Docker Expose Port 309

Expose Port 309 in Docker

docker run -p 309:309 your-image-name
		

This command maps container port 309 to the same port on the host machine.

Dockerfile Example

FROM node:18-alpine
WORKDIR /app
COPY . .
EXPOSE 309
CMD ["npm", "start"]
		

The EXPOSE instruction documents that the container listens on port 309.

EXPOSE vs Publish (-p)

• EXPOSE 309 documents the port inside the container.
• -p 309:309 makes the port accessible from the host.
• Exposed ports are not reachable unless published.

Docker Expose Port 309 Not Working

If Docker expose port 309 is not working, it is usually due to incorrect port publishing, an application not listening on the expected port, or a port conflict on the host machine.

Port 309 Already in Use

The error “port 309 already in use” means another process is already bound to that port on your host system.

# Find process using port 309
lsof -i :309

# OR use a different host port
docker run -p 8080:309 your-image-name
		

• Ensure your app is listening on port 309 inside the container
• Verify the port is published using -p
• Check firewall or network rules
• Avoid conflicts with existing services

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