EVDNS(3) 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_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, void *ptr);
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 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 of
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
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 calling
evdns_resolv_conf_parse() on UNIX and
evdns_config_windows_nameservers() on Windows.
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
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
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
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
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
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 integrate 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 Mathewson.
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.
NetBSD 6.1.5 May 14, 2010 NetBSD 6.1.5