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 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 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 anywhere.
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 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:
#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 you passed to evdns_resolve.
Searching:
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 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 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