evdns_nameserver_add(3) — Linux manual page


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

NAME         top

     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         top

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


     evdns_shutdown(int fail_requests);

     const char *
     evdns_err_to_string(int err);

     evdns_nameserver_add(unsigned long int address);




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

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

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

     evdns_resolv_conf_parse(int flags, const char *);


     evdns_search_add(const char *domain);

     evdns_search_ndots_set(const int ndots);

     evdns_set_log_fn(evdns_debug_log_fn_type fn);


DESCRIPTION         top

     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

     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
     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
           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
           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.


     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

     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

     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         top

     int evdns_init()
              Initializes support for non-blocking name resolution by call‐
              ing 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 net‐
              work 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

     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 dou‐
              ble-checking whether our calls to the various nameserver con‐
              figuration 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

     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 GetNet‐
              workParams; 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 information is parsed from
              this file:
                    DNS_OPTION_SEARCH         domain, search and ndots
                    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:         top

     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. 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 nameserver fails a probe we wait longer to try again with
     the next probe.

SEE ALSO         top

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

HISTORY         top

     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         top

     The evdns API and code was written by Adam Langley with significant
     contributions by Nick Mathewson.

BUGS         top

     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.

COLOPHON         top

     This page is part of the libevent (an event notification library)
     project.  Information about the project can be found at
     http://libevent.org/.  If you have a bug report for this manual page,
     see ⟨http://sourceforge.net/p/levent/bugs/⟩.  This page was obtained
     from the project's upstream Git repository
     ⟨https://github.com/libevent/libevent.git⟩ on 2020-08-13.  (At that
     time, the date of the most recent commit that was found in the reposi‐
     tory was 2020-08-11.)  If you discover any rendering problems in this
     HTML version of the page, or you believe there is a better or more up-
     to-date source for the page, or you have corrections or improvements to
     the information in this COLOPHON (which is not part of the original
     manual page), send a mail to man-pages@man7.org

BSD                            October 7, 2006                           BSD