The -p flag

Learn port mapping with the -p flag.

The -p flag

The -p (--publish) form is [host_ip:]host_port:container_port[/protocol]. The left side is the host, the right side is the container; mixing these up is the most common mistake:

docker run -d -p 3000:8080 api:1.0       # localhost:3000 -> container 8080
docker run -d -p 5353:53/udp dns-sunucu  # a UDP port

Port conflict

A host port can be published to only one container (or process). The second attempt errors; a different host port is chosen:

$ docker run -d -p 8080:80 nginx
$ docker run -d -p 8080:80 nginx
Error: Bind for 0.0.0.0:8080 failed: port is already allocated
$ docker run -d -p 8081:80 nginx      # the second copy on 8081

Exercise Two copies, two ports

Start two containers from the api:1.0 image in the background: api1 maps host port 3000 and api2 host port 3001 to container port 8080.

Show example solution

docker run -d --name api1 -p 3000:8080 api:1.0
docker run -d --name api2 -p 3001:8080 api:1.0

Test yourself

The application listens on 8080 inside the container and you want to open localhost:3000 in the browser. Which is right?
- --expose 3000
- -p 3000:8080
- -p 8080:3000
- -p 3000
Show answer

-p 3000:8080 — The host is on the left, the container on the right.

Publish a UDP port.

docker run -d -p 5353:53____ dns-sunucu

Show answer

docker run -d -p 5353:53/udp dns-sunucu

Match the mappings.

-p 3000:8080

localhost:3000 → 8080
-p 8081:80

localhost:8081 → 80
/udp

UDP protocol
port is already allocated

Host port taken

Next lesson: Multiple ports

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