我尝试创建iptables规则来防止我的服务器连接限制被未完成的 SYN 数据包填满，因为客户端没有返回任何 ACK 数据包（SYN 洪水攻击）。
我已经阅读了 [RFC 4987 TCP SYN 洪水攻击和常见缓解措施][1] 和 [使用 Red Hat Enterprise Linux 7 Beta 缓解 TCP SYN 洪水攻击][2]，最后尝试按照 [此处][3] 的说明进行操作：https://javapipe.com/blog/iptables-ddos-protection/
但是，我的网站仍然很容易受到以下命令的 DOS 攻击而关闭：
hping -S --flood -V -p 443 www.mydomain.com
在上面链接的 Javapipe.com 教程的最后，提到了判断 SYNPROXY 是否处于活动状态的命令。
要验证 SYNPROXY 是否正常工作，您可以执行 watch -n1 cat /proc/net/stat/synproxy。如果在您使用 SYNPROXY 的端口上建立新的 TCP 连接时值发生变化，则表示它正常工作。
命令是：
watch -n1 cat /proc/net/stat/synproxy
当我在服务器上使用该命令并尝试加载页面时，数据不会改变且全为零。
Every 1.0s: cat /proc/net/stat/synproxy Thu Jul 25 19:57:00 2019
entries syn_received cookie_invalid cookie_valid cookie_retrans conn_reopened
00000000 00000000 00000000 00000000 00000000 00000000
以下是 iptables -L 的输出
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere
INPUT_direct all -- anywhere anywhere
INPUT_ZONES_SOURCE all -- anywhere anywhere
INPUT_ZONES all -- anywhere anywhere
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
REJECT tcp -- anywhere anywhere #conn src/32 > 80 reject-with tcp-reset
ACCEPT tcp -- anywhere anywhere ctstate NEW limit: avg 20/sec burst 20
DROP tcp -- anywhere anywhere ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp flags:RST/RST limit: avg 2/sec burst 2
DROP tcp -- anywhere anywhere tcp flags:RST/RST
DROP all -- anywhere anywhere ctstate INVALID
SYNPROXY tcp -- anywhere anywhere tcp ctstate INVALID,UNTRACKED SYNPROXY sack-perm timestamp wscale 7 mss 1460
DROP all -- anywhere anywhere ctstate INVALID
DROP all -- anywhere anywhere state INVALID
Chain FORWARD (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere
FORWARD_direct all -- anywhere anywhere
FORWARD_IN_ZONES_SOURCE all -- anywhere anywhere
FORWARD_IN_ZONES all -- anywhere anywhere
FORWARD_OUT_ZONES_SOURCE all -- anywhere anywhere
FORWARD_OUT_ZONES all -- anywhere anywhere
DROP all -- anywhere anywhere ctstate INVALID
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
OUTPUT_direct all -- anywhere anywhere
Chain FORWARD_IN_ZONES (1 references)
target prot opt source destination
FWDI_public all -- anywhere anywhere [goto]
Chain FORWARD_IN_ZONES_SOURCE (1 references)
target prot opt source destination
Chain FORWARD_OUT_ZONES (1 references)
target prot opt source destination
FWDO_public all -- anywhere anywhere [goto]
Chain FORWARD_OUT_ZONES_SOURCE (1 references)
target prot opt source destination
Chain FORWARD_direct (1 references)
target prot opt source destination
Chain FWDI_public (1 references)
target prot opt source destination
FWDI_public_log all -- anywhere anywhere
FWDI_public_deny all -- anywhere anywhere
FWDI_public_allow all -- anywhere anywhere
REJECT icmp -- anywhere anywhere reject-with icmp-host-prohibited
Chain FWDI_public_allow (1 references)
target prot opt source destination
Chain FWDI_public_deny (1 references)
target prot opt source destination
Chain FWDI_public_log (1 references)
target prot opt source destination
Chain FWDO_public (1 references)
target prot opt source destination
FWDO_public_log all -- anywhere anywhere
FWDO_public_deny all -- anywhere anywhere
FWDO_public_allow all -- anywhere anywhere
Chain FWDO_public_allow (1 references)
target prot opt source destination
Chain FWDO_public_deny (1 references)
target prot opt source destination
Chain FWDO_public_log (1 references)
target prot opt source destination
Chain INPUT_ZONES (1 references)
target prot opt source destination
IN_public all -- anywhere anywhere [goto]
Chain INPUT_ZONES_SOURCE (1 references)
target prot opt source destination
Chain INPUT_direct (1 references)
target prot opt source destination
Chain IN_public (1 references)
target prot opt source destination
IN_public_log all -- anywhere anywhere
IN_public_deny all -- anywhere anywhere
IN_public_allow all -- anywhere anywhere
REJECT icmp -- anywhere anywhere reject-with icmp-host-prohibited
Chain IN_public_allow (1 references)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp dpt:http ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp dpt:https ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp dpt:smtp ctstate NEW
Chain IN_public_deny (1 references)
target prot opt source destination
Chain IN_public_log (1 references)
target prot opt source destination
Chain OUTPUT_direct (1 references)
target prot opt source destination
该服务器是使用 Digital Ocean 的 VPS，运行 Centos 7.6，内核：3.10.0-862.2.3.el7.x86_64
有人能帮忙解释一下为什么 SYNPROXY 可能没有运行，或者为什么/如何保护我的服务器免受 SYN 洪泛攻击吗？