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Registered port (1024–10000) High risk Community convention

Port 9042 CASSANDRA

Quick answer

Port 9042 is the community default for Cassandra, using TCP.

Check and block Is it safe? Troubleshooting FAQ
TransportTCP
RiskHigh
Typical useDatabases

What is port 9042 used for?

Port 9042 is widely used by Cassandra (TCP), a community convention rather than an IANA assignment. Apache Cassandra native client protocol port (community convention; not IANA-assigned). It sits in the registered range, the block used by databases, application servers and developer tooling. Port conflicts here are common when several local services start at once.

Ports are the transport-layer address a service listens on. A client that wants to reach Cassandra connects to port 9042. If nothing is listening, the connection is refused. That is why a port conflict appears whenever two processes try to bind the same number.

Use the commands below to confirm whether port 9042 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.

Because this number is not IANA-assigned, a different application may legitimately use it on a machine you do not control. Always identify the actual listening process before deciding that the port belongs to this technology.

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This port number is not assigned by IANA. The entry documents a widely adopted community convention.

Check and block port 9042

Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.

Check on Linux

sudo ss -tlpn 'sport = :9042'

Check on Windows

Get-NetTCPConnection -LocalPort 9042 -State Listen -ErrorAction SilentlyContinue

Block with UFW

sudo ufw deny 9042/tcp

Block with iptables

sudo iptables -A INPUT -p tcp --dport 9042 -j DROP

These commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.

Is port 9042 safe to expose?

High

Cassandra's native protocol is frequently deployed without authentication, exposing the whole database. Enable auth, TLS and private networking. Treat this port as sensitive. Restrict it to trusted networks or a VPN, require authentication and logging, and never expose it directly to the public internet without a hardened gateway in front.

Where port 9042 is usually used

Most traffic on port 9042 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.

Demo

Probe the Cassandra listener

A TCP probe confirms the native protocol port before you debug credentials or driver settings.

nc -vz 127.0.0.1 9042

Common problems with port 9042

Connection refused or timed out

Refused usually means the host is reachable but nothing is listening on port 9042. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.

Port already in use

Find the process holding the port with sudo ss -tlpn 'sport = :9042' or sudo lsof -i :9042. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.

Firewall rule did not survive a reboot

UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.

The port is blocked but still reachable

A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.

Port 9042 at a glance

Port number9042
Service nameCassandra
Transport protocolTCP
CategoryDatabases
RangeRegistered port (1024–10000)
Risk levelHigh
ReferenceCommunity convention (not IANA-assigned)
Registry descriptionApache Cassandra native client protocol port (community convention; not IANA-assigned)
SourceCommunity convention

Frequently asked questions

What is port 9042 used for?

Port 9042 (CASSANDRA) uses TCP. Apache Cassandra native client protocol port (community convention; not IANA-assigned) (community convention, not an IANA assignment).

How do I check if port 9042 is open?

On Linux run `sudo ss -tlpn 'sport = :9042'`. On Windows run `Get-NetTCPConnection -LocalPort 9042 -State Listen -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 9042; empty output usually means nothing is bound to it.

How do I block port 9042?

With UFW run `sudo ufw deny 9042/tcp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 9042 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.

Is port 9042 safe to expose?

Cassandra's native protocol is frequently deployed without authentication, exposing the whole database. Enable auth, TLS and private networking. Nevertheless, treat this port as sensitive. Restrict it to trusted networks or a VPN, require authentication and logging, and never expose it directly to the public internet without a hardened gateway in front.

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Primary data: IANA Service Name and Transport Protocol Port Number Registry. This number is a community convention, not an IANA assignment.

Last reviewed 2026-10-30 · Methodology · About · Privacy