Treat empty values of KRB5CCNAME (and other variables which can't usefully
be set to empty values) as if they were not set.  Incomplete and needs more
thought about whether or not it's even correct.

diff -ur krb5-1.6.1/src/appl/bsd/krshd.c krb5-1.6.1/src/appl/bsd/krshd.c
--- krb5-1.6.1/src/appl/bsd/krshd.c	2006-10-15 03:50:16.000000000 -0400
+++ krb5-1.6.1/src/appl/bsd/krshd.c	2007-05-18 19:51:18.000000000 -0400
@@ -1438,7 +1438,7 @@
      * child's environment.  This can't really have
      * a fixed position because tz may or may not be set.
      */
-    if (getenv("KRB5CCNAME")) {
+    if (getenv("KRB5CCNAME") && getenv("KRB5CCNAME")[0]) {
 	int i;
 	char *buf2 = (char *)malloc(strlen(getenv("KRB5CCNAME"))
 			 		   +strlen("KRB5CCNAME=")+1);
diff -ur krb5-1.6.1/src/appl/bsd/login.c krb5-1.6.1/src/appl/bsd/login.c
--- krb5-1.6.1/src/appl/bsd/login.c	2006-08-08 15:26:40.000000000 -0400
+++ krb5-1.6.1/src/appl/bsd/login.c	2007-05-18 19:49:32.000000000 -0400
@@ -528,7 +528,7 @@
     login_get_kconf(kcontext);
 
     /* Set up the credential cache environment variable */
-    if (!getenv(KRB5_ENV_CCNAME)) {
+    if (!getenv(KRB5_ENV_CCNAME) || !getenv(KRB5_ENV_CCNAME)[0]) {
 	sprintf(ccfile, "FILE:/tmp/krb5cc_p%ld", (long) getpid());
 	setenv(KRB5_ENV_CCNAME, ccfile, 1);
 	krb5_cc_set_default_name(kcontext, ccfile);
@@ -1763,7 +1763,7 @@
 	setenv ("TZ", tz, 1);
 #endif
 
-    if (ccname)
+    if (ccname && ccname[0])
 	setenv("KRB5CCNAME", ccname, 1);
 
     setenv("HOME", pwd->pw_dir, 1);
diff -ur krb5-1.6.1/src/appl/bsd/v4rcp.c krb5-1.6.1/src/appl/bsd/v4rcp.c
--- krb5-1.6.1/src/appl/bsd/v4rcp.c	2006-08-08 15:26:40.000000000 -0400
+++ krb5-1.6.1/src/appl/bsd/v4rcp.c	2007-05-18 19:50:40.000000000 -0400
@@ -1060,7 +1060,7 @@
 	  fprintf(stderr, "v4rcp: couldn't get local address (KRB5LOCALADDR)\n");
 	  exit(1);
 	}
-	if ((envaddr = getenv("KRB5REMOTEADDR"))) {
+	if ((envaddr = getenv("KRB5REMOTEADDR")) && envaddr[0]) {
 #ifdef HAVE_INET_ATON
 	  inet_aton(envaddr,  &foreign.sin_addr);
 #else
@@ -1068,7 +1068,7 @@
 #endif
 	  foreign.sin_family = AF_INET;
 	  envaddr = getenv("KRB5REMOTEPORT");
-	  if (envaddr)
+	  if (envaddr && envaddr[0])
 	    foreign.sin_port = htons(atoi(envaddr));
 	  else
 	    foreign.sin_port = 0;
diff -ur krb5-1.6.1/src/appl/telnet/libtelnet/kerberos5.c krb5-1.6.1/src/appl/telnet/libtelnet/kerberos5.c
--- krb5-1.6.1/src/appl/telnet/libtelnet/kerberos5.c	2006-12-01 16:27:28.000000000 -0500
+++ krb5-1.6.1/src/appl/telnet/libtelnet/kerberos5.c	2007-05-18 19:48:51.000000000 -0400
@@ -205,7 +205,7 @@
 	return;
 
     ccname = getenv("KRB5CCNAME");
-    if (ccname) {
+    if (ccname && ccname[0]) {
 	retval = krb5_cc_resolve(telnet_context, ccname, &ccache);
 	if (!retval)
 	    retval = krb5_cc_destroy(telnet_context, ccache);
diff -ur krb5-1.6.1/src/lib/kadm5/alt_prof.c krb5-1.6.1/src/lib/kadm5/alt_prof.c
--- krb5-1.6.1/src/lib/kadm5/alt_prof.c	2006-05-15 21:45:00.000000000 -0400
+++ krb5-1.6.1/src/lib/kadm5/alt_prof.c	2007-05-18 19:17:53.000000000 -0400
@@ -82,7 +82,8 @@
     if (i > 0)
 	krb5_config_len--;
     if (envname == NULL
-	|| (kdc_config = getenv(envname)) == NULL)
+	|| (kdc_config = getenv(envname)) == NULL
+	|| (kdc_config[0] == '\0'))
 	kdc_config = fname;
     if (kdc_config == NULL)
 	kdc_config_len = 0;
@@ -494,7 +495,8 @@
 	       !krb5_aprof_get_string(aprofile, hierarchy, TRUE, &svalue)) {
 	 params.mask |= KADM5_CONFIG_ADMIN_KEYTAB;
 	 params.admin_keytab = svalue;
-    } else if ((params.admin_keytab = (char *) getenv("KRB5_KTNAME"))) {
+    } else if ((params.admin_keytab = (char *) getenv("KRB5_KTNAME")) &&
+	       (params.admin_keytab[0] != '\0')) {
 	 params.admin_keytab = strdup(params.admin_keytab);
 	 if (params.admin_keytab)
 	      params.mask |= KADM5_CONFIG_ADMIN_KEYTAB;
diff -ur krb5-1.6.1/src/lib/krb4/g_cnffile.c krb5-1.6.1/src/lib/krb4/g_cnffile.c
--- krb5-1.6.1/src/lib/krb4/g_cnffile.c	2003-06-06 10:44:33.000000000 -0400
+++ krb5-1.6.1/src/lib/krb4/g_cnffile.c	2007-05-18 19:57:48.000000000 -0400
@@ -89,7 +89,7 @@
 
 	/* standard V4 override first */
 	s = getenv("KRB_CONF");
-	if (s) cnffile = fopen(s,"r");
+	if (s && s[0]) cnffile = fopen(s,"r");
 	/* if that's wrong, use V5 config */
 	if (!cnffile) cnffile = krb__v5_get_file("krb4_config");
 	/* and if V5 config doesn't have it, go to hard-coded values */
@@ -109,7 +109,7 @@
 
 	/* standard (not really) V4 override first */
 	s = getenv("KRB_REALMS");
-	if (s) realmsfile = fopen(s,"r");
+	if (s && s[0]) realmsfile = fopen(s,"r");
 	if (!realmsfile) realmsfile = krb__v5_get_file("krb4_realms");
 	if (!realmsfile) realmsfile = fopen(KRB_RLM_TRANS, "r");
 
diff -ur krb5-1.6.1/src/lib/krb4/tkt_string.c krb5-1.6.1/src/lib/krb4/tkt_string.c
--- krb5-1.6.1/src/lib/krb4/tkt_string.c	2006-03-11 17:23:28.000000000 -0500
+++ krb5-1.6.1/src/lib/krb4/tkt_string.c	2007-05-18 19:57:57.000000000 -0400
@@ -67,7 +67,7 @@
 
     if (!*krb_ticket_string) {
 	env = getenv("KRBTKFILE");
-        if (env) {
+        if (env && env[0]) {
 	    (void) strncpy(krb_ticket_string, env,
 			   sizeof(krb_ticket_string)-1);
 	    krb_ticket_string[sizeof(krb_ticket_string)-1] = '\0';
diff -ur krb5-1.6.1/src/lib/krb5/ccache/cccursor.c krb5-1.6.1/src/lib/krb5/ccache/cccursor.c
--- krb5-1.6.1/src/lib/krb5/ccache/cccursor.c	2006-11-07 15:18:31.000000000 -0500
+++ krb5-1.6.1/src/lib/krb5/ccache/cccursor.c	2007-05-18 19:19:11.000000000 -0400
@@ -136,7 +136,7 @@
 	/* fall through */
     case CCCURSOR_ENV:
 	name = getenv(KRB5_ENV_CCNAME);
-	if (name != NULL) {
+	if ((name != NULL) && (name[0] != '\0')) {
 	    cursor->pos = CCCURSOR_OS;
 	    ret = cccol_do_resolve(context, cursor, name, ccache);
 	    if (ret)
diff -ur krb5-1.6.1/src/lib/krb5/os/ccdefname.c krb5-1.6.1/src/lib/krb5/os/ccdefname.c
--- krb5-1.6.1/src/lib/krb5/os/ccdefname.c	2007-03-29 20:36:20.000000000 -0400
+++ krb5-1.6.1/src/lib/krb5/os/ccdefname.c	2007-05-18 18:32:20.000000000 -0400
@@ -292,7 +292,7 @@
             /* try the environment variable first */
             new_ccname = getenv(KRB5_ENV_CCNAME);
             
-            if (new_ccname == NULL) {
+            if ((new_ccname == NULL) || (new_ccname[0] == '\0')) {
                 /* fall back on the default ccache name for the OS */
                 new_ccname = new_ccbuf;
                 err = get_from_os (new_ccbuf, sizeof (new_ccbuf));
diff -ur krb5-1.6.1/src/lib/krb5/os/init_os_ctx.c krb5-1.6.1/src/lib/krb5/os/init_os_ctx.c
--- krb5-1.6.1/src/lib/krb5/os/init_os_ctx.c	2007-04-10 17:51:46.000000000 -0400
+++ krb5-1.6.1/src/lib/krb5/os/init_os_ctx.c	2007-05-18 18:32:20.000000000 -0400
@@ -196,7 +196,7 @@
     if (!secure)
     {
         char *env = getenv("KRB5_CONFIG");
-        if (env)
+        if (env && env[0])
         {
             name = malloc(strlen(env) + 1);
             if (!name) return ENOMEM;
@@ -250,7 +250,7 @@
 	filepath = DEFAULT_SECURE_PROFILE_PATH;
     } else { 
         filepath = getenv("KRB5_CONFIG");
-        if (!filepath) filepath = DEFAULT_PROFILE_PATH;
+        if (!filepath || !(filepath[0])) filepath = DEFAULT_PROFILE_PATH;
     }
 
     /* count the distinct filename components */
@@ -297,7 +297,7 @@
     profile_filespec_t *newfiles;
 
     file = getenv(KDC_PROFILE_ENV);
-    if (file == NULL)
+    if ((file == NULL) || (file[0] == '\0'))
 	file = DEFAULT_KDC_PROFILE;
 
     for (count = 0; (*pfiles)[count]; count++)
diff -ur krb5-1.6.1/src/lib/krb5/os/ktdefname.c krb5-1.6.1/src/lib/krb5/os/ktdefname.c
--- krb5-1.6.1/src/lib/krb5/os/ktdefname.c	2002-09-03 15:29:37.000000000 -0400
+++ krb5-1.6.1/src/lib/krb5/os/ktdefname.c	2007-05-18 19:19:28.000000000 -0400
@@ -47,7 +47,7 @@
 	    return KRB5_CONFIG_NOTENUFSPACE;
 	strcpy(name, krb5_overridekeyname);
     } else if ((context->profile_secure == FALSE) &&
-	(cp = getenv("KRB5_KTNAME"))) {
+	(cp = getenv("KRB5_KTNAME")) && (cp[0] != '\0')) {
 	if ((size_t) namesize < (strlen(cp)+1))
 	    return KRB5_CONFIG_NOTENUFSPACE;
 	strcpy(name, cp);
diff -ur krb5-1.6.1/src/lib/krb5/rcache/rc_base.c krb5-1.6.1/src/lib/krb5/rcache/rc_base.c
--- krb5-1.6.1/src/lib/krb5/rcache/rc_base.c	2006-06-08 16:01:44.000000000 -0400
+++ krb5-1.6.1/src/lib/krb5/rcache/rc_base.c	2007-05-18 19:18:48.000000000 -0400
@@ -94,7 +94,7 @@
 char * krb5_rc_default_type(krb5_context context)
 {
     char *s;
-    if ((s = getenv("KRB5RCACHETYPE")))
+    if ((s = getenv("KRB5RCACHETYPE")) && (s[0] != '\0'))
 	return s;
     else
 	return "dfl";
@@ -103,7 +103,7 @@
 char * krb5_rc_default_name(krb5_context context)
 {
     char *s;
-    if ((s = getenv("KRB5RCACHENAME")))
+    if ((s = getenv("KRB5RCACHENAME")) && (s[0] != '\0'))
 	return s;
     else
 	return (char *) 0;
diff -ur krb5-1.6.1/src/lib/krb5/rcache/rc_io.c krb5-1.6.1/src/lib/krb5/rcache/rc_io.c
--- krb5-1.6.1/src/lib/krb5/rcache/rc_io.c	2006-12-18 18:11:28.000000000 -0500
+++ krb5-1.6.1/src/lib/krb5/rcache/rc_io.c	2007-05-18 18:32:20.000000000 -0400
@@ -47,7 +47,7 @@
 {
     char *dir;
 
-    if (!(dir = getenv("KRB5RCACHEDIR"))) {
+    if (!(dir = getenv("KRB5RCACHEDIR")) || !dir[0]) {
 #if defined(_WIN32)
 	if (!(dir = getenv("TEMP")))
 	    if (!(dir = getenv("TMP")))