tl;dr
firewall-cmd --permanent --zone=public --add-port=2888/tcp
firewall-cmd --reload #重新载入服务
永久配置firewalld开启端口。
之前的一些坑
之前的一篇文章CentOS 7部署Node.js+MongoDB：在VPS上从安装到Hello world中，讲到了CentOS开启端口用这个命令
firewall-cmd --add-port=3000/tcp
这样是没错，开启了端口，但是后面发现这个端口会莫名其妙的被关闭
Google一番后了解到这样添加端口是运行时配置（Runtime configuration），在重载或重启firewalld后，这个配置就失效了。
自动关闭原因
CentOS 7 采用了firewalld作为防火墙服务，在Red Hat官方文档的Security Guide中有介绍firewalld
The dynamic firewall daemon firewalld provides a dynamically managed firewall with support for network “zones” to assign a level of trust to a network and its associated connections and interfaces. It has support for IPv4 and IPv6 firewall settings. It supports Ethernet bridges and has a separation of runtime and permanent configuration options. It also has an interface for services or applications to add firewall rules directly.
动态防火墙守护进程firewalld提供一个动态管理的防火墙，支持网络“区域”（zones），以用来给一个网络以及其关联的链接和接口分配一个信任层级。firewalld支持IPv4跟IPv6的防火墙设置。它还支持以太网桥，并且有运行时配置选项跟永久配置选项（runtime and permanent configuration options），二者相互分离。并且firewalld为服务或应用直接添加防火墙规则提供了接口。
有关Network Zones的概念这边不细讲，参照Security Guide。
firewalld有个图形化配置工具firewall-config，还有个命令行客户端，就是firewall-cmd了。
我们暂时还没用到图形化工具，所以这边就说一下firewall-cmd
Security Guide中关于firewall-cmd的介绍：
A command line client,firewall-cmd,is provided. It can be used to make permanent and non-permanent runtime changes as explained in man firewall-cmd(1). Permanent changes need to be made as explained in the firewalld(1) man page. Note that the firewall-cmd command can be run by the root user and also by an administrative user,in other words,a member of the wheel group. In the latter case the command will be authorized via the polkit mechanism.
具体就不翻译了，大概是说
firewall-cmd可以永久或非永久地改变配置，永久配置需要如man page中解释的那样改变（日了狗了）。
于是又去翻了翻firewalld(1)的man page，里面有两段关于Runtime configuration跟Permanent configuration的解释。
Runtime configuration
Runtime configuration is the actual active configuration and is not permanent. After
reload/restart of the service or a system reboot,runtime settings will be gone if they
haven’t been also in permanent configuration.Permanent configuration
The permanent configuration is stored in config files and will be loaded and become new
runtime configuration with every machine boot or service reload/restart.
运行时配置
运行时配置是实际上启用了但不是永久的配置。在服务重载/重启或系统重启之后，运行时的设置如果不存在永久配置中，就会失效。
永久配置
永久配置被保存在配置文件中，随着每次机器启动或服务重载/重启，永久配置都会被载入，变成新的运行时配置。
永久开启端口
好了，说了那么多，应该是理清楚端口被自动关闭的原委了，那怎么永久开启端口呢，万能的Security Guide中给出了答案
The rules can be made permanent by adding the –permanent option using the firewall-cmd –permanent –direct command or by modifying /etc/firewalld/direct.xml.
只要添加规则时加上–permanent参数或者修改/etc/firewalld/direct.xml就行了。
所以只需要两条命令
firewall-cmd --permanent --zone=public --add-port=2888/tcp
firewall-cmd --reload #重新载入服务
