MIT Kerberos Documentation

General plugin concepts

A krb5 dynamic plugin module is a Unix shared object or Windows DLL. Typically, the source code for a dynamic plugin module should live in its own project with a build system using automake and libtool, or tools with similar functionality.

A plugin module must define a specific symbol name, which depends on the pluggable interface and module name. For most pluggable interfaces, the exported symbol is a function named INTERFACE_MODULE_initvt, where INTERFACE is the name of the pluggable interface and MODULE is the name of the module. For these interfaces, it is possible for one shared object or DLL to implement multiple plugin modules, either for the same pluggable interface or for different ones. For example, a shared object could implement both KDC and client preauthentication mechanisms, by exporting functions named kdcpreauth_mymech_initvt and clpreauth_mymech_initvt.

A plugin module implementation should include the header file <krb5/INTERFACE_plugin.h>, where INTERFACE is the name of the pluggable interface. For instance, a ccselect plugin module implementation should use #include <krb5/ccselect_plugin.h>.

initvt functions have the following prototype:

krb5_error_code interface_modname_initvt(krb5_context context,
                                         int maj_ver, int min_ver,
                                         krb5_plugin_vtable vtable);

and should do the following:

  1. Check that the supplied maj_ver argument is supported by the module. If it is not supported, the function should return KRB5_PLUGIN_VER_NOTSUPP.
  2. Cast the supplied vtable pointer to the structure type corresponding to the major version, as documented in the pluggable interface header file.
  3. Fill in the structure fields with pointers to method functions and static data, stopping at the field indicated by the supplied minor version. Fields for unimplemented optional methods can be left alone; it is not necessary to initialize them to NULL.

In most cases, the context argument will not be used. The initvt function should not allocate memory; think of it as a glorified structure initializer. Each pluggable interface defines methods for allocating and freeing module state if doing so is necessary for the interface.

Pluggable interfaces typically include a name field in the vtable structure, which should be filled in with a pointer to a string literal containing the module name.

Here is an example of what an initvt function might look like for a fictional pluggable interface named fences, for a module named “wicker”:

fences_wicker_initvt(krb5_context context, int maj_ver,
                     int min_ver, krb5_plugin_vtable vtable)
    krb5_ccselect_vtable vt;

    if (maj_ver == 1) {
        krb5_fences_vtable vt = (krb5_fences_vtable)vtable;
        vt->name = "wicker";
        vt->slats = wicker_slats;
        vt->braces = wicker_braces;
    } else if (maj_ver == 2) {
        krb5_fences_vtable_v2 vt = (krb5_fences_vtable_v2)vtable;
        vt->name = "wicker";
        vt->material = wicker_material;
        vt->construction = wicker_construction;
        if (min_ver < 2)
            return 0;
        vt->footing = wicker_footing;
        if (min_ver < 3)
            return 0;
        vt->appearance = wicker_appearance;
    } else {
        return KRB5_PLUGIN_VER_NOTSUPP;
    return 0;

Logging from KDC and kadmind plugin modules

Plugin modules for the KDC or kadmind daemons can write to the configured logging outputs (see [logging]) by calling the com_err function. The first argument (whoami) is ignored. If the second argument (code) is zero, the formatted message is logged at informational severity; otherwise, the formatted message is logged at error severity and includes the error message for the supplied code. Here are examples:

com_err("", 0, "Client message contains %d items", nitems);
com_err("", retval, "while decoding client message");

(The behavior described above is new in release 1.17. In prior releases, the whoami argument is included for some logging output types, the logged message does not include the usual header for some output types, and the severity for syslog outputs is configured as part of the logging specification, defaulting to error severity.)