=== DNS server

Disabled by default; enable with `-DMG_ENABLE_DNS_SERVER`. 

==== mg_dns_create_reply()

[source,c]
----
struct mg_dns_reply mg_dns_create_reply(struct mbuf *io,
                                        struct mg_dns_message *msg);
----
Create a DNS reply.

The reply will be based on an existing query message `msg`.
The query body will be appended to the output buffer.
"reply + recursion allowed" will be added to the message flags and
message's num_answers will be set to 0.

Answer records can be appended with `mg_dns_send_reply` or by lower
level function defined in the DNS API.

In order to send the reply use `mg_dns_send_reply`.
It's possible to use a connection's send buffer as reply buffers,
and it will work for both UDP and TCP connections.

Example:

```c
reply = mg_dns_create_reply(&nc->send_mbuf, msg);
for (i = 0; i < msg->num_questions; i++) {
  rr = &msg->questions[i];
  if (rr->rtype == MG_DNS_A_RECORD) {
    mg_dns_reply_record(&reply, rr, 3600, &dummy_ip_addr, 4);
  }
}
mg_dns_send_reply(nc, &reply);
``` 

==== mg_dns_reply_record()

[source,c]
----
int mg_dns_reply_record(struct mg_dns_reply *reply,
                        struct mg_dns_resource_record *question,
                        const char *name, int rtype, int ttl, const void *rdata,
                        size_t rdata_len);
----
Append a DNS reply record to the IO buffer and to the DNS message.

The message num_answers field will be incremented. It's caller's duty
to ensure num_answers is propertly initialized.

Returns -1 on error. 

==== mg_dns_send_reply()

[source,c]
----
void mg_dns_send_reply(struct mg_connection *nc, struct mg_dns_reply *r);
----
Send a DNS reply through a connection.

The DNS data is stored in an IO buffer pointed by reply structure in `r`.
This function mutates the content of that buffer in order to ensure that
the DNS header reflects size and flags of the mssage, that might have been
updated either with `mg_dns_reply_record` or by direct manipulation of
`r->message`.

Once sent, the IO buffer will be trimmed unless the reply IO buffer
is the connection's send buffer and the connection is not in UDP mode.