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// Web

Port 6121: SPDY for a faster web

Port 6121 is assigned by IANA to spdy (SPDY for a faster web).

6121

How spdy works

Web
browser:6121spdy browser:6121spdy

A browser sends an HTTP request to port 6121 and the spdy server sends the page back.

Transport
TCP
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
Web
01 Security

Is port 6121 dangerous?

We have no specific security note for port 6121. Its risk depends on the program actually listening on it. General advice for web services:

  • Serve it over HTTPS only and check the HTTP security headers.
  • Keep admin consoles and development servers off the Internet.
  • Patch the application server and remove default credentials and sample apps.
02 Registry

IANA registry entries for port 6121

ServicePortTransportDescriptionReferenceUpdated
spdy 6121 TCP SPDY for a faster web - 2010-04-26
-6121-Reserved--

// source: IANA service name and port number registry

03 Check it

Which program uses port 6121, and is it reachable?

Linux

sudo ss -ltnup 'sport = :6121'      # process listening on 6121
sudo lsof -nP -i :6121

macOS

sudo lsof -nP -i :6121

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 6121 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 6121
netstat -ano | findstr :6121

From another machine

nmap -p 6121 -sV <host>
nc -zv <host> 6121

Block it in the firewall

sudo ufw deny 6121/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 6121" -Direction Inbound -LocalPort 6121 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 6121 used for?

Port 6121 is assigned by IANA to spdy (SPDY for a faster web).

Is port 6121 TCP or UDP?

Port 6121 is registered for TCP.

Is it safe to open port 6121?

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. Serve it over HTTPS only and check the HTTP security headers.

How do I check if port 6121 is open?

On Linux run "ss -ltnup 'sport = :6121'", on macOS "lsof -nP -i :6121", and on Windows "netstat -ano | findstr :6121" to see the local process. From another machine, "nmap -p 6121 <host>" shows whether it is reachable.

05 Related
:443 HTTPS tcp · udp · sctp · low risk :80 HTTP tcp · udp · sctp · moderate risk :8080 HTTP alternate tcp · udp · high risk :8443 HTTPS alternate tcp · udp · moderate risk :3000 Dev servers / Grafana tcp · udp · moderate risk :5000 Flask / Docker registry / UPnP tcp · udp · moderate risk :8000 HTTP alternate (dev) tcp · udp · moderate risk :8888 Jupyter / HTTP alternate tcp · udp · high risk

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