系统被SYN_RECV攻击，不求解决办法，只求找到原因

系统被SYN_RECV攻击，不求解决方法，只求找到原因
今天晚上，第三方监控网站发来邮件提醒，说服务器无法连接，立即ssh上去，卡得要死。

netstat一看，基本上很多如下：
tcp        0      0 1.2.3.4:80         61.130.51.186:63121     SYN_RECV    -               
tcp        0      0 1.2.3.4:80         218.25.197.162:27626    SYN_RECV    -               
tcp        0      0 1.2.3.4:80         222.78.251.82:2186      SYN_RECV    -               
tcp        0      0 1.2.3.4:80         60.190.0.30:2174        SYN_RECV    -               
tcp        0      0 1.2.3.4:80         118.254.9.13:1629       SYN_RECV    -               
tcp        0      0 1.2.3.4:80         218.90.220.174:2289     SYN_RECV    -               
tcp        0      0 1.2.3.4:80         121.13.225.58:1089      SYN_RECV    -               
tcp        0      0 1.2.3.4:80         59.56.178.213:4218      SYN_RECV    -               
tcp        0      0 1.2.3.4:80         112.112.4.66:2116       SYN_RECV    -