In DNS, recursion references the process where a name server will make DNS queries to other name servers on behalf of client systems. Most name servers are simply DNS clients that cache information for a specified amount of time. Recursion is disabled by default on most name servers. In Mac OS X recursion is enabled for subnets local to the server only.
In environments where you wish to provide recursive queries you can enable recursion by opening Server Admin, clicking on the disclosure triangle for the server you will be configuring and then clicking on the DNS service. From here, click on the Settings icon in the Server Admin toolbar and then in the section for Accept recursive queries from the following networks you would click on the plus sign (+). In this field provide the IP address or netmask that you would like to enable recursion for. For example, if you’re enabling recursion for all computers on the 192.168.0.0 subnet and the subnet mask for those clients is 255.255.255.0 then you would enter:
192.168.0.0/24
This will allow recursion for those clients by updating the /etc/dns/options.conf.apple file. Alternatively you can edit the setting by hand yourself, but don’t do so using the /etc/dns/options.conf.apple file or you could introduce instability into the DNS service and Server Admin could overwrite your settings. Rather, edit the /etc/named.conf file. In named.conf add the following line in the options section:
allow-recursion {192.168.0.0/24;};
Overall, this is a fairly straight forward technical note, but there is an underlying theme that Apple is doing a really good job of leveraging an include methodology with regards to configuration files. Inside the /etc/named.conf, also in the options section, you’ll notice that there is a line that begins with include and specifies the path of the Server managed file, which uses the word apple at the end of it. This is mirrored in zone files as well. While not all open source services use this method for allowing different configurations in the GUI and the command line, I hope they all will at some point.
