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BEST CURRENT PRACTICE

Network Working Group                                        M. Hamilton
Request for Comments: 2219                       Loughborough University
BCP: 17                                                        R. Wright
Category: Best Current Practice             Lawrence Berkeley Laboratory
                                                            October 1997


                Use of DNS Aliases for Network Services

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Abstract

   It has become a common practice to use symbolic names (usually
   CNAMEs) in the Domain Name Service (DNS - [RFC-1034, RFC-1035]) to
   refer to network services such as anonymous FTP [RFC-959] servers,
   Gopher [RFC-1436] servers, and most notably World-Wide Web HTTP
   [RFC-1945] servers.  This is desirable for a number of reasons.  It
   provides a way of moving services from one machine to another
   transparently, and a mechanism by which people or agents may
   programmatically discover that an organization runs, say, a World-
   Wide Web server.

   Although this approach has been almost universally adopted, there is
   no standards document or similar specification for these commonly
   used names.  This document seeks to rectify this situation by
   gathering together the extant 'folklore' on naming conventions, and
   proposes a mechanism for accommodating new protocols.

   It is important to note that these naming conventions do not provide
   a complete long term solution to the problem of finding a particular
   network service for a site.  There are efforts in other IETF working
   groups to address the long term solution to this problem, such as the
   Server Location Resource Records (DNS SRV) [RFC-2052] work.

1. Rationale

   In order to locate the network services offered at a particular
   Internet domain one is faced with the choice of selecting from a
   growing number of centralized databases - typically Web or Usenet
   News "wanderers", or attempting to infer the existence of network
   services from whatever DNS information may be available.  The former
   approach is not practical in some cases, notably when the entity
   seeking service information is a program.



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   Perhaps the most visible example of the latter approach at work is in
   the case of World-Wide Web HTTP servers.  It is common practice to
   try prefixing the domain name of an organization with "http://www."
   in order to reach its World-Wide Web site, e.g. taking "hivnet.fr"
   and arriving at "http://www.hivnet.fr."  Some popular World-Wide Web
   browsers have gone so far as to provide automatic support for this
   domain name expansion.

   Ideally, the DNS or some complementary directory service would
   provide a means for programs to determine automatically the network
   services which are offered at a particular Internet domain, the
   protocols which are used to deliver them, and other technical
   information.  Unfortunately, although much work has been done to
   develop said directory service technologies and to define new types
   of DNS resource record to provide this type of information, there is
   no widely agreed upon or widely deployed solution to the problem -
   except in a small number of cases.

   The first case is where the DNS already provides a lookup capability
   for the type of information being sought after.  For example: Mail
   Exchanger (MX) records specify how mail to a particular domain should
   be routed [RFC-974], the Start of Authority (SOA) records make it
   possible to determine who is responsible for a given domain, and Name
   Server (NS) records indicate which hosts provide DNS name service for
   a given domain.

   The second case is where the DNS does not provide an appropriate
   lookup capability, but there is some widely accepted convention for
   finding this information.  Some use has been made of Text (TXT)
   [RFC-1035] records in this scenario, but in the vast majority of
   cases a Canonical Name (CNAME) or Address (A) record pointer is used
   to indicate the host or hosts which provide the service.  This
   document proposes a slight formalization of this well-known alias
   approach.

   It should be noted that the DNS provides a Well Known Services (WKS)
   [RFC-1035] lookup capability, which makes it possible to determine
   the network services offered at a given domain name.  In practice
   this is not widely used, perhaps because of the absence of a suitable
   programming interface.  Use of WKS for mail routing was deprecated in
   the Host Requirements specification [RFC-1123] in favour of the MX
   record, and in the long term it is conceivable that SRV records will
   supersede both WKS and MX.








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2. A generic framework

   Our approach to dealing with aliases for protocols is
   straightforward. We define a standard set of DNS aliases for the most
   popular network services that currently exist (see the "Special
   Cases" section below). For protocols that are not explicitly listed
   in this document, the protocol specification must propose a name.

3. Special cases

   Special Cases:
        -----------------------------------------------------------
        Alias     Service
        -----------------------------------------------------------
        archie    archie [ARCHIE]
        finger    Finger [RFC-1288]
        ftp       File Transfer Protocol [RFC-959]
        gopher    Internet Gopher Protocol [RFC-1436]
        ldap      Lightweight Directory Access Protocol [RFC-1777]
        mail      SMTP mail [RFC-821]
        news      Usenet News via NNTP [RFC-977]
        ntp       Network Time Protocol [RFC-1305]
        ph        CCSO nameserver [PH]
        pop       Post Office Protocol [RFC-1939]
        rwhois    Referral WHOIS [RFC-1714]
        wais      Wide Area Information Server [RFC-1625]
        whois     NICNAME/WHOIS [RFC-954]
        www       World-Wide Web HTTP [RFC-1945]
        -----------------------------------------------------------

4. (Ab)Use of the DNS as a directory service

   The widespread use of these common aliases effectively means that it
   is sometimes possible to "guess" the domain names associated with an
   organization's network services, though this is becoming more
   difficult as the number of organizations registered in the DNS
   increases.

   It should be understood by implementors that the existence of a DNS
   entry such as

        www.hivnet.fr

   does not constitute a registration of a World-Wide Web service.
   There is no requirement that the domain name resolve to an IP address
   or addresses.  There is no requirement that a host be listening for





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   HTTP connections, or if it is, that the HTTP server be running on
   port 80.  Finally, even if all of these things are true, there can be
   no guarantee that the World-Wide Web server will be prepared to honor
   requests from arbitrary clients.

   Having said this, the aliases do provide useful "hints" about the
   services offered.  We propose that they be taken in this spirit.

   The conventions described in this document are, essentially, only
   useful when the organization's domain name can be determined - e.g.
   from some external database.  A number of groups, including the IETF,
   have been working on ways of finding domain names given a set of
   information such as organization name, location, and business type.
   It is hoped that one or more of these will eventually make it
   possible to augment the basic lookup service which the DNS provides
   with a more generalized search and retrieval capability.

5. DNS server configuration

   In the short term, whilst directory service technology and further
   types of DNS resource record are being developed, domain name
   administrators are encouraged to use these common names for the
   network services they run.  They will make it easier for outsiders to
   find information about your organization, and also make it easier for
   you to move services from one machine to another.

   There are two conventional approaches to creating these DNS entries.
   One is to add a single CNAME record to your DNS server's
   configuration, e.g.

        ph.hivnet.fr. IN CNAME baby.hivnet.fr.

   Note that in this scenario no information about ph.hivnet.fr should
   exist in the DNS other than the CNAME record. For example,
   ph.hivnet.fr could not contain a MX record.

   An alternative approach would be to create an A record for each of
   the IP addresses associated with ph.hivnet.fr, e.g.

        ph.hivnet.fr. IN A 194.167.157.2

   It isn't a simple matter of recommending CNAMEs over A records. Each
   site has it's own set of requirements that may make one approach
   better than the other. RFC 1912 [RFC-1912]  discusses some of the
   configuration issues involved in using CNAMEs.






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   Recent DNS server implementations provide a "round-robin" feature
   which causes the host's IP addresses to be returned in a different
   order each time the address is looked up.

   Network clients are starting to appear which, when they encounter a
   host with multiple addresses, use heuristics to determine the address
   to contact - e.g. picking the one which has the shortest round-trip-
   time.  Thus, if a server is mirrored (replicated) at a number of
   locations, it may be desirable to list the IP addresses of the mirror
   servers as A records of the primary server.  This is only likely to
   be appropriate if the mirror servers are exact copies of the original
   server.

6. Limitations of this approach

   Some services require that a client have more information than the
   server's domain name.  For example, an LDAP client needs to know a
   starting search base within the Directory Information Tree in order
   to have a meaningful dialogue with the server.  This document does
   not attempt to address this problem.

7. CCSO service name

   There are currently at least three different aliases in common use
   for the CCSO nameserver - e.g. "ph", "cso" and "ns".  It would appear
   to be in everyone's interest to narrow the choice of alias down to a
   single name.  "ns" would seem to be the best choice since it is the
   most commonly used name.  However, "ns" is also being used by DNS to
   point to the DNS server.  In fact, the most prevalent use of "ns" is
   to name DNS servers.  For this reason, we suggest the use of "ph" as
   the best name to use for CCSO nameservers.

   Sites with existing CCSO servers using some of these aliases may find
   it desirable to use all three.  This increases the likelihood of the
   service being found.

   As noted earlier, implementations should be resilient in the event
   that the name does not point to the expected service.

8. Security Considerations

   The DNS is open to many kinds of "spoofing" attacks, and it cannot be
   guaranteed that the result returned by a DNS lookup is indeed the
   genuine information.  Spoofing may take the form of denial of
   service, such as directing of the client to a non-existent address,
   or a passive attack such as an intruder's server which masquerades as
   the legitimate one.




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   Work is ongoing to remedy this situation insofar as the DNS is
   concerned [RFC-2065].  In the meantime it should be noted that
   stronger authentication mechanisms such as public key cryptography
   with large key sizes are a pre-requisite if the DNS is being used in
   any sensitive situations.  Examples of these would be on-line
   financial transactions, and any situation where privacy is a concern
   - such as the querying of medical records over the network.  Strong
   encryption of the network traffic may also be advisable, to protect
   against TCP connection "hijacking" and packet sniffing.

9. Conclusions

   The service names listed in this document provide a sensible set of
   defaults which may be used as an aid in determining the hosts which
   offer particular services for a given domain name.

   This document has noted some exceptions which are either inherently
   unsuitable for this treatment, or already have a substantial
   installed base using alternative aliases.

10. Acknowledgements

   Thanks to Jeff Allen, Tom Gillman, Renato Iannella, Thomas
   Lenggenhager, Bill Manning, Andy Powell, Sri Sataluri, Patrik
   Faltstrom, Paul Vixie and Greg Woods for their comments on draft
   versions of this document.

   This work was supported by UK Electronic Libraries Programme (eLib)
   grant 12/39/01, the European Commission's Telematics for Research
   Programme grant RE 1004, and U. S. Department of Energy Contract
   Number DE-AC03-76SF00098.

11. References

   Request For Comments (RFC) documents are available from
   <URL:ftp://ftp.internic.net/rfc> and numerous mirror sites.

   [ARCHIE]    A. Emtage, P. Deutsch. "archie - An Electronic
               Directory Service for the Internet", Winter Usenix
               Conference Proceedings 1992.  Pages 93-110.

   [PH]        R. Hedberg, S. Dorner, P. Pomes.  "The CCSO
               Nameserver (Ph) Architecture", Work in Progress.

   [RFC-768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
               August 1980.





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   [RFC-793]   Postel, J., "Transmission Control Protocol", STD 7,
               RFC 793, September 1981.

   [RFC-821]   Postel, J., "Simple Mail Transfer Protocol", STD 10,
               RFC 821, August 1982.

   [RFC-954]   Harrenstien, K., Stahl, M., and E. Feinler,
               "NICNAME/WHOIS", RFC 954, October 1985.

   [RFC-959]   Postel, J., and J.K. Reynolds, "File Transfer
               Protocol", STD 9, RFC 959, October 1985.

   [RFC-974]   Partridge, C., "Mail routing and the domain
               System", STD 14, RFC 974,  January 1986.

   [RFC-977]   Kantor, B., and P. Lapsley, "Network News Transfer
               Protocol", RFC 977, February 1986.

   [RFC-1034]  Mockapetris, P., "Domain names - concepts and
               facilities", STD 13, RFC 1034, November 1987.

   [RFC-1035]  Mockapetris, P., "Domain names - implementation
               and specification", STD 13, RFC 1035, November 1987.

   [RFC-1123]  Braden, R., "Requirements for Internet hosts -
               application and support", STD 3, RFC 1123, October 1989.

   [RFC-1288]  Zimmerman, D., "The Finger User Information
               Protocol", RFC 1288, December 1992.

   [RFC-1305]  Mills, D., "Network Time Protocol (Version 3)
               Specification, Implementation", RFC 1305,  March  1992.

   [RFC-1436]  Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
               Torrey, D., and B. Albert, "The Internet Gopher Protocol
               (a distributed document search and retrieval protocol)",
               RFC 1436, March 1993.

   [RFC-1590]  Postel, J., "Media Type Registration Procedure",
               RFC 1590, March 1994.

   [RFC-1625]  St. Pierre, M., Fullton, J., Gamiel, K., Goldman, J.,
               Kahle, B., Kunze, J., Morris, H., and F. Schiettecatte,
               "WAIS over Z39.50-1988", RFC 1625, June 1994.

   [RFC-1700]  Reynolds, J.K., and J. Postel,  "ASSIGNED NUMBERS",
               STD 2, RFC 1700, October 1994.




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   [RFC-1714]  Williamson, S., and M. Kosters, "Referral Whois
               Protocol (RWhois)", RFC 1714, November 1994.

   [RFC-1777]  Yeong, W., Howes, T., and S. Kille, "Lightweight
               Directory Access Protocol", RFC 1777, March 1995.

   [RFC-1912]  Barr, D., "Common DNS Operational and Configuration
               Errors", RFC 1912, Feburary 1996.

   [RFC-1939]  Myers, J., and M. Rose, "Post Office Protocol - Version
               3", STD 53, RFC 1939, May 1996.

   [RFC-1945]  Berners-Lee, T., Fielding, R., and H. Nielsen,
               "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945, May
               1996.

   [RFC-2052]  Gulbrandsen, A., and P. Vixie, "A DNS RR for specifying
               the location of services (DNS SRV)", RFC 2052, October
               1996.

   [RFC-2065]  Eastlake, D., and C. Kaufman, "Domain Name System
               Security Extensions", RFC 2065, January 1997.

12. Authors' Addresses

   Martin Hamilton
   Department of Computer Studies
   Loughborough University of Technology
   Leics. LE11 3TU, UK

   EMail: m.t.hamilton@lut.ac.uk


   Russ Wright
   Information & Computing Sciences Division
   Lawrence Berkeley National Laboratory
   1 Cyclotron Road, Berkeley
   Mail-Stop: 50A-3111
   CA 94720, USA

   EMail: wright@lbl.gov










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