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// Network services

Port 13: Daytime

Port 13 is assigned by IANA to daytime (Daytime).

13

How daytime works

Network services
client:13daytimeresponse client:13daytimeresponse

The client sends a request to port 13 and the daytime service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Network services
01 Security

Is port 13 dangerous?

We have no specific security note for port 13. Its risk depends on the program actually listening on it. General advice for network services services:

  • Answer only the networks that need the service (no open resolvers, relays or reflectors).
  • UDP services can be abused for DDoS amplification: rate-limit or block them at the edge.
  • Disable legacy test services that have no use in production.
02 Registry

IANA registry entries for port 13

ServicePortTransportDescriptionReferenceUpdated
daytime 13 TCP, UDP Daytime RFC 867 -

// source: IANA service name and port number registry

03 Check it

Which program uses port 13, and is it reachable?

Linux

sudo ss -ltnup 'sport = :13'      # process listening on 13
sudo lsof -nP -i :13

macOS

sudo lsof -nP -i :13

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 13 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 13
netstat -ano | findstr :13

From another machine

nmap -p 13 -sV <host>
sudo nmap -sU -p 13 <host>
nc -zv <host> 13

Block it in the firewall

sudo ufw deny 13/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 13" -Direction Inbound -LocalPort 13 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 13 used for?

Port 13 is assigned by IANA to daytime (Daytime).

Is port 13 TCP or UDP?

Port 13 is registered for TCP and UDP.

Is it safe to open port 13?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Answer only the networks that need the service (no open resolvers, relays or reflectors).

How do I check if port 13 is open?

On Linux run "ss -ltnup 'sport = :13'", on macOS "lsof -nP -i :13", and on Windows "netstat -ano | findstr :13" to see the local process. From another machine, "nmap -p 13 <host>" shows whether it is reachable.

05 Related
:53 DNS tcp · udp · moderate risk :135 MS RPC tcp · udp · high risk :123 NTP udp · tcp · moderate risk :67 DHCP server udp · tcp · moderate risk :111 rpcbind tcp · udp · high risk :9100 Raw printing (JetDirect) tcp · udp · high risk :631 IPP / CUPS tcp · udp · high risk :5353 mDNS udp · tcp · moderate risk

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