DNS Rebinding Protections
pfSense includes some built in methods of protection against DNS rebinding attacks. These measures are described below.
The DNS forwarder (dnsmasq) uses the option --stop-dns-rebind by default, which rejects and logs addresses from upstream nameservers which are in the private IP ranges. In the most common usage, this is filtering DNS responses received from the Internet to prevent DNS rebinding attacks. Internet DNS responses should never come back with a private IP, hence it's safest to block this.
There are some cases when public DNS servers have private IP address replies by default, though it is not recommended. In those cases, DNS rebinding can be disabled or an override may be placed in the DNS Forwarder Advanced Settings box as follows:
Note this is automatically overridden for domains in the DNS forwarder's domain override list, as the most common usage of that functionality is to resolve internal DNS hostnames.
DNS Resolver (Unbound)
Unbound has similar protections to dnsmasq, using its "Private Address support" option. With that option enabled RFC1918 addresses are stripped away from DNS answers. Additionally, the DNSSEC validator may mark the answers bogus.
In the package on 2.1 and earlier this option is located in the main "Unbound DNS Settings" tab. On 2.2 where Unbound is integrated into the base system, it is active by default and controlled by the DNS Rebinding option under System > Advanced.
Individual domains can be excluded from DNS rebinding protection using the Custom Options on the Unbound general settings. Enter one domain per line in the following format, preceded by the "server:" line.
server: private-domain: "example.com"
Web interface protection
For those not using the DNS forwarder, and as an additional layer of checks, the web interface will block attempts to access it via an unknown hostname. It will display "Potential DNS Rebind Attack Detected" and drop any request. By default, only the hostname and domain configured under System>General Setup are accepted. For instance if firewall.example.com is configured as the system's hostname, and it is loaded in a browser using fw1.example.com, that attempt will be rejected. Additional hostnames can be added under System>Advanced, "Alternate Hostnames".
Logging in using the IP address of the system rather than the hostname does work if this message is encountered when attempting to load by hostname. Once access has been obtained, configure the hostname(s) accordingly and then it is possible to log in using the desired hostname.
If this message is encountered when a client attempted to access a forwarded service (Port forward, 1:1 NAT, relayd, etc) it indicates that the request did not match any NAT rules. From the inside of the network, this would require NAT reflection or split DNS to accomplish. See Why can't I access forwarded ports on my WAN IP from my LAN/OPTx networks for more details.