draft-ietf-dnsext-mdns-43.txt

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INTERNET-DRAFT                    LLMNR                   29 August 2005


   will also reduce unnecessary LLMNR queries.

   [RFC1536] Section 6 describes name error bugs and recommended
   searchlist processing that will reduce unnecessary RCODE=3
   (authoritative name) errors, thereby also reducing unnecessary LLMNR
   queries.

3.1.  LLMNR Configuration

   Since IPv4 and IPv6 utilize distinct configuration mechanisms, it is
   possible for a dual stack host to be configured with the address of a
   DNS server over IPv4, while remaining unconfigured with a DNS server
   suitable for use over IPv6.

   In these situations, a dual stack host will send AAAA queries to the
   configured DNS server over IPv4.  However, an IPv6-only host
   unconfigured with a DNS server suitable for use over IPv6 will be
   unable to resolve names using DNS.  Automatic IPv6 DNS configuration
   mechanisms (such as [RFC3315] and [DNSDisc]) are not yet widely
   deployed, and not all DNS servers support IPv6.  Therefore lack of
   IPv6 DNS configuration may be a common problem in the short term, and
   LLMNR may prove useful in enabling link-local name resolution over
   IPv6.

   Where a DHCPv4 server is available but not a DHCPv6 server [RFC3315],
   IPv6-only hosts may not be configured with a DNS server.  Where there
   is no DNS server authoritative for the name of a host or the
   authoritative DNS server does not support dynamic client update over
   IPv6 or DHCPv6-based dynamic update, then an IPv6-only host will not
   be able to do DNS dynamic update, and other hosts will not be able to
   resolve its name.

   For example, if the configured DNS server responds to a AAAA RR query
   sent over IPv4 or IPv6 with an authoritative name error (RCODE=3) or
   RCODE=0 and an empty answer section, then a AAAA RR query sent using
   LLMNR over IPv6 may be successful in resolving the name of an
   IPv6-only host on the local link.

   Similarly, if a DHCPv4 server is available providing DNS server
   configuration, and DNS server(s) exist which are authoritative for
   the A RRs of local hosts and support either dynamic client update
   over IPv4 or DHCPv4-based dynamic update, then the names of local
   IPv4 hosts can be resolved over IPv4 without LLMNR.  However,  if no
   DNS server is authoritative for the names of local hosts, or the
   authoritative DNS server(s) do not support dynamic update, then LLMNR
   enables linklocal name resolution over IPv4.

   Where DHCPv4 or DHCPv6 is implemented, DHCP options can be used to



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   configure LLMNR on an interface.  The LLMNR Enable Option, described
   in [LLMNREnable], can be used to explicitly enable or disable use of
   LLMNR on an interface.  The LLMNR Enable Option does not determine
   whether or in which order DNS itself is used for name resolution.
   The order in which various name resolution mechanisms should be used
   can be specified using the Name Service Search Option (NSSO) for DHCP
   [RFC2937], using the LLMNR Enable Option code carried in the NSSO
   data.

   It is possible that DNS configuration mechanisms will go in and out
   of service.  In these circumstances, it is possible for hosts within
   an administrative domain to be inconsistent in their DNS
   configuration.

   For example, where DHCP is used for configuring DNS servers, one or
   more DHCP servers can fail.  As a result, hosts configured prior to
   the outage will be configured with a DNS server, while hosts
   configured after the outage will not.  Alternatively, it is possible
   for the DNS configuration mechanism to continue functioning while
   configured DNS servers fail.

   An outage in the DNS configuration mechanism may result in hosts
   continuing to use LLMNR even once the outage is repaired.  Since
   LLMNR only enables linklocal name resolution, this represents a
   degradation in capabilities.  As a result, hosts without a configured
   DNS server may wish to periodically attempt to obtain DNS
   configuration if permitted by the configuration mechanism in use.  In
   the absence of other guidance, a default retry interval of one (1)
   minute is RECOMMENDED.

4.  Conflict Resolution

   By default, a responder SHOULD be configured to behave as though its
   name is UNIQUE on each interface on which LLMNR is enabled.  However,
   it is also possible to configure multiple responders to be
   authoritative for the same name.  For example, multiple responders
   MAY respond to a query for an A or AAAA type record for a cluster
   name (assigned to multiple hosts in the cluster).

   To detect duplicate use of a name, an administrator can use a name
   resolution utility which employs LLMNR and lists both responses and
   responders.  This would allow an administrator to diagnose behavior
   and potentially to intervene and reconfigure LLMNR responders who
   should not be configured to respond to the same name.







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4.1.  Uniqueness Verification

   Prior to sending an LLMNR response with the 'T' bit clear, a
   responder configured with a UNIQUE name MUST verify that there is no
   other host within the scope of LLMNR query propagation that is
   authoritative for the same name on that interface.

   Once a responder has verified that its name is UNIQUE, if it receives
   an LLMNR query for that name, with the 'C' bit clear, it MUST
   respond, with the 'T' bit clear. Prior to verifying that its name is
   UNIQUE, a responder MUST set the 'T' bit in responses.

   Uniqueness verification is carried out when the host:

     - starts up or is rebooted
     - wakes from sleep (if the network interface was inactive
       during sleep)
     - is configured to respond to LLMNR queries on an interface
       enabled for transmission and reception of IP traffic
     - is configured to respond to LLMNR queries using additional
       UNIQUE resource records
     - verifies the acquisition of a new IP address and configuration
       on an interface

   To verify uniqueness, a responder MUST send an LLMNR query with the
   'C' bit clear, over all protocols on which it responds to LLMNR
   queries (IPv4 and/or IPv6).  It is RECOMMENDED that responders verify
   uniqueness of a name by sending a query for the name with type='ANY'.

   If no response is received, the sender retransmits the query, as
   specified in Section 2.7.  If a response is received, the sender MUST
   check if the source address matches the address of any of its
   interfaces; if so, then the response is not considered a conflict,
   since it originates from the sender.  To avoid triggering conflict
   detection, a responder that detects that it is connected to the same
   link on multiple interfaces SHOULD set the 'C' bit in responses.

   If a response is received with the 'T' bit clear, the responder MUST
   NOT use the name in response to LLMNR queries received over any
   protocol (IPv4 or IPv6).  If a response is received with the 'T' bit
   set, the responder MUST check if the source IP address in the
   response, interpreted as an unsigned integer, is less than the source
   IP address in the query.  If so, the responder MUST NOT use the name
   in response to LLMNR queries received over any protocol (IPv4 or
   IPv6).  For the purpose of uniqueness verification, the contents of
   the answer section in a response is irrelevant.

   Periodically carrying out uniqueness verification in an attempt to



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   detect name conflicts is not necessary, wastes network bandwidth, and
   may actually be detrimental.  For example, if network links are
   joined only briefly, and are separated again before any new
   communication is initiated, temporary conflicts are benign and no
   forced reconfiguration is required.  LLMNR responders SHOULD NOT
   periodically attempt uniqueness verification.

4.2.  Conflict Detection and Defense

   Hosts on disjoint network links may configure the same name for use
   with LLMNR.  If these separate network links are later joined or
   bridged together, then there may be multiple hosts which are now on
   the same link, trying to use the same name.

   In order to enable ongoing detection of name conflicts, when an LLMNR
   sender receives multiple LLMNR responses to a query, it MUST check if
   the 'C' bit is clear in any of the responses.  If so, the sender
   SHOULD send another query for the same name, type and class, this
   time with the 'C' bit set, with the potentially conflicting resource
   records included in the additional section.

   Queries with the 'C' bit set are considered advisory and responders
   MUST verify the existence of a conflict before acting on it.  A
   responder receiving a query with the 'C' bit set MUST NOT respond.

   If the query is for a UNIQUE name, then the responder MUST send its
   own query for the same name, type and class, with the 'C' bit clear.
   If a response is received, the sender MUST check if the source
   address matches the address of any of its interfaces; if so, then the
   response is not considered a conflict, since it originates from the
   sender.  To avoid triggering conflict detection, a responder that
   detects that it is connected to the same link on multiple interfaces
   SHOULD set the 'C' bit in responses.

   An LLMNR responder MUST NOT ignore conflicts once detected and SHOULD
   log them.  Upon detecting a conflict, an LLMNR responder MUST
   immediately stop using the conflicting name in response to LLMNR
   queries received over any supported protocol, if the source IP
   address in the response, interpreted as an unsigned integer, is less
   than the source IP address in the uniqueness verification query.

   After stopping the use of a name, the responder MAY elect to
   configure a new name.  However, since name reconfiguration may be
   disruptive, this is not required, and a responder may have been
   configured to respond to multiple names so that alternative names may
   already be available.  A host that has stopped the use of a name may
   attempt uniqueness verification again after the expiration of the TTL
   of the conflicting response.



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4.3.  Considerations for Multiple Interfaces

   A multi-homed host may elect to configure LLMNR on only one of its
   active interfaces.  In many situations this will be adequate.
   However, should a host need to configure LLMNR on more than one of
   its active interfaces, there are some additional precautions it MUST
   take.  Implementers who are not planning to support LLMNR on multiple
   interfaces simultaneously may skip this section.

   Where a host is configured to issue LLMNR queries on more than one
   interface, each interface maintains its own independent LLMNR
   resolver cache, containing the responses to LLMNR queries.

   A multi-homed host checks the uniqueness of UNIQUE records as
   described in Section 4.  The situation is illustrated in figure 1.

        ----------  ----------
         |      |    |      |
        [A]    [myhost]   [myhost]

      Figure 1.  Link-scope name conflict

   In this situation, the multi-homed myhost will probe for, and defend,
   its host name on both interfaces.  A conflict will be detected on one
   interface, but not the other.  The multi-homed myhost will not be
   able to respond with a host RR for "myhost" on the interface on the
   right (see Figure 1).  The multi-homed host may, however, be
   configured to use the "myhost" name on the interface on the left.

   Since names are only unique per-link, hosts on different links could
   be using the same name.  If an LLMNR client sends requests over
   multiple interfaces, and receives replies from more than one, the
   result returned to the client is defined by the implementation.  The
   situation is illustrated in figure 2.

        ----------  ----------
         |      |    |     |
        [A]    [myhost]   [A]


      Figure 2.  Off-segment name conflict

   If host myhost is configured to use LLMNR on both interfaces, it will
   send LLMNR queries on both interfaces.  When host myhost sends a
   query for the host RR for name "A" it will receive a response from
   hosts on both interfaces.

   Host myhost cannot distinguish between the situation shown in Figure



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   2, and that shown in Figure 3 where no conflict exists.

                [A]
               |   |
           -----   -----
               |   |
              [myhost]

      Figure 3.  Multiple paths to same host

   This illustrates that the proposed name conflict resolution mechanism
   does not support detection or resolution of conflicts between hosts
   on different links.  This problem can also occur with DNS when a
   multi-homed host is connected to two different networks with