iptables的CONNMARK与MARK是用于给数据连接和数据包打标记的两个target。一直没搞明白二者的区别。直到昨天花了不少时间解决openwrt下多路合并时工行网银登录问题，才大致弄清这两个target的用法。
两者的区别在于，同样是打标记，但CONNMARK是针对连接的，而MARK是针对单一数据包的。
这么说大家肯定还是不明白。但回想一下，这两种机制一般都要和ip rule中的fwmark联用，实现对满足某一类条件的数据包的策略路由。所以问题的关键在于两点：
1. 对连接打了标记，只是标记了连接，没有标记连接中的每个数据包。标记单个数据包，也不会对整条连接的标记有影响。二者是相对独立的。
2. 路由判定(routing decision)是以单一数据包为单位的。或者说，在netfilter框架之外，并没有连接标记的概念。或者说，ip命令只知道MARK, 而不知道CONNMARK是什么。
所以写相关的iptables规则，关键在于：给所有要进行ip rule匹配的单一数据包打上标记。方法一般有二：用MARK直接打，或者用CONNMARK –restore-mark把打在连接上的标记转移到数据包上。
理解了这些，要解决相关的问题就心里有数了。就比如网银登录的问题吧，在使用openwrt多拨时登录工行网银时，由于使用了多个IP，服务器检测到所认为有安全问题，就会阻止用户登录。解决思路是，由于网银用https协议，对相应的443端口做一些处理。让所有目的端口为443的outgoing包只走一条线路就行了。到iptables规则中，就是对这些包只打一种标记咯。假设打0×01吧。相应的规则像下面这样写：
<span style="font-family:inherit;color:#666666;border:0px; margin:0px; outline:0px; padding:0px; vertical-align:baseline;"><em># 第一个outgoing的包(tcp SYN)，打上标记</em></span>iptables <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-t</span> mangle <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-A</span> PREROUTING <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-p</span> tcp <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">--dport</span> <span style="font-family:inherit;color:#000000;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">443</span> <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-m</span> conntrack <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">--ctstate</span> NEW \<span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-j</span> MARK <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">--set-mark</span> 0x01 <span style="font-family:inherit;color:#666666;border:0px; margin:0px; outline:0px; padding:0px; vertical-align:baseline;"><em># routing decision时，会选择与0x01标记对应的那条路由。</em></span><span style="font-family:inherit;color:#666666;border:0px; margin:0px; outline:0px; padding:0px; vertical-align:baseline;"><em># 然后，我们把这个包上的标记(0x01)转存到与之对应的连接上。--save-mark功能就在于此。</em></span>iptables <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-t</span> mangle <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-A</span> POSTROUTING <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-m</span> conntrack <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">--ctstate</span> NEW <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-j</span> CONNMARK <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">--save-mark</span> <span style="font-family:inherit;color:#666666;border:0px; margin:0px; outline:0px; padding:0px; vertical-align:baseline;"><em># 这条连接后续的包，都用--restore-mark命令，把连接上的标记（上一条命令保存的）</em></span><span style="font-family:inherit;color:#666666;border:0px; margin:0px; outline:0px; padding:0px; vertical-align:baseline;"><em># 再打到每个单一数据包上。</em></span>iptables <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-t</span> mangle <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-A</span> PREROUTING <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-i</span> br-lan <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-m</span> conntrack <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">--ctstate</span> ESTABLISHED,RELATED \<span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">-j</span> CONNMARK <span style="font-family:inherit;color:#66033;border:0px; font-style:inherit; margin:0px; outline:0px; padding:0px; vertical-align:baseline;">--restore-mark</span> <span style="font-family:inherit;color:#666666;border:0px; margin:0px; outline:0px; padding:0px; vertical-align:baseline;"><em># 至此，所有（目的端口为443的outgoing包）对应连接上的每一个数据包都打上了0x01标记，</em></span><span style="font-family:inherit;color:#666666;border:0px; margin:0px; outline:0px; padding:0px; vertical-align:baseline;"><em># 可以被正确地路由。</em></span>
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2 THOUGHTS ON “IPTABLES的CONNMARK与MARK”