Linux下原始套接字的原理
创建原始套接字：
socket(AF_NET, SOCK_RAW, protocol);
1. 参数protocol用来致命所接收的协议包，如果是像IPPROTO_TCP(6)这种非0、非255的协议，能接收ip头为protocol域的数据包，包括IP头，协议头以及数据；发送数据时，默认只需构建protocol协议头及数据，不需构建IP头。可以通过设置原始套接字的IP_HDRINCL属性，使用户自己构建IP头。
setsockopt (rawsock, IP, IP_HDRINCL, “1”, sizeof (“1”));
2. 如果protocol为IPPROTO_RAW，创建的原始套接字只能用来发送IP数据包，且默认开启IP_HDRINCL属性，需要用户自己构建IP包头，计算校验和。
3. 对于protocol为IPPROTO_IP的原始套接字，可以接收任何的IP数据包。其中的校验和验证和协议分析由程序自己完成。
4. 若要监测所有输入与输出的数据包，而且不仅限制于IP包(tcp/udp/icmp)，监测 arp/rarp包，以及以太网头部，需要通过以下语句建立原始套接字：
sock_raw = socket( AF_PACKET , SOCK_RAW , htons(ETH_P_ALL)) ;
原始套接字在windows下的局限
Limitations on Raw Sockets
On Windows 7, Windows Vista, Windows XP with Service Pack 2 (SP2), and Windows XP with Service Pack 3 (SP3), the ability to send traffic over raw sockets has been restricted in several ways:
- TCP data cannot be sent over raw sockets.
- UDP datagrams with an invalid source address cannot be sent over raw sockets. The IP source address for any outgoing UDP datagram must exist on a network interface or the datagram is dropped. This change was made to limit the ability of malicious code to create distributed denial-of-service attacks and limits the ability to send spoofed packets (TCP/IP packets with a forged source IP address).
- A call to the bind function with a raw socket for the IPPROTO_TCP protocol is not allowed.
Note The bind function with a raw socket is allowed for other protocols (IPPROTO_IP, IPPROTO_UDP, or IPPROTO_SCTP, for example).
These above restrictions do not apply to Windows Server 2008 R2, Windows Server 2008 , Windows Server 2003, or to versions of the operating system earlier than Windows XP with SP2.
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