To setup a Synology NAS as a secondary DNS to provide a fall back if/when the primary DNS is off line. To setup a DNS and DHCP serve see my post for DNS with bind9 and DHCP on Ubuntu 14.04.

In the Synology DSM, open the Package Center, and install the DNS package.

You should enable the Synology DSM Firewall to allow DNS traffic in and out. In the Synology DSM, open the Control Panel -> Security Tab.
Highlight the interface desired, and click Create.
Select Select from a built in list of applications and make sure DNS Server is enabled.
Click OK, and then Save the rule.

Open up DNS Server from the Synology launch menu.
Click on Zones and then on Create > Slave Zone.
Choose Forward Zone from the drop down.
Enter the domain name for your DNS, and Master DNS IP address.
Click apply.

Select Edit -> Resource Records and you should see a bunch of records have been pulled over.

If you are using a rndc.key with your bind9 server you will need to makes some changes to be able to use it. Synology, annoyingly, stop you from using the default name used by Ubuntu, rndc.key. So you have to change the bind9 install on the master to use a different key name. It needs to be changed in these files:

• /etc/bind/rndc.key
• /etc/bind/named.conf.local
• /etc/dhcp/dhcpd.conf

With that done import the key file. Go to Keys on the left hand list.
Click Create but select Import.
The dialog displayed is too small to display the browse button. A rather stupid mistake on behalf of Synology! so scroll down and click browse, find the key file and import it.
You should see it listed in the main window.
Back to Zones, highlight your new zone, and click edit selecting zone settings.
Enable TSIG and select the new key from the list.
While you are here, you can tighten up security by limiting the IP addresses that can use this DNS to IPs for just your LAN.

Click OK.

Now select Resolution. Tick the box Enable resolution services.
Also tick the box for Limit source IP service, enter your LAN IP and netmask.
Click OK.