* Copyright (c) 2006-2007 Niels Provos <provos@citi.umich.edu>
* Copyright (c) 2007-2012 Niels Provos and Nick Mathewson
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
* The original DNS code is due to Adam Langley with heavy
* modifications by Nick Mathewson. Adam put his DNS software in the
* public domain. You can find his original copyright below. Please,
* aware that the code as part of Libevent is governed by the 3-clause
* This software is Public Domain. To view a copy of the public domain dedication,
* visit http://creativecommons.org/licenses/publicdomain/ or send a letter to
* Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
* I ask and expect, but do not require, that all derivative works contain an
* attribution similar to:
* Parts developed by Adam Langley <agl@imperialviolet.org>
* You may wish to replace the word "Parts" with something else depending on
* the amount of original code.
* (Derivative works does not include programs which link against, run or include
* the source verbatim in their source distributions)
* 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,
* The library keeps track of the state of nameservers and will avoid
* them when they go down. Otherwise it will round robin between them.
* 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
* 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
* 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
* Order: www.myhome.net, www.
* Order: www.abc., www.abc.myhome.net
* 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
#ifndef EVENT2_DNS_H_INCLUDED_
#define EVENT2_DNS_H_INCLUDED_
#include <event2/visibility.h>
/** Error codes 0-5 are as described in RFC 1035. */
/** The name server was unable to interpret the query */
/** The name server was unable to process this query due to a problem with the
#define DNS_ERR_SERVERFAILED 2
/** The domain name does not exist */
#define DNS_ERR_NOTEXIST 3
/** The name server does not support the requested kind of query */
#define DNS_ERR_NOTIMPL 4
/** The name server refuses to reform the specified operation for policy
#define DNS_ERR_REFUSED 5
/** The reply was truncated or ill-formatted */
#define DNS_ERR_TRUNCATED 65
/** An unknown error occurred */
#define DNS_ERR_UNKNOWN 66
/** Communication with the server timed out */
#define DNS_ERR_TIMEOUT 67
/** The request was canceled because the DNS subsystem was shut down. */
#define DNS_ERR_SHUTDOWN 68
/** The request was canceled via a call to evdns_cancel_request */
#define DNS_ERR_CANCEL 69
/** There were no answers and no error condition in the DNS packet.
* This can happen when you ask for an address that exists, but a record
#define DNS_ERR_NODATA 70
#define DNS_QUERY_NO_SEARCH 1
#define DNS_OPTION_SEARCH 1
#define DNS_OPTION_NAMESERVERS 2
#define DNS_OPTION_MISC 4
#define DNS_OPTION_HOSTSFILE 8
#define DNS_OPTIONS_ALL 15
/* Obsolete name for DNS_QUERY_NO_SEARCH */
#define DNS_NO_SEARCH DNS_QUERY_NO_SEARCH
* The callback that contains the results from a lookup.
* - result is one of the DNS_ERR_* values (DNS_ERR_NONE for success)
* - 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. It will be an array of
* 4-byte sequences for ipv4, or an array of 16-byte sequences for ipv6,
* or a nul-terminated string for PTR.
typedef void (*evdns_callback_type) (int result, char type, int count, int ttl, void *addresses, void *arg);
/** Flag for evdns_base_new: process resolv.conf. */
#define EVDNS_BASE_INITIALIZE_NAMESERVERS 1
/** Flag for evdns_base_new: Do not prevent the libevent event loop from
* exiting when we have no active dns requests. */
#define EVDNS_BASE_DISABLE_WHEN_INACTIVE 0x8000
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.
@param event_base the event base to associate the dns client with
@param flags any of EVDNS_BASE_INITIALIZE_NAMESERVERS|
EVDNS_BASE_DISABLE_WHEN_INACTIVE
@return evdns_base object if successful, or NULL if an error occurred.
struct evdns_base * evdns_base_new(struct event_base *event_base, int initialize_nameservers);
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.
@param evdns_base the evdns base to free
@param fail_requests if zero, active requests will be aborted; if non-zero,
active requests will return DNS_ERR_SHUTDOWN.
void evdns_base_free(struct evdns_base *base, int fail_requests);
Remove all hosts entries that have been loaded into the event_base via
evdns_base_load_hosts or via event_base_resolv_conf_parse.
@param evdns_base the evdns base to remove outdated host addresses from
void evdns_base_clear_host_addresses(struct evdns_base *base);
Convert a DNS error code to a string.
@param err the DNS error code
@return a string containing an explanation of the error code
const char *evdns_err_to_string(int err);
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.
@param base the evdns_base to which to add the name server
@param address an IP address in network byte order
@return 0 if successful, or -1 if an error occurred
@see evdns_base_nameserver_ip_add()
int evdns_base_nameserver_add(struct evdns_base *base,
unsigned long int address);
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
@param base the evdns_base to which to apply this operation
@return the number of configured nameservers
@see evdns_base_nameserver_add()
int evdns_base_count_nameservers(struct evdns_base *base);
Remove all configured nameservers, and suspend all pending resolves.
Resolves will not necessarily be re-attempted until evdns_base_resume() is called.
@param base the evdns_base to which to apply this operation
@return 0 if successful, or -1 if an error occurred
int evdns_base_clear_nameservers_and_suspend(struct evdns_base *base);
Resume normal operation and continue any suspended resolve requests.
Re-attempt resolves left in limbo after an earlier call to
evdns_base_clear_nameservers_and_suspend().
@param base the evdns_base to which to apply this operation
@return 0 if successful, or -1 if an error occurred
@see evdns_base_clear_nameservers_and_suspend()
int evdns_base_resume(struct evdns_base *base);
Add a nameserver by string address.
This function parses a n IPv4 or IPv6 address from a string and adds it as a
nameserver. It supports the following formats:
If no port is specified, it defaults to 53.
@param base the evdns_base to which to apply this operation
@return 0 if successful, or -1 if an error occurred
@see evdns_base_nameserver_add()
int evdns_base_nameserver_ip_add(struct evdns_base *base,
const char *ip_as_string);
Add a nameserver by sockaddr.
evdns_base_nameserver_sockaddr_add(struct evdns_base *base,
const struct sockaddr *sa, ev_socklen_t len, unsigned flags);
Lookup an A record for a given name.
@param base the evdns_base to which to apply this operation
@param name a DNS hostname
@param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
@param callback a callback function to invoke when the request is completed
@param ptr an argument to pass to the callback function
@return an evdns_request object if successful, or NULL if an error occurred.
@see evdns_resolve_ipv6(), evdns_resolve_reverse(), evdns_resolve_reverse_ipv6(), evdns_cancel_request()
struct evdns_request *evdns_base_resolve_ipv4(struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *ptr);
Lookup an AAAA record for a given name.
@param base the evdns_base to which to apply this operation
@param name a DNS hostname
@param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
@param callback a callback function to invoke when the request is completed
@param ptr an argument to pass to the callback function
@return an evdns_request object if successful, or NULL if an error occurred.
@see evdns_resolve_ipv4(), evdns_resolve_reverse(), evdns_resolve_reverse_ipv6(), evdns_cancel_request()
struct evdns_request *evdns_base_resolve_ipv6(struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *ptr);
Lookup a PTR record for a given IP address.
@param base the evdns_base to which to apply this operation
@param in an IPv4 address
@param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
@param callback a callback function to invoke when the request is completed
@param ptr an argument to pass to the callback function
@return an evdns_request object if successful, or NULL if an error occurred.
@see evdns_resolve_reverse_ipv6(), evdns_cancel_request()
struct evdns_request *evdns_base_resolve_reverse(struct evdns_base *base, const struct in_addr *in, int flags, evdns_callback_type callback, void *ptr);
Lookup a PTR record for a given IPv6 address.
@param base the evdns_base to which to apply this operation
@param in an IPv6 address
@param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
@param callback a callback function to invoke when the request is completed
@param ptr an argument to pass to the callback function
@return an evdns_request object if successful, or NULL if an error occurred.
@see evdns_resolve_reverse_ipv6(), evdns_cancel_request()
struct evdns_request *evdns_base_resolve_reverse_ipv6(struct evdns_base *base, const struct in6_addr *in, int flags, evdns_callback_type callback, void *ptr);
Cancels a pending DNS resolution request.
@param base the evdns_base that was used to make the request
@param req the evdns_request that was returned by calling a resolve function
@see evdns_base_resolve_ipv4(), evdns_base_resolve_ipv6, evdns_base_resolve_reverse
void evdns_cancel_request(struct evdns_base *base, struct evdns_request *req);
Set the value of a configuration option.
The currently available configuration options are:
ndots, timeout, max-timeouts, max-inflight, attempts, randomize-case,
bind-to, initial-probe-timeout, getaddrinfo-allow-skew.
In versions before Libevent 2.0.3-alpha, the option name needed to end with
@param base the evdns_base to which to apply this operation
@param option the name of the configuration option to be modified
@param val the value to be set
@return 0 if successful, or -1 if an error occurred
int evdns_base_set_option(struct evdns_base *base, const char *option, const char *val);
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
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
@param base the evdns_base to which to apply this operation
@param flags any of DNS_OPTION_NAMESERVERS|DNS_OPTION_SEARCH|DNS_OPTION_MISC|
DNS_OPTION_HOSTSFILE|DNS_OPTIONS_ALL
@param filename the path to the resolv.conf file
@return 0 if successful, or various positive error codes if an error
@see resolv.conf(3), evdns_config_windows_nameservers()
int evdns_base_resolv_conf_parse(struct evdns_base *base, int flags, const char *const filename);
Load an /etc/hosts-style file from 'hosts_fname' into 'base'.
If hosts_fname is NULL, add minimal entries for localhost, and nothing
Note that only evdns_getaddrinfo uses the /etc/hosts entries.
This function does not replace previously loaded hosts entries; to do that,
call evdns_base_clear_host_addresses first.
Return 0 on success, negative on failure.
int evdns_base_load_hosts(struct evdns_base *base, const char *hosts_fname);
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,
@return 0 if successful, or -1 if an error occurred
@see evdns_resolv_conf_parse()
int evdns_base_config_windows_nameservers(struct evdns_base *);
#define EVDNS_BASE_CONFIG_WINDOWS_NAMESERVERS_IMPLEMENTED
Clear the list of search domains.
It is recommended that you Edit text format, this type of Fix handles quite a lot in one request
