NTP Server behind NAT - c

Question

I have a self implemented NTP server working on a Ubuntu machine.
NTP Client is another Ubuntu machine with both ntpdate and ntpd utilities installed. Server has has address 192.168.2.200 and the client is 192.168.2.74 . If I run:
ntpdate 192.168.2.200
Or
ntpd -c /root/NTP.conf
With NTP.conf
server 192.168.2.200 minpoll 0 maxpoll 0 prefer
The client OS will synchronize with my server.
Next step is to put server behind a NAT.
NTP Client still has address 192.168.2.74 but now the server is behind a Windows 10 machine with address 192.168.2.75 .
I set the port forwarding in order to forward UDP packets directed to 192.168.2.75:123 to the internal address 192.168.2.200:123, where the server is located. For some reason, even if the client (2.74) receives the same answer from the server (2.200 natted as 2.75), it won't synchronize.
This is what I've done so far:
Define a custom struct for NTP message as defined in rfc5905:
typedef struct
{
uint8_t hdrhead[4];
uint32_t delay;
uint32_t dispersion;
uint32_t reference_id;
uint64_t reference_ts;
uint64_t origin_ts;
uint64_t receive_ts;
uint64_t transmit_ts;
} ntp_msg5905;
In main function, I have a simple while loop:
// socket initialization...
while (1)
{
uint8_t recv_msg[NTP_MSGLEN];
while (recvfrom(s, recv_msg, NTP_MSGLEN, 0, (struct sockaddr*) &src_addr, &src_addrlen) < NTP_MSGLEN)
{
_log_("Arrived wrong NTP request: message too short", stdout);
}
_log_("Arrived NTP request", stdout);
uint64_t recvtime;
timestamp_64(&recvtime);
ntp_reply(s, &src_addr , src_addrlen, *(ntp_msg5905*)recv_msg, recvtime);
}
timestamp_64() function just build a 64bit timestamp obtained with gettimeofday(), nothing special or ntp related.
The ntp_reply function is:
void ntp_reply(int clisock, struct sockaddr_in* saddr_p, socklen_t saddrlen, ntp_msg5905 request, uint64_t recvtime)
{
ntp_msg5905 reply;
memset(&reply, 0x00, NTP_MSGLEN);
reply.hdrhead[STRATUM] = NTP_STRATUM_PRIMARY_SERVER;
reply.hdrhead[PREC] = NTP_HOST_PRECISION;
reply.delay = NTP_NODELAY;
reply.dispersion = NTP_NODISPERSION;
reply.reference_id = NTP_REF_ID_GPS;
reply.hdrhead[LIVN] = request.hdrhead[LIVN] & 0x38 + NTP_MODE_SERVER;
reply.hdrhead[POLL] = request.hdrhead[POLL];
reply.receive_ts = recvtime;
reply.origin_ts = request.transmit_ts;
uint64_t* rts = &reply.reference_ts;
uint64_t* tts = &reply.transmit_ts;
timestamp_64(rts);
timestamp_64(tts);
if (sendto(clisock, (char*) &reply, NTP_MSGLEN, 0, (struct sockaddr*) saddr_p, saddrlen) < NTP_MSGLEN)
{
_log_("Cannot send NTP reply", stdout);
}
else
{
_log_("Request reply sent back to client", stdout);
}
}
Global defines below:
/* Global NTP parameters */
#define NTP_UTC 2208988800U
#define NTP_PORT 123
#define NTP_MSGLEN 48
/* NTP header's first 4 bytes */
#define LIVN 0
#define STRATUM 1
#define POLL 2
#define PREC 3
/* NTP header field values */
#define NTP_MODE_SERVER 4
#define NTP_STRATUM_PRIMARY_SERVER 1
#define NTP_HOST_PRECISION -6
#define NTP_NODELAY 0
#define NTP_NODISPERSION 0
#define NTP_REF_ID_GPS *(uint32_t*)("GPS")
Where am I wrong?