RFC868 Time Protocol Server: Simple Server Example in C and PythonThe RFC868 Time Protocol is a simple, decades‑old protocol designed to provide the current time from a server to a client. It dates from the early Internet era and was widely used before more sophisticated protocols such as NTP (Network Time Protocol) became standard. RFC868 defines a minimal service: a server listens on TCP or UDP port 37 and returns a 32‑bit unsigned integer containing the number of seconds since 00:00 (midnight) January 1, 1900 GMT. This article explains the protocol, discusses important implementation details, and provides straightforward example servers in both C and Python, with notes on portability, byte order, and security considerations.
Protocol summary
- Protocol: RFC868 Time Protocol
- Port: 37 (both TCP and UDP)
- Payload: 32‑bit unsigned integer, seconds since 1900‑01‑01 00:00:00 GMT
- Transport: TCP (connection-oriented) or UDP (datagram)
- Byte order: big-endian (network byte order)
The client initiates a connection (TCP) or sends a datagram (UDP). For TCP, the server sends the 4‑byte timestamp and then closes the connection. For UDP, the server sends a 4‑byte reply to the client’s address/port.
Because RFC868 uses a 32‑bit seconds count from 1900, it will (and did) wrap around every 2^32 seconds (~136 years). The first wrap occurs in 2036 if interpreted as unsigned seconds relative to 1900. Modern systems typically use NTP or 64‑bit time representations to avoid ambiguity.
Time conversion details
Most modern systems represent time as seconds since the Unix epoch (00:00:00 UTC, January 1, 1970). RFC868 uses seconds since 1900. To convert Unix time (seconds since 1970) to RFC868 seconds:
RFC868_time = Unix_time + OFFSET
where OFFSET = number of seconds between 1900‑01‑01 and 1970‑01‑01.
Compute OFFSET:
- 70 years including 17 leap years = 70*365 + 17 = 25567 days
- BUT it’s easier to use the constant: OFFSET = 2208988800 seconds
Thus: RFC868_seconds = unix_seconds + 2208988800
When implementing, ensure you convert to unsigned 32‑bit and send in big‑endian order.
Implementation considerations
- Byte order: Always send the 4 bytes in network byte order (big‑endian). Use htonl() in C or struct.pack(“!I”, value) in Python.
- 32‑bit wrap: If your system time is beyond the 32‑bit range (e.g., 2036+), a plain 32‑bit unsigned value will wrap; consider whether you want to implement range checks or return modulo 2^32 as RFC868 specifies.
- Leap seconds: RFC868 is a simple epoch‑based seconds count and does not account for leap seconds. If you require high precision, use NTP.
- Transport choice: UDP is stateless and simpler but unreliable; TCP is reliable and easier to test with telnet/netcat.
- Permissions: Binding to port 37 (<1024) requires elevated privileges on many systems. For testing use an unprivileged port (e.g., 1037) or run as root (not recommended).
- Security: Exposing a time server can be abused (e.g., reflection/amplification attacks using UDP). Limit access with firewall rules and rate limiting.
- Logging and error handling: Keep logs concise. For production, consider monitoring and failover.
Simple example: TCP server in C
This example is minimal, portable to POSIX systems, and demonstrates key steps: open socket, bind, listen, accept, send 4‑byte RFC868 timestamp, close connection. It omits advanced error recovery and robustness for clarity.
// rfc868_tcp_server.c // Compile: gcc -O2 -Wall rfc868_tcp_server.c -o rfc868_tcp_server // Run (unprivileged port for testing): ./rfc868_tcp_server 1037 #include <arpa/inet.h> #include <errno.h> #include <netinet/in.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/socket.h> #include <sys/time.h> #include <time.h> #include <unistd.h> #define DEFAULT_PORT 1037 #define RFC868_OFFSET 2208988800UL int main(int argc, char *argv[]) { int port = (argc > 1) ? atoi(argv[1]) : DEFAULT_PORT; int listen_fd = -1; listen_fd = socket(AF_INET, SOCK_STREAM, 0); if (listen_fd < 0) { perror("socket"); return 1; } int opt = 1; setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)); struct sockaddr_in addr; memset(&addr, 0, sizeof(addr)); addr.sin_family = AF_INET; addr.sin_addr.s_addr = INADDR_ANY; addr.sin_port = htons(port); if (bind(listen_fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) { perror("bind"); close(listen_fd); return 1; } if (listen(listen_fd, 16) < 0) { perror("listen"); close(listen_fd); return 1; } printf("RFC868 TCP time server listening on port %d ", port); for (;;) { struct sockaddr_in client; socklen_t clientlen = sizeof(client); int conn = accept(listen_fd, (struct sockaddr*)&client, &clientlen); if (conn < 0) { perror("accept"); continue; } // Get current time time_t now = time(NULL); if (now == (time_t)-1) now = 0; uint32_t ts = (uint32_t)( (uint64_t)now + RFC868_OFFSET ); uint32_t net_ts = htonl(ts); ssize_t w = write(conn, &net_ts, sizeof(net_ts)); if (w != sizeof(net_ts)) { // partial or failed write } close(conn); } close(listen_fd); return 0; } Notes:
- For production, add signal handling, resource limits, and robust error handling.
- If your platform’s time_t is 64‑bit, the cast to uint32_t intentionally truncates to RFC868’s 32‑bit field.
Simple example: UDP server in Python
This Python example uses the standard library only and is easy to run. It binds to a port, receives any datagram, and replies with the 4‑byte RFC868 timestamp.
# rfc868_udp_server.py # Run: python3 rfc868_udp_server.py 1037 import socket import struct import sys import time RFC868_OFFSET = 2208988800 # seconds between 1900 and 1970 def run(port=1037): sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) sock.bind(("0.0.0.0", port)) print(f"RFC868 UDP time server listening on port {port}") try: while True: data, addr = sock.recvfrom(1024) # client may send anything or empty payload unix_sec = int(time.time()) rfc868 = (unix_sec + RFC868_OFFSET) & 0xFFFFFFFF payload = struct.pack("!I", rfc868) sock.sendto(payload, addr) except KeyboardInterrupt: print("Stopping server") finally: sock.close() if __name__ == "__main__": port = int(sys.argv[1]) if len(sys.argv) > 1 else 1037 run(port) Notes:
- This server replies to every incoming datagram. You can optionally ignore empty/invalid packets, or implement rate limiting.
- For UDP reflection mitigation, restrict access via firewall or only bind to localhost for testing.
Simple example: TCP server in Python
A short Python TCP server using the socket module, suitable for quick testing:
# rfc868_tcp_server.py # Run: python3 rfc868_tcp_server.py 1037 import socket import struct import sys import time RFC868_OFFSET = 2208988800 def run(port=1037): sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) sock.bind(("0.0.0.0", port)) sock.listen(5) print(f"RFC868 TCP time server listening on port {port}") try: while True: conn, addr = sock.accept() with conn: unix_sec = int(time.time()) rfc868 = (unix_sec + RFC868_OFFSET) & 0xFFFFFFFF conn.sendall(struct.pack("!I", rfc868)) except KeyboardInterrupt: print("Stopping server") finally: sock.close() if __name__ == "__main__": port = int(sys.argv[1]) if len(sys.argv) > 1 else 1037 run(port) Testing the server
- TCP: Use netcat or telnet:
- nc localhost 1037
- The connection will close after 4 bytes; view raw bytes with hexdump: nc localhost 1037 | xxd
- UDP: Use netcat in UDP mode:
- echo | nc -u -w1 localhost 1037
- Or write a small Python client to send a datagram and unpack the 4 bytes:
- struct.unpack(“!I”, data)[0] – RFC868 value; subtract OFFSET to see Unix seconds.
Example Python client snippet:
import socket, struct s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.connect(("localhost", 1037)) data = s.recv(4) val = struct.unpack("!I", data)[0] unix = val - 2208988800 print(val, unix) s.close() Security and deployment notes
- Avoid running as root. Use capabilities or bind to higher ports for testing.
- Limit exposure: Only bind to required interfaces (e.g., internal network), use firewall rules to restrict clients.
- UDP servers can be abused for reflection if they reply to spoofed source addresses; restrict access and consider rate limiting.
- Use NTP for accurate time synchronization in production; RFC868 is mainly of historical or interoperability interest.
- Logging: avoid logging raw client payloads—may contain malicious data.
Troubleshooting
- Wrong byte order: If clients get wildly incorrect times, ensure your server sends big‑endian (network) order.
- Offset mistake: If returned times are ~70 years off, you likely omitted or miscalculated the 2208988800 offset.
- Permission denied on bind: Choose a non‑privileged port (>1024) or run with appropriate privileges.
- Partial writes on TCP: Ensure send/write is retried until all 4 bytes are sent, especially in nonblocking code.
Conclusion
RFC868 is intentionally simple: a four‑byte timestamp over TCP or UDP. The examples above show minimal, functional servers in both C and Python suitable for learning, testing, or lightweight internal use. For production time services, prefer NTP or authenticated time protocols; use the RFC868 server only when its simplicity fits your constraints (legacy compatibility, constrained environments, or educational demonstrations).
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