为什么仅配置了"expose"的服务能够与互联网通信? [英] Why a service configured with only `expose` is able to communicate with internet?

问题描述

某些事情使我对Docker网络感到困惑.我有一个 docker-compose.yml 文件，可以这样简化:

Something confuse me about docker networking. I've a docker-compose.yml file which can be simplified like this:

version: '3.8'
services:
    foo:
        ...
        networks:
            - main_network
        ports:
            - "3000:3000"
    bar:
        ...
        networks:
            - main_network
        expose:
          - "5000"
networks:
    main_network:

根据此答案，公开 ...

暴露端口而不将其发布到主机上-只有链接的服务才能访问它们.只能指定内部端口.

Expose ports without publishing them to the host machine - they’ll only be accessible to linked services. Only the internal port can be specified.

如果这是真的， bar 应该只将5000端口公开给 foo 服务.而且它似乎按预期工作.如果可以，我将 bash 运行到 bar 服务中并执行:

If this is true, bar should only expose the 5000 port to foo service. And It seems to work as expected. If, I run bash into bar service and execute:

$ ss -lntu 

5000端口已正确打开:

The 5000 port is opened correctly:

Netid    State     Recv-Q  Send-Q   Local Address:Port    Peer Address:Port
...
tcp      LISTEN    0       128      0.0.0.0:5000          0.0.0.0:*
...

按预期，从容器外部，例如，使用Web浏览器，我无法连接到该主机.另外，如果我跑步

As expected, from outside of my container, using a web browser for example, I cannot connect to this host. Also, If I run

$ nmap -p1-65535 127.0.0.1

我可以验证仅 foo 服务的3000/TCP端口已打开:

I can verify that only the 3000/TCP port of foo service is opened:

PORT     STATE SERVICE
3000/tcp open  ppp

所以，我不明白的是，IRL，我的 bar 服务能够在线连接到Mongo Atlas或ping Internet.如果未暴露/未打开端口以接收端口，它将如何得到答案?

So, what I don't understand is that IRL, my bar service is able to connect to Mongo Atlas online or ping internet. How does it get it's answer if the ports aren't Exposed/Opened in order to receive it?

推荐答案

您所链接的答案是古老且错误的.

The answer to which you have linked is old and incorrect.

1. EXPOSE 关键字主要是禁止操作的关键字.它提供了很多信息(此映像将在这些端口上提供服务")，但是对操作没有任何影响.

1. The EXPOSE keyword is primarily a no-op. It's informative ("this image will offer services on these ports"), but it doesn't have any operational impact.

在较旧的Docker版本中， EXPOSE 关键字可以用于链接容器的服务发现，但是(a)它仍然没有任何操作影响-端口是否可用，无论是否没有匹配的 EXPOSE -容器链接"已弃用了相当长一段时间.

In older versions of Docker, the EXPOSE keyword could be used for service discovery by linked containers, but (a) it still didn't have any operational impact -- the ports were available whether or not there was a matching EXPOSE -- and container "linking" has been deprecated for a quite some time.

默认情况下，容器具有出站互联网访问权限.出站访问通过主机防火墙 nat 表中的简单NAT规则(以及 filter FORWARD 链中的相应规则)进行管理.例如，在运行Docker 20.10.2的系统上，在 FORWARD 链中，我有:

Containers have outbound internet access by default. Outbound access is managed with a simple NAT rule in the host firewall nat table (and corresponding rules in the filter FORWARD chain). E.g., on a system where I am running Docker 20.10.2, in the FORWARD chain I have:

-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -o docker0 -j DOCKER
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT

第一个规则传递作为现有TCP连接一部分的数据包.这样可以为您的出站连接返回数据包.

The first rule passes packets that are part of an existing TCP connection. This permits the return packets for your outbound connections.

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