Detect and error loops in --cname configuration.

CHANGELOG

 version 2.77
+	    Generate an error when configured with a CNAME loop,
+	    rather than a crash. Thnaks to George Metz for
+	    spotting this problem.
+
 	    Calculate the length of TFTP error reply packet 
 	    correctly. This fixes a problem when the error 
 	    message in a TFTP packet exceeds the arbitrary 

src/dnsmasq.h

@@ -310,9 +310,9 @@ struct ptr_record {
 };
 
 struct cname {
-  int ttl;
+  int ttl, flag;
   char *alias, *target;
-  struct cname *next;
+  struct cname *next, *targetp;
 }; 
 
 struct ds_config {

src/option.c

@@ -4663,11 +4663,44 @@ void read_opts(int argc, char **argv, char *compile_opts)
 
   if (daemon->cnames)
     {
-      struct cname *cn;
+      struct cname *cn, *cn2, *cn3;
+
+#define NOLOOP 1
+#define TESTLOOP 2      
+
+      /* Fill in TTL for CNAMES noe we have local_ttl.
+	 Also prepare to do loop detection. */
+      for (cn = daemon->cnames; cn; cn = cn->next)
+	{
+	  if (cn->ttl == -1)
+	    cn->ttl = daemon->local_ttl;
+	  cn->flag = 0;
+	  cn->targetp = NULL;
+	  for (cn2 = daemon->cnames; cn2; cn2 = cn2->next)
+	    if (hostname_isequal(cn->target, cn2->alias))
+	      {
+		cn->targetp = cn2;
+		break;
+	      }
+	}
       
+      /* Find any CNAME loops.*/
       for (cn = daemon->cnames; cn; cn = cn->next)
-	if (cn->ttl == -1)
-	  cn->ttl = daemon->local_ttl;
+	{
+	  for (cn2 = cn->targetp; cn2; cn2 = cn2->targetp)
+	    {
+	      if (cn2->flag == NOLOOP)
+		break;
+	      
+	      if (cn2->flag == TESTLOOP)
+		die(_("CNAME loop involving %s"), cn->alias, EC_BADCONF);
+	      
+	      cn2->flag = TESTLOOP;
+	    }
+	  
+	  for (cn3 = cn->targetp; cn3 != cn2; cn3 = cn3->targetp)
+	    cn3->flag = NOLOOP;
+	}
     }
 
   if (daemon->if_addrs)