TcpDump可以将网络中传送的数据包的“头”完全截获下来提供分析。它支持针对网络层、协议、主机、网络或端口的过滤，并提供and、or、not等逻辑语句来帮助你去掉无用的信息。tcpdump就是一种免费的网络分析工具，尤其其提供了源代码，公开了接口，因此具备很强的可扩展性，对于网络维护和入侵者都是非常有用的工具。tcpdump存在于基本的FreeBSD系统中，由于它需要将网络界面设置为混杂模式，普通用户不能正常执行，但具备root权限的用户可以直接执行它来获取网络上的信息。因此系统中存在网络分析工具主要不是对本机安全的威胁，而是对网络上的其他计算机的安全存在威胁。-TcpDump the " head"　of the transmission of data packets in the network can be completely intercepted off providing analysis. It supports filtering for network layer protocol, host, network, or port, and to provide and, or, not a logical statement to help you get rid of useless information. tcpdump is a free network analysis tools, in particular, provided the source code, open interfaces, and therefore has strong scalability, are useful tools for network maintenance and intruders. tcpdump exist in the basic FreeBSD system, because it requires the network interface into promiscuous mode, normal users can not run properly, but users with root privileges can be executed directly on the network to get information. System network analysis tools are not a threat to the safety of the machine, but the other computers on the network security threats.