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:
void callback(int result, char type, int count, int ttl, void
*addresses, void *arg);
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
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
Order: www.myhome.net, www.
Order: www.abc., www.abc.myhome.net
Initializes support for non-blocking name resolution by call‐
ing evdns_resolv_conf_parse() on UNIX and
evdns_config_windows_nameservers() on Windows.
intevdns_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.
intevdns_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
intevdns_resolve(const char *name, int flags, evdns_callback_typecallback, 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
Returns non-zero on error
Clears the list of search domains
voidevdns_search_add(const char *domain)
Add a domain to the list of search domains
Set the number of dots which, when found in a name, causes the
first query to be without any search domain.
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.
Remove all currently configured nameservers, and suspend all
pending resolves. Resolves will not necessarily be re-
attempted until evdns_resume() is called.
Re-attempt resolves left in limbo after an earlier call to
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.
intevdns_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
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
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.
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.
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 firstname.lastname@example.org
BSD October 7, 2006 BSD