ClickHouse's server-to-server port: replicas fetch data parts from each other here.
ClickHouse uses 9009 for inter-server communication — replication and data exchange between replicas at the low-level data access layer — separate from the 9000 native client protocol and the 8123 HTTP interface. On a ClickHouse cluster every node has it open to its peers. IANA registers 9009 as pichat, which you will not meet in practice.
Do not expose
This is an internal cluster path that moves raw data parts between replicas; keep it on the cluster network and never route it from the internet.
$ sudo lsof -nP -iTCP:9009 -sTCP:LISTENSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.