net: dns: fix for DNS queries sent to the wrong MAC address

When a DNS query is sent out, the ethernet packet can get directed to
the MAC address of a server that was communicated to before.  This is
wrong when the previously stored MAC address corresponds to a different
server's IP address, i.e. when the IP address of the previous and the
current communication are different.

The error can get reproduced by running a sequence of e.g. a TFTP
download and a DNS query, where the TFTP and DNS servers reside on
individual machines.

The fix is to clear the server's MAC address that might be left from a
previous operation, and to fetch the peer's MAC address in a new ARP
lookup, before the DNS query is sent.  This is the approach taken in
other network services, like 8e52533d ("net: tftpsrv: Get correct
client MAC address").

Reported-by: Dirk Zimoch <dirk.zimoch@psi.ch>
Signed-off-by: Gerhard Sittig <gsi@denx.de>

net/dns.c

@@ -202,5 +202,8 @@ DnsStart(void)
 	NetSetTimeout(DNS_TIMEOUT, DnsTimeout);
 	net_set_udp_handler(DnsHandler);
 
+	/* Clear a previous MAC address, the server IP might have changed. */
+	memset(NetServerEther, 0, sizeof(NetServerEther));
+
 	DnsSend();
 }