AFG/dump1090
3
0
Fork 0
Code Issues Pull requests Packages Projects Releases Wiki Activity

BUGFIX : Beast Binary Escape characters

Browse source 
Thanks to Blort on the PP list.

The Beast binary message stream uses the 0x1a character a and escape to
mark the beginning of a new message. However, the 0x1a character could
occur in the body of a message since the message is binary. Therefore,
the 0x1a is repeated  -as Blort put it :

This 56-bit Mode S Frame (containing a 1a) is supposed to look like
this:

1a  32 00 00 48 7b a6 1a 1a 0c  20 28 17 b0 c0 c3 b0

What is actually looks (looked) like (from wireshark) is this:

1a  32 00 00 48 7b a6 1a 0c  20 28 17 b0 c0 c3 b0

There are supposed to be two 1A’s (Gunter’s Escape Character) in a row
(1A 1A) whenever a data byte contains 1A, after the initial two 1A 3x
characters.
This commit is contained in:
Malcolm Robb 2014-02-23 01:55:34 +00:00
parent 75a4c6ee21
commit 260b955f47
1 changed files with 35 additions and 9 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

44
net_io.c
View file

@ -189,7 +189,9 @@ void modesSendBeastOutput(struct modesMessage *mm) {
char *p = &Modes.beastOut[Modes.beastOutUsed];
int msgLen = mm->msgbits / 8;
char * pTimeStamp;
char ch;
int j;
int iOutLen = msgLen + 9; // Escape, message type, timestamp, sigLevel and msg
*p++ = 0x1a;
if (msgLen == MODES_SHORT_MSG_BYTES)
@ -203,14 +205,19 @@ void modesSendBeastOutput(struct modesMessage *mm) {
pTimeStamp = (char *) &mm->timestampMsg;
for (j = 5; j >= 0; j--) {
*p++ = pTimeStamp[j];
*p++ = (ch = pTimeStamp[j]);
if (0x1A == ch) {*p++ = ch; iOutLen++;}
}
*p++ = mm->signalLevel;
*p++ = (ch = mm->signalLevel);
if (0x1A == ch) {*p++ = ch; iOutLen++;}
memcpy(p, mm->msg, msgLen);
for (j = 0; j < msgLen; j++) {
*p++ = (ch = mm->msg[j]);
if (0x1A == ch) {*p++ = ch; iOutLen++;}
}
Modes.beastOutUsed += (msgLen + 9);
Modes.beastOutUsed += iOutLen;
if (Modes.beastOutUsed >= Modes.net_output_raw_size)
{
modesSendAllClients(Modes.bos, Modes.beastOut, Modes.beastOutUsed);
@ -427,7 +434,9 @@ void modesQueueOutput(struct modesMessage *mm) {
// case where we want broken messages here to close the client connection.
//
int decodeBinMessage(struct client *c, char *p) {
int msgLen = 0;
int msgLen = 0;
int j;
char ch;
unsigned char msg[MODES_LONG_MSG_BYTES];
struct modesMessage mm;
MODES_NOTUSED(c);
@ -445,9 +454,17 @@ int decodeBinMessage(struct client *c, char *p) {
// Mark messages received over the internet as remote so that we don't try to
// pass them off as being received by this instance when forwarding them
mm.remote = 1;
p += 7; // Skip the timestamp
mm.signalLevel = *p++; // Grab the signal level
memcpy(msg, p, msgLen); // and the data
for (j = 0; j < 7; j++) { // Skip the message type and timestamp
ch = *p++;
if (0x1A == ch) {p++;}
}
mm.signalLevel = ch = *p++; // Grab the signal level
if (0x1A == ch) {p++;}
for (j = 0; j < msgLen; j++) { // and the data
msg[j] = ch = *p++;
if (0x1A == ch) {p++;}
}
if (msgLen == MODEAC_MSG_BYTES) { // ModeA or ModeC
decodeModeAMessage(&mm, ((msg[0] << 8) | msg[1]));
@ -767,7 +784,7 @@ void modesReadFromClient(struct client *c, char *sep,
int nread;
int fullmsg;
int bContinue = 1;
char *s, *e;
char *s, *e, *p;
while(bContinue) {
@ -817,6 +834,15 @@ void modesReadFromClient(struct client *c, char *sep,
left = &(c->buf[c->buflen]) - e;
continue;
}
// we need to be careful of double escape characters in the message body
for (p = s; p < e; p++) {
if (0x1A == *p) {
p++; e++;
if (e > &(c->buf[c->buflen])) {
break;
}
}
}
left = &(c->buf[c->buflen]) - e;
if (left < 0) { // Incomplete message in buffer
e = s - 1; // point back at last found 0x1a.