When using dynamic port mapping or multiple services, it’s not always obvious which host port maps to which container port. docker compose port tells you exactly.

Basic usage

# Which host port maps to container port 80 on the web service?
docker compose port web 80
# Output: 0.0.0.0:8080

Why dynamic ports matter

When you let Docker assign ports automatically, the host port changes on every docker compose up:

services:
  web:
    image: nginx
    ports:
      - "80"   # Dynamic host port, container port 80

docker compose port web 80
# Output: 0.0.0.0:55432  (assigned dynamically)

This is common in CI or when running multiple instances of the same project.

Scaled services with –index

When you scale a service, each replica gets its own dynamic port. Use --index to query a specific replica:

services:
  web:
    image: nginx
    ports:
      - "80"   # Dynamic host port for each replica

# Scale to 3 replicas
docker compose up -d --scale web=3

# Get the port for each replica
docker compose port --index=1 web 80
# Output: 0.0.0.0:55001

docker compose port --index=2 web 80
# Output: 0.0.0.0:55002

docker compose port --index=3 web 80
# Output: 0.0.0.0:55003

Without --index, the command returns the port for the first replica.

Specifying protocol

By default, docker compose port looks for TCP mappings. If a service exposes both TCP and UDP on the same port, use --protocol to pick the right one:

# TCP (default)
docker compose port myservice 53

# UDP
docker compose port --protocol=udp myservice 53

Combining with other debug commands

docker compose port is one of several useful commands for network debugging:

# List all port mappings for all services
docker compose ps --format "table {{.Name}}\t{{.Ports}}"

# Get port mappings as JSON for a specific service
docker compose ps web --format json | jq '.[].Ports'

# Check connectivity from one container to another
docker compose exec app wget -qO- http://web:80/health

Further reading