Port 37: Time Protocol
Port 37 is the Time Protocol, an early way for a client to fetch the current time from a server as a raw count of seconds since 1900. NTP on port 123 replaced it for anything that needs real accuracy.
| Port number | 37 |
|---|---|
| Service | Time Protocol |
| Protocol | TCP/UDP |
| Category | Legacy protocols |
| Default encryption | No |
How do I check whether port 37 is open?
Port 37 carries TCP, so a connection either completes its handshake or it does not, which makes the check definitive.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | nc -vz example.com 37 | A TCP handshake either completes or it does not, so netcat gives a definitive answer in one line. |
| Same check on Windows | Test-NetConnection example.com -Port 37 | Built into PowerShell. TcpTestSucceeded in the output is the answer; telnet is not installed by default on modern Windows. |
| What is listening locally? | ss -tlnp | grep ':37 ' | Lists the process bound to port 37 on Linux. On macOS use lsof -nP -iTCP:37 -sTCP:LISTEN, on Windows netstat -ano | findstr :37 |
| Is it open on a remote host? | sudo nmap -sU -p 37 example.com | UDP has no handshake, so there is nothing to complete. nmap infers the state from whether an ICMP port-unreachable comes back, needs root, and is slow and easy to misread. A silent port may be open, filtered, or simply not replying to a probe it does not recognise. |
| What is listening locally? | ss -ulnp | grep ':37 ' | Shows the UDP socket bound to port 37. This is the reliable way to answer the question, because you are asking the host itself rather than guessing from outside. |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 37 used for?
Port 37 is the Time Protocol, an early way for a client to fetch the current time from a server as a raw count of seconds since 1900. NTP on port 123 replaced it for anything that needs real accuracy.
Is port 37 TCP or UDP?
Port 37 (Time Protocol) uses TCP/UDP.
Is port 37 secure?
Port 37 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 37 on my firewall?
Only if you specifically need Time Protocol. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.