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Versions: 00 01 02 03 rfc4367                                           
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

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.

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   This Internet-Draft will expire on July 6, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   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

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   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

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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.

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2.  Modeling Usage of the DNS

                       |        |
                       |        |
                       |  DNS   |
                       |Service |
                       |        |
                         ^   |
                         |   |
                         |   |
                         |   |
          /--\           |   |
         |    |          |   V
         |    |        +--------+                     +--------+
          \--/         |        |                     |        |
            |          |        |                     |        |
         ---+---       | Client |-------------------->| Server |
            |          |        |                     |        |
            |          |        |                     |        |
           /\          +--------+                     +--------+
          /  \
         /    \


                                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

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   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].

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   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.

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   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

   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

   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

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      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

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   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

   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

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   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.

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      A corollary is that protocol designers should include such
      mechanisms when evolution is expected in the usage of the

   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

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

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:

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      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.

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   [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

   [9]   Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April

   [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

   [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

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Author's Address

   Jonathan Rosenberg, Editor
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Phone: +1 973 952-5000
   EMail: jdrosen@cisco.com
   URI:   http://www.jdrosen.net

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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.  Information
   on the procedures with respect to rights in RFC documents can be
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