Port 11211 Memcached · TCP and UDP

What runs on port 11211

Memcached: an in-memory cache with no authentication at all. Reachable from the internet it hands over whatever was cached (sessions, API responses), and its UDP listener was the amplifier behind the record 1.3 Tbps DDoS of 2018 (51,000× amplification). Bind to localhost or a private address and disable UDP (-U 0), which is the default since 1.5.6.

Software that uses it: memcached · ElastiCache for Memcached · PHP/Django/Rails session stores

Should port 11211 be open?

Loopback or the host's own network only. The service should listen on 127.0.0.1 (or a Unix socket) and be reached by a reverse proxy or by the application on the same host. If it answers on a public address, that is a misconfiguration.

• echo stats | nc host 11211 from another machine should time out; if it answers, close it.
• OPTIONS="-l 127.0.0.1 -U 0" in /etc/sysconfig/memcached or /etc/memcached.conf.

Check port 11211

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :11211'
ss -lunp 'sport = :11211'
sudo lsof -i :11211

# from another machine: is it reachable?
nc -zv host 11211
nmap -p 11211 host
nc -zuv host 11211
sudo nmap -sU -p 11211 host
curl -v telnet://host:11211      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 11211

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 11211 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 11211 proto udp
sudo ufw deny 11211/tcp
sudo ufw deny 11211/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 11211 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 11211 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 11211 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 11211 -j DROP
iptables -A INPUT -p udp --dport 11211 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 11211 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=11211 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 11211" with no source is just "open".

Related ports

• 6379 Redis — No auth by default; never expose
• 27017 MongoDB

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions