Try harder to make keytab-based AS requests work

When making a keytab-based AS request, a client has to choose between
sending its reply key enctype preference list (the enctypes it has in
the keytab) and its session key enctype preference list (all of the
enctypes it supports).  Heimdal and MIT krb5 1.11 clients send the
reply key preference list.  If this list doesn't overlap with the
server principal keys (say, because the krbtgt principal has only a
DES key), then the AS request will fail.

Try to make this work by making the KDC optimistically pick the first
permitted enctype in the request as the session key, even though it
can't be certain that other KDCs in the realm support that enctype.

Make sure to exercise this case in t_keytab.py by doing a multipass
keytab kinit test.

ticket: 7190 (new)

diff --git a/src/kdc/kdc_util.c b/src/kdc/kdc_util.c
index 3c2169e0df322c9ae7bd13ebf0cc9e47b93f994d..48947c64822376e3310b8d5b80ab67c0e9ae6b87 100644 (file)
--- a/src/kdc/kdc_util.c
+++ b/src/kdc/kdc_util.c
@@ -1517,7 +1517,7 @@ validate_tgs_request(register krb5_kdc_req *request, krb5_db_entry server,
 }
 
 /* Return true if we believe server can support enctype as a session key. */
-krb5_boolean
+static krb5_boolean
 dbentry_supports_enctype(krb5_context context, krb5_db_entry *server,
                          krb5_enctype enctype)
 {
@@ -1571,6 +1571,7 @@ select_session_keytype(krb5_context context, krb5_db_entry *server,
                        int nktypes, krb5_enctype *ktype)
 {
     int         i;
+    krb5_enctype first_permitted = 0;
 
     for (i = 0; i < nktypes; i++) {
         if (!krb5_c_valid_enctype(ktype[i]))
@@ -1579,9 +1580,23 @@ select_session_keytype(krb5_context context, krb5_db_entry *server,
         if (!krb5_is_permitted_enctype(context, ktype[i]))
             continue;
 
+        if (first_permitted == 0)
+            first_permitted = ktype[i];
+
         if (dbentry_supports_enctype(context, server, ktype[i]))
             return ktype[i];
     }
+
+    /*
+     * If we didn't find a match and the server is the local TGS server, this
+     * could be a keytab-based AS request where the keytab enctypes don't
+     * overlap the TGT principal enctypes.  Try to make this work by using the
+     * first permitted enctype in the request, even though we can't be certain
+     * that other KDCs in the realm support it.
+     */
+    if (krb5_principal_compare(context, server->princ, tgs_server))
+        return first_permitted;
+
     return 0;
 }
 

diff --git a/src/tests/t_keytab.py b/src/tests/t_keytab.py
index f56c7bba3535a095c26e92e418dc8a94c799d1f7..ef303f197571a8678c7950f767f05370deab2038 100644 (file)
--- a/src/tests/t_keytab.py
+++ b/src/tests/t_keytab.py
@@ -1,10 +1,11 @@
 #!/usr/bin/python
 from k5test import *
 
-realm = K5Realm()
+for realm in multipass_realms(create_user=False):
+    # Test kinit with a keytab.
+    realm.kinit(realm.host_princ, flags=['-k'])
 
-# Test kinit with a keytab.
-realm.kinit(realm.host_princ, flags=['-k'])
+realm = K5Realm(get_creds=False)
 
 # Test kinit with a partial keytab.
 pkeytab = realm.keytab + '.partial'