Port 9042: Cassandra / ScyllaDB

What uses port 9042

Applications and cqlsh connect to Cassandra and ScyllaDB with the CQL native protocol on 9042 (9142 with TLS in some setups). Nodes gossip with each other on 7000/7001, and JMX monitoring uses 7199. Amazon Keyspaces and Azure Cosmos DB for Apache Cassandra speak CQL too.

Should port 9042 be open to the internet?

Keep private. Never expose this port to the internet. Keep it on localhost, a private network or behind a VPN or SSH tunnel.

Check what is listening on port 9042

sudo ss -ltnp 'sport = :9042'

sudo lsof -nP -iTCP:9042 -sTCP:LISTEN

Get-NetTCPConnection -LocalPort 9042 -State Listen
netstat -ano | findstr :9042

No output means nothing is listening. If a service should be, check that it is running and which address it binds to: 127.0.0.1 only accepts local connections, 0.0.0.0 or :: accepts them on every interface.

Fix “address already in use” on port 9042

Another process already holds the port. Find it with the commands above, then stop that service properly, or end the process:

sudo kill $(sudo lsof -t -iTCP:9042 -sTCP:LISTEN)

Stop-Process -Id (Get-NetTCPConnection -LocalPort 9042 -State Listen).OwningProcess

Allow port 9042 from trusted networks only

The examples allow 9042 only from 10.0.0.0/8; replace it with your own private network, office IP or VPN range.

sudo ufw allow from 10.0.0.0/8 to any port 9042 proto tcp
# Close it again:
sudo ufw delete allow from 10.0.0.0/8 to any port 9042 proto tcp

sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/8" port port="9042" protocol="tcp" accept'
sudo firewall-cmd --reload

New-NetFirewallRule -DisplayName "Cassandra / ScyllaDB 9042/TCP" -Direction Inbound -Protocol TCP -LocalPort 9042 -Action Allow -RemoteAddress 10.0.0.0/8

On AWS, Google Cloud and Azure, also allow the port in the security group or network firewall rules; host firewalls and cloud firewalls are separate.

Port 9042 with Docker

# Publish on localhost only. Docker's port rules bypass ufw and firewalld,
# so -p 9042:9042 would expose the port to the whole network.
docker run -d -p 127.0.0.1:9042:9042 cassandra

The format is -p host_port:container_port. Containers on the same Docker network reach each other on the container port directly, without publishing it. Convert a long docker run command into a Compose file with docker run to compose.

Test port 9042 from another machine

nc -zv example.com 9042

Test-NetConnection example.com -Port 9042

Related ports

• Port 7000 (AirPlay (macOS) / Cassandra): used by the macOS AirPlay Receiver and by Cassandra inter-node traffic.

More databases ports

1433 (Microsoft SQL Server) , 1434 (SQL Server Browser) , 1521 (Oracle Database) , 3306 (MySQL / MariaDB) , 5432 (PostgreSQL) , 6379 (Redis) , 8086 (InfluxDB) , 9200 (Elasticsearch / OpenSearch) . See all ports in the port lookup.

Sources

• IANA port registry (not registered for this use)

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Frequently asked questions