IPv4报文分片代码

IP报文内包装了一个ICMP报文，不过还是没法重组报文。想要练习重组报文似乎得直接抓链路层的包才行。

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/uio.h>
#include <netdb.h>
#include <linux/icmp.h>
#include <linux/ip.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <netinet/in.h>
#include <arpa/inet.h>

struct iphdr ip;     //IP报头

char buf[128];     //ICMP报头
//校验和，网上挖过来的代码
unsigned short checksum(unsigned short *buffer, int size) {
    unsigned long cksum = 0;
    while (size > 1) {
        cksum += *buffer++;
        size -= sizeof(unsigned short);
    }
    if (size)
        cksum += *(unsigned short*) buffer;
    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >> 16);
    return (unsigned short) (~cksum);
}
//获取网络地址
int getaddr(const char *name, struct sockaddr_in *addr) {
    struct addrinfo hint = { 0, AF_INET, SOCK_RAW, IPPROTO_ICMP };
    struct addrinfo *res;
    int rv;
    if ((rv = getaddrinfo(name, 0, &hint, &res)) != 0) {
        fprintf(stderr, "error at gethostinfo:%s\n", gai_strerror(rv));
        return -1;
    }
    memcpy(addr, res->ai_addr, res->ai_addrlen);
    freeaddrinfo(res);
    return 0;
}
//获取原始套接字
int ip_socket() {
    int rv, sock;
    if ((sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) == -1) {
        fprintf(stderr, "error at socket:%s\n", strerror(errno));
        return -1;
    }
    setuid(getuid());
    rv = 1;
    //自己操纵IP报头
    if ((rv = setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &rv, sizeof rv) == -1)) {
        fprintf(stderr, "error at setsockopt:%s\n", strerror(errno));
        return -1;
    }
    return sock;
}

void init(const struct sockaddr_in *src, const struct sockaddr_in *dest) {
    char c = 0, *p;
    struct icmphdr *icmp = (struct icmphdr *) buf;
    for (p = (char *) (icmp + 1); p < buf + sizeof buf; ++p) {
        *p = c++;
    }
    //配置ICMP报头
    icmp->type = ICMP_ECHO;
    icmp->code = 0;
    icmp->checksum = 0;
    icmp->un.echo.id = getpid();
    icmp->un.echo.sequence = 0;
    icmp->checksum = checksum((unsigned short *) buf, sizeof buf);
    //配置IP报头，所有单元皆为大端序
    ip.version = 4;
    ip.ihl = 5;
    ip.tos = 0;
    ip.tot_len = htons((ip.ihl << 2) + sizeof buf);
    ip.id = getpid();
    ip.frag_off = 0;
    ip.ttl = 223;
    ip.protocol = IPPROTO_ICMP;
    ip.check = 0;
    memcpy(&ip.saddr, &src->sin_addr, sizeof(src->sin_addr));
    memcpy(&ip.daddr, &dest->sin_addr, sizeof(dest->sin_addr));
}
//发送分片，last为1时结束
int sendfrag(int sock, const struct sockaddr_in *dest, char *base, uint off,
        uint size, int last) {
    char flag;
    if (last) {
        flag = 0x40;     //DF位，   0100 0000，大端序
    } else {
        flag = 0x60;     //MF|DF位，0110 0000，大端序
    }
    int tot_len = sizeof(ip) + size;
    ip.tot_len = htons(tot_len);
    ip.frag_off = htons(off >> 3);
    ip.frag_off |= flag;
    ip.check = checksum((unsigned short *) &ip, sizeof ip);
    char sendb[tot_len];    //自己设置IP报头时，MSG_MORE似乎无效，所以还是得拷贝数据
    memcpy(sendb, &ip, sizeof ip);
    memcpy(sendb + sizeof ip, base + off, size);
    long int rv;
    if ((rv = sendto(sock, sendb, tot_len, 0, (struct sockaddr*) dest,
            sizeof(struct sockaddr_in))) == -1) {
        fprintf(stderr, "error at sendto1:%s\n", strerror(errno));
        return -1;
    }
    return rv;
}

int main(int argc, char *argv[]) {
    struct sockaddr_in src, dest;
    char buf2[256];
    getaddr("127.0.0.1", &src);
    getaddr(argv[1], &dest);
    printf("from host:%s --->", inet_ntoa(src.sin_addr));
    printf(" to host: %s\n", inet_ntoa(dest.sin_addr));
    int sock = ip_socket();
    if (sock == -1) {
        fprintf(stderr, "sock err and exit.\n");
        return -1;
    }
    init(&src, &dest);
    uint i, len = sizeof dest;
    for (i = 0; i < 1024; ++i) {
        printf("ping dest: %s...", inet_ntoa(dest.sin_addr));
        sendfrag(sock, &dest, buf, 0, 32, 0);
        sendfrag(sock, &dest, buf, 32, 32, 0);
        sendfrag(sock, &dest, buf, 64, 32, 0);
        sendfrag(sock, &dest, buf, 96, 32, 1);
        recvfrom(sock, buf2, 256, 0, (struct sockaddr *) &dest, &len);
        printf("recved.\n");
        sleep(1);
    }
    printf("done.\n");
    return 0;
}