EVDNS(3) BSD Programmer's 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_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);
DESCRIPTION
Welcome, gentle reader.
Asynchronous 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 event_init() must be called before any
of the APIs in this file.
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 was passed to evdns_resolve().
Searching:
In order for this library to be a good replacement for the libc 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 evdns_resolv_conf_parse(), evdns_init(),
and evdns_search_add() are never called 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(5), 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().
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 ad-
dress and adds it as a nameserver. Returns non-zero on error
int evdns_resolve(const char *name, int flags, evdns_callback_type call-
back, 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 the definition 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 check-
ing whether calls to the various nameserver configuration func-
tions 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_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(5) 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: lookup
and sortlist. Additionally, the following "options" are ignored:
debug, edns0, inet6, insecure1, and insecure2.
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.
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 limits the
number of concurrent requests so that the nameserver does not get flood-
ed. Several algorithms require a full walk of the inflight queue so lim-
iting its size keeps thing going nicely under huge (many thousands of re-
quests) loads.
If a nameserver loses too many requests it is considered down and we try
not to use it. After a while a probe is sent 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 ALSOevent(3), gethostbyname(3), resolv.conf(5)HISTORY
The evdns API was developed by Adam Langley on top of the libevent API.
The code was integrated 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.
MirOS BSD #10-current October 7, 2006 3