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 8e52533d1095 ("net: tftpsrv: Get correct
client MAC address").

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

diff --git a/net/dns.c b/net/dns.c
index ff9ddffc9d0a1f15e7febf808c5606f69ab48a3d..dd45320150452299e2d2adcdfcd96eb84d06c7f7 100644 (file)
--- a/net/dns.c
+++ b/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();
 }