Iptables can use extended packet matching modules.
These are loaded in two ways: implicitly, when -p or --protocol is specified,or with the -m or --match options, followed by the matching module name; after these, various extra command line options become available, depending on the specific module.
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You can specify multiple extended match modules in one line, and you can use the -h or --help options after the module has been specified to receive help specific to that module.
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两个例子：
tos：
This module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence bits).
tos --tos
The argument is either a standard name,(use ‘iptables -m tos -h’ to see the list), or a numeric value to match.
length:
-m length --length 100
其中，100指定的是IP数据包的大小。而在ping中，用-l(windows下)或-s（linux下）指定的是要发送的数据data的大小。这样的数据要先后加上ICMP头--8字节、IP头--20字节...
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关于tos：
tos.doc
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