Port 11211 (Memcached)
Port 11211 is Memcached, an in-memory cache that web applications use to store query results and sessions.
- Service
- Memcached
- Protocol
- TCP and UDPTCP. UDP 11211 exists too but is disabled by default since Memcached 1.5.6.
- IANA registry
- Officially assigned: memcache (Memory cache service), TCP/UDP
Check if a port is open on your IP
We only check the address you are visiting from, one TCP port at a time. You cannot enter someone else’s address: this is not a port scanner.
Is it safe to open port 11211?
Memcached has no authentication by default. Its UDP port was abused for huge reflection attacks (CVE-2018-1000115), after which version 1.5.6 turned UDP off by default. Bind Memcached to localhost or a private network and never expose 11211.
Sources
“Port 11211 is already in use”: how to fix it
The error means another program is already listening on port 11211. Find its process ID (PID), then stop that program or start yours on a different port.
Windows (Command Prompt or PowerShell as administrator)
netstat -ano | findstr :11211taskkill /PID <PID> /FmacOS
lsof -nP -iTCP:11211 -sTCP:LISTENkill <PID>Linux
sudo ss -ltnp | grep :11211sudo lsof -i :11211sudo kill <PID>Replace <PID> with the number from the last column (Windows) or the PID column (macOS, Linux).
Why a port can look closed when your server is running
Most home networks sit behind a router doing NAT: the router owns the public address, and your computer has a private one such as 192.168.1.10. A connection from the internet reaches the router and stops there unless you add a port forwarding rule to your computer.
Many mobile and some home providers go further and use carrier-grade NAT (CGNAT): several customers share one public address, and your router itself gets an address from 100.64.0.0/10. Then no rule on your side can open a port; ask the provider for a public IP or use a tunnel or VPN service.
Even with forwarding in place, the firewall on the computer (Windows Defender Firewall, ufw, firewalld) must allow the port, and the program must listen on all interfaces (0.0.0.0), not only on 127.0.0.1.
UDP cannot be checked this way: a UDP service usually stays silent to packets it does not understand, so “no answer” means nothing. This check uses TCP only.
Frequently asked questions
What is port 11211 used for?
Port 11211 is Memcached, an in-memory cache that web applications use to store query results and sessions.
Is port 11211 TCP or UDP?
TCP. UDP 11211 exists too but is disabled by default since Memcached 1.5.6.
Is it safe to open port 11211?
Memcached has no authentication by default. Its UDP port was abused for huge reflection attacks (CVE-2018-1000115), after which version 1.5.6 turned UDP off by default. Bind Memcached to localhost or a private network and never expose 11211.
How do I free port 11211 if it is already in use?
Find the process: on Windows run netstat -ano | findstr :11211 and stop it with taskkill /PID <pid> /F; on macOS or Linux run lsof -i :11211 and stop it with kill <pid>.