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.

man/dnsmasq.8

@@ -136,6 +136,8 @@ Log the results of DNS queries handled by dnsmasq. Enable a full cache dump on r
 then the log has extra information at the start of each line.
 This consists of a serial number which ties together the log lines associated with an individual query, and the IP address of the requestor. If the argument "proto" is supplied, this shows everything that "extra" does and also the network protocol used to communicate the queries. Logging of only queries to the authoritative server can be configured with
 .B --log-queries=auth
+and logging only failed replies (Error, NXDOMAIN or NODATA) can be configured with
+.B --log-queries=only_failed
 .TP
 .B \-8, --log-facility=<facility>
 Set the facility to which dnsmasq will send syslog entries, this

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;

src/dnsmasq.h

@@ -289,7 +289,8 @@ struct event_desc {
 #define OPT_DO_0x20        75
 #define OPT_AUTH_LOG       76
 #define OPT_LEASEQUERY     77
-#define OPT_LAST           78
+#define OPT_LOG_ONLY_FAILED  78
+#define OPT_LAST           79
 
 #define OPTION_BITS (sizeof(unsigned int)*8)
 #define OPTION_SIZE ( (OPT_LAST/OPTION_BITS)+((OPT_LAST%OPTION_BITS)!=0) )

src/option.c

@@ -3463,6 +3463,8 @@ static int one_opt(int option, char *arg, char *errstr, char *gen_err, int comma
 	    }
 	  else if (strcmp(arg, "auth") == 0)
 	    set_option_bool(OPT_AUTH_LOG);
+	  else if (strcmp(arg, "only_failed") == 0)
+	    set_option_bool(OPT_LOG_ONLY_FAILED);
 	}
       break;