Linux 防火墙的基本认识
Linux防火墙是由Netfilter组件提供的,Netfilter工作在内核空间,集成在linux内核中
Netfilter 是Linux2.4.x之后新一代的Linux防火墙机制,是linux内核的一个子系统。Netfilter采用模块化设计,具有良好的可扩充性,提供扩展各种网络服务的结构化底层框架。Netfilter与IP协议栈是无缝契合,并允许对数据报进行过滤、地址转换、处理等操作
Netfilter官网文档:https://netfilter.org/documentation/
2.2 防火墙工具介绍
2.2.1 iptables
由软件包iptables提供的命令行工具,工作在用户空间,用来编写规则,写好的规则被送往netfilter,告诉内核如何去处理信息包
[root@centos8 ~]#rpm -qi iptables Name : iptables Version : 1.8.2 Release : 9.el8 Architecture: x86_64 Install Date: Wed 25 Sep 2019 09:29:06 PM CST Group : Unspecified Size : 2050086 License : GPLv2 and Artistic 2.0 and ISC Signature : RSA/SHA256, Tue 02 Jul 2019 06:50:00 AM CST, Key ID 05b555b38483c65d Source RPM : iptables-1.8.2-9.el8.src.rpm Build Date : Sat 11 May 2019 10:21:57 PM CST Build Host : x86-01.mbox.centos.org Relocations : (not relocatable) Packager : CentOS Buildsys <bugs@centos.org> Vendor : CentOS URL : http://www.netfilter.org/ Summary : Tools for managing Linux kernel packet filtering capabilities Description : The iptables utility controls the network packet filtering code in the Linux kernel. If you need to set up firewalls and/or IP masquerading, you should either install nftables or this package. Note: This package contains the nftables-based variants of iptables and ip6tables, which are drop-in replacements of the legacy tools. [root@centos8 ~]# iptables --version iptables v1.8.2 (nf_tables) [root@centos8 ~]#ll /usr/sbin/iptables lrwxrwxrwx. 1 root root 17 May 11 2019 /usr/sbin/iptables -> xtables-nft-multi [root@centos7 ~]#ll /usr/sbin/iptables lrwxrwxrwx. 1 root root 13 Dec 9 2018 /usr/sbin/iptables -> xtables-multi [root@centos7 ~]# iptables --version iptables v1.4.21 [root@centos6 ~]#iptables --version iptables v1.4.7 [root@centos6 ~]#ll /sbin/iptables lrwxrwxrwx. 1 root root 33 Dec 12 2018 /sbin/iptables -> /etc/alternatives/iptables.x86_64 [root@centos6 ~]#ll /etc/alternatives/iptables.x86_64 lrwxrwxrwx. 1 root root 20 Dec 12 2018 /etc/alternatives/iptables.x86_64 -> /sbin/iptables-1.4.7 [root@centos6 ~]#ll /sbin/iptables lrwxrwxrwx. 1 root root 33 Dec 12 2018 /sbin/iptables -> /etc/alternatives/iptables.x86_64
范例:安装iptables的service包
[root@centos8 ~]#dnf -y install iptables-services
[root@centos8 ~]#rpm -ql iptables-services
/etc/sysconfig/ip6tables
/etc/sysconfig/iptables
/usr/lib/systemd/system/ip6tables.service
/usr/lib/systemd/system/iptables.service
/usr/libexec/initscripts/legacy-actions/ip6tables
/usr/libexec/initscripts/legacy-actions/ip6tables/panic
/usr/libexec/initscripts/legacy-actions/ip6tables/save
/usr/libexec/initscripts/legacy-actions/iptables
/usr/libexec/initscripts/legacy-actions/iptables/panic
/usr/libexec/initscripts/legacy-actions/iptables/save
/usr/libexec/iptables
/usr/libexec/iptables/ip6tables.init
/usr/libexec/iptables/iptables.init
2.2.2 firewalld
从CentOS 7 版开始引入了新的前端管理工具
软件包:
-
firewalld
-
firewalld-config
管理工具:
-
firewall-cmd 命令行工具
-
firewall-config 图形工作
2.2.3 nftables
此软件是CentOS 8 新特性,Nftables最初在法国巴黎的Netfilter Workshop 2008上发表,然后由长期的netfilter核心团队成员和项目负责人Patrick McHardy于2009年3月发布。它在2013年末合并到Linux内核中,自2014年以来已在内核3.13中可用。
它重用了netfilter框架的许多部分,例如连接跟踪和NAT功能。它还保留了命名法和基本iptables设计的几个部分,例如表,链和规则。就像iptables一样,表充当链的容器,并且链包含单独的规则,这些规则可以执行操作,例如丢弃数据包,移至下一个规则或跳至新链。
从用户的角度来看,nftables添加了一个名为nft的新工具,该工具替代了iptables,arptables和ebtables中的所有其他工具。从体系结构的角度来看,它还替换了内核中处理数据包过滤规则集运行时评估的那些部分。
范例:查看软件包
[root@centos8 ~]#rpm -qi nftables
Name : nftables
Epoch : 1
Version : 0.9.0
Release : 8.el8
Architecture: x86_64
Install Date: Wed 25 Sep 2019 09:29:06 PM CST
Group : Unspecified
Size : 758622
License : GPLv2
Signature : RSA/SHA256, Tue 02 Jul 2019 08:19:09 AM CST, Key ID 05b555b38483c65d
Source RPM : nftables-0.9.0-8.el8.src.rpm
Build Date : Sat 11 May 2019 11:06:46 PM CST
Build Host : x86-01.mbox.centos.org
Relocations : (not relocatable)
Packager : CentOS Buildsys <bugs@centos.org>
Vendor : CentOS
URL : http://netfilter.org/projects/nftables/
Summary : Netfilter Tables userspace utillites
Description :
Netfilter Tables userspace utilities.
范例:CentOS 8 支持三种防火墙工具
[root@centos8 ~]#systemctl status iptables.service ● iptables.service - IPv4 firewall with iptables Loaded: loaded (/usr/lib/systemd/system/iptables.service; disabled; vendor preset: disabled) Active: inactive (dead) [root@centos8 ~]#systemctl status firewalld.service ● firewalld.service - firewalld - dynamic firewall daemon Loaded: loaded (/usr/lib/systemd/system/firewalld.service; disabled; vendor preset: enabled) Active: inactive (dead) Docs: man:firewalld(1) [root@centos8 ~]#systemctl status nftables.service ● nftables.service - Netfilter Tables Loaded: loaded (/usr/lib/systemd/system/nftables.service; disabled; vendor preset: disabled) Active: inactive (dead) Docs: man:nft(8)
2.3 netfilter 中五个勾子函数和报文流向
Netfilter在内核中选取五个位置放了五个hook(勾子) function(INPUT、OUTPUT、FORWARD、PREROUTING、POSTROUTING),而这五个hook function向用户开放,用户可以通过一个命令工具(iptables)向其写入规则 由信息过滤表(table)组成,包含控制IP包处理的规则集(rules),规则被分组放在链(chain)上
提示:从 Linux kernel 4.2 版以后,Netfilter 在prerouting 前加了一个 ingress 勾子函数。可以使用这个新的入口挂钩来过滤来自第2层的流量,这个新挂钩比预路由要早,基本上是 tc 命令(流量控制工具)的替代品
三种报文流向
-
流入本机:PREROUTING –> INPUT–>用户空间进程
-
流出本机:用户空间进程 –>OUTPUT–> POSTROUTING
-
转发:PREROUTING –> FORWARD –> POSTROUTING
2.4 iptables的组成
iptables由五个表table和五个链chain以及一些规则组成
五个表table:filter、nat、mangle、raw、security
filter表:过滤规则表,根据预定义的规则过滤符合条件的数据包 nat表:network address translation 地址转换规则表 mangle:修改数据标记位规则表 raw:关闭启用的连接跟踪机制,加快封包穿越防火墙速度 security:用于强制访问控制(MAC)网络规则,由Linux安全模块(如SELinux)实现
优先级由高到低的顺序为:
security -->raw-->mangle-->nat-->filter
五个内置链chain:
INPUT,OUTPUT,FORWARD,PREROUTING,POSTROUTING
链 chain:
-
内置链:每个内置链对应于一个钩子函数
-
自定义链:用于对内置链进行扩展或补充,可实现更灵活的规则组织管理机制;只有Hook钩子调用自定义链时,才生效
表和链对应关系
数据包过滤匹配流程
内核中数据包的传输过程
-
当一个数据包进入网卡时,数据包首先进入PREROUTING链,内核根据数据包目的IP判断是否需要转送出去
-
如果数据包是进入本机的,数据包就会沿着图向下移动,到达INPUT链。数据包到达INPUT链后,任何进程都会收到它。本机上运行的程序可以发送数据包,这些数据包经过OUTPUT链,然后到达POSTROUTING链输出
-
如果数据包是要转发出去的,且内核允许转发,数据包就会向右移动,经过FORWARD链,然后到达POSTROUTING链输出
范例:
root@centos8 ~]#iptables -vnL -t filter
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
[root@centos8 ~]#iptables -vnL -t nat
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
[root@centos8 ~]#iptables -vnL -t mangle
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
[root@centos8 ~]#iptables -vnL -t raw
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
[root@centos8 ~]#iptables -vnL -t security
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
[root@centos8 ~]#
#CentOS 6 nat表不支持INPUT链
[root@centos6 ~]#iptables -vnL -t nat
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
2.5 netfilter 完整流程
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