Add S4U2Proxy feature:

http://sourceforge.net/mailarchive/forum.php?thread_name=4EE665D1.3000308%40redhat.com&forum_name=modauthkerb-help

The attached patches add support for using s4u2proxy 
(http://k5wiki.kerberos.org/wiki/Projects/Services4User) to allow the 
web service to obtain credentials on behalf of the authenticated user.

The first patch adds basic support for s4u2proxy. This requires the web 
administrator to manually create and manage the credentails cache for 
the apache user (via a cron job, for example).

The second patch builds on this and makes mod_auth_kerb manage the 
ccache instead.

These are patches against the current CVS HEAD (mod_auth_krb 5.4).

I've added a new module option to enable this support, 
KrbConstrainedDelegation. The default is off.

--- a/README
+++ b/README
@@ -122,4 +122,16 @@ KrbSaveCredentials, the tickets will be
 credential cache that will be available for the request handler. The ticket
 file will be removed after request is handled.
 
+Constrained Delegation
+----------------------
+S4U2Proxy, or constrained delegation, enables a service to use a client's
+ticket to itself to request another ticket for delegation. The KDC
+checks krbAllowedToDelegateTo to decide if it will issue a new ticket.
+If KrbConstrainedDelegation is enabled the server will use its own credentials
+to retrieve a delegated ticket for the user. For this to work the user must
+have a forwardable ticket (though the delegation flag need not be set).
+The server needs a valid credentials cache for this to work.
+
+The module itself will obtain and manage the necessary credentials.
+
 $Id: README,v 1.12 2008/09/17 14:01:55 baalberith Exp $
--- a/src/mod_auth_kerb.c
+++ b/src/mod_auth_kerb.c
@@ -42,6 +42,31 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+/*
+ * Locking mechanism inspired by mod_rewrite.
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * S4U2Proxy code
+ *
+ * Copyright (C) 2012  Red Hat
+ */
+
 #ident "$Id: mod_auth_kerb.c,v 1.150 2008/12/04 10:14:03 baalberith Exp $"
 
 #include "config.h"
@@ -49,6 +74,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdarg.h>
+#include <unixd.h>
 
 #define MODAUTHKERB_VERSION "5.4"
 
@@ -131,6 +157,12 @@ module AP_MODULE_DECLARE_DATA auth_kerb_
 module auth_kerb_module;
 #endif
 
+#ifdef STANDARD20_MODULE_STUFF
+/* s4u2proxy only supported in 2.0+ */
+static const char *lockname;
+static apr_global_mutex_t *s4u2proxy_lock = NULL;
+#endif
+
 /*************************************************************************** 
  Macros To Ease Compatibility
  ***************************************************************************/
@@ -165,6 +197,7 @@ typedef struct {
 	int krb_method_gssapi;
 	int krb_method_k5pass;
 	int krb5_do_auth_to_local;
+       int krb5_s4u2proxy;
 #endif
 #ifdef KRB4
 	char *krb_4_srvtab;
@@ -185,6 +218,11 @@ set_kerb_auth_headers(request_rec *r, co
 
 static const char*
 krb5_save_realms(cmd_parms *cmd, void *sec, const char *arg);
+static const char *
+cmd_delegationlock(cmd_parms *cmd, void *dconf, const char *a1);
+
+static int
+obtain_server_credentials(request_rec *r, const char *service_name);
 
 #ifdef STANDARD20_MODULE_STUFF
 #define command(name, func, var, type, usage)           \
@@ -237,6 +275,12 @@ static const command_rec kerb_auth_cmds[
 
    command("KrbLocalUserMapping", ap_set_flag_slot, krb5_do_auth_to_local,
      FLAG, "Set to 'on' to have Kerberos do auth_to_local mapping of principal names to system user names."),
+
+   command("KrbConstrainedDelegation", ap_set_flag_slot, krb5_s4u2proxy,
+     FLAG, "Set to 'on' to have Kerberos use S4U2Proxy delegation."),
+
+    AP_INIT_TAKE1("KrbConstrainedDelegationLock", cmd_delegationlock, NULL,
+     RSRC_CONF, "the filename of a lockfile used for inter-process synchronization"),
 #endif 
 
 #ifdef KRB4
@@ -302,6 +346,7 @@ static void *kerb_dir_create_config(MK_P
 #endif
 #ifdef KRB5
   ((kerb_auth_config *)rec)->krb5_do_auth_to_local = 0;
+	((kerb_auth_config *)rec)->krb5_s4u2proxy = 0;
 	((kerb_auth_config *)rec)->krb_method_k5pass = 1;
 	((kerb_auth_config *)rec)->krb_method_gssapi = 1;
 #endif
@@ -319,6 +364,24 @@ krb5_save_realms(cmd_parms *cmd, void *v
    return NULL;
 }
 
+static const char *
+cmd_delegationlock(cmd_parms *cmd, void *dconf, const char *a1)
+{
+    const char *error;
+
+    if ((error = ap_check_cmd_context(cmd, GLOBAL_ONLY)) != NULL)
+        return error;
+
+    /* fixup the path, especially for s4u2proxylock_remove() */
+    lockname = ap_server_root_relative(cmd->pool, a1);
+
+    if (!lockname) {
+        return apr_pstrcat(cmd->pool, "Invalid KrbConstrainedDelegationLock path ", a1, NULL);
+    }
+
+    return NULL;
+}
+
 static void
 log_rerror(const char *file, int line, int level, int status,
            const request_rec *r, const char *fmt, ...)
@@ -1170,6 +1233,7 @@ get_gss_creds(request_rec *r,
    gss_buffer_desc token = GSS_C_EMPTY_BUFFER;
    OM_uint32 major_status, minor_status, minor_status2;
    gss_name_t server_name = GSS_C_NO_NAME;
+   gss_cred_usage_t usage = GSS_C_ACCEPT;
    char buf[1024];
    int have_server_princ;
 
@@ -1212,10 +1276,14 @@ get_gss_creds(request_rec *r,
 
    log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r, "Acquiring creds for %s",
 	      token.value);
+   if (conf->krb5_s4u2proxy) {
+       usage = GSS_C_BOTH;
+       obtain_server_credentials(r, conf->krb_service_name);
+   }
    gss_release_buffer(&minor_status, &token);
    
    major_status = gss_acquire_cred(&minor_status, server_name, GSS_C_INDEFINITE,
-			           GSS_C_NO_OID_SET, GSS_C_ACCEPT,
+			           GSS_C_NO_OID_SET, usage,
 				   server_creds, NULL, NULL);
    gss_release_name(&minor_status2, &server_name);
    if (GSS_ERROR(major_status)) {
@@ -1257,6 +1325,302 @@ cmp_gss_type(gss_buffer_t token, gss_OID
 }
 #endif
 
+/* Renew the ticket if it will expire in under a minute */
+#define RENEWAL_TIME 60
+
+/*
+ * Services4U2Proxy lets a server prinicipal request another service
+ * principal on behalf of a user. To do this the Apache service needs
+ * to have its own ccache. This will ensure that the ccache has a valid
+ * principal and will initialize or renew new credentials when needed.
+ */
+
+static int
+verify_server_credentials(request_rec *r,
+                          krb5_context kcontext,
+                          krb5_ccache ccache,
+                          krb5_principal princ,
+                          int *renew
+)
+{
+    krb5_creds match_cred;
+    krb5_creds creds;
+    char * princ_name = NULL;
+    char *tgs_princ_name = NULL;
+    krb5_timestamp now;
+    krb5_error_code kerr = 0;
+
+    *renew = 0;
+
+    memset (&match_cred, 0, sizeof(match_cred));
+    memset (&creds, 0, sizeof(creds));
+
+    if (NULL == ccache || NULL == princ) {
+        /* Nothing to verify */
+        *renew = 1;
+        goto cleanup;
+    }
+
+    if ((kerr = krb5_unparse_name(kcontext, princ, &princ_name))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                   "Could not unparse principal %s (%d)",
+                   error_message(kerr), kerr);
+        goto cleanup;
+    }
+
+    log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+        "Using principal %s for s4u2proxy", princ_name);
+
+    tgs_princ_name = apr_psprintf(r->pool, "%s/%.*s@%.*s", KRB5_TGS_NAME,
+                                  krb5_princ_realm(kcontext, princ)->length,
+                                  krb5_princ_realm(kcontext, princ)->data,
+                                  krb5_princ_realm(kcontext, princ)->length,
+                                  krb5_princ_realm(kcontext, princ)->data);
+
+    if ((kerr = krb5_parse_name(kcontext, tgs_princ_name, &match_cred.server))) 
+    {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                        "Could not parse principal %s: %s (%d)",
+                        tgs_princ_name, error_message(kerr), kerr);
+        goto cleanup;
+    }
+
+    match_cred.client = princ;
+
+    if ((kerr = krb5_cc_retrieve_cred(kcontext, ccache, 0, &match_cred, &creds)))
+    {
+        krb5_unparse_name(kcontext, princ, &princ_name);
+        log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+                   "Could not unparse principal %s: %s (%d)",
+                   princ_name, error_message(kerr), kerr);
+        goto cleanup;
+    }
+
+    if ((kerr = krb5_timeofday(kcontext, &now))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                        "Could not get current time: %d (%s)",
+                        kerr, error_message(kerr));
+        goto cleanup;
+    }
+
+    if (now > (creds.times.endtime + RENEWAL_TIME)) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                   "Credentials for %s have expired or will soon "
+                   "expire - now %d endtime %d",
+                   princ_name, now, creds.times.endtime);
+        *renew = 1;
+    } else {
+        log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+                   "Credentials for %s will expire at "
+                   "%d, it is now %d", princ_name, creds.times.endtime, now);
+    }
+
+cleanup:
+    /* Closing context, ccache, etc happens elsewhere */
+    if (match_cred.server) {
+        krb5_free_principal(kcontext, match_cred.server);
+    }
+    if (creds.client) {
+        krb5_free_cred_contents(kcontext, &creds);
+    }
+
+    return kerr;
+}
+
+static int
+obtain_server_credentials(request_rec *r,
+                          const char *service_name)
+{
+    krb5_context kcontext = NULL;
+    krb5_keytab keytab = NULL;
+    krb5_ccache ccache = NULL;
+    char * princ_name = NULL;
+    char *tgs_princ_name = NULL;
+    krb5_error_code kerr = 0;
+    krb5_principal princ = NULL;
+    krb5_creds creds;
+    krb5_get_init_creds_opt gicopts;
+    int renew = 0;
+    apr_status_t rv = 0;
+
+    memset(&creds, 0, sizeof(creds));
+
+    if ((kerr = krb5_init_context(&kcontext))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+            "Kerberos context initialization failed: %s (%d)", error_message(kerr), kerr);
+        goto done;
+    }
+
+    if ((kerr = krb5_cc_default(kcontext, &ccache))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                        "Could not get default Kerberos ccache: %s (%d)",
+                        error_message(kerr), kerr);
+        goto done;
+    }
+
+    if ((kerr = krb5_cc_get_principal(kcontext, ccache, &princ))) {
+        char * name = NULL;
+
+        if ((asprintf(&name, "%s:%s", krb5_cc_get_type(kcontext, ccache),
+          krb5_cc_get_name(kcontext, ccache))) == -1) {
+            kerr = KRB5_CC_NOMEM;
+            goto done;
+        }
+
+        if (KRB5_FCC_NOFILE == kerr) {
+            log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+                       "Credentials cache %s not found, create one", name);
+            krb5_cc_close(kcontext, ccache);
+            ccache = NULL;
+            free(name);
+        } else {
+            log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                       "Failure to open credentials cache %s: %s (%d)",
+                       name, error_message(kerr), kerr);
+            free(name);
+            goto done;
+        }
+    }
+
+    kerr = verify_server_credentials(r, kcontext, ccache, princ, &renew);
+
+    if (kerr || !renew) {
+        goto done;
+    }
+
+#ifdef STANDARD20_MODULE_STUFF
+    if (s4u2proxy_lock) {
+        rv = apr_global_mutex_lock(s4u2proxy_lock);
+        if (rv != APR_SUCCESS) {
+            ap_log_rerror(APLOG_MARK, APLOG_ERR, rv, r,
+                          "apr_global_mutex_lock(s4u2proxy_lock) "
+                          "failed");
+        }
+    }
+#endif
+
+    /* We have the lock, check again to be sure another process hasn't already
+     * renewed the ticket.
+     */
+    kerr = verify_server_credentials(r, kcontext, ccache, princ, &renew);
+    if (kerr || !renew) {
+        goto unlock;
+    }
+
+    if (NULL == princ) {
+        if (strchr(service_name, '/') != NULL)
+           kerr = krb5_parse_name(kcontext, service_name, &princ);
+        else
+           kerr = krb5_sname_to_principal(kcontext, ap_get_server_name(r),
+                                         (service_name) ? service_name : SERVICE_NAME,
+                                         KRB5_NT_SRV_HST, &princ);
+
+        if (kerr) {
+            log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                       "Could not parse principal: %s (%d) ",
+                       error_message(kerr), kerr);
+            goto unlock;
+        }
+
+        if ((kerr = krb5_unparse_name(kcontext, princ, &princ_name))) {
+            log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+                       "Could not unparse principal %s: %s (%d)",
+                       princ_name, error_message(kerr), kerr);
+        }
+    } else if (NULL == princ_name) {
+        if ((kerr = krb5_unparse_name(kcontext, princ, &princ_name))) {
+            log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+                       "Could not unparse principal %s: %s (%d)",
+                       princ_name, error_message(kerr), kerr);
+            goto unlock;
+        }
+    }
+
+    if ((kerr = krb5_kt_default(kcontext, &keytab))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                   "Unable to get default keytab: %s (%d)",
+                   error_message(kerr), kerr);
+        goto unlock;
+    }
+
+    log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+               "Obtaining new credentials for %s", princ_name);
+    krb5_get_init_creds_opt_init(&gicopts);
+    krb5_get_init_creds_opt_set_forwardable(&gicopts, 1);
+
+    tgs_princ_name = apr_psprintf(r->pool, "%s/%.*s@%.*s", KRB5_TGS_NAME,
+                                  krb5_princ_realm(kcontext, princ)->length,
+                                  krb5_princ_realm(kcontext, princ)->data,
+                                  krb5_princ_realm(kcontext, princ)->length,
+                                  krb5_princ_realm(kcontext, princ)->data);
+
+    if ((kerr = krb5_get_init_creds_keytab(kcontext, &creds, princ, keytab,
+         0, tgs_princ_name, &gicopts))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                   "Failed to obtain credentials for principal %s: "
+                   "%s (%d)", princ_name, error_message(kerr), kerr);
+        goto unlock;
+    }
+
+    krb5_kt_close(kcontext, keytab);
+    keytab = NULL;
+
+    if (NULL == ccache) {
+        if ((kerr = krb5_cc_default(kcontext, &ccache))) {
+            log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                      "Failed to open default ccache: %s (%d)",
+                      error_message(kerr), kerr);
+            goto unlock;
+        }
+    }
+
+    if ((kerr = krb5_cc_initialize(kcontext, ccache, princ))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                   "Failed to initialize ccache for %s: %s (%d)",
+                   princ_name, error_message(kerr), kerr);
+        goto unlock;
+    }
+
+    if ((kerr = krb5_cc_store_cred(kcontext, ccache, &creds))) {
+        log_rerror(APLOG_MARK, APLOG_ERR, 0, r,
+                   "Failed to store %s in ccache: %s (%d)",
+                   princ_name, error_message(kerr), kerr);
+        goto unlock;
+    }
+
+unlock:
+#ifdef STANDARD20_MODULE_STUFF
+    if (s4u2proxy_lock) {
+        apr_global_mutex_unlock(s4u2proxy_lock);
+        if (rv != APR_SUCCESS) {
+            ap_log_rerror(APLOG_MARK, APLOG_ERR, rv, r,
+                          "apr_global_mutex_unlock(s4u2proxy_lock) "
+                          "failed");
+        }
+    }
+#endif
+
+done:
+    if (0 == kerr)
+        log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+                   "Done obtaining credentials for s4u2proxy");
+    else
+        log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r,
+                   "Failed to obtain credentials for s4u2proxy");
+
+    if (creds.client) {
+        krb5_free_cred_contents(kcontext, &creds);
+    }
+    if (ccache) {
+        krb5_cc_close(kcontext, ccache);
+    }
+    if (kcontext) {
+        krb5_free_context(kcontext);
+    }
+
+    return kerr;
+}
+
 static int
 authenticate_user_gss(request_rec *r, kerb_auth_config *conf,
 		      const char *auth_line, char **negotiate_ret_value)
@@ -1697,10 +2061,60 @@ have_rcache_type(const char *type)
 /*************************************************************************** 
  Module Setup/Configuration
  ***************************************************************************/
+#ifdef STANDARD20_MODULE_STUFF
+static apr_status_t
+s4u2proxylock_create(server_rec *s, apr_pool_t *p)
+{
+    apr_status_t rc;
+
+    /* only operate if a lockfile is used */
+    if (lockname == NULL || *(lockname) == '\0') {
+        return APR_SUCCESS;
+    }
+
+    /* create the lockfile */
+    rc = apr_global_mutex_create(&s4u2proxy_lock, lockname,
+                                 APR_LOCK_DEFAULT, p);
+    if (rc != APR_SUCCESS) {
+        ap_log_error(APLOG_MARK, APLOG_CRIT, rc, s,
+                     "Parent could not create lock file %s", lockname);
+        return rc;
+    }
+
+#ifdef AP_NEED_SET_MUTEX_PERMS
+    rc = ap_unixd_set_global_mutex_perms(s4u2proxy_lock);
+    if (rc != APR_SUCCESS) {
+        ap_log_error(APLOG_MARK, APLOG_CRIT, rc, s,
+                     "mod_auth_kerb: Parent could not set permissions "
+                     "on lock; check User and Group directives");
+        return rc;
+    }
+#endif
+
+    return APR_SUCCESS;
+}
+
+static apr_status_t
+s4u2proxylock_remove(void *unused)
+{
+    /* only operate if a lockfile is used */
+    if (lockname == NULL || *(lockname) == '\0') {
+        return APR_SUCCESS;
+    }
+
+    /* destroy the rewritelock */
+    apr_global_mutex_destroy(s4u2proxy_lock);
+    s4u2proxy_lock = NULL;
+    lockname = NULL;
+    return APR_SUCCESS;
+}
+#endif
+
 #ifndef STANDARD20_MODULE_STUFF
 static void
 kerb_module_init(server_rec *dummy, pool *p)
 {
+   apr_status_t status;
 #ifndef HEIMDAL
    /* Suppress the MIT replay cache.  Requires MIT Kerberos 1.4.0 or later.
       1.3.x are covered by the hack overiding the replay calls */
@@ -1741,6 +2155,7 @@ static int
 kerb_init_handler(apr_pool_t *p, apr_pool_t *plog,
       		  apr_pool_t *ptemp, server_rec *s)
 {
+   apr_status_t rv;
    ap_add_version_component(p, "mod_auth_kerb/" MODAUTHKERB_VERSION);
 #ifndef HEIMDAL
    /* Suppress the MIT replay cache.  Requires MIT Kerberos 1.4.0 or later.
@@ -1748,14 +2163,41 @@ kerb_init_handler(apr_pool_t *p, apr_poo
    if (getenv("KRB5RCACHETYPE") == NULL && have_rcache_type("none"))
       putenv(strdup("KRB5RCACHETYPE=none"));
 #endif
+#ifdef STANDARD20_MODULE_STUFF
+    rv = s4u2proxylock_create(s, p);
+    if (rv != APR_SUCCESS) {
+        return HTTP_INTERNAL_SERVER_ERROR;
+    }
+
+    apr_pool_cleanup_register(p, (void *)s, s4u2proxylock_remove,
+                              apr_pool_cleanup_null);
+#endif
    
    return OK;
 }
 
 static void
+initialize_child(apr_pool_t *p, server_rec *s)
+{
+    apr_status_t rv = 0;
+
+#ifdef STANDARD20_MODULE_STUFF
+    if (lockname != NULL && *(lockname) != '\0') {
+        rv = apr_global_mutex_child_init(&s4u2proxy_lock, lockname, p);
+        if (rv != APR_SUCCESS) {
+            ap_log_error(APLOG_MARK, APLOG_CRIT, rv, s,
+                         "mod_auth_kerb: could not init s4u2proxy_lock"
+                         " in child");
+        }
+    }
+#endif
+}
+
+static void
 kerb_register_hooks(apr_pool_t *p)
 {
    ap_hook_post_config(kerb_init_handler, NULL, NULL, APR_HOOK_MIDDLE);
+   ap_hook_child_init(initialize_child, NULL, NULL, APR_HOOK_MIDDLE);
    ap_hook_check_user_id(kerb_authenticate_user, NULL, NULL, APR_HOOK_MIDDLE);
 }