[2704] Use the selectSubnet() subnet function to pick the suitable subnet.

The selectSubnet() function offers two ways to find a subnet for a
particular interface: using the name of the interface that the subnet is
configured for; and using the packet source address. Also, the redundant
warning is removed if suitable subnet was not found as this warning is
already issued elsewhere.

src/bin/dhcp6/dhcp6_messages.mes

@@ -132,18 +132,6 @@ IPv6 DHCP server but it is not running.
 During startup the IPv6 DHCP server failed to detect any network
 interfaces and is therefore shutting down.
 
-% DHCP6_NO_SUBNET_DEF_OPT failed to find subnet for address %1 when adding default options
-This warning message indicates that when attempting to add default options
-to a response, the server found that it was not configured to support
-the subnet from which the DHCPv6 request was received.  The packet has
-been ignored.
-
-% DHCP6_NO_SUBNET_REQ_OPT failed to find subnet for address %1 when adding requested options
-This warning message indicates that when attempting to add requested
-options to a response, the server found that it was not configured
-to support the subnet from which the DHCPv6 request was received.
-The packet has been ignored.
-
 % DHCP6_OPEN_SOCKET opening sockets on port %1
 A debug message issued during startup, this indicates that the IPv6 DHCP
 server is about to open sockets on the specified port.

src/bin/dhcp6/dhcp6_srv.cc

@@ -393,29 +393,21 @@ Dhcpv6Srv::copyDefaultOptions(const Pkt6Ptr& question, Pkt6Ptr& answer) {
 }
 
 void
-Dhcpv6Srv::appendDefaultOptions(const Pkt6Ptr& question, Pkt6Ptr& answer) {
+Dhcpv6Srv::appendDefaultOptions(const Pkt6Ptr&, Pkt6Ptr& answer) {
     // add server-id
     answer->addOption(getServerID());
-
-    // Get the subnet object. It holds options to be sent to the client
-    // that belongs to the particular subnet.
-    Subnet6Ptr subnet = CfgMgr::instance().getSubnet6(question->getRemoteAddr());
-    // Warn if subnet is not supported and quit.
-    if (!subnet) {
-        LOG_WARN(dhcp6_logger, DHCP6_NO_SUBNET_DEF_OPT)
-            .arg(question->getRemoteAddr().toText());
-        return;
-    }
-
 }
 
 void
 Dhcpv6Srv::appendRequestedOptions(const Pkt6Ptr& question, Pkt6Ptr& answer) {
-    // Get the subnet for a particular address.
-    Subnet6Ptr subnet = CfgMgr::instance().getSubnet6(question->getRemoteAddr());
+    // Get the configured subnet suitable for the incoming packet.
+    Subnet6Ptr subnet = selectSubnet(question);
+    // Leave if there is no subnet matching the incoming packet.
+    // There is no need to log the error message here because
+    // it will be logged in the assignLease() when it fails to
+    // pick the suitable subnet. We don't want to duplicate
+    // error messages in such case.
     if (!subnet) {
-        LOG_WARN(dhcp6_logger, DHCP6_NO_SUBNET_REQ_OPT)
-            .arg(question->getRemoteAddr().toText());
         return;
     }