What uses port 1521
The Oracle TNS listener accepts connections on 1521 and hands them to the database. Connection strings look like host:1521/SERVICE_NAME. Oracle Cloud Autonomous Database uses 1522 for TLS connections. Like other database ports, 1521 belongs on a private network.
Should port 1521 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 1521
sudo ss -ltnp 'sport = :1521'sudo lsof -nP -iTCP:1521 -sTCP:LISTENGet-NetTCPConnection -LocalPort 1521 -State Listen
netstat -ano | findstr :1521No 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 1521
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:1521 -sTCP:LISTEN)Stop-Process -Id (Get-NetTCPConnection -LocalPort 1521 -State Listen).OwningProcessAllow port 1521 from trusted networks only
The examples allow 1521 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 1521 proto tcp
# Close it again:
sudo ufw delete allow from 10.0.0.0/8 to any port 1521 proto tcpsudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.0/8" port port="1521" protocol="tcp" accept'
sudo firewall-cmd --reloadNew-NetFirewallRule -DisplayName "Oracle Database 1521/TCP" -Direction Inbound -Protocol TCP -LocalPort 1521 -Action Allow -RemoteAddress 10.0.0.0/8On 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 1521 with Docker
# Publish on localhost only. Docker's port rules bypass ufw and firewalld,
# so -p 1521:1521 would expose the port to the whole network.
docker run -d -p 127.0.0.1:1521:1521 gvenzl/oracle-free
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 1521 from another machine
nc -zv example.com 1521Test-NetConnection example.com -Port 1521Related ports
- Port 1433 (Microsoft SQL Server): the default port for Microsoft SQL Server.
- Port 3306 (MySQL / MariaDB): the default port for MySQL and MariaDB.
- Port 5432 (PostgreSQL): the default port for PostgreSQL.
More databases ports
1434 (SQL Server Browser) , 6379 (Redis) , 8086 (InfluxDB) , 9042 (Cassandra / ScyllaDB) , 9200 (Elasticsearch / OpenSearch) , 9300 (Elasticsearch transport) , 11211 (Memcached) , 27017 (MongoDB) . See all ports in the port lookup.
Sources
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Frequently asked questions
What is port 1521 used for?
Port 1521 is the default port for the Oracle Database listener. The Oracle TNS listener accepts connections on 1521 and hands them to the database. Connection strings look like host:1521/SERVICE_NAME. Oracle Cloud Autonomous Database uses 1522 for TLS connections. Like other database ports, 1521 belongs on a private network.
Is it safe to open port 1521?
Never expose this port to the internet. Keep it on localhost, a private network or behind a VPN or SSH tunnel. Allow it only from the networks that need it, as in the firewall examples on this page.
How do I check if port 1521 is open?
On the server, list listening sockets with "sudo ss -ltnp 'sport = :1521'" on Linux or "sudo lsof -nP -i :1521" on macOS. From another machine, run "nc -zv host 1521" or, on Windows, "Test-NetConnection host -Port 1521". Connection refused means nothing is listening; a timeout means a firewall is dropping the traffic.