KRB5ENVVAR(7) Standards, Environments, and Macros KRB5ENVVAR(7)

NAME

krb5envvar - Kerberos environment variables

DESCRIPTION

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:

KRB5_KTNAME

Used by the mechanism to specify the location of the key table
file. The variable can be set to the following value:

[[<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.

r

If KRB5_KTNAME is not defined, the default value is:

FILE:/etc/krb5/krb5.keytab

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(8) 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.


KRB5CCNAME

Used by the mechanism to specify the location of the credential
cache. The variable can be set to the following value:

[[<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:

FILE:/tmp/krb5cc_<uid>

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.


KRB5RCNAME

Used by the mechanism to specify the type and location of the
replay cache. The variable can be set to the following value:

[[<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:

FILE:/var/krb5/rcache/root/rc_<service>

...if the process is owned by root, or:

FILE:/var/krb5/rcache/rc_<service>

...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:

o The duration from the last write to the replay cache
before reboot to the point when the Kerberized server
applications are running is less than the Kerberos
clockskew (see krb5.conf(5)).
When specifying the NONE replay cache time you need to understand
that this disables the replay cache, and all security risks that
this presents. This includes all the risks outlined in this
section of the man page.

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.


KRB5_CONFIG

Allows you to change the default location of the
/etc/krb5/krb5.conf file to enable the Kerberos library code to
read configuration parameters from another file specified by
KRB5_CONFIG. For example (using kinit from ksh(1)):

KRB5_CONFIG=/var/tmp/krb5.conf kinit

ATTRIBUTES

See attributes(7) for a description of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Uncommitted |
+--------------------+-----------------+

SEE ALSO

chmod(1), kinit(1), klist(1), ksh(1), krb5.conf(5), attributes(7),
kerberos(7), kadmin(8), kadmind(8)

February 13, 2008 KRB5ENVVAR(7)