backend/ipp.c: Fix infinite loop with Kerberos

If IP address was used with Kerberos, IPP backend retried connection
indefinitely. The fix is to abort when we find out hostname is an IP
address and we require Kerberos.

Fixes #838

Author: zdohnal

CHANGES.md

@@ -40,6 +40,7 @@ Changes in CUPS v2.5b1 (TBA)
 - Fixed memory leak when creating color profiles (Issue #814)
 - Fixed crash in `scan_ps()` if incoming argument is NULL (Issue #831)
 - Fixed setting job state reasons for successful jobs (Issue #832)
+- Fixed infinite loop in IPP backend if hostname is IP address with Kerberos (Issue #838)
 - Fixed crash in `ppdEmitString()` if there is no record for page size `Custom`
   (Issue #849)
 - Fixed reporting `media-source-supported` when sharing printer which has numbers as strings

backend/ipp.c

@@ -409,6 +409,19 @@ main(int  argc,				/* I - Number of command-line args */
   else
     cupsSetEncryption(HTTP_ENCRYPTION_IF_REQUESTED);
 
+  if (!strcmp(auth_info_required, "negotiate") &&
+      (isdigit(hostname[0] & 255) || hostname[0] == '['))
+  {
+   /*
+    * IP addresses are not allowed with Kerberos...
+    */
+
+    _cupsLangPrintFilter(stderr, "ERROR",
+			 _("IP address is not allowed as hostname when using Negotiate - use FQDN."));
+    update_reasons(NULL, "-connecting-to-device");
+    return (CUPS_BACKEND_FAILED);
+  }
+
  /*
   * See if there are any options...
   */