[PATCH] Fix UDP DNS queries in addns

[Timur I. Bakeyev]

Simple testing shows up that `net ads dns gethostbyname` is broken at least
on FreeBSD, although shouldn't work on Linux either:

# net ads dns gethostbyname 8.8.8.8 samba.org
do_gethostbyname returned ERROR_DNS_SOCKET_ERROR (10)

Strace shows for the AD server:

socket(PF_INET,SOCK_DGRAM,IPPROTO_UDP)         = 8 (0x8)
> connect(8,{ AF_INET 10.10.0.20:53 },16)         = 0 (0x0)
> getpeername(8,{ AF_INET 10.10.0.20:53 },0x7fffffffd34c) = 0 (0x0)
> read(5,":\\",2)                     = 2 (0x2)
> sendto(8,"\\:\0\0\0\^A\0\0\0\0\0\0\vcliffw"...,29,0,{ AF_INET
> 10.10.0.20:53 },128) ERR#56 'Socket is already connected'
> close(8)                     = 0 (0x0)
>

Which explains why this error occurs - in source3/utils/net_dns.c in
do_gethostbyname() we call  dns_open_connection(server, DNS_UDP, NULL,
&conn) and then, through other wrapping functions - dns_send_udp() from
lib/addns/dnssock.c.

dns_open_connection() for both UDP and TCP is calling dns_open_helper(),
which, among other staff calls connect() on the created socket.

For UDP to send a packet you can use two similar approaches, with slightly
different effects. One is to use connect()/send() sequence, another is just
to use sendto(). Both methods should work, but you shouldn't combine
connect() together with the sendto() - that would create the EISCONN error.
Even Linux manpage says:

If sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET)
> socket, the arguments dest_addr and addrlen are ignored (and the  error
> EISCONN  may be returned  when they are not NULL and 0).
>

That doesn't say anything about SOCK_DGRAM, but it should be an error
also(and it's a bug in the implementation if it's not produced).

Another issue with `net ads dns gethostbyname` is that it queries only
A(IPv4) records, ignoring AAAA(IPv6). IMHO that's also a bug, taking into
account that we are trying to support IPv6 on all levels.

With best regards,
Timur Bakeyev.
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