--devel=forwardMinMessages,2

require minimum number of messages for a plane needed to forward
messages for it
will not affect non-aircraft messages

net_io.c

@@ -5765,17 +5765,24 @@ static void outputMessage(struct modesMessage *mm) {
     if (Modes.filterDF && (mm->sbs_in || !(Modes.filterDFbitset & (1 << mm->msgtype)))) {
         return;
     }
+
+    struct aircraft *ac = mm->aircraft;
+
+    if (ac && ac->messages < Modes.net_forward_min_messages) {
+        return;
+    }
+
     int noforward = (mm->timestamp == MAGIC_NOFORWARD_TIMESTAMP) && !Modes.beast_forward_noforward;
     int64_t orig_ts = mm->timestamp;
     if (Modes.beast_set_noforward_timestamp) {
         mm->timestamp = MAGIC_NOFORWARD_TIMESTAMP;
     }
 
-    struct aircraft *ac = mm->aircraft;
-
     // Suppress the first message when using an SDR
     // messages with crc 0 have an explicit checksum and are more reliable, don't suppress them when there was no CRC fix performed
-    if (Modes.net && !mm->sbs_in && (Modes.net_only || Modes.net_verbatim || (mm->crc == 0 && mm->correctedbits == 0) || (ac && ac->messages > 1) || mm->msgtype == DFTYPE_MODEAC)) {
+    if (Modes.net && !mm->sbs_in
+            && (Modes.net_only || Modes.net_verbatim || (mm->crc == 0 && mm->correctedbits == 0) || (ac && ac->messages > 1) || mm->msgtype == DFTYPE_MODEAC)
+       ) {
         int is_mlat = (mm->source == SOURCE_MLAT);
 
         if (mm->jsonPositionOutputEmit && Modes.json_out.connections) {

readsb.c

@@ -1932,6 +1932,10 @@ static error_t parse_opt(int key, char *arg, struct argp_state *state) {
                 if (strcasecmp(token[0], "messageRateMult") == 0 && token[1]) {
                     Modes.messageRateMult = atof(token[1]);
                 }
+                if (strcasecmp(token[0], "forwardMinMessages") == 0 && token[1]) {
+                    Modes.net_forward_min_messages = atoi(token[1]);
+                    fprintf(stderr, "forwardMinMessages: %u\n", Modes.net_forward_min_messages);
+                }
                 if (strcasecmp(token[0], "incrementId") == 0) {
                     Modes.incrementId = 1;
                 }

readsb.h

@@ -728,6 +728,7 @@ struct _Modes
     uint64_t receiver_focus;
 
     uint32_t preambleThreshold;
+    uint32_t net_forward_min_messages;
     int net_output_flush_size; // Minimum Size of output data
     int32_t net_output_beast_reduce_interval; // Position update interval for data reduction
     int32_t ping_reduce;