我有一个负载相当重的 Web 服务器，使用：
Ubuntu server
nginx
php-fpm + apc
昨天我的服务器发生了一些奇怪的事情。它崩溃了并且停止了响应，在我重新启动它之后，网页开始加载得非常非常慢，在大多数情况下都显示“请求超时”。
我检查了一下/var/log/syslog，发现有很多类似的消息：TCP: Possible SYN flooding on port 80. Sending cookies.
页面本地加载大约需要 2 分钟：
time wget -O /dev/null mysite.net
--2012-12-21 13:17:15-- http://mysite.net/
Resolving ficbook.net... 85.254.49.180
Connecting to mysite.net|85.254.49.180|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1311 (1.3K) [text/html]
Saving to: `/dev/null'
100%[========================================================================================================>] 1,311 --.-K/s in 0s
2012-12-21 13:19:18 (181 MB/s) - `/dev/null' saved [1311/1311]
real 2m2.438s
user 0m0.000s
sys 0m0.000s
我不确定这是否真的是 SYN Flood 攻击。如果是，为什么 cookies 没有帮助？以下是来自 netstat 的信息：
netstat -tuna | grep :80 | grep SYN_RECV
tcp 0 0 85.254.49.180:80 92.37.173.66:3214 SYN_RECV
tcp 0 0 85.254.49.180:80 81.26.91.4:49471 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4940 SYN_RECV
tcp 0 0 85.254.49.180:80 213.87.140.242:23259 SYN_RECV
tcp 0 0 85.254.49.180:80 94.139.229.219:49827 SYN_RECV
tcp 0 0 85.254.49.180:80 95.67.233.125:51267 SYN_RECV
tcp 0 0 85.254.49.180:80 83.149.2.69:7051 SYN_RECV
tcp 0 0 85.254.49.180:80 95.67.239.40:54497 SYN_RECV
tcp 0 0 85.254.49.180:80 195.91.229.193:58981 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4925 SYN_RECV
tcp 0 0 85.254.49.180:80 88.154.3.228:59086 SYN_RECV
tcp 0 0 85.254.49.180:80 92.113.26.124:3887 SYN_RECV
tcp 0 0 85.254.49.180:80 77.34.83.254:26963 SYN_RECV
tcp 0 0 85.254.49.180:80 195.208.64.130:3542 SYN_RECV
tcp 0 0 85.254.49.180:80 81.26.91.4:49480 SYN_RECV
tcp 0 0 85.254.49.180:80 87.253.29.234:53130 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4939 SYN_RECV
tcp 0 0 85.254.49.180:80 88.154.3.228:41696 SYN_RECV
tcp 0 0 85.254.49.180:80 178.45.39.169:41758 SYN_RECV
tcp 0 0 85.254.49.180:80 217.118.66.37:51534 SYN_RECV
tcp 0 0 85.254.49.180:80 83.149.9.197:8249 SYN_RECV
tcp 0 0 85.254.49.180:80 37.29.88.202:3531 SYN_RECV
tcp 0 0 85.254.49.180:80 178.34.206.52:3409 SYN_RECV
tcp 0 0 85.254.49.180:80 193.188.254.93:50317 SYN_RECV
tcp 0 0 85.254.49.180:80 217.66.152.162:8883 SYN_RECV
tcp 0 0 85.254.49.180:80 109.198.235.10:56382 SYN_RECV
tcp 0 0 85.254.49.180:80 95.53.159.39:2256 SYN_RECV
tcp 0 0 85.254.49.180:80 188.232.13.175:49819 SYN_RECV
tcp 0 0 85.254.49.180:80 88.203.2.27:64080 SYN_RECV
tcp 0 0 85.254.49.180:80 217.118.64.52:12382 SYN_RECV
tcp 0 0 85.254.49.180:80 92.124.76.189:3416 SYN_RECV
tcp 0 0 85.254.49.180:80 37.29.88.202:30532 SYN_RECV
tcp 0 0 85.254.49.180:80 87.253.29.234:53131 SYN_RECV
tcp 0 0 85.254.49.180:80 213.87.123.1:44943 SYN_RECV
tcp 0 0 85.254.49.180:80 176.51.255.3:1642 SYN_RECV
tcp 0 0 85.254.49.180:80 85.26.165.112:56906 SYN_RECV
tcp 0 0 85.254.49.180:80 88.203.2.27:64081 SYN_RECV
tcp 0 0 85.254.49.180:80 217.118.66.37:51533 SYN_RECV
tcp 0 0 85.254.49.180:80 176.51.211.131:1699 SYN_RECV
tcp 0 0 85.254.49.180:80 37.29.88.202:22233 SYN_RECV
tcp 0 0 85.254.49.180:80 211.167.112.18:58353 SYN_RECV
tcp 0 0 85.254.49.180:80 217.118.66.32:38640 SYN_RECV
tcp 0 0 85.254.49.180:80 217.144.185.150:64421 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4928 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4927 SYN_RECV
tcp 0 0 85.254.49.180:80 94.153.254.218:1084 SYN_RECV
tcp 0 0 85.254.49.180:80 37.29.88.202:30384 SYN_RECV
tcp 0 0 85.254.49.180:80 46.201.3.189:51032 SYN_RECV
tcp 0 0 85.254.49.180:80 109.187.107.41:50565 SYN_RECV
tcp 0 0 85.254.49.180:80 91.146.60.86:49266 SYN_RECV
tcp 0 0 85.254.49.180:80 87.253.29.234:53134 SYN_RECV
tcp 0 0 85.254.49.180:80 80.83.238.25:2515 SYN_RECV
tcp 0 0 85.254.49.180:80 176.102.16.8:54291 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4918 SYN_RECV
tcp 0 0 85.254.49.180:80 95.153.164.165:26752 SYN_RECV
tcp 0 0 85.254.49.180:80 80.83.239.76:46519 SYN_RECV
tcp 0 0 85.254.49.180:80 94.139.229.219:49826 SYN_RECV
tcp 0 0 85.254.49.180:80 188.239.193.48:49418 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4919 SYN_RECV
tcp 0 0 85.254.49.180:80 217.118.66.32:38639 SYN_RECV
tcp 0 0 85.254.49.180:80 95.67.233.125:51266 SYN_RECV
tcp 0 0 85.254.49.180:80 85.26.235.172:59092 SYN_RECV
tcp 0 0 85.254.49.180:80 213.87.136.21:44804 SYN_RECV
tcp 0 0 85.254.49.180:80 95.109.193.247:1206 SYN_RECV
tcp 0 0 85.254.49.180:80 217.112.11.130:2714 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4941 SYN_RECV
tcp 0 0 85.254.49.180:80 88.154.3.228:52640 SYN_RECV
tcp 0 0 85.254.49.180:80 37.79.93.27:64801 SYN_RECV
tcp 0 0 85.254.49.180:80 91.203.96.76:45132 SYN_RECV
tcp 0 0 85.254.49.180:80 80.83.238.25:2513 SYN_RECV
tcp 0 0 85.254.49.180:80 85.26.235.172:60092 SYN_RECV
tcp 0 0 85.254.49.180:80 188.239.193.48:49416 SYN_RECV
tcp 0 0 85.254.49.180:80 178.130.42.68:60373 SYN_RECV
tcp 0 0 85.254.49.180:80 80.239.243.181:58110 SYN_RECV
tcp 0 0 85.254.49.180:80 87.253.29.234:53128 SYN_RECV
tcp 0 0 85.254.49.180:80 83.149.9.197:18870 SYN_RECV
tcp 0 0 85.254.49.180:80 88.154.3.228:53380 SYN_RECV
tcp 0 0 85.254.49.180:80 88.135.63.40:58845 SYN_RECV
tcp 0 0 85.254.49.180:80 80.239.243.110:52234 SYN_RECV
tcp 0 0 85.254.49.180:80 46.201.3.189:51028 SYN_RECV
tcp 0 0 85.254.49.180:80 88.154.3.228:53457 SYN_RECV
tcp 0 0 85.254.49.180:80 85.235.176.138:12101 SYN_RECV
tcp 0 0 85.254.49.180:80 109.187.107.41:50567 SYN_RECV
tcp 0 0 85.254.49.180:80 83.149.48.29:4172 SYN_RECV
tcp 0 0 85.254.49.180:80 188.232.13.175:49820 SYN_RECV
tcp 0 0 85.254.49.180:80 37.29.88.202:6651 SYN_RECV
tcp 0 0 85.254.49.180:80 91.198.143.6:45591 SYN_RECV
tcp 0 0 85.254.49.180:80 85.235.176.138:50667 SYN_RECV
tcp 0 0 85.254.49.180:80 176.209.98.72:53653 SYN_RECV
tcp 0 0 85.254.49.180:80 80.83.239.71:49701 SYN_RECV
tcp 0 0 85.254.49.180:80 188.232.13.175:49817 SYN_RECV
tcp 0 0 85.254.49.180:80 188.239.193.48:49417 SYN_RECV
tcp 0 0 85.254.49.180:80 88.154.3.228:54175 SYN_RECV
tcp 0 0 85.254.49.180:80 61.147.79.111:51039 SYN_RECV
tcp 0 0 85.254.49.180:80 88.154.3.228:58854 SYN_RECV
tcp 0 0 85.254.49.180:80 87.253.29.234:53135 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4938 SYN_RECV
tcp 0 0 85.254.49.180:80 62.122.51.139:4942 SYN_RECV
tcp 0 0 85.254.49.180:80 176.209.98.72:53662 SYN_RECV
tcp 0 0 85.254.49.180:80 2.74.51.158:1092 SYN_RECV
tcp 0 0 85.254.49.180:80 213.87.140.242:48178 SYN_RECV
tcp 0 0 85.254.49.180:80 213.87.129.42:29549 SYN_RECV
tcp 0 0 85.254.49.180:80 37.29.88.202:28428 SYN_RECV
tcp 0 0 85.254.49.180:80 85.26.235.172:50983 SYN_RECV
tcp 0 0 85.254.49.180:80 217.118.64.52:12381 SYN_RECV
tcp 0 0 85.254.49.180:80 85.26.235.172:55459 SYN_RECV
tcp 0 0 85.254.49.180:80 84.244.12.209:64975 SYN_RECV
tcp 0 0 85.254.49.180:80 83.149.2.121:10768 SYN_RECV
tcp 0 0 85.254.49.180:80 84.240.248.206:3494 SYN_RECV
tcp 0 0 85.254.49.180:80 195.91.229.193:52428 SYN_RECV
tcp 0 0 85.254.49.180:80 95.109.193.247:1202 SYN_RECV
tcp 0 0 85.254.49.180:80 79.105.204.56:56822 SYN_RECV
tcp 0 0 85.254.49.180:80 85.15.184.141:56335 SYN_RECV
tcp 0 0 85.254.49.180:80 164.177.225.31:50584 SYN_RECV
tcp 0 0 85.254.49.180:80 80.83.238.25:2511 SYN_RECV
tcp 0 0 85.254.49.180:80 84.240.248.206:3493 SYN_RECV
tcp 0 0 85.254.49.180:80 80.83.239.76:26950 SYN_RECV
tcp 0 0 85.254.49.180:80 84.240.248.206:3495 SYN_RECV
tcp 0 0 85.254.49.180:80 217.144.185.150:58141 SYN_RECV
tcp 0 0 85.254.49.180:80 178.215.97.15:13346 SYN_RECV
我尝试禁用 syn cookies，但没有效果。似乎该服务器正在限制连接数，如果您查看“每分钟点击数”指标，它看起来如下：
昨天，在崩溃之前，一切都运行良好。我会感谢任何有关问题可能是什么或如何诊断的信息或建议。
更新
我非常确定这不是一次攻击。当我重新启动 nginx 时，一切都正常工作了几个小时，然后，系统日志再次充满了：
Possible SYN flooding on port 80
Possible SYN flooding on port 9000
然后nginx的错误日志首先得到很多104错误：
2013/01/08 20:28:24 [error] 959#0: *2387458 recv() failed (104: Connection reset by peer) while reading response header from upstream
然后是 110：
2013/01/08 21:27:19 [error] 30349#0: *760749 upstream timed out (110: Connection timed out) while connecting to upstream
这种情况发生在晚上，当负载达到一定量（每秒约 800 次）时就会出现问题。
关闭 syn cookies 和调整积压没有任何效果。
互联网上有很多类似的说法，但找不到真正的答案。请帮忙！
答案1
听起来你的上游服务器出了问题，导致 nginx 显得非常慢。
当 nginx 速度缓慢时，请求是否仅当代理到您的 php-fpm + apc 时才会花费很长时间？您是否尝试过定义非代理location，并查看是否会出现任何问题？
您的 php-fpm + apc 是否内存不足、连接/文件描述符不足或工作线程/进程不足？您是否在使用 OpenVZ？或者任何其他可能因设计不当而导致的内核级虚拟化？如果没有，您是否可能遇到任何其他进程或内存限制？您可以通过进入su运行 php-fpm / apc 的用户并运行limit来检查限制tcsh。
您可能应该发布 nginx 和 php-fpm + apc 的整个配置，否则，这将是一场大猜谜游戏。我不是 php 专家，但我有根据的猜测是，您的 php-fpm+apc 端存在某种连接或工作线程限制，而您的 nginx 代理超出了该限制。
另外，我看到你有一张很好的图表，显示了某天情况突然变糟的情况；在这件事发生之前的几天内，你是否做过任何更改或升级？
答案2
答案3
我强烈推荐让你设置一些默认的 Iptables 链。尝试丢弃 SYN 泛洪并记录所有丢弃的数据包。你甚至不知道你的网络层。
- 阻止洪水攻击，丢弃包裹
- 记录所有丢弃的数据包
- 请告诉我们此事的最新进展
最后，对于具有网络分析经验的人来说，可以轻松检测到服务器崩溃并立即阻止攻击。