Add --net-verbatim, which forwards the original uncorrected message to net clients,

not the corrected version. Then the client can make its own policy decisions about
whether to accept and correct the damaged messages.

dump1090.c

@@ -514,6 +514,7 @@ void showHelp(void) {
 "--net-ro-interval <rate> TCP output memory flush rate in seconds (default: 0)\n"
 "--net-heartbeat <rate>   TCP heartbeat rate in seconds (default: 60 sec; 0 to disable)\n"
 "--net-buffer <n>         TCP buffer size 64Kb * (2^n) (default: n=0, 64Kb)\n"
+"--net-verbatim           Do not apply CRC corrections to messages we forward; send unchanged\n"
 "--lat <latitude>         Reference/receiver latitude for surface posn (opt)\n"
 "--lon <longitude>        Reference/receiver longitude for surface posn (opt)\n"
 "--max-range <distance>   Absolute maximum range for position decoding (in nm, default: 300)\n"
@@ -826,6 +827,8 @@ int main(int argc, char **argv) {
             Modes.net_output_sbs_port = atoi(argv[++j]);
         } else if (!strcmp(argv[j],"--net-buffer") && more) {
             Modes.net_sndbuf_size = atoi(argv[++j]);
+        } else if (!strcmp(argv[j],"--net-verbatim")) {
+            Modes.net_verbatim = 1;
         } else if (!strcmp(argv[j],"--onlyaddr")) {
             Modes.onlyaddr = 1;
         } else if (!strcmp(argv[j],"--metric")) {

dump1090.h

@@ -349,6 +349,7 @@ struct {                             // Internal state
     int   net_http_port;             // HTTP port
     int   net_fatsv_port;            // FlightAware TSV port
     int   net_sndbuf_size;           // TCP output buffer size (64Kb * 2^n)
+    int   net_verbatim;              // if true, send the original message, not the CRC-corrected one
     int   quiet;                     // Suppress stdout
     int   interactive;               // Interactive mode
     int   interactive_rows;          // Interactive mode: max number of rows
@@ -394,6 +395,7 @@ struct {                             // Internal state
 struct modesMessage {
     // Generic fields
     unsigned char msg[MODES_LONG_MSG_BYTES];      // Binary message.
+    unsigned char verbatim[MODES_LONG_MSG_BYTES]; // Binary message, as originally received before correction
     int           msgbits;                        // Number of bits in message 
     int           msgtype;                        // Downlink format #
     uint32_t      crc;                            // Message CRC

mode_s.c

@@ -417,8 +417,12 @@ static char *ais_charset = "@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_ !\"#$%&'()*+,-./01
 
 int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
 {
-    // Work on our local copy
+    // Work on our local copy.
     memcpy(mm->msg, msg, MODES_LONG_MSG_BYTES);
+    if (Modes.net_verbatim) {
+        // Preserve the original uncorrected copy for later forwarding
+        memcpy(mm->verbatim, msg, MODES_LONG_MSG_BYTES);
+    }
     msg = mm->msg;
 
     // Get the message type ASAP as other operations depend on this

net_io.c

@@ -273,6 +273,7 @@ void modesSendBeastOutput(struct modesMessage *mm) {
     char * pTimeStamp;
     char ch;
     int  j;
+    unsigned char *msg = (Modes.net_verbatim ? mm->verbatim : mm->msg);
 
     if (!p)
         return;
@@ -297,7 +298,7 @@ void modesSendBeastOutput(struct modesMessage *mm) {
     if (0x1A == ch) {*p++ = ch; }
 
     for (j = 0; j < msgLen; j++) {
-        *p++ = (ch = mm->msg[j]);
+        *p++ = (ch = msg[j]);
         if (0x1A == ch) {*p++ = ch; }
     }
 
@@ -314,6 +315,7 @@ void modesSendRawOutput(struct modesMessage *mm) {
     char *p = prepareWrite(&Modes.raw_out, msgLen*2 + 15);
     int j;
     unsigned char * pTimeStamp;
+    unsigned char *msg = (Modes.net_verbatim ? mm->verbatim : mm->msg);
 
     if (!p)
         return;
@@ -329,7 +331,7 @@ void modesSendRawOutput(struct modesMessage *mm) {
         *p++ = '*';
 
     for (j = 0; j < msgLen; j++) {
-        sprintf(p, "%02X", mm->msg[j]);
+        sprintf(p, "%02X", msg[j]);
         p += 2;
     }