Docker: port is already allocated

Then a host process holds it. ss -lntp shows the PID and program.

Docker: Bind for 0.0.0.0:80 failed – port is already allocated

Docker cannot bind the host port because something already holds it: another container, a host service, or a stuck docker-proxy process.

daniele@mikweb: ~/errors — less docker-port-already-allocated.md

What the message looks like

Error response from daemon: driver failed programming external connectivity on endpoint web: Bind for 0.0.0.0:80 failed: port is already allocated

Another container publishes the same host port, often after switching Compose projects.
A host service uses the port, such as nginx, Apache or Pi-hole.
An old container is still running because docker compose down was never executed.
A container uses network_mode: host and also declares port mappings.
A docker-proxy process from a removed container still holds the port.

Who listens on the port?
ss -lntp | grep ':80 '
Which container publishes it?
docker ps --format 'table {{.Names}}\t{{.Ports}}' | grep ':80'
All ports including stopped containers
docker ps -a --format 'table {{.Names}}\t{{.Status}}\t{{.Ports}}'
Stuck proxy processes
ps aux | grep -i '[d]ocker-proxy'

Find and stop the container with docker stop NAME, then run docker compose up -d again.
Identify the host service and either stop it or move the container to another port, for example 8080:80.
With several Compose projects, assign ports deliberately instead of mapping 80 and 443 twice.
Clean up old containers with docker compose down --remove-orphans.
docker restart does not help: the port stays taken while the other process runs.

Not directly. Only one holds the host port. Put a reverse proxy in front and use different internal ports.

Then a host process holds it. ss -lntp shows the PID and program.