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// Other services

Port 8121: Apollo Data Port

Port 8121 is assigned by IANA to apollo-data (Apollo Data Port).

8121

How apollo-data works

Other services
client:8121apollo-dataresponse client:8121apollo-dataresponse

The client sends a request to port 8121 and the apollo-data 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
Registered (1024-49151)
Category
Other services
01 Security

Is port 8121 dangerous?

We have no specific security note for port 8121. 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 8121

ServicePortTransportDescriptionReferenceUpdated
apollo-data 8121 TCP, UDP Apollo Data Port - 2003-08

// source: IANA service name and port number registry

03 Check it

Which program uses port 8121, and is it reachable?

Linux

sudo ss -ltnup 'sport = :8121'      # process listening on 8121
sudo lsof -nP -i :8121

macOS

sudo lsof -nP -i :8121

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 8121 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 8121
netstat -ano | findstr :8121

From another machine

nmap -p 8121 -sV <host>
sudo nmap -sU -p 8121 <host>
nc -zv <host> 8121

Block it in the firewall

sudo ufw deny 8121/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 8121" -Direction Inbound -LocalPort 8121 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 8121 used for?

Port 8121 is assigned by IANA to apollo-data (Apollo Data Port).

Is port 8121 TCP or UDP?

Port 8121 is registered for TCP and UDP.

Is it safe to open port 8121?

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 8121 is open?

On Linux run "ss -ltnup 'sport = :8121'", on macOS "lsof -nP -i :8121", and on Windows "netstat -ano | findstr :8121" to see the local process. From another machine, "nmap -p 8121 <host>" shows whether it is reachable.

05 Related
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