Скачать или смотреть How to Retrieve the Bridge's IP Address from a Docker Container or Set it via Environment Variable

Learn how to easily access the bridge's IP address inside a Docker container or provide it as an environment variable using simple commands and scripts.
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How to Retrieve the Bridge's IP Address from a Docker Container or Set it via Environment Variable

When working with Docker, developers often create networks to manage communication between containers. A common scenario involves needing to access the bridge IP address associated with a Docker network. For instance, you might want this IP for whitelisting purposes in your application. This guide will guide you through the methods available to retrieve the bridge's IP address from within a Docker container or set it via an environment variable.

Understanding the Challenge

When you create a Docker network, Docker automatically creates a bridge that acts as a virtual router enabling the containers on that network to communicate. Each bridge gets an IP address assigned to it, which can be critical for certain configurations. For example, a developer may need to whitelist the bridge's IP address in application settings to allow external services to communicate properly.

The question is: How can we retrieve this bridge's IP address while running a container inside the Docker network? Or, alternatively, can we provision this IP address as an environment variable when starting the container?

Solution: Retrieving the Bridge's IP Address

To access the bridge IP address from within a Docker container, you can follow these straightforward steps:

Step 1: Create a Docker Network

First, create a Docker network (for instance, DUMMY):

[[See Video to Reveal this Text or Code Snippet]]

Once you have done that, you can confirm the creation of the bridge network using ifconfig:

[[See Video to Reveal this Text or Code Snippet]]

The output will yield something like this:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Run a Container on the Network

Launch a new container attached to your network. Here's an example of running an Alpine Linux shell:

[[See Video to Reveal this Text or Code Snippet]]

Once inside the container, you can check the routing table using the command:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Extract the Bridge IP Address

To find the IP address of the bridge easily, you can parse the output of the ip route command:

[[See Video to Reveal this Text or Code Snippet]]

This command does the following:

Looks for the line where the first column is "default".

Prints the third column, which gives you the bridge's IP address.

Example Output

You should see something like:

[[See Video to Reveal this Text or Code Snippet]]

This indicates that the bridge's IP address 172.20.0.1 is retrieved successfully inside the container.

Alternative: Setting the Bridge IP as an Environment Variable

If you find it more convenient to pass the bridge's IP address as an environment variable, consider the following method:

Step 1: Assign the IP to an Environment Variable

Before starting your container, you can retrieve the bridge's IP address and assign it to an environment variable directly in the Docker run command:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Access the Environment Variable in Your Application

Inside your container, you can now access the BRIDGE_IP variable like so:

[[See Video to Reveal this Text or Code Snippet]]

This command will return the bridge IP address that you can use for your application's configuration.

Conclusion

Retrieving the bridge's IP address in a Docker network is a straightforward process, whether you use the ip route command directly inside the container or set it as an environment variable before launching the container. This flexibility allows for better configuration management and communication in complex Docker applications.

Armed with this knowledge, you can