Port 11211 — Memcached

TCP/UDP registered

What runs on port 11211

Distributed memory cache.

Memcached, which has no authentication in its standard configuration. Beyond the obvious risk of an exposed cache, the UDP interface was used for record-breaking amplification attacks in 2018 — a small forged request produced a response thousands of times larger, aimed at a victim. UDP is disabled by default now as a direct result. Bind it to localhost.

Security considerations

The UDP interface was used for record-breaking amplification attacks in 2018. Disable UDP and bind to localhost.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :11211
sudo ss -lntp | grep :11211

# Windows
netstat -ano | findstr :11211
Get-NetTCPConnection -LocalPort 11211

# is it reachable from outside?
nc -zv example.com 11211
curl -v telnet://example.com:11211

Freeing the port

# find the process, then stop it
sudo lsof -ti :11211 | xargs kill        # Linux / macOS
netstat -ano | findstr :11211            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Should this port be open to the internet?

No. Port 11211 should never be reachable from a public address. Bind it to localhost or a private network, and reach it through a VPN or bastion host if remote access is genuinely needed. Internet-wide scanners find newly exposed instances of this service within minutes.

Opening or blocking port 11211

There is no blanket-allow rule on this page, because there is no case for one. Memcached on port 11211 should be reachable from named addresses or not at all, so what follows restricts rather than opens:

# ufw — Debian, Ubuntu
sudo ufw allow from 10.0.0.0/8 to any port 11211 proto tcp/udp
sudo ufw deny 11211/tcp/udp                    # everything else

# firewalld — RHEL, Fedora, CentOS
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=11211 protocol=tcp/udp accept'
sudo firewall-cmd --reload

# iptables, if you are still there
sudo iptables -A INPUT -p tcp/udp --dport 11211 -s 10.0.0.0/8 -j ACCEPT
sudo iptables -A INPUT -p tcp/udp --dport 11211 -j DROP

# Windows
New-NetFirewallRule -DisplayName "Memcached 11211" -Direction Inbound -Protocol TCP/UDP -LocalPort 11211 -RemoteAddress 10.0.0.0/8 -Action Allow

Change 10.0.0.0/8 to whatever your private range actually is. A firewall rule is the second line: the first is binding the service to a private interface, because a service that only listens on 127.0.0.1 cannot be reached however the firewall is configured.

Quick reference

Frequently asked questions

Other common ports

• Port 27017 — MongoDB
• Port 20 — FTP data
• Port 21 — FTP control
• Port 22 — SSH / SFTP
• Port 23 — Telnet
• Port 25 — SMTP
• Port 53 — DNS
• Port 67 — DHCP server
• Port 68 — DHCP client
• Port 69 — TFTP
• Port 80 — HTTP
• Port 110 — POP3
• Port 123 — NTP
• Port 143 — IMAP
• Port 161 — SNMP
• Port 389 — LDAP
• Port 443 — HTTPS
• Port 445 — SMB

All port numbers