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discard ntp answers with inconsistent mode/stratum/values #147

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discard ntp answers with inconsistent mode/stratum/values #147

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16 changes: 13 additions & 3 deletions NTPClient.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -102,16 +102,26 @@ bool NTPClient::forceUpdate() {
timeout++;
} while (cb == 0);

this->_lastUpdate = millis() - (10 * (timeout + 1)); // Account for delay in reading the time

this->_udp->read(this->_packetBuffer, NTP_PACKET_SIZE);

unsigned long highWord = word(this->_packetBuffer[40], this->_packetBuffer[41]);
unsigned long lowWord = word(this->_packetBuffer[42], this->_packetBuffer[43]);
byte mode = this->_packetBuffer[0] & 0b00000111;
byte vn = (this->_packetBuffer[0] & 0b00111000) >> 3;
byte stratum = this->_packetBuffer[1];

if(mode != 4 || stratum >= 16 || highWord == 0 || lowWord == 0 ) {
#ifdef DEBUG_NTPClient
Serial.println("Answer from NTP looks weird. Discarding it.");
#endif
return false;
}


// combine the four bytes (two words) into a long integer
// this is NTP time (seconds since Jan 1 1900):
unsigned long secsSince1900 = highWord << 16 | lowWord;

this->_lastUpdate = millis() - (10 * (timeout + 1)); // Account for delay in reading the time
this->_currentEpoc = secsSince1900 - SEVENZYYEARS;

return true; // return true after successful update
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