Random ports

Learn to assign random ports with -P.

Random port

If no host port is written (-p 80), Docker picks a free high port. -P (uppercase) publishes all EXPOSEd ports in the image to random ports:

$ docker run -d --name w1 -p 80 nginx
$ docker port w1
80/tcp -> 0.0.0.0:32768
$ docker run -d --name w2 -P nginx
$ docker port w2
80/tcp -> 0.0.0.0:32769

When is it used?

Random ports are handy for opening many copies of the same service without conflicts (tests, CI). The chosen port is then read with a script:

docker run -d --name test-db -P postgres:16
PORT=$(docker port test-db 5432 | cut -d: -f2)
echo "Bağlantı: localhost:$PORT"

Exercise Test database

Write two commands: start a container named test-db from postgres:16 in the background with POSTGRES_PASSWORD=test, publishing all EXPOSEd ports randomly; then show which host port 5432 is mapped to.

Show example solution

docker run -d --name test-db -e POSTGRES_PASSWORD=test -P postgres:16
docker port test-db 5432

Test yourself

Which ports does the -P flag publish?
- Only 80
- None
- All host ports
- All ports declared with EXPOSE in the image
Show answer

All ports declared with EXPOSE in the image — This is a concrete effect of EXPOSE.

Publish all EXPOSEd ports randomly.

docker run -d ____ nginx

Show answer

docker run -d -P nginx

Match.

-p 80

Random host port
-P

All EXPOSE ports
32768

Chosen high port
docker port

Find the chosen one

Next lesson: IP binding

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