参考：http://docs.mongodb.org/manual/tutorial/generate-key-file/
Generate a Key File
Overview
This section describes how to generate a key file to store authentication information. After generating a key file, specify the key file using the keyFile option when starting a mongod or mongos instance.
A key’s length must be between 6 and 1024 characters and may only contain characters in the base64 set. The key file must not have group or world permissions on UNIX systems. Key file permissions are not checked on Windows systems.
1、Create a key file.
Create the key file your deployment will use to authenticate servers to each other.
To generate pseudo-random data to use for a keyfile, issue the following openssl command:
openssl rand -base64 741 > mongodb-keyfile
chmod 600 mongodb-keyfile
chown mongod.mongod mongodb-keyfile
You may generate a key file using any method you choose. Always ensure that the password stored in the key file is both long and contains a high amount of entropy. Using openssl in this manner helps generate such a key.
2、Specify the key file when starting a MongoDB instance.
Specify the path to the key file with the keyFile option.