我是从 Linux 迁移过来的 FreeBSD 新用户，Linux 中使用的是 netfilter 防火墙框架（LCTT 译注：netfilter 是由 Rusty Russell 提出的 Linux 2.4 内核防火墙框架）。那么在 FreeBSD 上，我该如何设置 PF 防火墙，来保护只有一个公共 IP 地址和端口的 web 服务器呢？
PF 是包过滤器的简称。它是为 OpenBSD 开发的，但是已经被移植到了 FreeBSD 以及其它操作系统上。PF 是一个包状态过滤引擎。在这篇教程中，我将向你展示如何在 FreeBSD 10.x 以及 11.x 中设置 PF 防火墙，从而来保护 web 服务器。
第一步：开启 PF 防火墙
你需要把下面这几行内容添加到文件 /etc/rc.conf 文件中：
# echo 'pf_enable="YES"' >> /etc/rc.conf# echo 'pf_rules="/usr/local/etc/pf.conf"' >> /etc/rc.conf# echo 'pflog_enable="YES"' >> /etc/rc.conf# echo 'pflog_logfile="/var/log/pflog"' >> /etc/rc.conf
在这里：
pf_enable="YES"- 开启 PF 服务pf_rules="/usr/local/etc/pf.conf"- 从文件/usr/local/etc/pf.conf中读取 PF 规则pflog_enable="YES"- 为 PF 服务打开日志支持pflog_logfile="/var/log/pflog"- 存储日志的文件，即日志存于文件/var/log/pflog中
第二步：在 /usr/local/etc/pf.conf 文件中创建防火墙规则
输入下面这个命令打开文件（超级用户模式下）：
# vi /usr/local/etc/pf.conf
在文件中添加下面这些 PF 规则集：
# vim: set ft=pf# /usr/local/etc/pf.conf## 设置公共端口 ##ext_if="vtnet0"## 设置服务器公共 IP 地址 ##ext_if_ip="172.xxx.yyy.zzz"## 设置并删除下面这些公共端口上的 IP 范围 ##martians = "{ 127.0.0.0/8, 192.168.0.0/16, 172.16.0.0/12, \10.0.0.0/8, 169.254.0.0/16, 192.0.2.0/24, \0.0.0.0/8, 240.0.0.0/4 }"## 设置 http(80)/https (443) 端口 ##webports = "{http, https}"## 启用下面这些服务 ##int_tcp_services = "{domain, ntp, smtp, www, https, ftp, ssh}"int_udp_services = "{domain, ntp}"## 跳过回环端口 - 跳过端口上的所有 PF 处理 ##set skip on lo## 设置 PF 应该统计的端口信息，如发送/接收字节数，通过/禁止的包的数目 ##set loginterface $ext_if## 设置默认策略 ##block return in log allblock out all# 基于 IP 分片的错误处理来防御攻击scrub in all# 删除所有不可达路由地址block drop in quick on $ext_if from $martians to anyblock drop out quick on $ext_if from any to $martians## 禁止欺骗包antispoof quick for $ext_if# 打开 SSH 端口，SSH 服务仅从 VPN IP 139.xx.yy.zz 监听 22 号端口# 出于安全原因，我不允许/接收 SSH 流量pass in quick on $ext_if inet proto tcp from 139.xxx.yyy.zzz to $ext_if_ip port = ssh flags S/SA keep state label "USER_RULE: Allow SSH from 139.xxx.yyy.zzz"## 使用下面这些规则来为所有来自任何 IP 地址的用户开启 SSH 服务 ### pass in inet proto tcp to $ext_if port ssh### [ OR ] ##### pass in inet proto tcp to $ext_if port 22# Allow Ping-Pong stuff. Be a good sysadminpass inet proto icmp icmp-type echoreq# All access to our Nginx/Apache/Lighttpd Webserver portspass proto tcp from any to $ext_if port $webports# 允许重要的发送流量pass out quick on $ext_if proto tcp to any port $int_tcp_servicespass out quick on $ext_if proto udp to any port $int_udp_services# 在下面添加自定义规则
保存并关闭文件。欢迎来参考我的规则集。如果要检查语法错误，可以运行：
# service pf check
或
/etc/rc.d/pf check
或
# pfctl -n -f /usr/local/etc/pf.conf
第三步：开始运行 PF 防火墙
命令如下。请小心，如果是基于 SSH 的会话，你可能会和服务器断开连接。
开启 PF 防火墙：
# service pf start
停用 PF 防火墙：
# service pf stop
检查语法错误：
# service pf check
重启服务：
# service pf restart
查看 PF 状态：
# service pf status
示例输出：
Status: Enabled for 0 days 00:02:18 Debug: UrgentInterface Stats for vtnet0 IPv4 IPv6Bytes In 19463 0Bytes Out 18541 0Packets InPassed 244 0Blocked 3 0Packets OutPassed 136 0Blocked 12 0State Table Total Ratecurrent entries 1searches 395 2.9/sinserts 4 0.0/sremovals 3 0.0/sCountersmatch 19 0.1/sbad-offset 0 0.0/sfragment 0 0.0/sshort 0 0.0/snormalize 0 0.0/smemory 0 0.0/sbad-timestamp 0 0.0/scongestion 0 0.0/sip-option 0 0.0/sproto-cksum 0 0.0/sstate-mismatch 0 0.0/sstate-insert 0 0.0/sstate-limit 0 0.0/ssrc-limit 0 0.0/ssynproxy 0 0.0/smap-failed 0 0.0/s
开启/关闭/重启 pflog 服务的命令
输入下面这些命令：
# service pflog start# service pflog stop# service pflog restart
第四步：pfctl 命令的简单介绍
你需要使用 pfctl 命令来查看 PF 规则集和参数配置，包括来自包过滤器的状态信息。让我们来看一下所有常见命令：
显示 PF 规则信息
# pfctl -s rules
示例输出：
block return in log allblock drop out allblock drop in quick on ! vtnet0 inet from 172.xxx.yyy.zzz/24 to anyblock drop in quick inet from 172.xxx.yyy.zzz/24 to anypass in quick on vtnet0 inet proto tcp from 139.aaa.ccc.ddd to 172.xxx.yyy.zzz/24 port = ssh flags S/SA keep state label "USER_RULE: Allow SSH from 139.aaa.ccc.ddd"pass inet proto icmp all icmp-type echoreq keep statepass out quick on vtnet0 proto tcp from any to any port = domain flags S/SA keep statepass out quick on vtnet0 proto tcp from any to any port = ntp flags S/SA keep statepass out quick on vtnet0 proto tcp from any to any port = smtp flags S/SA keep statepass out quick on vtnet0 proto tcp from any to any port = http flags S/SA keep statepass out quick on vtnet0 proto tcp from any to any port = https flags S/SA keep statepass out quick on vtnet0 proto tcp from any to any port = ftp flags S/SA keep statepass out quick on vtnet0 proto tcp from any to any port = ssh flags S/SA keep statepass out quick on vtnet0 proto udp from any to any port = domain keep statepass out quick on vtnet0 proto udp from any to any port = ntp keep state
显示每条规则的详细内容
# pfctl -v -s rules
在每条规则的详细输出中添加规则编号：
# pfctl -vvsr show
显示状态信息
# pfctl -s state# pfctl -s state | more# pfctl -s state | grep 'something'
如何在命令行中禁止 PF 服务
# pfctl -d
如何在命令行中启用 PF 服务
# pfctl -e
如何在命令行中刷新 PF 规则/NAT/路由表
# pfctl -F all
示例输出：
rules clearednat cleared0 tables deleted.2 states clearedsource tracking entries clearedpf: statistics clearedpf: interface flags reset
如何在命令行中仅刷新 PF 规则
# pfctl -F rules
如何在命令行中仅刷新队列
# pfctl -F queue
如何在命令行中刷新统计信息（它不是任何规则的一部分）
# pfctl -F info
如何在命令行中清除所有计数器
# pfctl -z clear
第五步：查看 PF 日志
PF 日志是二进制格式的。使用下面这一命令来查看：
# tcpdump -n -e -ttt -r /var/log/pflog
示例输出：
Aug 29 15:41:11.757829 rule 0/(match) block in on vio0: 86.47.225.151.55806 > 45.FOO.BAR.IP.23: S 757158343:757158343(0) win 52206 [tos 0x28]Aug 29 15:41:44.193309 rule 0/(match) block in on vio0: 5.196.83.88.25461 > 45.FOO.BAR.IP.26941: S 2224505792:2224505792(0) ack 4252565505 win 17520 (DF) [tos 0x24]Aug 29 15:41:54.628027 rule 0/(match) block in on vio0: 45.55.13.94.50217 > 45.FOO.BAR.IP.465: S 3941123632:3941123632(0) win 65535Aug 29 15:42:11.126427 rule 0/(match) block in on vio0: 87.250.224.127.59862 > 45.FOO.BAR.IP.80: S 248176545:248176545(0) win 28200 <mss 1410,sackOK,timestamp 1044055305 0,nop,wscale 8> (DF)Aug 29 15:43:04.953537 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7475: S 1164335542:1164335542(0) win 1024Aug 29 15:43:05.122156 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7475: R 1164335543:1164335543(0) win 1200Aug 29 15:43:37.302410 rule 0/(match) block in on vio0: 94.130.12.27.18080 > 45.FOO.BAR.IP.64857: S 683904905:683904905(0) ack 4000841729 win 16384 <mss 1460>Aug 29 15:44:46.574863 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7677: S 3451987887:3451987887(0) win 1024Aug 29 15:44:46.819754 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.7677: R 3451987888:3451987888(0) win 1200Aug 29 15:45:21.194752 rule 0/(match) block in on vio0: 185.40.4.130.55910 > 45.FOO.BAR.IP.80: S 3106068642:3106068642(0) win 1024Aug 29 15:45:32.999219 rule 0/(match) block in on vio0: 185.40.4.130.55910 > 45.FOO.BAR.IP.808: S 322591763:322591763(0) win 1024Aug 29 15:46:30.157884 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6511: S 2412580953:2412580953(0) win 1024 [tos 0x28]Aug 29 15:46:30.252023 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6511: R 2412580954:2412580954(0) win 1200 [tos 0x28]Aug 29 15:49:44.337015 rule 0/(match) block in on vio0: 189.219.226.213.22640 > 45.FOO.BAR.IP.23: S 14807:14807(0) win 14600 [tos 0x28]Aug 29 15:49:55.161572 rule 0/(match) block in on vio0: 5.196.83.88.25461 > 45.FOO.BAR.IP.40321: S 1297217585:1297217585(0) ack 1051525121 win 17520 (DF) [tos 0x24]Aug 29 15:49:59.735391 rule 0/(match) block in on vio0: 36.7.147.209.2545 > 45.FOO.BAR.IP.3389: SWE 3577047469:3577047469(0) win 8192 <mss 1460,nop,wscale 8,nop,nop,sackOK> (DF) [tos 0x2 (E)]Aug 29 15:50:00.703229 rule 0/(match) block in on vio0: 36.7.147.209.2546 > 45.FOO.BAR.IP.3389: SWE 1539382950:1539382950(0) win 8192 <mss 1460,nop,wscale 8,nop,nop,sackOK> (DF) [tos 0x2 (E)]Aug 29 15:51:33.880334 rule 0/(match) block in on vio0: 45.55.22.21.53510 > 45.FOO.BAR.IP.2362: udp 14Aug 29 15:51:34.006656 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6491: S 151489102:151489102(0) win 1024 [tos 0x28]Aug 29 15:51:34.274654 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6491: R 151489103:151489103(0) win 1200 [tos 0x28]Aug 29 15:51:36.393019 rule 0/(match) block in on vio0: 60.191.38.78.4249 > 45.FOO.BAR.IP.8000: S 3746478095:3746478095(0) win 29200 (DF)Aug 29 15:51:57.213051 rule 0/(match) block in on vio0: 24.137.245.138.7343 > 45.FOO.BAR.IP.5358: S 14134:14134(0) win 14600Aug 29 15:52:37.852219 rule 0/(match) block in on vio0: 122.226.185.125.51128 > 45.FOO.BAR.IP.23: S 1715745381:1715745381(0) win 5840 <mss 1420,sackOK,timestamp 13511417 0,nop,wscale 2> (DF)Aug 29 15:53:31.309325 rule 0/(match) block in on vio0: 189.218.148.69.377 > 45.FOO.BAR.IP5358: S 65340:65340(0) win 14600 [tos 0x28]Aug 29 15:53:31.809570 rule 0/(match) block in on vio0: 13.93.104.140.53184 > 45.FOO.BAR.IP.1433: S 39854048:39854048(0) win 1024Aug 29 15:53:32.138231 rule 0/(match) block in on vio0: 13.93.104.140.53184 > 45.FOO.BAR.IP.1433: R 39854049:39854049(0) win 1200Aug 29 15:53:41.459088 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6028: S 168338703:168338703(0) win 1024Aug 29 15:53:41.789732 rule 0/(match) block in on vio0: 77.72.82.22.47218 > 45.FOO.BAR.IP.6028: R 168338704:168338704(0) win 1200Aug 29 15:54:34.993594 rule 0/(match) block in on vio0: 212.47.234.50.5102 > 45.FOO.BAR.IP.5060: udp 408 (DF) [tos 0x28]Aug 29 15:54:57.987449 rule 0/(match) block in on vio0: 51.15.69.145.5100 > 45.FOO.BAR.IP.5060: udp 406 (DF) [tos 0x28]Aug 29 15:55:07.001743 rule 0/(match) block in on vio0: 190.83.174.214.58863 > 45.FOO.BAR.IP.23: S 757158343:757158343(0) win 27420Aug 29 15:55:51.269549 rule 0/(match) block in on vio0: 142.217.201.69.26112 > 45.FOO.BAR.IP.22: S 757158343:757158343(0) win 22840 <mss 1460>Aug 29 15:58:41.346028 rule 0/(match) block in on vio0: 169.1.29.111.29765 > 45.FOO.BAR.IP.23: S 757158343:757158343(0) win 28509Aug 29 15:59:11.575927 rule 0/(match) block in on vio0: 187.160.235.162.32427 > 45.FOO.BAR.IP.5358: S 22445:22445(0) win 14600 [tos 0x28]Aug 29 15:59:37.826598 rule 0/(match) block in on vio0: 94.74.81.97.54656 > 45.FOO.BAR.IP.3128: S 2720157526:2720157526(0) win 1024 [tos 0x28]statefulAug 29 15:59:37.991171 rule 0/(match) block in on vio0: 94.74.81.97.54656 > 45.FOO.BAR.IP.3128: R 2720157527:2720157527(0) win 1200 [tos 0x28]Aug 29 16:01:36.990050 rule 0/(match) block in on vio0: 182.18.8.28.23299 > 45.FOO.BAR.IP.445: S 1510146048:1510146048(0) win 16384
如果要查看实时日志，可以运行：
# tcpdump -n -e -ttt -i pflog0
如果你想了解更多信息，可以访问 PF FAQ 和 FreeBSD HANDBOOK 以及下面这些 man 页面：
# man tcpdump# man pfctl# man pf
