docker multiple hosts 1 router

Guri Bee

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20-10-2024

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Connecting Multiple Hosts to Docker Swarm Using a Single Router: A Practical Guide

Docker Swarm allows you to create a cluster of Docker nodes, making it possible to run distributed applications across multiple hosts. But how do you connect multiple hosts to a Docker Swarm network when you only have one router? This article will provide a detailed guide to achieve this, using insights from real-world scenarios and GitHub discussions.

The Challenge:

Connecting Docker hosts to a Swarm network typically involves overlay networks, which encapsulate communication within the Swarm. However, overlay networks require each host to have its own unique IP address, creating a challenge when using a single router with limited IP addresses.

Solution: Port Forwarding and Network Isolation

The solution lies in a combination of port forwarding on your router and network isolation within your Docker Swarm. Here's a breakdown:

1. Configure Router for Port Forwarding:

   • Your router needs to be configured to forward specific ports from the public network to the internal network used by your Docker hosts. This allows external clients to access services running within your Docker Swarm.

   • Example: Forwarding port 80 to Docker host 1's internal IP address (e.g., 192.168.1.10) will allow external access to a web server running on port 80 within the container on that host.

2. Isolate Docker Networks:

   • Within your Docker Swarm, use different overlay networks for each host. This allows you to have independent Docker networks for each host, effectively isolating them from each other.

   • Example: Host 1 could use the overlay network web-network, while Host 2 uses database-network.

3. Map Docker Ports to External Ports:

   • When you deploy your Docker containers, map the container ports to specific external ports. This is crucial for port forwarding to work correctly.

   • Example: If your web server container exposes port 80, you can map it to port 8080 on Host 1 and port 8081 on Host 2, making them accessible from the public network.

Example Implementation (Based on GitHub Discussion):

Let's consider a scenario where two Docker hosts (Host1 and Host2) need to communicate with each other through a single router. Both hosts run a different web server application:

• Host1: Web server running on port 80, container named web1
• Host2: Web server running on port 80, container named web2

Here's how you would implement this setup:

1. Router Configuration:

   • Forward external port 8080 to Host 1's internal IP address and port 80.
   • Forward external port 8081 to Host 2's internal IP address and port 80.

2. Docker Swarm Setup:

   • Create two overlay networks: web1-network for Host 1 and web2-network for Host 2.
   • Deploy the web1 container on Host 1 connected to web1-network.
   • Deploy the web2 container on Host 2 connected to web2-network.
   • Map the container port 80 to external port 8080 for web1 and external port 8081 for web2.

3. Communication:

   • Now, you can access the web servers from the public network at http://your-router-ip:8080 for web1 and http://your-router-ip:8081 for web2.

Additional Considerations:

• Security: Use a firewall to protect your Docker Swarm network from unauthorized access.
• Load Balancing: For more advanced scenarios, consider using a load balancer to distribute traffic across your Docker hosts.

Conclusion:

By combining port forwarding on your router with network isolation in your Docker Swarm, you can effectively connect multiple Docker hosts to a single router. This approach allows you to manage your Docker applications across multiple hosts without relying on a dedicated network for each, providing flexibility and efficiency.