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// Other services

Port 9: Discard

Port 9 is assigned by IANA to discard (Discard).

9

How discard works

Other services
client:9discardresponse client:9discardresponse

The client sends a request to port 9 and the discard service answers.

Transport
TCP · UDP · SCTP · DCCP
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
Other services
01 Security

Is port 9 dangerous?

We have no specific security note for port 9. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 9

ServicePortTransportDescriptionReferenceUpdated
discard 9 TCP, UDP, SCTP, DCCP Discard - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 9, and is it reachable?

Linux

sudo ss -ltnup 'sport = :9'      # process listening on 9
sudo lsof -nP -i :9

macOS

sudo lsof -nP -i :9

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 9 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 9
netstat -ano | findstr :9

From another machine

nmap -p 9 -sV <host>
sudo nmap -sU -p 9 <host>
nc -zv <host> 9

Block it in the firewall

sudo ufw deny 9/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 9" -Direction Inbound -LocalPort 9 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 9 used for?

Port 9 is assigned by IANA to discard (Discard).

Is port 9 TCP or UDP?

Port 9 is registered for TCP and UDP and SCTP and DCCP.

Is it safe to open port 9?

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. Close the port if nothing on the host needs it.

How do I check if port 9 is open?

On Linux run "ss -ltnup 'sport = :9'", on macOS "lsof -nP -i :9", and on Windows "netstat -ano | findstr :9" to see the local process. From another machine, "nmap -p 9 <host>" shows whether it is reachable.

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