How to fix ERROR: Network mynetwork declared as external, but could not be found in Docker

The most direct fix is to create the network that Docker Compose is looking for:

# Create a basic bridge network
docker network create mynetwork

# Verify it was created
docker network ls | grep mynetwork

Replace mynetwork with the actual network name from your error message.

For Swarm mode overlay networks:

docker network create -d overlay --attachable mynetwork

Check your docker-compose.yml to see how the network is defined:

networks:
  mynetwork:
    external: true

Or with an explicit name:

networks:
  mynetwork:
    name: my-actual-network-name
    external: true

List existing networks to find the correct name:

docker network ls

If there's a name mismatch, either:
1. Update the Compose file to use the correct network name
2. Create a new network with the name Compose expects

If you don't actually need to share this network with other projects or manually-created containers, remove the external: true flag:

Before (external network):

networks:
  mynetwork:
    external: true

After (Compose-managed network):

networks:
  mynetwork:
    driver: bridge

Or simply don't define the network at all - Docker Compose creates a default network automatically:

services:
  web:
    image: nginx
  api:
    image: myapi
# No networks section needed - services can communicate by service name

Docker Compose prefixes auto-created networks with the project name. If you're referencing a network created by another Compose project, include the full name:

# List networks to see full names
docker network ls

# Output might show:
# myproject_default
# otherapp_shared

Update your Compose file to use the full network name:

networks:
  shared:
    name: otherapp_shared
    external: true

The name field specifies the actual Docker network name, while the key (shared) is just the alias within your Compose file.

If the network is created by another docker-compose project, start that project first:

# Start the project that creates the network
cd /path/to/other-project
docker-compose up -d

# Verify the network exists
docker network ls

# Now start your project
cd /path/to/your-project
docker-compose up -d

For a more robust solution, create a shared network in a startup script rather than depending on another Compose project.

If you're using Docker Swarm, overlay networks require special handling:

# Initialize swarm if not already done
docker swarm init

# Create an attachable overlay network
docker network create -d overlay --attachable mynetwork

# Verify it's an overlay network
docker network inspect mynetwork | grep Driver

In your Compose file:

networks:
  mynetwork:
    external: true
    # driver: overlay is inferred from the existing network

Note: On Swarm worker nodes, overlay networks may not be immediately visible until a service uses them. The network is created on the manager but only materializes on workers when needed.