evdns_set_transaction_id_fn

evdns.h(3)                 Library Functions Manual                 evdns.h(3)



NAME
       evdns.h -

       Welcome, gentle reader.


SYNOPSIS
       #include <evutil.h>


   Data Structures
       struct evdns_server_question
       struct evdns_server_request

   Defines
       #define DNS_ERR_FORMAT   1
           The name server was unable to interpret the query.
       #define DNS_ERR_NONE   0
           Error codes 0-5 are as described in RFC 1035.
       #define DNS_ERR_NOTEXIST   3
           The domain name does not exist.
       #define DNS_ERR_NOTIMPL   4
           The name server does not support the requested kind of query.
       #define DNS_ERR_REFUSED   5
           The name server refuses to reform the specified operation for
           policy reasons.
       #define DNS_ERR_SERVERFAILED   2
           The name server was unable to process this query due to a problem
           with the name server.
       #define DNS_ERR_SHUTDOWN   68
           The request was canceled because the DNS subsystem was shut down.
       #define DNS_ERR_TIMEOUT   67
           Communication with the server timed out.
       #define DNS_ERR_TRUNCATED   65
           The reply was truncated or ill-formated.
       #define DNS_ERR_UNKNOWN   66
           An unknown error occurred.
       #define DNS_IPv4_A   1
       #define DNS_IPv6_AAAA   3
       #define DNS_NO_SEARCH   1
       #define DNS_OPTION_MISC   4
       #define DNS_OPTION_NAMESERVERS   2
       #define DNS_OPTION_SEARCH   1
       #define DNS_OPTIONS_ALL   7
       #define DNS_PTR   2
       #define DNS_QUERY_NO_SEARCH   1
       #define dns_question_class   class
       #define EVDNS_ADDITIONAL_SECTION   2
       #define EVDNS_ANSWER_SECTION   0
       #define EVDNS_AUTHORITY_SECTION   1
       #define EVDNS_CLASS_INET   1
       #define EVDNS_QTYPE_ALL   255
       #define EVDNS_QTYPE_AXFR   252
       #define EVDNS_TYPE_A   1
       #define EVDNS_TYPE_AAAA   28
       #define EVDNS_TYPE_CNAME   5
       #define EVDNS_TYPE_MX   15
       #define EVDNS_TYPE_NS   2
       #define EVDNS_TYPE_PTR   12
       #define EVDNS_TYPE_SOA   6
       #define EVDNS_TYPE_TXT   16

   Typedefs
       typedef void(* evdns_callback_type )(int result, char type, int count,
           int ttl, void *addresses, void *arg)
           The callback that contains the results from a lookup.
       typedef void(* evdns_debug_log_fn_type )(int is_warning, const char
           *msg)
           A callback that is invoked when a log message is generated.
       typedef void(* evdns_request_callback_fn_type )(struct
           evdns_server_request *, void *)

   Functions
       struct evdns_server_port * evdns_add_server_port (int socket, int
           is_tcp, evdns_request_callback_fn_type callback, void *user_data)
       int evdns_clear_nameservers_and_suspend (void)
           Remove all configured nameservers, and suspend all pending
           resolves.
       void evdns_close_server_port (struct evdns_server_port *port)
       int evdns_count_nameservers (void)
           Get the number of configured nameservers.
       const char * evdns_err_to_string (int err)
           Convert a DNS error code to a string.
       int evdns_init (void)
           Initialize the asynchronous DNS library.
       int evdns_nameserver_add (unsigned long int address)
           Add a nameserver.
       int evdns_nameserver_ip_add (const char *ip_as_string)
           Add a nameserver.
       int evdns_resolv_conf_parse (int flags, const char *const filename)
           Parse a resolv.conf file.
       int evdns_resolve_ipv4 (const char *name, int flags,
           evdns_callback_type callback, void *ptr)
           Lookup an A record for a given name.
       int evdns_resolve_ipv6 (const char *name, int flags,
           evdns_callback_type callback, void *ptr)
           Lookup an AAAA record for a given name.
       int evdns_resolve_reverse (const struct in_addr *in, int flags,
           evdns_callback_type callback, void *ptr)
           Lookup a PTR record for a given IP address.
       int evdns_resolve_reverse_ipv6 (const struct in6_addr *in, int flags,
           evdns_callback_type callback, void *ptr)
           Lookup a PTR record for a given IPv6 address.
       int evdns_resume (void)
           Resume normal operation and continue any suspended resolve
           requests.
       void evdns_search_add (const char *domain)
           Add a domain to the list of search domains.
       void evdns_search_clear (void)
           Obtain nameserver information using the Windows API.
       void evdns_search_ndots_set (const int ndots)
           Set the 'ndots' parameter for searches.
       int evdns_server_request_add_a_reply (struct evdns_server_request *req,
           const char *name, int n, void *addrs, int ttl)
       int evdns_server_request_add_aaaa_reply (struct evdns_server_request
           *req, const char *name, int n, void *addrs, int ttl)
       int evdns_server_request_add_cname_reply (struct evdns_server_request
           *req, const char *name, const char *cname, int ttl)
       int evdns_server_request_add_ptr_reply (struct evdns_server_request
           *req, struct in_addr *in, const char *inaddr_name, const char
           *hostname, int ttl)
       int evdns_server_request_add_reply (struct evdns_server_request *req,
           int section, const char *name, int type, int dns_class, int ttl,
           int datalen, int is_name, const char *data)
       int evdns_server_request_drop (struct evdns_server_request *req)
       int evdns_server_request_get_requesting_addr (struct
           evdns_server_request *_req, struct sockaddr *sa, int addr_len)
       int evdns_server_request_respond (struct evdns_server_request *req, int
           err)
       void evdns_set_log_fn (evdns_debug_log_fn_type fn)
           Set the callback function to handle log messages.
       int evdns_set_option (const char *option, const char *val, int flags)
           Set the value of a configuration option.
       void evdns_set_transaction_id_fn (ev_uint16_t(*fn)(void))
           Set a callback that will be invoked to generate transaction IDs.
       void evdns_shutdown (int fail_requests)
           Shut down the asynchronous DNS resolver and terminate all active
           requests.

Detailed 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 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 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 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

       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. 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.

Define Documentation
   #define DNS_ERR_NONE   0
       Error codes 0-5 are as described in RFC 1035.

   #define DNS_ERR_SHUTDOWN   68
       The request was canceled because the DNS subsystem was shut down.

Typedef Documentation
   typedef void(* evdns_callback_type)(int result, char type, int count, int
       ttl, void *addresses, void *arg)
       The callback that contains the results from a lookup. .IP "·" 2 type is
       either DNS_IPv4_A or DNS_PTR or DNS_IPv6_AAAA

       · count contains the number of addresses of form type

       · ttl is the number of seconds the resolution may be cached for.

       · addresses needs to be cast according to type

   typedef void(* evdns_debug_log_fn_type)(int is_warning, const char *msg)
       A callback that is invoked when a log message is generated. Parameters:
           is_warning indicates if the log message is a 'warning'
           msg the content of the log message

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

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_resume()

   int evdns_count_nameservers (void)
       Get the number of configured nameservers. This returns 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.

       Returns:
           the number of configured nameservers

       See also:
           evdns_nameserver_add()

   const char* evdns_err_to_string (int err)
       Convert a DNS error code to a string. Parameters:
           err the DNS error code

       Returns:
           a string containing an explanation of the error code

   int evdns_init (void)
       Initialize the asynchronous DNS library. This function initializes
       support for non-blocking name resolution by calling
       evdns_resolv_conf_parse() on UNIX and
       evdns_config_windows_nameservers() on Windows.

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_shutdown()

   int evdns_nameserver_add (unsigned long int address)
       Add a nameserver. The address should be an IPv4 address in network byte
       order. The type of address is chosen so that it matches in_addr.s_addr.

       Parameters:
           address an IP address in network byte order

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_nameserver_ip_add()

   int evdns_nameserver_ip_add (const char * ip_as_string)
       Add a nameserver. This wraps the evdns_nameserver_add() function by
       parsing a string as an IP address and adds it as a nameserver.

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_nameserver_add()

   int evdns_resolv_conf_parse (int flags, const char *const  filename)
       Parse a resolv.conf file. The 'flags' parameter determines what
       information is parsed from the resolv.conf file. See the man page for
       resolv.conf for the format of this file.

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

       If this function encounters an error, the possible return values are: 1
       = failed to open file, 2 = failed to stat file, 3 = file too large, 4 =
       out of memory, 5 = short read from file, 6 = no nameservers listed in
       the file

       Parameters:
           flags any of
           DNS_OPTION_NAMESERVERS|DNS_OPTION_SEARCH|DNS_OPTION_MISC|
           DNS_OPTIONS_ALL
           filename the path to the resolv.conf file

       Returns:
           0 if successful, or various positive error codes if an error
           occurred (see above)

       See also:
           resolv.conf(3), evdns_config_windows_nameservers()

   int evdns_resolve_ipv4 (const char * name, int flags, evdns_callback_type
       callback, void * ptr)
       Lookup an A record for a given name. Parameters:
           name a DNS hostname
           flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for
           this query.
           callback a callback function to invoke when the request is
           completed
           ptr an argument to pass to the callback function

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_resolve_ipv6(), evdns_resolve_reverse(),
           evdns_resolve_reverse_ipv6()

   int evdns_resolve_ipv6 (const char * name, int flags, evdns_callback_type
       callback, void * ptr)
       Lookup an AAAA record for a given name. Parameters:
           name a DNS hostname
           flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for
           this query.
           callback a callback function to invoke when the request is
           completed
           ptr an argument to pass to the callback function

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_resolve_ipv4(), evdns_resolve_reverse(),
           evdns_resolve_reverse_ipv6()

   int evdns_resolve_reverse (const struct in_addr * in, int flags,
       evdns_callback_type callback, void * ptr)
       Lookup a PTR record for a given IP address. Parameters:
           in an IPv4 address
           flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for
           this query.
           callback a callback function to invoke when the request is
           completed
           ptr an argument to pass to the callback function

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_resolve_reverse_ipv6()

   int evdns_resolve_reverse_ipv6 (const struct in6_addr * in, int flags,
       evdns_callback_type callback, void * ptr)
       Lookup a PTR record for a given IPv6 address. Parameters:
           in an IPv6 address
           flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for
           this query.
           callback a callback function to invoke when the request is
           completed
           ptr an argument to pass to the callback function

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_resolve_reverse_ipv6()

   int evdns_resume (void)
       Resume normal operation and continue any suspended resolve requests.
       Re-attempt resolves left in limbo after an earlier call to
       evdns_clear_nameservers_and_suspend().

       Returns:
           0 if successful, or -1 if an error occurred

       See also:
           evdns_clear_nameservers_and_suspend()

   void evdns_search_add (const char * domain)
       Add a domain to the list of search domains. Parameters:
           domain the domain to be added to the search list

   void evdns_search_clear (void)
       Obtain nameserver information using the Windows API. 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 if successful, or -1 if an error occurred

       See also:
           evdns_resolv_conf_parse() Clear the list of search domains.

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

       Parameters:
           ndots the new ndots parameter

   void evdns_set_log_fn (evdns_debug_log_fn_type fn)
       Set the callback function to handle log messages. Parameters:
           fn the callback to be invoked when a log message is generated

   int evdns_set_option (const char * option, const char * val, int flags)
       Set the value of a configuration option. The currently available
       configuration options are:

       ndots, timeout, max-timeouts, max-inflight, and attempts

       Parameters:
           option the name of the configuration option to be modified
           val the value to be set
           flags either 0 | DNS_OPTION_SEARCH | DNS_OPTION_MISC

       Returns:
           0 if successful, or -1 if an error occurred

   void evdns_set_transaction_id_fn (ev_uint16_t(*)(void) fn)
       Set a callback that will be invoked to generate transaction IDs. By
       default, we pick transaction IDs based on the current clock time.

       Parameters:
           fn the new callback, or NULL to use the default.

   void evdns_shutdown (int fail_requests)
       Shut down the asynchronous DNS resolver and terminate all active
       requests. If the 'fail_requests' option is enabled, all active requests
       will return an empty result with the error flag set to
       DNS_ERR_SHUTDOWN. Otherwise, the requests will be silently discarded.

       Parameters:
           fail_requests if zero, active requests will be aborted; if non-
           zero, active requests will return DNS_ERR_SHUTDOWN.

       See also:
           evdns_init()

Author
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libevent                          22 Jun 2012                       evdns.h(3)