EVDNS(3) BSD Library Functions Manual EVDNS(3)
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.
const char *
evdns_nameserver_add(unsigned long int address);
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,
evdns_resolv_conf_parse(int flags, const char *);
evdns_search_add(const char *domain);
evdns_search_ndots_set(const int ndots);
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 getad-
drinfo_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
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 unpre-
dictable 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 gettime-
ofday. 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 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
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 search-
ing. 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 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
For example, with ndots set to 1 (the default) and a search domain list of ["myhome.net"]:
Order: www.myhome.net, www.
Order: www.abc., www.abc.myhome.net
Initializes support for non-blocking name resolution by calling
evdns_resolv_conf_parse() on UNIX and evdns_config_windows_nameservers() on Win-
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
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 error
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 disables 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
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.
Return 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.
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 evdns_clear_name-
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 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 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:
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
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.
event(3), gethostbyname(3), resolv.conf(5)
The evdns API was developed by Adam Langley on top of the libevent API. The code was inte-
grate into Tor by Nick Mathewson and finally put into libevent itself by Niels Provos.
The evdns API and code was written by Adam Langley with significant contributions by Nick
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.
BSD May 14, 2010 BSD