=== Asynchronouns DNS resolver 

==== struct mg_resolve_async_opts

[source,c]
----
struct mg_resolve_async_opts {
  const char *nameserver_url;
  int max_retries;    /* defaults to 2 if zero */
  int timeout;        /* in seconds; defaults to 5 if zero */
  int accept_literal; /* pseudo-resolve literal ipv4 and ipv6 addrs */
  int only_literal;   /* only resolves literal addrs; sync cb invocation */
  struct mg_connection **dns_conn; /* return DNS connection */
};
----
Options for `mg_resolve_async_opt`. 

==== mg_resolve_async()

[source,c]
----
int mg_resolve_async(struct mg_mgr *mgr, const char *name, int query,
                     mg_resolve_callback_t cb, void *data);
----
See `mg_resolve_async_opt()` 

==== mg_resolve_async_opt()

[source,c]
----
int mg_resolve_async_opt(struct mg_mgr *mgr, const char *name, int query,
                         mg_resolve_callback_t cb, void *data,
                         struct mg_resolve_async_opts opts);
----
Resolved a DNS name asynchronously.

Upon successful resolution, the user callback will be invoked
with the full DNS response message and a pointer to the user's
context `data`.

In case of timeout while performing the resolution the callback
will receive a NULL `msg`.

The DNS answers can be extracted with `mg_next_record` and
`mg_dns_parse_record_data`:

[source,c]
----
struct in_addr ina;
struct mg_dns_resource_record *rr = mg_next_record(msg, MG_DNS_A_RECORD,
  NULL);
mg_dns_parse_record_data(msg, rr, &ina, sizeof(ina));
---- 

==== mg_resolve_from_hosts_file()

[source,c]
----
int mg_resolve_from_hosts_file(const char *host, union socket_address *usa);
----
Resolve a name from `/etc/hosts`.

Returns 0 on success, -1 on failure.