NAT64 policy and DNS64 (DNS proxy)

NAT64 policy translates IPv6 addresses to IPv4 addresses so that a client on an IPv6 network can communicate transparently with a server on an IPv4 network.

NAT64 policy is usually implemented in combination with the DNS proxy called DNS64. DNS64 synthesizes AAAA records from A records and is used to synthesize IPv6 addresses for hosts that only have IPv4 addresses. DNS proxy and DNS64 are interchangeable terms.

Sample topology

In this example, a host on the internal IPv6 network communicates with ControlPC.qa.fortinet.com that only has IPv4 address on the Internet. Central NAT is disabled.

1. The host on the internal network does a DNS lookup for ControlPC.qa.fortinet.com by sending a DNS query for an AAAA record for ControlPC.qa.fortinet.com.
2. The DNS query is intercepted by the FortiGate DNS proxy. The DNS proxy performs an A-record query for ControlPC.qa.fortinet.com and gets back an RRSet containing a single A record with the IPv4 address 172.16.200.55.
3. The DNS proxy then synthesizes an AAAA record. The IPv6 address in the AAAA record begins with the configured NAT64 prefix in the upper 96 bits and the received IPv4 address in the lower 32 bits. By default, the resulting IPv6 address is 64:ff9b::172.16.200.55.
4. The host on the internal network receives the synthetic AAAA record and sends a packet to the destination address 64:ff9b::172.16.200.55.
5. The packet is routed to the FortiGate internal interface (port10) where it is accepted by the NAT64 security policy.
6. The FortiGate translates the destination address of the packets from IPv6 address 64:ff9b::172.16.200.55 to IPv4 address 172.16.200.55 and translates the source address of the packets to 172.16.200.200 (or another address in the IP pool range) and forwards the packets out the port9 interface to the Internet.

Sample configuration

To configure a NAT64 policy with DNS64 in the GUI:

1. Enable IPv6 and DNS database:

   1. Go to System > Feature Visibility.
   2. In the Core Features section, enable IPv6.
   3. In the Additional Features section, enable DNS Database.
   4. Click Apply.

2. Enable DNS proxy on the IPv6 interface:

   1. Go to Network > DNS Servers.
   2. In the DNS Service on Interface table, click Create New.
   3. For Interface, select port10.
   4. For Mode, select Forward to System DNS.
   5. Click OK.

3. Configure the IPv6 DHCP server:

   1. Go to Network > Interfaces and edit port10.

   2. Enable DHCPv6 Server and enter the following:

      IPv6 subnet	2001:db8:1::/64
      DNS service	Specify
      DNS server 1	2001:db8:1::10

   3. Click OK.

4. Configure the IPv6 VIP for the destination IPv6 addresses:

   These are all of the IPv6 addresses that the FortiGate DNS proxy synthesizes when an IPv6 device performs a DNS query that resolves to an IPv4 Address. In this example, the synthesized IPv6 address in the AAAA record begins with the configured NAT64 prefix in the upper 96 bits, so the VIP is for all the IPv6 addresses that begin with 64:ff9b.

   1. Go to Policy & Objects > Virtual IPs and click Create New > Virtual IP.
   2. Enter the following:

      VIP type	IPv6
      Name	vip6
      Eternal IP address/range	64:ff9b::-64:ff9b:ffff:ffff
      Map to IPv4 address/range	Use Embedded

   3. Click OK.

5. Configure the IPv6 firewall address for the internal network:

   1. Click Create New > Address.
   2. Enter the following:

      Category	IPv6 Address
      Name	internal-net6
      Type	IPv6 Subnet
      IP/Netmask	2001:db8:1::/48

   3. Click OK.

6. Configure the IP pool containing the IPv4 address that is used as the source address of the packets exiting port9:

   1. Go to Policy & Objects > IP Pools and click Create New.
   2. Enter the following:

      IP Pool Type	IPv4 Pool
      Name	exit-pool4
      Type	Overload
      External IP address/range	172.16.200.200-172.16.200.207
      NAT64	Enable

   3. Click OK.

7. Configure the NAT64 policy:

   1. Go to Policy & Objects > Firewall Policy and click Create New.

   2. Enter the following:

      Name	policy64-1
      Incoming Interface	port10
      Outgoing Interface	port9
      Source	internal-net6
      Destination	vip6
      Schedule	always
      Service	ALL
      Action	ACCEPT
      NAT	NAT64
      IP Pool Configuration	exit-pool4

   3. Click OK.

To configure a NAT64 policy with DNS64 in the CLI:

1. Enable IPv6 and DNS database:

   config system global
      set gui-ipv6 enable
   end

   config system settings
      set gui-dns-database enable
   end

2. Enable DNS proxy on the IPv6 interface:

   config system dns-server
      edit "port10"
          set mode forward-only
      next
   end

3. Configure the IPv6 DHCP server:

   config system dhcp6 server
      edit 1
          set subnet 2001:db8:1::/64
          set interface "port10"
          set dns-server1 2001:db8:1::10
      next
   end

4. Configure the IPv4 firewall address for the external network:

   config firewall address
       edit "external-net4"
           set associated-interface "port9"
           set subnet 172.16.200.0 255.255.255.0
       next
   end

5. Configure the IPv6 firewall address for the internal network:

   config firewall address6
       edit "internal-net6"
           set ip6 2001:db8:1::/48
       next
   end

6. Configure the IP pool containing the IPv4 address that is used as the source address of the packets exiting port9:

   config firewall ippool
       edit "exit-pool4"
           set startip 172.16.200.200
           set endip 172.16.200.207
           set nat64 enable
       next
   end

7. Configure the NAT64 policy:

   config firewall policy
       edit 1
           set name "policy64-1"
           set srcintf "port10"
           set dstintf "port9"
           set action accept
           set nat64 enable
           set srcaddr "all"
           set dstaddr "all"
           set srcaddr6 internal-net6
           set dstaddr6 vip6
           set schedule "always"
           set service "ALL"
           set ippool enable
           set poolname "exit-pool4"
       next
   end

To enable DNS64 and related settings using the CLI:

Enabling DNS64 means that all IPv6 traffic received by the current VDOM can be subject to NAT64 if the source and destination address matches an NAT64 security policy.

By default, the setting always-synthesize-aaaa-record is enabled. If you disable this setting, the DNS proxy (DNS64) will attempt to find an AAAA records for queries to domain names and therefore resolve the host names to IPv6 addresses. If the DNS proxy cannot find an AAAA record, it synthesizes one by adding the NAT64 prefix to the A record.

config system dns64
    set status {enable | disable}
    set dns64-prefix <ipv6-prefix>
    set always-synthesize-aaaa-record {enable | disable}
end

By default, the dns64-prefix is 64:ff9b::/96.