Domain Name System Operations                                  J. Massar
(dnsop)                                                    Unfix / SixXS
Internet-Draft                                              July 9, 2005
Expires: January 10, 2006

                     The _service domain and prefix

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document defines a new domain, _service., which can be used for
   automatic service configuration and discovery.  The associated
   anycast prefixes can be used to configure a default DNS server, which
   provides lookups for a local _service. domain but also acts as a
   (caching) recursive DNS server, thus allowing DNS clients to use this
   well-known address as their default DNS server as well as to use it
   to find various well known services, thus avoiding manual

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Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The _service domain  . . . . . . . . . . . . . . . . . . . . .  3
     3.1   DNS Search Path  . . . . . . . . . . . . . . . . . . . . .  5
     3.2   Browsing for services  . . . . . . . . . . . . . . . . . .  5
   4.  The _service anycast address . . . . . . . . . . . . . . . . .  5
     4.1   The _service prefix  . . . . . . . . . . . . . . . . . . .  5
     4.2   Monitoring . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.3   Discovery and failover . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     7.1   Normative References . . . . . . . . . . . . . . . . . . .  7
     7.2   Informative References . . . . . . . . . . . . . . . . . .  7
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  8

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1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Introduction

   Currently there are a number of methods of configuring a (caching)
   recursive DNS server into the resolver code of various clients.
   Amongst these methods are static configuration, DHCP, IPv6 Router
   Advertisement options, PPP configuration and a multitude of others.

   Next to the configuration of a DNS server, the user of the client
   will also have to configure various other services, eg Outgoing SMTP
   server, incoming POP3 or IMAP4 server, HTTP proxy server, FTP proxy
   server or with the advent of IPv6, one of the various IPv6 tunneling
   techiques and one day one of the various IPv4 tunneling techniques,
   to allow IPv6 hosts to contact IPv4 hosts.

   All these services now have to be manually configured or using some
   kind of automation, which is likely to be different for each type of

   SRV records [RFC2782] defines a method of defining services, it does
   not however define where these records are located nor handles the
   case when a service has multiple protocols.

   This document specifies a method, which allows vendors to hardcode a
   well known anycast prefix into their resolving clients.  This anycast
   prefix contains a single well known IP address that runs a (caching)
   recursive DNS server.  [RFC1034] [RFC1035].  This server thus allows
   looking up of all available domains (, etc).
   Additionally it is provides lookups for the _service. domain which
   contains the in this document described domain to be used for

3.  The _service domain

   The _service domain contains PTR records to their respective SRV
   records.  Service names should abide SRV naming rules, they are
   protocol independent though.

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   Typical contents of a _service domain.

   $ORIGIN _service.
   @            TXT "Example Networks"
   @            RP
   _website     PTR
   _pgpkey      PTR
   _imap        PTR
   _mailsubmit  PTR
   _ntp         PTR

   The TXT record specifies which organisation is announcing this
   prefix.  This record can be shown in browser functions or to the
   enduser.  The RP record specifies a contact for this service zone.

   The above defines a website, available over HTTP and HTTPS, based on
   the priority and weights given by their SRV records.  This allows one
   to specify that a client must first try the HTTPS variant, if it does
   not work, or the client does not understand this protocol it can try
   the HTTP variant.  Due to the nature of SRV, these services might be
   located on different hosts.  The NTP service is defined to use either
   UDP or NTP.

   The actual _service.  Top Level Domain (TLD) can actually also be a
   DNAME to the organisations domain, this way the organisation only has
   to maintain one .service domain.

   _service. DNAME

   This also allows an organisation have multiple domain names or
   clients domains, simply adding the DNAME to the service domain allows
   the users to pick any of the domains and the configuration
   information is already available to the user.  This is naturally only
   required when the user is not inside the anycast range of the

   A _service domain SHOULD be AXFR'able [RFC1995] this to facilitate
   browsing of the service zone.  An organisation MAY opt to decline
   AXFR's based on their policy.

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3.1  DNS Search Path

   Lookups in the _service domain should be done according to the DNS
   search path.  Thus if the DNS search path of host is: then the resolver should try the items in:


   This allows the _service domain to be located anywhere in the search
   path of the client.  Additionally this also allows one to specify a
   remote domain and thus having components be configured based on the
   service values given for that domain.

3.2  Browsing for services

   In case the user wants to know what kind of services are available
   for her, as provided by the local organisation, an application could
   try to AXFR the _service zones of the domains in the search path and
   then displaying the available services.

4.  The _service anycast address

   x.x.x.1 and xxxx::1 (*IANA UPDATE!*) can be hardcoded into any client
   resolver.  These addresses point to the IPv4 and IPv6 variant of a
   resolver which also provides access to the _service prefix.

   On the _service anycast address a full recursive DNS server is
   responding.  It must also provide the lookup facility for the
   _service. domain.  This domain might reside on another DNS server.

   This address can be passed to the client using DHCP [RFC2131], other
   automatic configuration methods or manual configuration.

4.1  The _service prefix

   x.x.x.1 is part of x.x.x.1/32 and xxxx::1 is part of xxxx::/64.

   The prefix SHOULD only be announced in the local IGP of the
   organisation.  The prefix MAY be announced to other organisations
   when the two parties agree on this setup.  The prefix MUST not be
   seen globally around the world.  Though it is of course possible,
   having the prefix available to the global internet would not have
   much of a function as the services are most likely only provided for
   users of the organisation.

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4.2  Monitoring

   Any organisation corresponding to this specification must include a
   monitoring function, to check that the _service is operational.  The
   router must stop injecting the route leading to the server
   immediately if it detects that the DNS function is not operational.

   A remote host should try to query either the TXT or RP record of the
   to be monitored service zone to see if the DNS server still answers
   queries.  Other methods can of course also be used.

4.3  Discovery and failover

   The DNS client resolver send packets to the DNS service by sending
   them to the anycast address.  These packets will reach the closest
   service provided by their organisation or by another organisation.

   When a client does not have connectivity to this prefix, there will
   be no routing entry for the anycast prefix and thus a destinatation
   unreachable will be sent to the host.  The resolver then learns that
   the DNS service in question is not available.

   When a _service server somehow breaks it should stop announcing the
   anycast prefix to the local network.  At that point, the local IGP
   will automatically compute a route towards the "next best" _service
   server.  We expect that adequate monitoring tools will be used to
   guarantee timely discovery of connectivity losses and should allow
   seamless functionality for the endusers.

5.  Security Considerations

   This anycast technique introduces an risk, that a rogue router or a
   rogue AS could introduce a bogus route to their own resolver setup
   providing rogue _service entries, thereby diverting the traffic to
   the service they want.  Any service using cleartext passwords and
   having no additional security, eg TLS/SSL, can thus be easily
   transformed into password collection setups.  Care must be taken that
   nobody can insert a faked _service server into a network.

6.  IANA Considerations

   IANA will need to mark the _service. domain and the _service sub
   domain as reserved to be used solely for this purpose.

   IANA will need to allocate a IPv6 /64 and a IPv4 /32 for the purposes
   of having a well known anycast address in which the (caching)
   recursive DNS server can operate.

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   The prefix is only required in the organisation itself and should not
   be carried in intra-domain routing tables.  A global prefix is
   required so that the prefix can be shared between organisations. eg
   an organisation providing this service to other autonomous systems.

7.  References

7.1  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

7.2  Informative References

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              August 1996.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

Author's Address

   Jeroen Massar
   Unfix / SixXS
   Hofpoldersingel 45
   Gouda  2807 LW


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