Host configuration

All hosts running Kerberos software, whether they are clients, application servers, or KDCs, can be configured using krb5.conf. Here we describe some of the behavior changes you might want to make.

Plugin module configuration

Many aspects of Kerberos behavior, such as client preauthentication and KDC service location, can be modified through the use of plugin modules. For most of these behaviors, you can use the [plugins] section of krb5.conf to register third-party modules, and to switch off registered or built-in modules.

A plugin module takes the form of a Unix shared object (modname.so) or Windows DLL (modname.dll). If you have installed a third-party plugin module and want to register it, you do so using the module relation in the appropriate subsection of the [plugins] section. The value for module must give the module name and the path to the module, separated by a colon. The module name will often be the same as the shared object’s name, but in unusual cases (such as a shared object which implements multiple modules for the same interface) it might not be. For example, to register a client preauthentication module named mypreauth installed at /path/to/mypreauth.so, you could write:

[plugins]
    clpreauth = {
        module = mypreauth:/path/to/mypreauth.so
    }

Many of the pluggable behaviors in MIT krb5 contain built-in modules which can be switched off. You can disable a built-in module (or one you have registered) using the disable directive in the appropriate subsection of the [plugins] section. For example, to disable the use of .k5identity files to select credential caches, you could write:

[plugins]
    ccselect = {
        disable = k5identity
    }

If you want to disable multiple modules, specify the disable directive multiple times, giving one module to disable each time.

Alternatively, you can explicitly specify which modules you want to be enabled for that behavior using the enable_only directive. For example, to make kadmind check password quality using only a module you have registered, and no other mechanism, you could write:

[plugins]
    pwqual = {
        module = mymodule:/path/to/mymodule.so
        enable_only = mymodule
    }

Again, if you want to specify multiple modules, specify the enable_only directive multiple times, giving one module to enable each time.

Some Kerberos interfaces use different mechanisms to register plugin modules.

KDC location modules

For historical reasons, modules to control how KDC servers are located are registered simply by placing the shared object or DLL into the “libkrb5” subdirectory of the krb5 plugin directory, which defaults to LIBDIR/krb5/plugins. For example, Samba’s winbind krb5 locator plugin would be registered by placing its shared object in LIBDIR/krb5/plugins/libkrb5/winbind_krb5_locator.so.

GSSAPI mechanism modules

GSSAPI mechanism module are registered using the file /etc/gss/mech. Each line in this file has the form:

oid  pathname  [options]  <type>

Only the oid and pathname are required. oid is the object identifier of the GSSAPI mechanism to be registered. pathname is a path to the module shared object or DLL. options (if present) are options provided to the plugin module, surrounded in square brackets. type (if present) can be used to indicate a special type of module. Currently the only special module type is “interposer”, for a module designed to intercept calls to other mechanisms.

Configuration profile modules

A configuration profile module replaces the information source for krb5.conf itself. To use a profile module, begin krb5.conf with the line:

module PATHNAME:STRING

where PATHNAME is a path to the module shared object or DLL, and STRING is a string to provide to the module. The module will then take over, and the rest of krb5.conf will be ignored.