#section-5">Section 5</a>
   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.  <a href="#section-6">Section 6</a> 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.

   <a href="#section-7">Section 7</a> 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.
      [<a href="#ref-18" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">18</a>]

   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.



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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Target Audience</span>

   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.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Modeling Usage of the DNS</span>


                       +--------+
                       |        |
                       |        |
                       |  DNS   |
                       |Service |
                       |        |
                       +--------+
                         ^   |
                         |   |
                         |   |
                         |   |
          /--\           |   |
         |    |          |   V
         |    |        +--------+                     +--------+
          \--/         |        |                     |        |
            |          |        |                     |        |
         ---+---       | Client |--------------------&gt;| Server |
            |          |        |                     |        |
            |          |        |                     |        |
           /\          +--------+                     +--------+
          /  \
         /    \

         User
                                 Figure 1




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   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 [<a href="#ref-5" title="&quot;Hypertext Transfer Protocol -- HTTP/1.1&quot;">5</a>], SIP
   [<a href="#ref-7" title="&quot;SIP: Session Initiation Protocol&quot;">7</a>], RTSP [<a href="#ref-6" title="&quot;Real Time Streaming Protocol (RTSP)&quot;">6</a>], and SMTP [<a href="#ref-9" title="&quot;Simple Mail Transfer Protocol&quot;">9</a>].

   &gt;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.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Possible Assumptions</span>

   For each of the three elements, there are many types of false
   assumptions that can be made.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  By the User</span>

   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 [<a href="#ref-8" title="&quot;.sex Considered Dangerous&quot;">8</a>].  These assumptions are
   unavoidable, may all be false, and are not the focus of this
   document.









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<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  By the Client</span>

   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 [<a href="#ref-10" title="&quot;Hypertext Transfer Protocol (HTTP) Digest Authentication Using Authentication and Key Agreement (AKA)&quot;">10</a>], 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) [<a href="#ref-16" title="&quot;HTTP Over TLS&quot;">16</a>].

   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 [<a href="#ref-11" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies&quot;">11</a>] 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 (<a href="./rfc3262">RFC 3262</a> [<a href="#ref-17" title="&quot;Reliability of Provisional Responses in Session Initiation Protocol (SIP)&quot;">17</a>]) might assume
      that this extension is supported on servers in the domain



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      sip.example.telecom.  Furthermore, the client would not implement
      the fallback behavior defined in <a href="./rfc3262">RFC 3262</a>, 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.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  By the Server</span>

   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.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Consequences of False Assumptions</span>

   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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   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.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Reasons Why the Assumptions Can Be False</span>

   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.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Evolution</span>

   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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   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.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Leakage</span>

   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 [<a href="#ref-12" title="&quot;IAB Technical Comment on the Unique DNS Root&quot;">12</a>], 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.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Sub-Delegation</span>

   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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   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 [<a href="#ref-8" title="&quot;.sex Considered Dangerous&quot;">8</a>] and in <a href="#section-3.3">Section 3.3</a> of [<a href="#ref-20" title="&quot;Design Choices When Expanding DNS&quot;">20</a>].

   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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<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  Mobility</span>

   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.

<span class="h3"><a class="selflink" id="section-6.5" href="#section-6.5">6.5</a>.  Human Error</span>

   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.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Recommendations</span>

   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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      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 [<a href="#ref-13" title="&quot;Indicating Media Features for MIME Content&quot;">13</a>] [<a href="#ref-14" title="&quot;A Syntax for Describing Media Feature Sets&quot;">14</a>] [<a href="#ref-15" title="&quot;Protocol-independent Content Negotiation Framework&quot;">15</a>].  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"
      [<a href="#ref-18" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">18</a>]:  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.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  A Note on <a href="./rfc2219">RFC 2219</a> and <a href="./rfc2782">RFC 2782</a></span>

   Based on the definition of an assumption given here, the behavior
   hinted at by records in the DNS also represents an assumption.  <a href="./rfc2219">RFC</a>
   <a href="./rfc2219">2219</a> [<a href="#ref-19" title="&quot;Use of DNS Aliases for Network Services&quot;">19</a>] 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 [<a href="#ref-2" title="&quot;A DNS RR for specifying the location of services (DNS SRV)&quot;">2</a>], 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 <a href="./rfc2219">RFC 2219</a> was not
   known by, or not followed by, the owner of domain.com.  With <a href="./rfc2782">RFC</a>
   <a href="./rfc2782">2782</a>, an SRV record for a particular service would be present only by
   explicit choice of the domain administrator, and thus a client that




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   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.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Security Considerations</span>

   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.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Acknowledgements</span>

   The IAB would like to thank John Klensin, Keith Moore and Peter Koch
   for their comments.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  IAB Members</span>

   Internet Architecture Board members at the time of writing of this
   document are:

      Bernard Aboba

      Loa Andersson

      Brian Carpenter

      Leslie Daigle

      Patrik Faltstrom




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      Bob Hinden

      Kurtis Lindqvist

      David Meyer

      Pekka Nikander

      Eric Rescorla

      Pete Resnick

      Jonathan Rosenberg

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Informative References</span>

   [<a id="ref-1">1</a>]   Mockapetris, P., "Domain names - concepts and facilities",
         STD 13, <a href="./rfc1034">RFC 1034</a>, November 1987.

   [<a id="ref-2">2</a>]   Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
         specifying the location of services (DNS SRV)", <a href="./rfc2782">RFC 2782</a>,
         February 2000.

   [<a id="ref-3">3</a>]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Three: The Domain Name System (DNS) Database", <a href="./rfc3403">RFC 3403</a>,
         October 2002.

   [<a id="ref-4">4</a>]   Davis, C., Vixie, P., Goodwin, T., and I. Dickinson, "A Means
         for Expressing Location Information in the Domain Name System",
         <a href="./rfc1876">RFC 1876</a>, January 1996.

   [<a id="ref-5">5</a>]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", <a href="./rfc2616">RFC 2616</a>, June 1999.

   [<a id="ref-6">6</a>]   Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", <a href="./rfc2326">RFC 2326</a>, April 1998.

   [<a id="ref-7">7</a>]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>, June 2002.

   [<a id="ref-8">8</a>]   Eastlake, D., ".sex Considered Dangerous", <a href="./rfc3675">RFC 3675</a>,
         February 2004.

   [<a id="ref-9">9</a>]   Klensin, J., "Simple Mail Transfer Protocol", <a href="./rfc2821">RFC 2821</a>,
         April 2001.




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   [<a id="ref-10">10</a>]  Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer
         Protocol (HTTP) Digest Authentication Using Authentication and
         Key Agreement (AKA)", <a href="./rfc3310">RFC 3310</a>, September 2002.

   [<a id="ref-11">11</a>]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         <a href="./rfc2045">RFC 2045</a>, November 1996.

   [<a id="ref-12">12</a>]  Internet Architecture Board, "IAB Technical Comment on the
         Unique DNS Root", <a href="./rfc2826">RFC 2826</a>, May 2000.

   [<a id="ref-13">13</a>]  Klyne, G., "Indicating Media Features for MIME Content",
         <a href="./rfc2912">RFC 2912</a>, September 2000.

   [<a id="ref-14">14</a>]  Klyne, G., "A Syntax for Describing Media Feature Sets",
         <a href="./rfc2533">RFC 2533</a>, March 1999.

   [<a id="ref-15">15</a>]  Klyne, G., "Protocol-independent Content Negotiation
         Framework", <a href="./rfc2703">RFC 2703</a>, September 1999.

   [<a id="ref-16">16</a>]  Rescorla, E., "HTTP Over TLS", <a href="./rfc2818">RFC 2818</a>, May 2000.

   [<a id="ref-17">17</a>]  Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
         Responses in Session Initiation Protocol (SIP)", <a href="./rfc3262">RFC 3262</a>,
         June 2002.

   [<a id="ref-18">18</a>]  Braden, R., "Requirements for Internet Hosts - Communication
         Layers", STD 3, <a href="./rfc1122">RFC 1122</a>, October 1989.

   [<a id="ref-19">19</a>]  Hamilton, M. and R. Wright, "Use of DNS Aliases for Network
         Services", <a href="https://www.rfc-editor.org/bcp/bcp17">BCP 17</a>, <a href="./rfc2219">RFC 2219</a>, October 1997.

   [<a id="ref-20">20</a>]  Faltstrom, P., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22Design+Choices+When+Expanding+DNS%22'>"Design Choices When Expanding DNS"</a>, 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:   <a href="http://www.jdrosen.net">http://www.jdrosen.net</a>





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Full Copyright Statement

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