Memcached listens on port 11211 and serves PHP applications as a fast key-value cache. Nextcloud supports Memcached as a distributed cache but recommends APCu locally plus Redis for locking.
Never public. Memcached has no authentication (except SASL in the binary protocol). Open servers with UDP 11211 were used in 2018 for amplification attacks above 1 Tbps; since version 1.5.6 UDP is off by default. Start it with -l 127.0.0.1 -U 0.
Typical: high miss rate from too little memory (-m) – visible as evictions in stats.
Check
printf 'stats\r\nquit\r\n' | nc 127.0.0.1 11211