Port 13: Daytime
Port 13 is the Daytime Protocol, which returns the current date and time as human-readable text when a client connects. It predates NTP (port 123) and has been superseded by it for anything that needs accurate time.
| Port number | 13 |
|---|---|
| Service | Daytime Protocol |
| Protocol | TCP/UDP |
| Category | Legacy protocols |
| Default encryption | No |
How do I check whether port 13 is open?
Port 13 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 13 | 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 13 | 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 ':13 ' | Lists the process bound to port 13 on Linux. On macOS use lsof -nP -iTCP:13 -sTCP:LISTEN, on Windows netstat -ano | findstr :13 |
| Is it open on a remote host? | sudo nmap -sU -p 13 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 ':13 ' | Shows the UDP socket bound to port 13. 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 13 used for?
Port 13 is the Daytime Protocol, which returns the current date and time as human-readable text when a client connects. It predates NTP (port 123) and has been superseded by it for anything that needs accurate time.
Is port 13 TCP or UDP?
Port 13 (Daytime) uses TCP/UDP.
Is port 13 secure?
Port 13 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 13 on my firewall?
Only if you specifically need Daytime. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.