client.py

import socket, sntp, time
from _datetime import datetime, timedelta
import conversions

OS_LINUX = 'clock_settime' in dir(time) # Interoperability between OS'es
if not OS_LINUX: import win32api

sync_time = 0 # Last synchronization time, NTP timestamp

def set_sntp_time ():

    server_addr = ("time.google.com", 123)

    # the below function sets the clock of the local system
    def setsystime(time_str):
        '''takes time in string and sets it to the locale system ->use time.ctime()'''

        ntp_dt = datetime.strptime(time_str, "%a %b %d %H:%M:%S %Y")
        ist_dt = ntp_dt - timedelta(hours=5, minutes=30)
        time_t = (ist_dt.year, ist_dt.month, ist_dt.day,
                  ist_dt.hour, ist_dt.minute, ist_dt.second, 0)
        dayofweek = datetime(*time_t).isocalendar()[2]
        t = time_t[:2] + (dayofweek,) + time_t[2:]
        win32api.SetSystemTime(*t)


    # Create a UDP socket
    with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as c_sock:
        hserver_req = sntp.SNTPMsg()
        hserver_req["LI"] = 3  # alarm condition clock not synchronized
        hserver_req["VN"] = 4
        hserver_req["Mode"] = 3  # 3-> client coz its sending request to (hserver)higher server
        hserver_req["Stratum"] = 15  # 2-15 -> secondary reference server
        hserver_req["OriginateTimestamp"] = conversions.posix_to_ntp(time.time())  # time when the message departs
        hserver_req["TransmitTimestamp"] = conversions.posix_to_ntp(time.time())  # time when the message departs

        c_sock.sendto(hserver_req, server_addr)
        hserver_res, addr = c_sock.recvfrom(48)
        hserver_res = sntp.SNTPMsg(hserver_res)
        if hserver_res["OriginateTimestamp"] == hserver_req["TransmitTimestamp"]:
            # the above check is done for reply attacks
            if hserver_res["Stratum"] == 0 or hserver_res["TransmitTimestamp"] == 0 or (
                    hserver_res["Mode"] != 4 and hserver_res["Mode"] != 5):
                print("Server reply is discarded\nDoes not comply with message standard specified by rfc")
            else:
                print("Processing server reply")
                print(f"The current primary server:{server_addr[0]}\nThe server response message :\n{hserver_res}")
                # calculating round trip delay
                t1 = hserver_req["OriginateTimestamp"]
                t2 = hserver_res["RecieveTimestamp"]
                t3 = hserver_res["TransmitTimestamp"]
                t4 = conversions.posix_to_ntp(time.time())
                # d is the round trip delay
                d = (t4 - t1) - (t3 - t2)
                # t is the clock offset
                t = ((t2 - t1) + (t3 - t4)) / 2

                # the below transmit time is same as client local time
                accurate_time = (hserver_req["TransmitTimestamp"] + int(t)) - int((d / 2))
                ist_accurate_time = conversions.ntp_to_posix(accurate_time)

                print("Time is: "+time.ctime(ist_accurate_time))
                if OS_LINUX: # If running on Linux
                    time.clock_settime (0, ist_accurate_time)
                else:        # Running on Windows
                    setsystime(time.ctime(ist_accurate_time))

                global sync_time
                sync_time = accurate_time


if __name__ == "__main__":
    set_sntp_time ()
    print ("System time is corrected")