Try harder to avoid password change replay errors

Commit d7b3018 (ticket 7905) changed
change_set_password() to prefer TCP.  However, because UDP_LAST falls
back to UDP after one second, we can still get a replay error due to a
dropped packet, before the TCP layer has a chance to retry.

Instead, try k5_sendto() with NO_UDP, and only fall back to UDP after
TCP fails completely without reaching a server.  In sendto_kdc.c,
implement an ONLY_UDP transport strategy to allow the UDP fallback.

ticket: 9037

src/lib/krb5/os/changepw.c

@@ -255,9 +255,16 @@ change_set_password(krb5_context context,
     callback_info.pfn_cleanup = kpasswd_sendto_msg_cleanup;
     krb5_free_data_contents(callback_ctx.context, &chpw_rep);
 
+    /* UDP retransmits may be seen as replays.  Only try UDP after other
+     * transports fail completely. */
     code = k5_sendto(callback_ctx.context, NULL, &creds->server->realm,
-                     &sl, UDP_LAST, &callback_info, &chpw_rep,
+                     &sl, NO_UDP, &callback_info, &chpw_rep,
                      ss2sa(&remote_addr), &addrlen, NULL, NULL, NULL);
+    if (code == KRB5_KDC_UNREACH) {
+        code = k5_sendto(callback_ctx.context, NULL, &creds->server->realm,
+                         &sl, ONLY_UDP, &callback_info, &chpw_rep,
+                         ss2sa(&remote_addr), &addrlen, NULL, NULL, NULL);
+    }
     if (code)
         goto cleanup;
 

src/lib/krb5/os/os-proto.h

@@ -49,6 +49,7 @@ typedef enum {
     UDP_FIRST = 0,
     UDP_LAST,
     NO_UDP,
+    ONLY_UDP
 } k5_transport_strategy;
 
 /* A single server hostname or address. */

src/lib/krb5/os/sendto_kdc.c

@@ -804,11 +804,14 @@ resolve_server(krb5_context context, const krb5_data *realm,
     int err, result;
     char portbuf[PORT_LENGTH];
 
-    /* Skip UDP entries if we don't want UDP. */
+    /* Skip entries excluded by the strategy. */
     if (strategy == NO_UDP && entry->transport == UDP)
         return 0;
+    if (strategy == ONLY_UDP && entry->transport != UDP &&
+        entry->transport != TCP_OR_UDP)
+        return 0;
 
-    transport = (strategy == UDP_FIRST) ? UDP : TCP;
+    transport = (strategy == UDP_FIRST || strategy == ONLY_UDP) ? UDP : TCP;
     if (entry->hostname == NULL) {
         /* Added by a module, so transport is either TCP or UDP. */
         ai.ai_socktype = socktype_for_transport(entry->transport);
@@ -852,8 +855,9 @@ resolve_server(krb5_context context, const krb5_data *realm,
     }
 
     /* For TCP_OR_UDP entries, add each address again with the non-preferred
-     * transport, unless we are avoiding UDP.  Flag these as deferred. */
-    if (retval == 0 && entry->transport == TCP_OR_UDP && strategy != NO_UDP) {
+     * transport, if there is one.  Flag these as deferred. */
+    if (retval == 0 && entry->transport == TCP_OR_UDP &&
+        (strategy == UDP_FIRST || strategy == UDP_LAST)) {
         transport = (strategy == UDP_FIRST) ? TCP : UDP;
         for (a = addrs; a != 0 && retval == 0; a = a->ai_next) {
             a->ai_socktype = socktype_for_transport(transport);