The Kerberos mechanism provides a number of environment variables to configure different behavior in order to meet applications' needs. Environment variables used within the Kerberos mechanism are:
[[<kt type>:]<file name>]
where <kt type> can be FILE or WRFILE. FILE is for read operations; WRFILE is for write operations. <file name> is the location of the keytab file.
If KRB5_KTNAME is not defined, the default value is:
The keytab file is used to store credentials persistently and is used commonly for service daemons.
Specifying the FILE type assumes that the subsequent operations on the associated file are readable by the invoking process. Care must be taken to ensure that the file is readable only by the set of principals that need to retrieve their unencrypted keys.
The WRFILE type is used by the kadmin(1M) command. Specifying this type allows the administrator to designate an alternate keytab file to write to without using extra command line arguments for file location.
[[<cc type>:]<file name>]
where <cc type> can be FILE or MEMORY. <file name> is the location of the principal's credential cache.
If KRB5CCNAME is not defined, the default value is:
where <uid> is the user id of the process that created the cache file.
The credential cache file is used to store tickets that have been granted to the principal.
Specifying the FILE types assumes that subsequent operations on the associated file are readable and writable by the invoking process. Care must be taken to ensure that the file is accessible only by the set of principals that need to access their credentials. If the credential file is in a directory to which other users have write access, you need to set that directory's sticky bit (see chmod(1)).
The MEMORY credential cache type is used only in special cases, such as when making a temporary cache for the life of the invoking process.
[[<rc type>:]<file name>]
where <rc type> can be either FILE, MEMORY, or NONE. <file name> is relevant only when specifying the replay cache file type.
If not defined, the default value is:
...if the process is owned by root, or:
...if the process is owned by a user other than root. <service> is the service process name associated with the replay cache file.
The replay cache is used by Kerberos to detect the replay of authentication data. This prevents people who capture authentication messages on the network from authenticating to the server by resending these messages.
When specifying the FILE replay cache type, care must be taken to prevent the replay cache file from being deleted by another user. Make sure that every directory in the replay cache path is either writable only by the owner of the replay cache or that the sticky bit ("t") is set on every directory in the replay cache path to which others have write permission.
When specifying the MEMORY replay cache type you need to weigh the trade-off of performance against the slight security risk created by using a non-persistent cache. The risk occurs during system reboots when the following condition obtains:
Under this condition, the server applications can accept a replay of Kerberos authentication data (up to the difference between the time of the last write and the clockskew). Typically, this is a small window of time. If the server applications take longer than the clockskew to start accepting connections there is no replay risk.
The risk described above is the same when using FILE replay cache types when the replay cache resides on swap file systems, such as /tmp and /var/run.
The performance improvement in MEMORY replay cache types over FILE types is derived from the absence of disk I/O. This is true even if the FILE replay cache is on a memory-backed file system, such as swap (/tmp and /var/run).
Note that MEMORY-type caches are per-process caches, therefore use of these types of caches must be carefully considered. One example of where MEMORY-type caches can be problematic is when an application uses more than one process for establishing security contexts. In such a case, memory replay caches are not shared across the processes, thus allowing potential for replay attacks.
See attributes(5) for a description of the following attributes:
chmod(1), kinit(1), klist(1), ksh(1), kadmin(1M), kadmind(1M), krb5.conf(4), attributes(5), kerberos(5)