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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.


 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.

Hamilton & Wright Best Current Practice [Page 1] RFC 2219 DNS Aliases October 1997

 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 ""
 and arriving at ""  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
 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.

Hamilton & Wright Best Current Practice [Page 2] RFC 2219 DNS Aliases October 1997

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

Hamilton & Wright Best Current Practice [Page 3] RFC 2219 DNS Aliases October 1997

 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. IN CNAME
 Note that in this scenario no information about should
 exist in the DNS other than the CNAME record. For example, could not contain a MX record.
 An alternative approach would be to create an A record for each of
 the IP addresses associated with, e.g. IN A
 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.

Hamilton & Wright Best Current Practice [Page 4] RFC 2219 DNS Aliases October 1997

 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

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.

Hamilton & Wright Best Current Practice [Page 5] RFC 2219 DNS Aliases October 1997

 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:> 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.

Hamilton & Wright Best Current Practice [Page 6] RFC 2219 DNS Aliases October 1997

 [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.

Hamilton & Wright Best Current Practice [Page 7] RFC 2219 DNS Aliases October 1997

 [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
 [RFC-2052]  Gulbrandsen, A., and P. Vixie, "A DNS RR for specifying
             the location of services (DNS SRV)", RFC 2052, October
 [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
 Russ Wright
 Information & Computing Sciences Division
 Lawrence Berkeley National Laboratory
 1 Cyclotron Road, Berkeley
 Mail-Stop: 50A-3111
 CA 94720, USA

Hamilton & Wright Best Current Practice [Page 8]

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