From inside of a Docker container, how do I connect to the localhost of the machine

When running applications in Docker containers, you often need to connect from the container to services running on the host machine's localhost. For example, if you have an Nginx container that needs to access a MySQL database running on your host's localhost (127.0.0.1), the default container networking won't allow this connection since the container has its own isolated network namespace.

This article explains different methods to establish connectivity between Docker containers and host services, with solutions varying by operating system and networking requirements.

Method 1 − Host Network Mode (Linux)

The simplest solution on Linux is using the --network="host" option with the Docker run command. This makes the container share the host's network stack directly.

docker run -it --network=host ubuntu:latest

With host networking, the container's localhost (127.0.0.1) points directly to the host machine. You can verify this by comparing IP addresses:

# On host machine
ip addr show eth0

# Inside container with host networking
docker run -it --network=host ubuntu:latest ip addr show eth0

Important: Any ports exposed by the container are directly accessible on the host without port mapping. This bypasses Docker's port isolation.

Method 2 − Special Hostnames (Windows/Mac)

Docker Desktop for Windows and Mac provides special hostnames to access the host:

Docker Desktop (Current Versions)

# Use this hostname in your container
host.docker.internal

Example MySQL connection from container:

mysql -uroot -h host.docker.internal -p

Docker Desktop (Legacy Mac)

# Older Mac versions
docker.for.mac.localhost
# or
docker.for.mac.host.internal

Method 3 − Bridge Network with Host IP

For bridge networking, configure your host service to listen on the Docker bridge network IP address. Find the Docker bridge IP:

ip route | grep docker0
# Typically shows 172.17.0.1

Configure your service (e.g., MySQL) to bind to this address by modifying the configuration file:

# MySQL config example
bind-address = 172.17.0.1

Then connect from the container using the host's Docker bridge IP:

mysql -uroot -h 172.17.0.1 -p

Method 4 − Custom Host Entry (Linux)

On Linux, you can add a custom host entry to make host.docker.internal work like on Windows/Mac:

docker run --add-host host.docker.internal:host-gateway my-container

This allows using the same connection strings across different platforms.

Comparison

Conclusion

Connecting from Docker containers to host services depends on your platform and networking requirements. Use --network=host on Linux for simplicity, host.docker.internal on Docker Desktop, or configure bridge networking for production environments. Choose the method that best balances ease of use with network security for your specific use case.