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CentOS's userspace tools are also incompatible with the connlimit module in our latest 3.x series kernels because the version of 'iptables' that is installed is too old:
//CentOS的用户空间工具在我们最新的3.x系列核心中与connlimit模块不兼容因为所安装的iptables版本太旧了：
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# iptables -V
iptables v1.4.7
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Please issue these commands, and again, remember that building from source means that this package will no longer be managed by your package management system (yum):
//输入以下指令，后面的懒得翻译，注意这会覆盖掉你的东西就行了
yum update
rpm -e --nodeps iptables-1.4.7-9.el6.i686
rpm -e --nodeps iptables-ipv6-1.4.7-9.el6.i686
yum groupinstall 'Development Tools'
wget http://www.netfilter.org/projects/iptables/files/iptables-1.4.19.tar.bz2
tar jxvf iptables-1.4.19.tar.bz2
cd iptables-1.4.19
LDFLAGS="-L$PWD/libiptc/.libs" ./configure --prefix=/usr --exec-prefix= --bindir=/usr/bin --with-xtlibdir=/lib/xtables --with-pkgconfigdir=/usr/lib/pkgconfig --enable-libipq --enable-devel
make
make install

When the installation is complete, run "iptables -V" and you should see the following output:
//当你完成安装时，输入“iptables -V” 然后你会看到下面的内容
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iptables v1.4.19
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Now try to load that iptables rule:
//现在你可以用了

iptables -I FORWARD -p tcp --syn --dport 80 -m connlimit --connlimit-above 20 -j DROP

It should succeed with no error message. You can verify that the rule was loaded with either of these commands:
//下载应该能运行的了，你可以通过以下任意一个指令来验证你的规矩是否已经加载

iptables-save
iptables -L -n -v





搞定，感谢Linode客服