The only_failed argument has been added to the log-queries parameter.

This may be useful for embedded systems for example:
log-queries without only_failed places a heavy load on the
NAND flash memory. At the same time, logging requests
is necessary to troubleshoot network issues.

src/cache.c

@@ -2254,6 +2254,15 @@ static char *edestr(int ede)
     }
 }
 
+static int error_occured(unsigned int flags) {
+  if (flags & F_RCODE)
+    return 1;
+  else if (flags & F_NEG)
+    return 1;
+  else
+    return 0;
+}
+
 void log_query(unsigned int flags, char *name, union all_addr *addr, char *arg, unsigned short type)
 {
   char *source, *dest;
@@ -2266,6 +2275,9 @@ void log_query(unsigned int flags, char *name, union all_addr *addr, char *arg,
   if (!option_bool(OPT_LOG))
     return;
 
+  if(option_bool(OPT_LOG_ONLY_FAILED) && !error_occured(flags))
+    return;
+
   /* F_NOERR is reused here to indicate logs arrising from auth queries */ 
   if (!(flags & F_NOERR) && option_bool(OPT_AUTH_LOG))
     return;