[PATCH] Fix UDP DNS queries in addns

[Andreas Schneider]

On Tuesday, 12 June 2018 02:56:38 CEST Timur I. Bakeyev via samba-technical 
wrote:
> 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).

RB+

-- 
Andreas Schneider                   GPG-ID: CC014E3D
Samba Team                             asn at samba.org
www.samba.org