Specify the callback function to use in the server to inform the
client of the largest buffer the server is able to receive when
using the DIGEST-MD5 "auth-int" or "auth-conf" Quality of
Protection (qop). If this directive is missing, the default value
65536 will be assumed. The function can be later retrieved using
This function is part of the old callback interface.
The new interface uses gsasl_callback_set() to set the application
callback, and uses gsasl_callback() or gsasl_property_get() to
invoke the callback for certain properties.