原始套接字编程(AF_PACKET+SOCK_RAW)模拟一个PING

补丁牛仔裤 2024-05-14 发布于广东

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1. 背景

最近看一个客户的代码片段，发现他在用原始套接字编程，一般学习套接字都是流式套接字和数据报套接字，本来也不是搞网络的，原始套接字了解得很少，借着这次机会，自己来学习一下原始套接字编程。

2. 原始套接字是什么

我去翻看<Linux_Unix系统编程手册>第58章(TCP/IP网络基础)中只有一个泛泛的介绍：

但是这个描述并不完全，于是我参考: 信息安全课程9：raw socket编程 - 知乎

另外还了解到原始套接字在socket的创建上有不同的组合，例如: AF_INET+SOCK_RAW最多只能允许用户层与IP层直接通信，而AF_PACKET+SOCK_RAW就可以允许用户层与数据链路层直接通信了(这一点也是Linux_Unix系统编程手册说得不准确的地方)

另外，关于AF_PACKET+SOCK_RAW可以参考man packet：

3. 封装与PING包格式

同样参考 <Linux_Unix系统编程手册>中的封装基本概念:

我自己用wireshark抓了一个ping包的格式如下:

上图是执行ping 192.168.0.103 -c 1抓的包，可以看到一个ping操作实际分为两个包，一个是由本机发出的包(echo ping request)，另一个是收到对端发来的ack包(echo ping reply)，不论怎样，这两个包组成都是相同的:

98byte = EthernetII(以太网头部14byte)+Internet Protocol Version4(IP包20byte)+ICMP(64byte)

4. 实验代码

实验代码实际上是受到下面博客的启发:Linux 网络编程——原始套接字实例：MAC 地址扫描器_siocgifhwaddr_Mike江的博客-CSDN博客

在这篇博客中，作者用AF_PACKET+SOCK_RAW的原始套接字在数据链路层模拟了一个地址解析协议的操作(Address Resolution Protocol)，其中作者没有使用繁杂的包数据结构去构造发送数据，转而使用了直接赋值的方式，非常直观与暴力，可以对着wireshark的数据来构造自己的数据包，非常便于理解与学习，所以我自己模仿了一个PING操作,代码如下:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include <errno.h>
#include <net/if.h>           //struct ifreq
#include <sys/ioctl.h>        //ioctl、SIOCGIFADDR
#include <sys/socket.h>
#include <netinet/ether.h>    //ETH_P_ALL
#include <netpacket/packet.h> //struct sockaddr_ll
#include <netinet/in.h>
#include <netinet/ip_icmp.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define BUFFSIZE (1024)
#define IPPACKETLEN (20)
#define ICMPHEADERLEN (16)
#define ICMPDATALEN (48)
#define PINGPACKETLEN (98)
//#define DEBUG
static void test(void)
{
    unsigned short a = 0x3623; /* in memory should be 0x23 0x36 */
    unsigned short b = htons(a);
    printf("network order b = %#x\n", b); /* in memory should be 0x36 0x23 */
    printf("local order b' = %#x\n", ntohs(b));
    return;
}
/* 2 bytes was a group
 * totally lens group
 */
static unsigned short calc_checksum(unsigned short *addr, int len)
{
    int i;
    unsigned int sum = 0;
    unsigned short tmp = 0;
    /* in this algorithm,
     * we use ntohs to convert the already constructed big endian data
     */
    for (i = 0; i < len; i++) {
        tmp = ntohs(*addr);
        sum += tmp;
        addr++;
    }
    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >>16);
    tmp = ~sum;
    return tmp;
}
int main(int argc,char *argv[])
{
    int i = 1;
#ifdef DEBUG
    test();
    return 0;
#endif
    unsigned char send_msg[BUFFSIZE] = {
        //--------------Ethernet II------------------------14--------------------------
        0xf8, 0x94, 0xc2, 0xdb, 0x72, 0x43, //dst_mac: F8-94-C2-DB-72-43
        0x00, 0x0c, 0x29, 0x7e, 0x2f, 0x86, //src_mac: 00-0C-29-7E-2F-86
        0x08, 0x00,                         //type: 0x0800 IPV4
        //--------------Internet Protocol Version 4--------20--------------------------
        0x45,                               //0100-version4, 0101-5*4byte=20byte
        0x00,                               //DSCP:0, ECN-Not-ECT
        0x00, 0x54,                         //total length: 0x54==84==20(IP)+64(ICMP)
        0x36, 0x23,                         //identification: 0x36 0x23
        0x40, 0x00,                         //flags: 0x4000, do not fragment
        0x40,                               //time to live(TTL): 64
        0x01,                               //protocol: ICMP(1)
        0x00, 0x00,                         //checksum(need to calc later)
        192, 168, 0, 104,                   //src ip addr
        192, 168, 0, 103,                   //dst ip addr
        //--------------ICMP-Header------------------------16--------------------------
        0x08,                               //type: 8(echo ping request)
        0x00,                               //code:0
        0x00, 0x00,                         //checksum(need to calc later)
        0x20, 0x09,                         //identifier: 0x20 0x09
        0x00, 0x00,                         //sequence number
        0x12, 0xfc, 0xd4, 0x64,
        0x00, 0x00, 0x00, 0x00,             //timestamp from icmp data
        //--------------ICMP-Data--------------------------48--------------------------
        0x12, 0x34, 0x56, 0x78,
        0x12, 0x34, 0x56, 0x78,
        0x12, 0x34, 0x56, 0x78,
        0x12, 0x34, 0x56, 0x78,
    };
    unsigned short checksum = 0;
    unsigned short checksum_be = 0;
    /* handle checksum of IP packet */
    checksum = calc_checksum((unsigned short *)(&send_msg[14]), IPPACKETLEN/2);
    printf("IP packet checksum = %#x\n", checksum);
    checksum_be = htons(checksum);
    memcpy(&send_msg[24], &checksum_be, sizeof(checksum_be));
    /* handle checksum of ICMP packet */
    checksum = calc_checksum((unsigned short *)(&send_msg[34]), (ICMPHEADERLEN + ICMPDATALEN)/2);
    printf("ICMP packet checksum = %#x\n", checksum);
    checksum_be = htons(checksum);
    memcpy(&send_msg[36], &checksum_be, sizeof(checksum_be));
    /* socket create failed, why: Operation not permitted
     * should have root permission to create the socket raw
     */
    int sock_raw_fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
    if (sock_raw_fd < 0) {
        printf("socket create failed, why: %s\n", strerror(errno));
        return -1;
    }
    printf("socket create success.\n");
    struct sockaddr_ll sll;
    struct ifreq req;
    strncpy(req.ifr_name, "ens33", IFNAMSIZ);
    ioctl(sock_raw_fd, SIOCGIFINDEX, &req);
    bzero(&sll, sizeof(sll));
    sll.sll_ifindex = req.ifr_ifindex;
    int len = sendto(sock_raw_fd, send_msg, PINGPACKETLEN, 0 , (struct sockaddr *)&sll, sizeof(sll));
    if (len == -1) {
        printf("sendto failed, why: %s\n", strerror(errno));
        close(sock_raw_fd);
        return -1;
    }
    printf("\n%d bytes sended.\n", len);
    unsigned char recv_msg[BUFFSIZE] = {0};
    len = recvfrom(sock_raw_fd, recv_msg, sizeof(recv_msg), 0, NULL, NULL);
    if (len < 0) {
        printf("recvfrom failed, why: %s\n", strerror(errno));
        close(sock_raw_fd);
        return -1;
    }
    printf("\n%d bytes received, check the wireshark.\n", len);
    close(sock_raw_fd);
    return 0;
}

需要注意的是send_msg的数据全部是依据wireshark包来构造的，自己机器的ip和mac，对端机器的ip和mac需要自行适配，另外，IP包和ICMP包是有 checksum需要计算的，一个偷懒的办法是先胡乱写一个checksum进去，发包的时候wireshark会识别到错误并告诉你正确的checksum是什么，再回填进去即可，通常，IP包的checksum wireshark不检查(显示validation disabled)，但是如果构造不对的话是收不到echo ping reply包的，可以按照下图的方法打开wireshark IP包的checksum检查结果: