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EVDNS(3)		 BSD Library Functions Manual		      EVDNS(3)

NAME
     evdns_init evdns_shutdown evdns_err_to_string evdns_nameserver_add
     evdns_count_nameservers evdns_clear_nameservers_and_suspend evdns_resume
     evdns_nameserver_ip_add evdns_resolve_ipv4 evdns_resolve_reverse
     evdns_resolv_conf_parse evdns_config_windows_nameservers
     evdns_search_clear evdns_search_add evdns_search_ndots_set
     evdns_set_log_fn — asynchronous functions for DNS resolution.

SYNOPSIS
     #include <sys/time.h>
     #include <event.h>
     #include <evdns.h>

     int
     evdns_init();

     void
     evdns_shutdown(int fail_requests);

     const char *
     evdns_err_to_string(int err);

     int
     evdns_nameserver_add(unsigned long int address);

     int
     evdns_count_nameservers();

     int
     evdns_clear_nameservers_and_suspend();

     int
     evdns_resume();

     int
     evdns_nameserver_ip_add(const(char, *ip_as_string););

     int
     evdns_resolve_ipv4(const char *name, int flags,
	 evdns_callback_type callback, void *ptr);

     int
     evdns_resolve_reverse(struct in_addr *in, int flags,
	 evdns_callback_type callback, void *ptr);

     int
     evdns_resolv_conf_parse(int flags, const char *);

     void
     evdns_search_clear();

     void
     evdns_search_add(const char *domain);

     void
     evdns_search_ndots_set(const int ndots);

     void
     evdns_set_log_fn(evdns_debug_log_fn_type fn);

     int
     evdns_config_windows_nameservers();

DESCRIPTION
     Welcome, gentle reader

     Async DNS lookups are really a whole lot harder than they should be,
     mostly stemming from the fact that the libc resolver has never been very
     good at them. Before you use this library you should see if libc can do
     the job for you with the modern async call getaddrinfo_a (see
     http://www.imperialviolet.org/page25.html#e498). Otherwise, please con‐
     tinue.

     This code is based on libevent and you must call event_init before any of
     the APIs in this file. You must also seed the OpenSSL random source if
     you are using OpenSSL for ids (see below).

     This library is designed to be included and shipped with your source
     code. You statically link with it. You should also test for the existence
     of strtok_r and define HAVE_STRTOK_R if you have it.

     The DNS protocol requires a good source of id numbers and these numbers
     should be unpredictable for spoofing reasons. There are three methods for
     generating them here and you must define exactly one of them. In increas‐
     ing order of preference:

	   DNS_USE_GETTIMEOFDAY_FOR_ID	Using the bottom 16 bits of the usec
					result from gettimeofday. This is a
					pretty poor solution but should work
					anywhere.
	   DNS_USE_CPU_CLOCK_FOR_ID	Using the bottom 16 bits of the nsec
					result from the CPU's time counter.
					This is better, but may not work
					everywhere. Requires POSIX realtime
					support and you'll need to link
					against -lrt on glibc systems at
					least.
	   DNS_USE_OPENSSL_FOR_ID	Uses the OpenSSL RAND_bytes call to
					generate the data. You must have
					seeded the pool before making any
					calls to this library.

     The library keeps track of the state of nameservers and will avoid them
     when they go down. Otherwise it will round robin between them.

     Quick start guide:

	   #include <evdns.h>
	   void callback(int result, char type, int count, int ttl,
		    void *addresses, void *arg);
	   evdns_resolv_conf_parse(DNS_OPTIONS_ALL, "/etc/resolv.conf");
	   evdns_resolve("www.hostname.com", 0, callback, NULL);

     When the lookup is complete the callback function is called. The first
     argument will be one of the DNS_ERR_* defines in evdns.h.	Hopefully it
     will be DNS_ERR_NONE, in which case type will be DNS_IPv4_A, count will
     be the number of IP addresses, ttl is the time which the data can be
     cached for (in seconds), addresses will point to an array of uint32_t's
     and arg will be whatever you passed to evdns_resolve.

     Searching:

     In order for this library to be a good replacement for glibc's resolver
     it supports searching. This involves setting a list of default domains,
     in which names will be queried for. The number of dots in the query name
     determines the order in which this list is used.

     Searching appears to be a single lookup from the point of view of the
     API, although many DNS queries may be generated from a single call to
     evdns_resolve. Searching can also drastically slow down the resolution of
     names.

     To disable searching:
	   1.	Never set it up. If you never call evdns_resolv_conf_parse,()
		evdns_init,() or evdns_search_add() then no searching will
		occur.
	   2.	If you do call evdns_resolv_conf_parse() then don't pass
		DNS_OPTION_SEARCH (or DNS_OPTIONS_ALL, which implies it).
	   3.	When calling evdns_resolve,() pass the DNS_QUERY_NO_SEARCH
		flag.

     The order of searches depends on the number of dots in the name. If the
     number is greater than the ndots setting then the names is first tried
     globally. Otherwise each search domain is appended in turn.

     The ndots setting can either be set from a resolv.conf, or by calling
     evdns_search_ndots_set.

     For example, with ndots set to 1 (the default) and a search domain list
     of ["myhome.net"]:
      Query: www
      Order: www.myhome.net, www.

      Query: www.abc
      Order: www.abc., www.abc.myhome.net

API reference
     int evdns_init()
	      Initializes support for non-blocking name resolution by calling
	      evdns_resolv_conf_parse() on UNIX and
	      evdns_config_windows_nameservers() on Windows.

     int evdns_nameserver_add(unsigned long int address)
	      Add a nameserver. The address should be an IP address in network
	      byte order. The type of address is chosen so that it matches
	      in_addr.s_addr.  Returns non-zero on error.

     int evdns_nameserver_ip_add(const char *ip_as_string)
	      This wraps the above function by parsing a string as an IP
	      address and adds it as a nameserver.  Returns non-zero on error

     int evdns_resolve(const char *name, int flags, evdns_callback_type
	      callback, void *ptr)
	      Resolve a name. The name parameter should be a DNS name.	The
	      flags parameter should be 0, or DNS_QUERY_NO_SEARCH which dis‐
	      ables searching for this query. (see defn of searching above).

	      The callback argument is a function which is called when this
	      query completes and ptr is an argument which is passed to that
	      callback function.

	      Returns non-zero on error

     void evdns_search_clear()
	      Clears the list of search domains

     void evdns_search_add(const char *domain)
	      Add a domain to the list of search domains

     void evdns_search_ndots_set(int ndots)
	      Set the number of dots which, when found in a name, causes the
	      first query to be without any search domain.

     int evdns_count_nameservers(void)
	      Return the number of configured nameservers (not necessarily the
	      number of running nameservers).  This is useful for double-
	      checking whether our calls to the various nameserver configura‐
	      tion functions have been successful.

     int evdns_clear_nameservers_and_suspend(void)
	      Remove all currently configured nameservers, and suspend all
	      pending resolves.	 Resolves will not necessarily be re-attempted
	      until evdns_resume() is called.

     int evdns_resume(void)
	      Re-attempt resolves left in limbo after an earlier call to
	      evdns_clear_nameservers_and_suspend().

     int evdns_config_windows_nameservers(void)
	      Attempt to configure a set of nameservers based on platform set‐
	      tings on a win32 host.  Preferentially tries to use GetNetwork‐
	      Params; if that fails, looks in the registry.  Returns 0 on suc‐
	      cess, nonzero on failure.

     int evdns_resolv_conf_parse(int flags, const char *filename)
	      Parse a resolv.conf like file from the given filename.

	      See the man page for resolv.conf for the format of this file.
	      The flags argument determines what information is parsed from
	      this file:
		    DNS_OPTION_SEARCH	      domain, search and ndots options
		    DNS_OPTION_NAMESERVERS    nameserver lines
		    DNS_OPTION_MISC	      timeout and attempts options
		    DNS_OPTIONS_ALL	      all of the above

	      The following directives are not parsed from the file:
		sortlist, rotate, no-check-names, inet6, debug

	      Returns non-zero on error:
		    0	no errors
		    1	failed to open file
		    2	failed to stat file
		    3	file too large
		    4	out of memory
		    5	short read from file

Internals:
     Requests are kept in two queues. The first is the inflight queue. In this
     queue requests have an allocated transaction id and nameserver.  They
     will soon be transmitted if they haven't already been.

     The second is the waiting queue. The size of the inflight ring is limited
     and all other requests wait in waiting queue for space. This bounds the
     number of concurrent requests so that we don't flood the nameserver. Sev‐
     eral algorithms require a full walk of the inflight queue and so bounding
     its size keeps thing going nicely under huge (many thousands of requests)
     loads.

     If a nameserver loses too many requests it is considered down and we try
     not to use it. After a while we send a probe to that nameserver (a lookup
     for google.com) and, if it replies, we consider it working again. If the
     nameserver fails a probe we wait longer to try again with the next probe.

SEE ALSO
     event(3), gethostbyname(3), resolv.conf(5)

HISTORY
     The evdns API was developed by Adam Langley on top of the libevent API.
     The code was integrate into Tor by Nick Mathewson and finally put into
     libevent itself by Niels Provos.

AUTHORS
     The evdns API and code was written by Adam Langley with significant con‐
     tributions by Nick Mathewson.

BUGS
     This documentation is neither complete nor authoritative.	If you are in
     doubt about the usage of this API then check the source code to find out
     how it works, write up the missing piece of documentation and send it to
     me for inclusion in this man page.

BSD				 May 14, 2010				   BSD
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