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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 continue.

     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 rea-
     sons. There are three methods for generating them here and you must define exactly one of them. In increasing
     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 set-
     ting a list of default domains, in which names will be queried for. The number of dots in the query name deter-
     mines 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 gen-
     erated 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 cho-
              sen 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 disables 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 configuration functions have
              been successful.

     int evdns_clear_nameservers_and_suspend(void)
              Remove all currently configured nameservers, and suspend all pending resolves.  Resolves will not neces-
              sarily 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 settings on a win32 host.  Preferentially
              tries to use GetNetworkParams; if that fails, looks in the registry.  Returns 0 on success, 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 infor-
              mation 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 trans-
     action 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. Several
     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 name-
     server 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 contributions 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                             October 7, 2006                            BSD