What does expose mean in docker?

In docker, expose means marking information and is a way of document expression; expose can allow developers to quickly understand which services are opened by Image in the dockerfile. It cannot directly open the port, which is convenient for operation. When maintenance personnel use containers, they know which ports to open. If there is no expose port, services between Containers can also access each other.

The operating environment of this tutorial: linux7.3 system, docker version 19.03, Dell G3 computer.

What does expose mean in docker?

The expose in Dockerfile is tag information. The port cannot be opened directly, so that the operation and maintenance personnel know to open it when using the container. which ports. When using docker run -itd -P xxx:xxx , capital -P will map the port defined using EXPOSE in the Dockerfile file, and there is no need to manually -p: this form.

EXPOSE is just a way of document expression. In the dockerfile, developers can quickly understand which services are opened by Image. It has no practical significance. If there is no EXPOSE port, services between Containers can also access each other.

The difference between expose and publish (run -p) in Docker

The main record here is a small doubt I discovered recently using docker. To sum up:

In docker, what are the differences and connections between the docker run -p exposed interface when the command line is started and the expose interface in the dockerfile.

The specific description is as follows:

For example, I first Expose a port 8080 in the Dockerfile

EXPOSE 8080

Then I build this image for later use Use

docker build -t myimage - Dockerfile

The next step is to use docker run to start a container instance of this image, and publish a port

docker container run -d -p 8080 myimage

There is a question I mentioned here, since I have already added it in the Dockerfile Expose port 8080 is declared here. Why, I still need to publish this port (-p). What is the difference and connection between the two? Can I

docker container run -d myimage

directly? In this case, are there any ports exposed?

After testing, (the testing process is omitted, interested friends can test it by themselves. In fact, there are essential differences between the two, and there is also a certain degree of correlation.

I list these four combinations here

A, neither Expose nor run -p

B in Dockerfile, only Expose this port in Dockerfile

C, at the same time Expose in the Dockerfile, and run -p

D, only run -p

1------For case A, start in this container The service cannot be accessed by the host host and external network, nor by the link container. It can only be used inside this container.

2--------For case B, start it in this container. The service cannot be accessed by the outside world of docker (host and other hosts), but can be accessed by containers of other links through container

link

3--------For In case C, the started cotnainer can be accessed by both the outside world of docker and the container of the link.

4---------For case D, in fact, docker has made special implicit Conversion, equivalent to case C, can be accessed by the outside world or by the link's container (this is true because docker believes that since you have to open port

to the outside world, it is equivalent to other containers that can definitely be accessed, so docker does automatic Expose

Recommended learning: "docker video tutorial"

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