原始套接字

在计算机网络中, 原始套接字（raw socket）是一种网络套接字，允许直接发送/接收IP协议数据包而不需要任何传输层协议格式。

简介

对于标准的套接字，通常数据按照选定的传输层协议(例如TCP、UDP)自动封装，socket用户并不知道在网络介质上广播的数据包含了这种协议包头。
从原始套接字读取数据包含了传输层协议包头。用原始套接字发送数据，是否自动增加传输层协议包头是可选的。
原始套接字用于安全相关的应用程序，如nmap。原始套接字一种可能的用例是在用户空间实现新的传输层协议。 原始套接字常在网络设备上用于路由协议，例如IGMPv4、开放式最短路径优先协议 (OSPF)、互联网控制消息协议 (ICMP)。Ping就是发送一个ICMP响应请求包然后接收ICMP响应回复.

实现

大部分套接字API都支持原始套接字功能。Winsock自2001年起在Windows XP上支持原始套接字。但由于安全原因，2004年微软限制了Winsock的原始套接字功能。

例子

下例在Linux上实现了Ping程序的主要功能：
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <unistd.h>#include <sys/types.h>#include <sys/stat.h>#include <sys/socket.h>#include <netinet/in.h>#include <netinet/ip.h>#include <netinet/ip_icmp.h>#include <arpa/inet.h>#include <netdb.h>/* 校验和計算*/u_int16_t checksum(unsigned short *buf, int size){
unsigned long sum = 0;
while (size > 1) {
sum += *buf;
buf++;
size -= 2;
}
if (size == 1)
sum += *(unsigned char *)buf;
sum = (sum & 0xffff) + (sum >> 16);
sum = (sum & 0xffff) + (sum >> 16);
return ~sum;}/* protocol指定的raw socket创建 */int make_raw_socket(int protocol){
int s = socket(AF_INET, SOCK_RAW, protocol);
if (s < 0) {
perror("socket");
exit(EXIT_FAILURE);
}
return s;}/* ICMP头部的作成*/void setup_icmphdr(u_int8_t type, u_int8_t code, u_int16_t id, u_int16_t seq, struct icmphdr *icmphdr){
memset(icmphdr, 0, sizeof(struct icmphdr));
icmphdr->type = type;
icmphdr->code = code;
icmphdr->checksum = 0;
icmphdr->un.echo.id = id;
icmphdr->un.echo.sequence = seq;
icmphdr->checksum = checksum((unsigned short *)icmphdr, sizeof(struct icmphdr));}int main(int argc, char **argv){
int n, soc;
char buf[1500];
struct sockaddr_in addr;
struct in_addr insaddr;
struct icmphdr icmphdr;
struct iphdr *recv_iphdr;
struct icmphdr *recv_icmphdr;

if (argc < 2) {
printf("Usage : %s IP_ADDRESS\n", argv[0]);
return 1;
}

addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(argv[1]);
soc = make_raw_socket(IPPROTO_ICMP);
setup_icmphdr(ICMP_ECHO, 0, 0, 0, &icmphdr);

/* ICMP包的送信*/
n = sendto(soc, (char *)&icmphdr, sizeof(icmphdr), 0, (struct sockaddr *)&addr, sizeof(addr));
if (n < 1) {
perror("sendto");
return 1;
}

/* ICMP包的受信*/
n = recv(soc, buf, sizeof(buf), 0);
if (n < 1) {
perror("recv");
return 1;
}

recv_iphdr = (struct iphdr *)buf;
/* 从IP包头获取IP包的长度，从而确定icmp包头的开始位置*/
recv_icmphdr = (struct icmphdr *)(buf + (recv_iphdr->ihl << 2));
insaddr.s_addr = recv_iphdr->saddr;
/* 检查送信包的源地址匹配受信包的目的地址*/
if (!strcmp(argv[1], inet_ntoa(insaddr)) && recv_icmphdr->type == ICMP_ECHOREPLY)
printf("icmp echo reply from %s\n", argv[1]);
close(soc);
return 0;}
下例是binding一个原始套接字并使用：
#include<stdio.h>#include<stdlib.h>#include<sys/socket.h>#include<features.h>#include<linux/if_packet.h>#include<linux/if_ether.h>#include<errno.h>#include<sys/ioctl.h>#include<net/if.h>
 
#define PACKET_LENGTH1024
 
int CreateRawSocket(int protocol_to_sniff){
int rawsock;
if((rawsock = socket(PF_PACKET, SOCK_RAW, htons(protocol_to_sniff)))== -1) {
perror("Error creating raw socket: ");
exit(-1);
}
return rawsock;}
 
int BindRawSocketToInterface(char *device, int rawsock, int protocol){
struct sockaddr_ll sll;
struct ifreq ifr;

memset(&sll, 0, sizeof(sll));
memset(&ifr, 0, sizeof(ifr));
/* First Get the Interface Index  */
strncpy((char *)ifr.ifr_name, device, IFNAMSIZ);
if((ioctl(rawsock, SIOCGIFINDEX, &ifr)) == -1) {
printf("Error getting Interface index !\n");
exit(-1);
}
/* Bind our raw socket to this interface */
sll.sll_family = AF_PACKET;
sll.sll_ifindex = ifr.ifr_ifindex;
sll.sll_protocol = htons(protocol);
if((bind(rawsock, (struct sockaddr *)&sll, sizeof(sll)))== -1) {
perror("Error binding raw socket to interface\n");
exit(-1);
}
return 1;}
 
int SendRawPacket(int rawsock, unsigned char *pkt, int pkt_len){
int sent= 0;
 /* A simple write on the socket ..thats all it takes ! */
 if((sent = write(rawsock, pkt, pkt_len)) != pkt_len) {
return 0;
}
return 1;}/* argv[1] is the device e.g. eth0   argv[2] is the number of packets to send */
 
main(int argc, char **argv){
int raw;
unsigned char packet[PACKET_LENGTH];
int num_of_pkts;

/* Set the packet to all A's */
memset(packet, 'A', PACKET_LENGTH);
/* Create the raw socket */
raw = CreateRawSocket(ETH_P_ALL);
/* Bind raw socket to interface */
BindRawSocketToInterface(argv[1], raw, ETH_P_ALL);
num_of_pkts = atoi(argv[2]);
while((num_of_pkts--)>0) {
 if(!SendRawPacket(raw, packet, PACKET_LENGTH)) {
perror("Error sending packet");
} else {
printf("Packet sent successfully\n");
}
}
close(raw);
return 0;}