resolver revision 9bff67898d55cddfcec9ce30cc2b1bb6211ec691
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Copyright (C) 1999, 2000 Internet Software Consortium.
5f7ca73d88db8aedeef501b1a791ea61e48a17f9Mark AndrewsSee COPYRIGHT in the source root or http://www.isc.org/copyright for terms.
ec5347e2c775f027573ce5648b910361aa926c01Automatic Updater$Id: resolver,v 1.2 2000/06/21 23:43:42 tale Exp $
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Multi-target Resolver
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉=====================
0c865fa57d3b9b072c2878b09fad17c732be2eb9Mark AndrewsDue to IPv6 and other multiple-path resolving needs, the resolver in
0c865fa57d3b9b072c2878b09fad17c732be2eb9Mark AndrewsBIND 9 needs to be able to handle more than one simulaneous resolving
0c865fa57d3b9b072c2878b09fad17c732be2eb9Mark Andrewstask for any single request.
0c865fa57d3b9b072c2878b09fad17c732be2eb9Mark AndrewsFor instance, following an A6 chain could result in multiple "forks"
0c865fa57d3b9b072c2878b09fad17c732be2eb9Mark Andrewsfor each network providor.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Resolver Overview
ec5347e2c775f027573ce5648b910361aa926c01Automatic Updater=================
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉The resolver core is divided into several parts, each of which has a
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉fairly simple function (with some exceptions) to simplify the problem.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉The most complicated portion is the search algorithm "state machine."
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Flow of a Request
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉-----------------
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Query Management.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Each query is associated with a given view. The view-based management
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉allows different clients to be provided with different data based on
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉administrative requirements.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉This is also the layer where EDNS is handled first. EDNS knowledge is
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉handled in some ways throughout the resolver, but several constraints
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉need to be handled if the receiving client cannot handle the EDNS
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉format. (XXX need more)
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉When a client (caller) issues a query, the first thing that happens is
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉the list of currently outstanding query is checked. If another query
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉is already being processed for another client, the new request
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉attaches to the running query and shares the result when it
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉If no outstanding requests for this query exist, a new query context
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉is created and an event is passed to an internal "resolver task" for
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉In either case, when the internal resolver task completes the request
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉the waiting clients are sent an event. Note that the calling clients
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉can remove their request for various reasons, such as a timeout. The
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉internal task will continue to work on the request, however. This
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉will let other clients attach to a query that some work was done for,
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉and it will help to populate the cache in hopes this query will be
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Calling into the resolver may cause the currently running task to do
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉some amount of work, rather than handing off to an internal task
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉immediately. This is more efficient (fewer context switches) but the
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉client's task will not be blocked for any long-term purposes.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉The query management layer is also used for internal queries, from the
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉search algorithm state machine. In this case, the "client" is an
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉internal resolver task, and all callbacks are within the resolver
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉The next stage in the resolving process is the search algorithm state
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Search Algorithm State Machine.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉This is where the guts of the resolving takes place. The cache is
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉consulted for an answer, and if deemed good, returned. The
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉authoritative data is also consulted here.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Many internal queries may be launched to perform recursive queries on
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉behalf of clients or the resolver itself. The algorithm is described
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉The output of this stage is either to return to query management with
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉an answer, or to move to the address selection phase.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Address Selection.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Given that the resolver may have a forwarder list, or may have a
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉number of possible IP addresses to consult for more information, some
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉selection of which is the best address to use needs to be performed.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉This layer does this.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉It will select from a possible set of IP addresses to send a query
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉to. They are ranked in various ways (round-trip time, reliability,
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉and lameness for a given zone are a few) and the best is selected.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Some state is maintained to allow retransmission in the case of a
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉The output of this goes to message formatting.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Message Formatting.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉This section will construct a wire message to perform a query.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉The output of this section is what is transmitted to the wire in the
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉external DNS query section.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉External DNS Query.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉This is where a wire message is transmitted to the "best" socket
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉address. Timeouts are handled here, and timing information is
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉gathered when requests complete.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Received Message Handling.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉When a message arrives from an internal query, the result is evaluated
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉here. Things like message ID matching the query, query answers
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉question, TSIG, DNSSEC, etc are performed here.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Caching and Evaluation of Result.
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉Once the message format and envelope are examined, some bits are
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉cached (including hints that a given rrset has a zero TTL). Other
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉bits feed back into the search algorithm state machine to continue
d0eb2cc33c5db3366a16b1cb0abcca6ec7c8ee3cTatuya JINMEI 神明達哉[ XXX need examples? ]