Munge daemon needs a secret key file /etc/munge/munge.key.

If not existing previously, the package generates a new key at
installation time. In order to minimize the generation time, the
generator does not use a random number generator with a very good
source of entropy. In order to generate stronger key with better
entropy, you are advised to run the following command as root:

  # create-munge-key -f -r

If the secret key file is created or copyed before the package is
installed, the installation script will accept the key instead of
generating a new one.

Then follow the instructions in QUICKSTART.

You need to securely propagate the secret key file (eg, via ssh) to
other hosts within the same security realm. Credentials are valid only
within a security realm (ie, on hosts sharing the same secret key). All
munged daemons exchanging credentials need to share the same secret key.
