rndc.conf is the configuration file for rndc, the BIND 9 name server control utility. This file has a similar structure and syntax to named.conf. Statements are enclosed in braces and terminated with a semi-colon. Clauses in the statements are also semi-colon terminated. The usual comment styles are supported:

C style: /* */

C++ style: // to end of line

Unix style: # to end of line

rndc.conf is much simpler than named.conf. The file uses three statements: an options statement, a server statement and a key statement.

The options statement contains three clauses. The default-server clause is followed by the name or address of a name server. This host will be used when no name server is given as an argument to rndc. The default-key clause is followed by the name of a key which is identified by a key statement. If no keyid is provided on the rndc command line, and no key clause is found in a matching server statement, this default key will be used to authenticate the server's commands and responses. The default-port clause is followed by the port to connect to on the remote name server. If no port option is provided on the rndc command line, and no port clause is found in a matching server statement, this default port will be used to connect.

After the server keyword, the server statement includes a string which is the hostname or address for a name server. The statement has two possible clauses: key and port. The key name must match the name of a key statement in the file. The port number specifies the port to connect to.

The key statement begins with an identifying string, the name of the key. The statement has two clauses. algorithm identifies the encryption algorithm for rndc to use; currently only HMAC-MD5 is supported. This is followed by a secret clause which contains the base-64 encoding of the algorithm's encryption key. The base-64 string is enclosed in double quotes.

There are two common ways to generate the base-64 string for the secret. The BIND 9 program dnssec-keygen can be used to generate a random key, or the mmencode program, also known as mimencode, can be used to generate a base-64 string from known input. mmencode does not ship with BIND 9 but is available on many systems. See the EXAMPLE section for sample command lines for each.

 options {
 default-server localhost;
 default-key samplekey;
 };

 server localhost {
 key samplekey;
 };

 key samplekey {
 algorithm hmac-md5;
 secret "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
 };
 

In the above example, rndc will by default use the server at localhost (127.0.0.1) and the key called samplekey. Commands to the localhost server will use the samplekey key, which must also be defined in the server's configuration file with the same name and secret. The key statement indicates that samplekey uses the HMAC-MD5 algorithm and its secret clause contains the base-64 encoding of the HMAC-MD5 secret enclosed in double quotes.

To generate a random secret with dnssec-keygen:

$ dnssec-keygen -a hmac-md5 -b 128 -n user rndc

The base-64 string will appear in two files, Krndc.+157.+{random}.key and Krndc.+157.+{random}.private. After extracting the key to be placed in the rndc.conf and named.conf key statements, the .key and .private files can be removed.

To generate a random secret with mmencode:

$ echo "known plaintext for a secret" | mmencode

The name server must be configured to accept rndc connections and to recognize the key specified in the rndc.conf file, using the controls statement in named.conf. See the sections on the controls statement in the BIND 9 Administrator Reference Manual for details.

rndc(8), dnssec-keygen(8), mmencode(1), BIND 9 Administrator Reference Manual.

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