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Part 2 - How to Perform Packet Filtering, Network Address Translation and Set Kernel Runtime Parameters
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================================================================================
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As promised in Part 1 (“[Setup Static Network Routing][1]”), in this article (Part 2 of RHCE series) we will begin by introducing the principles of packet filtering and network address translation (NAT) in Red Hat Enterprise Linux 7, before diving into setting runtime kernel parameters to modify the behavior of a running kernel if certain conditions change or needs arise.
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RHCE: Network Packet Filtering – Part 2
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### Network Packet Filtering in RHEL 7 ###
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When we talk about packet filtering, we refer to a process performed by a firewall in which it reads the header of each data packet that attempts to pass through it. Then, it filters the packet by taking the required action based on rules that have been previously defined by the system administrator.
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As you probably know, beginning with RHEL 7, the default service that manages firewall rules is [firewalld][2]. Like iptables, it talks to the netfilter module in the Linux kernel in order to examine and manipulate network packets. Unlike iptables, updates can take effect immediately without interrupting active connections – you don’t even have to restart the service.
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Another advantage of firewalld is that it allows us to define rules based on pre-configured service names (more on that in a minute).
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In Part 1, we used the following scenario:
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Static Routing Network Diagram
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However, you will recall that we disabled the firewall on router #2 to simplify the example since we had not covered packet filtering yet. Let’s see now how we can enable incoming packets destined for a specific service or port in the destination.
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First, let’s add a permanent rule to allow inbound traffic in enp0s3 (192.168.0.19) to enp0s8 (10.0.0.18):
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# firewall-cmd --permanent --direct --add-rule ipv4 filter FORWARD 0 -i enp0s3 -o enp0s8 -j ACCEPT
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The above command will save the rule to /etc/firewalld/direct.xml:
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# cat /etc/firewalld/direct.xml
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Check Firewalld Saved Rules
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Then enable the rule for it to take effect immediately:
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# firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -i enp0s3 -o enp0s8 -j ACCEPT
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Now you can telnet to the web server from the RHEL 7 box and run [tcpdump][3] again to monitor the TCP traffic between the two machines, this time with the firewall in router #2 enabled.
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# telnet 10.0.0.20 80
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# tcpdump -qnnvvv -i enp0s3 host 10.0.0.20
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What if you want to only allow incoming connections to the web server (port 80) from 192.168.0.18 and block connections from other sources in the 192.168.0.0/24 network?
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In the web server’s firewall, add the following rules:
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.18/24" service name="http" accept'
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.18/24" service name="http" accept' --permanent
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.0/24" service name="http" drop'
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.0/24" service name="http" drop' --permanent
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Now you can make HTTP requests to the web server, from 192.168.0.18 and from some other machine in 192.168.0.0/24. In the first case the connection should complete successfully, whereas in the second it will eventually timeout.
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To do so, any of the following commands will do the trick:
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# telnet 10.0.0.20 80
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# wget 10.0.0.20
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I strongly advise you to check out the [Firewalld Rich Language][4] documentation in the Fedora Project Wiki for further details on rich rules.
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### Network Address Translation in RHEL 7 ###
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Network Address Translation (NAT) is the process where a group of computers (it can also be just one of them) in a private network are assigned an unique public IP address. As result, they are still uniquely identified by their own private IP address inside the network but to the outside they all “seem” the same.
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In addition, NAT makes it possible that computers inside a network sends requests to outside resources (like the Internet) and have the corresponding responses be sent back to the source system only.
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Let’s now consider the following scenario:
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Network Address Translation
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In router #2, we will move the enp0s3 interface to the external zone, and enp0s8 to the internal zone, where masquerading, or NAT, is enabled by default:
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# firewall-cmd --list-all --zone=external
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# firewall-cmd --change-interface=enp0s3 --zone=external
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# firewall-cmd --change-interface=enp0s3 --zone=external --permanent
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# firewall-cmd --change-interface=enp0s8 --zone=internal
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# firewall-cmd --change-interface=enp0s8 --zone=internal --permanent
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For our current setup, the internal zone – along with everything that is enabled in it will be the default zone:
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# firewall-cmd --set-default-zone=internal
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Next, let’s reload firewall rules and keep state information:
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# firewall-cmd --reload
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Finally, let’s add router #2 as default gateway in the web server:
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# ip route add default via 10.0.0.18
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You can now verify that you can ping router #1 and an external site (tecmint.com, for example) from the web server:
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# ping -c 2 192.168.0.1
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# ping -c 2 tecmint.com
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Verify Network Routing
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### Setting Kernel Runtime Parameters in RHEL 7 ###
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In Linux, you are allowed to change, enable, and disable the kernel runtime parameters, and RHEL is no exception. The /proc/sys interface (sysctl) lets you set runtime parameters on-the-fly to modify the system’s behavior without much hassle when operating conditions change.
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To do so, the echo shell built-in is used to write to files inside /proc/sys/<category>, where <category> is most likely one of the following directories:
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- dev: parameters for specific devices connected to the machine.
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- fs: filesystem configuration (quotas and inodes, for example).
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- kernel: kernel-specific configuration.
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- net: network configuration.
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- vm: use of the kernel’s virtual memory.
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To display the list of all the currently available values, run
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# sysctl -a | less
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In Part 1, we changed the value of the net.ipv4.ip_forward parameter by doing
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# echo 1 > /proc/sys/net/ipv4/ip_forward
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in order to allow a Linux machine to act as router.
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Another runtime parameter that you may want to set is kernel.sysrq, which enables the Sysrq key in your keyboard to instruct the system to perform gracefully some low-level functions, such as rebooting the system if it has frozen for some reason:
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# echo 1 > /proc/sys/kernel/sysrq
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To display the value of a specific parameter, use sysctl as follows:
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# sysctl <parameter.name>
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For example,
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# sysctl net.ipv4.ip_forward
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# sysctl kernel.sysrq
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Some parameters, such as the ones mentioned above, require only one value, whereas others (for example, fs.inode-state) require multiple values:
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Check Kernel Parameters
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In either case, you need to read the kernel’s documentation before making any changes.
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Please note that these settings will go away when the system is rebooted. To make these changes permanent, we will need to add .conf files inside the /etc/sysctl.d as follows:
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# echo "net.ipv4.ip_forward = 1" > /etc/sysctl.d/10-forward.conf
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(where the number 10 indicates the order of processing relative to other files in the same directory).
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and enable the changes with
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# sysctl -p /etc/sysctl.d/10-forward.conf
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### Summary ###
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In this tutorial we have explained the basics of packet filtering, network address translation, and setting kernel runtime parameters on a running system and persistently across reboots. I hope you have found this information useful, and as always, we look forward to hearing from you!
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Don’t hesitate to share with us your questions, comments, or suggestions using the form below.
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--------------------------------------------------------------------------------
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via: http://www.tecmint.com/perform-packet-filtering-network-address-translation-and-set-kernel-runtime-parameters-in-rhel/
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作者：[Gabriel Cánepa][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
|
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|
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](https://linux.cn/) 荣誉推出
|
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[a]:http://www.tecmint.com/author/gacanepa/
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[1]:http://www.tecmint.com/how-to-setup-and-configure-static-network-routing-in-rhel/
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[2]:http://www.tecmint.com/firewalld-rules-for-centos-7/
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[3]:http://www.tecmint.com/12-tcpdump-commands-a-network-sniffer-tool/
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[4]:https://fedoraproject.org/wiki/Features/FirewalldRichLanguage
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@ -0,0 +1,175 @@
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RHCE 第二部分 - 如何进行包过滤、网络地址转换和设置内核运行时参数
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================================================================================
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正如第一部分（“[设置静态网络路由][1]”）承诺的，在这篇文章（RHCE 系列第二部分），我们首先介绍红帽企业版 Linux 7中包过滤和网络地址转换原理，然后再介绍某些条件发送变化或者需要激活时设置运行时内核参数以改变运行时内核行为。
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RHCE 第二部分：网络包过滤
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### RHEL 7 中的网络包过滤 ###
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当我们讨论数据包过滤的时候，我们指防火墙读取每个尝试通过它的数据包的包头所进行的处理。然后，根据系统管理员之前定义的规则，通过采取所要求的动作过滤数据包。
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正如你可能知道的，从 RHEL 7 开始，管理防火墙的默认服务是 [firewalld][2]。类似 iptables，它和 Linux 内核的 netfilter 模块交互以便检查和操作网络数据包。不像 iptables，Firewalld 的更新可以立即生效，而不用中断活跃的连接 - 你甚至不需要重启服务。
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Firewalld 的另一个优势是它允许我们定义基于预配置服务名称的规则（之后会详细介绍）。
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在第一部分，我们用了下面的场景：
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静态路由网络示意图
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然而，你应该记得，由于还没有介绍包过滤，为了简化例子，我们停用了路由器 2号 的防火墙。现在让我们来看看如何可以使接收的数据包发送到目的地的特定服务或端口。
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首先，让我们添加一条永久规则允许从 enp0s3 (192.168.0.19) 到 enp0s8 (10.0.0.18) 的绑定流量：
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# firewall-cmd --permanent --direct --add-rule ipv4 filter FORWARD 0 -i enp0s3 -o enp0s8 -j ACCEPT
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上面的命令会把规则保存到 /etc/firewalld/direct.xml：
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# cat /etc/firewalld/direct.xml
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检查 Firewalld 保存的规则
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然后启用规则使其立即生效：
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# firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -i enp0s3 -o enp0s8 -j ACCEPT
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现在你可以从 RHEL 7 中通过 telnet 登录到 web 服务器并再次运行 [tcpdump][3] 监视两台机器之间的 TCP 流量，这次路由器 2号已经启用了防火墙。
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# telnet 10.0.0.20 80
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# tcpdump -qnnvvv -i enp0s3 host 10.0.0.20
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如果你想只允许从 192.168.0.18 到 web 服务器（80 号端口）的连接而阻塞 192.168.0.0/24 网络中的其它来源呢？
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在 web 服务器的防火墙中添加以下规则：
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.18/24" service name="http" accept'
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.18/24" service name="http" accept' --permanent
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.0/24" service name="http" drop'
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# firewall-cmd --add-rich-rule 'rule family="ipv4" source address="192.168.0.0/24" service name="http" drop' --permanent
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现在你可以从 192.168.0.18 和 192.168.0.0/24 中的其它机器发送到 web 服务器的 HTTP 请求。第一种情况连接会成功完成，但第二种情况最终会超时。
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任何下面的命令可以验证这个结果：
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# telnet 10.0.0.20 80
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# wget 10.0.0.20
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我强烈建议你看看 Fedora Project Wiki 中的 [Firewalld Rich Language][4] 文档更详细地了解关于富规则的内容。
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### RHEL 7 中的网络地址转换 ###
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网络地址转换（NAT）是为专用网络中的一组计算机（也可能是其中的一台）分配一个独立的公共 IP 地址的过程。结果，在内部网络中仍然可以用它们自己的私有 IP 地址区别，但外部“看来”它们是一样的。
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另外，网络地址转换使得内部网络中的计算机发送请求到外部资源（例如因特网）然后只有源系统能接收到对应的响应成为可能。
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现在让我们考虑下面的场景：
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网络地址转换
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在路由器 2 中，我们会把 enp0s3 接口移动到外部区域，enp0s8 到内部区域，伪装或者说 NAT 默认是启用的：
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# firewall-cmd --list-all --zone=external
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# firewall-cmd --change-interface=enp0s3 --zone=external
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# firewall-cmd --change-interface=enp0s3 --zone=external --permanent
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# firewall-cmd --change-interface=enp0s8 --zone=internal
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# firewall-cmd --change-interface=enp0s8 --zone=internal --permanent
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对于我们当前的设置，内部区域 - 以及和它一起启用的任何东西都是默认区域：
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# firewall-cmd --set-default-zone=internal
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下一步，让我们重载防火墙规则并保持状态信息：
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# firewall-cmd --reload
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最后，在 web 服务器中添加路由器 2 为默认网关：
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# ip route add default via 10.0.0.18
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现在你会发现在 web 服务器中你可以 ping 路由器 1 和外部网站（例如 tecmint.com）：
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# ping -c 2 192.168.0.1
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# ping -c 2 tecmint.com
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验证网络路由
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### 在 RHEL 7 中设置内核运行时参数 ###
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在 Linux 中，允许你更改、启用以及停用内核运行时参数，RHEL 也不例外。/proc/sys 接口允许你当操作条件发生变化时实时设置运行时参数以改变系统行为而不需太多麻烦。
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|
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为了实现这个目的，会用内建的 echo shell 写 /proc/sys/<category\> 中的文件，其中 <category\> 很可能是以下目录中的一个：
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- dev: 连接到机器中的特定设备的参数。
|
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- fs: 文件系统配置（例如 quotas 和 inodes）。
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- kernel: 内核配置。
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- net: 网络配置。
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- vm: 内核虚拟内存的使用。
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要显示所有当前可用值的列表，运行
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# sysctl -a | less
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|
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在第一部分中，我们通过以下命令改变了 net.ipv4.ip_forward 参数的值以允许 Linux 机器作为一个路由器。
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# echo 1 > /proc/sys/net/ipv4/ip_forward
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另一个你可能想要设置的运行时参数是 kernel.sysrq，它会启用你键盘上的 Sysrq 键，以使系统更好的运行一些底层函数，例如如果由于某些原因冻结了后重启系统：
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||||
# echo 1 > /proc/sys/kernel/sysrq
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||||
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||||
要显示特定参数的值，可以按照下面方式使用 sysctl：
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||||
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# sysctl <parameter.name>
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例如，
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# sysctl net.ipv4.ip_forward
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# sysctl kernel.sysrq
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一些参数，例如上面提到的一个，只需要一个值，而其它一些（例如 fs.inode-state）要求多个值：
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||||
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查看内核参数
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不管什么情况下，做任何更改之前你都需要阅读内核文档。
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||||
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请注意系统重启后这些设置会丢失。要使这些更改永久生效，我们需要添加内容到 /etc/sysctl.d 目录的 .conf 文件，像下面这样：
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||||
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||||
# echo "net.ipv4.ip_forward = 1" > /etc/sysctl.d/10-forward.conf
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||||
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||||
（其中数字 10 表示相对同一个目录中其它文件的处理顺序）。
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并用下面命令启用更改
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||||
# sysctl -p /etc/sysctl.d/10-forward.conf
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||||
### 总结 ###
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||||
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||||
在这篇指南中我们解释了基本的包过滤、网络地址变换和在运行的系统中设置内核运行时参数并使重启后能持久化。我希望这些信息能对你有用，如往常一样，我们期望收到你的回复！
|
||||
别犹豫，在下面的表格中和我们分享你的疑问、评论和建议吧。
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: http://www.tecmint.com/perform-packet-filtering-network-address-translation-and-set-kernel-runtime-parameters-in-rhel/
|
||||
|
||||
作者：[Gabriel Cánepa][a]
|
||||
译者：[ictlyh](https://github.com/ictlyh)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:http://www.tecmint.com/author/gacanepa/
|
||||
[1]:http://www.tecmint.com/how-to-setup-and-configure-static-network-routing-in-rhel/
|
||||
[2]:http://www.tecmint.com/firewalld-rules-for-centos-7/
|
||||
[3]:http://www.tecmint.com/12-tcpdump-commands-a-network-sniffer-tool/
|
||||
[4]:https://fedoraproject.org/wiki/Features/FirewalldRichLanguage
|
||||
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Reference in New Issue
Block a user