Linux实用技巧 用ICMP实现简单的Ping功能

#include "unp.h"

void send_echo_req(int sockfd, struct sockaddr_in *dstaddr);
uint16_t in_cksum(uint16_t *addr, int len);
void recv_echo_reply(int sockfd);

int main(int argc, char **argv)
{
        int sockfd;
        struct sockaddr_in dstaddr;

        if ((sockfd = socket(PF_INET, SOCK_RAW, IPPROTO_ICMP)) == -1)
                err_sys("socket");

        bzero(&dstaddr, sizeof(dstaddr));
        dstaddr.sin_family = AF_INET;
        dstaddr.sin_port = htons(0);
        if (inet_pton(AF_INET, argv[1], &dstaddr.sin_addr) <= 0)
                err_sys("inet_pton");

        send_echo_req(sockfd, &dstaddr);
        recv_echo_reply(sockfd);

        exit(0);
}

void send_echo_req(int sockfd, struct sockaddr_in *dstaddr)
{
        char buf[100];
        size_t len = sizeof(struct icmp);
        struct icmp *icmp;
        socklen_t dstlen = sizeof(struct sockaddr_in);

        bzero(buf, sizeof(buf));
        icmp = (struct icmp *)buf;
        icmp->icmp_type = ICMP_ECHO;
        icmp->icmp_code = 0;
        icmp->icmp_id = getpid();
        icmp->icmp_seq = 1;
        icmp->icmp_cksum = in_cksum((uint16_t *) icmp, sizeof(struct icmp));

        if (sendto(sockfd, buf, len, 0, (SA *)dstaddr, dstlen) == -1)
                err_sys("sendto");
}

void recv_echo_reply(int sockfd)
{
        char buf[100];
        ssize_t n;
        struct ip *ip;
        struct icmp *icmp;
        while (1) {
                alarm(5); /* set timeout */
                if ((n = read(sockfd, buf, sizeof(buf))) == -1)
                        err_sys("read");

                ip = (struct ip *)buf;
                if (ip->ip_p != IPPROTO_ICMP) {
                        fprintf(stderr, "protocol error.\r\n");
                        exit(1);
                }
                icmp = (struct icmp *)(buf + sizeof(struct ip));
                if (icmp->icmp_type == ICMP_ECHOREPLY) {
                        if (icmp->icmp_id != getpid()) {
                                fprintf(stderr, "not this process.\r\n");
                                exit(1);
                        } else {
                                printf("destination host is alive.\r\n");
                                break;
                        }
                }
        }
}

uint16_t in_cksum(uint16_t *addr, int len)
{
        int nleft = len;
        uint32_t sum = 0;
        uint16_t *w = addr;
        uint16_t answer = 0;

        while (nleft > 1) {
                sum += *w++;
                nleft -= 2;
        }

        if (nleft == 1) {
                *(unsigned char *)(&answer) = *(unsigned char *)w ;
                sum += answer;
        }

        sum = (sum >> 16) + (sum & 0xffff);
        sum += (sum >> 16);
        answer = ~sum;
        return(answer);
}

void err_sys(const char *errmsg)
{
        perror(errmsg);
        exit(1);
}