fallback_realm
krb5_hostrealm_fallback_realm_fn fallback_realm;
if (h->vt.fallback_realm == NULL)
return h->vt.fallback_realm(context, h->data, host, realms_out);
ret = fallback_realm(context, *hp, cleanname, &realms);
vt->fallback_realm = dns_fallback_realm;
vt->fallback_realm = domain_fallback_realm;
vt->fallback_realm = split_comps;