pingcat

Makefile

all: pingcat.c
	gcc -o pingcat pingcat.c

pingcat.c

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>

#define fail_if(v, msg) \
    if (v) { \
    fprintf(stderr, "error: %s\n", msg); \
    return 1;\
    }

int receive_ping(int icmp_sock, int expect, int cookie, int count,
                 int tries, int *id, u_char **data, int *datalen_out) {
    static u_char buf[0x10000];
    struct msghdr msg;
    int datalen;
    struct iovec iov = { buf, sizeof(buf) };
    struct iphdr *ip = (struct iphdr *)buf;
    struct icmphdr *icp;
    while (tries-- > 0) {
        memset(&msg, 0, sizeof(msg));
        msg.msg_name = 0;
        msg.msg_namelen = 0;
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;
        errno = 0;
        datalen = recvmsg(icmp_sock, &msg, MSG_DONTWAIT);
        if (errno == EAGAIN) {
            struct timeval one_sec = {1, 0};
            select(0, 0, 0, 0, &one_sec);
            continue;
        } else {
            fail_if(datalen < 0, "recvmsg");
        }
        int hlen = ip->ihl*4;
        datalen -= hlen;
        if (datalen >= 4) {
            icp = (struct icmphdr *)(buf + hlen);
            char *payload = (char*)icp + 8;
            int seqnum = ntohs(icp->un.echo.sequence);
            if (icp->type == expect) {
                if (*(uint32_t *)payload == cookie
                    && seqnum == count) {
                    break;
                }
                fprintf(stderr, "wrong cookie (%c%c%c%c) or seqnum (%d != %d)\n",
                        payload[0], payload[1], payload[2], payload[3],
                        seqnum, count);
            }
        }
    }
    if (tries < 0) {
        fprintf(stderr, "ran out of retries: %d", tries);
        return 1;
    }

    /* printf("payload_len=%d: [", datalen); */
    /* int i; */
    /* for (i = 0; i < datalen; ++i) { */
    /*     printf("%x ", buf[i]); */
    /* } */
    /* printf("]\n"); */
	/* printf(" icmp_seq=%u\n", ntohs(icp->un.echo.sequence)); */

    if (data != 0) {
        *data = ((u_char*)icp) + 12;
        *datalen_out = datalen - 12;
    }

    if (id != 0) {
        *id = icp->un.echo.id;
    }

    return 0;
}

int server(uint32_t cookie) {
    int icmp_sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
    fail_if(icmp_sock < 0, "socket()");

    int i;
    for (i = 0; 1; ++i) {
        u_char *data;
        int datalen;
        fail_if(receive_ping(icmp_sock, ICMP_ECHOREPLY,
                             cookie, i, 10, 0, &data, &datalen) != 0,
                "receive_ping");
        if (datalen == 1) {
            break;
        }
        fwrite(data, 1, datalen, stdout);
        fflush(stdout);
    }

    return 0;
}

/* Original code from iputils.
   http://www.linuxfoundation.org/collaborate/workgroups/networking/iputils
*/
u_short in_cksum(const u_short *addr, register int len, u_short csum)
{
	register int nleft = len;
	const u_short *w = addr;
	register u_short answer;
	register int sum = csum;
	while (nleft > 1)  {
		sum += *w++;
		nleft -= 2;
	}
	if (nleft == 1) {
		sum += htons((u_short)(*(u_char *)w) << 8);
    }
	sum = (sum >> 16) + (sum & 0xffff);
	sum += (sum >> 16);
	answer = ~sum;
	return (answer);
}

int send_ping(int icmp_sock, uint32_t cookie, int id, int count,
              struct sockaddr_in addr, char *data, int datalen) {
    static u_char buf[0x10000];
    static struct icmphdr *icp = (struct icmphdr *)buf;
    icp->type = ICMP_ECHO;
	icp->code = 0;
	icp->checksum = 0;
	icp->un.echo.sequence = htons(count);
	icp->un.echo.id = id;
    memset(buf + 8, 0, datalen + 5);
    fail_if(0x10000 < datalen + 13, "too much data");
    *(uint32_t *)(buf + 8) = cookie;
    memcpy(buf + 12, data, datalen);
    icp->checksum = in_cksum((u_short*)icp, datalen + 13, 0);

    struct iovec iov = { buf, 0 };
    iov.iov_len = datalen + 13;
    struct msghdr hdr = { &addr, sizeof(addr),
                          &iov, 1, 0, 0, 0 };
    fail_if(sendmsg(icmp_sock, &hdr, 0) < 0, "sendmsg");
    return 0;
}

int client(uint32_t cookie, char *target) {
    int icmp_sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
    fail_if(icmp_sock < 0, "socket()");

    struct sockaddr_in addr;
    memset((char *)&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    fail_if(inet_aton(target, &addr.sin_addr) != 1, "inet_aton");

    int id = htons(getpid() & 0xFFFF);
    int i;
    int terminating = 0;
    for (i = 0; 1; ++i) {
        char buf[1024];
        int size = 0;
        if (terminating) {
            break;
        }
        if ((size = fread(buf, 1, 1024, stdin)) <= 0) {
            terminating = 1;
            size = 0;
        }
        for (;;) {
            fail_if(send_ping(icmp_sock, cookie, id,
                              i, addr, buf, size) != 0,
                    "send_ping");
            int re_id;
            if (0 == receive_ping(icmp_sock, ICMP_ECHO, cookie,
                                  i, 2, &re_id, 0, 0)
                && re_id == id)
            {
                break;
            }
            fprintf(stderr, "ping lost\n");
            struct timeval one_sec = {1, 0};
            select(0, 0, 0, 0, &one_sec);
        }
    }

    close(icmp_sock);

    return 0;
}

uint32_t cookie_int(char *cookie) {
    if (strlen(cookie) != 4) {
        printf("cookie must be 4 bytes: %s\n", cookie);
        exit(1);
    }
    return *(uint32_t*)cookie;
}

int main(int argc, char *argv[]) {
    if (2 == argc) {
        return server(cookie_int(argv[1]));
    } else if (3 == argc) {
        return client(cookie_int(argv[1]), argv[2]);
    } else {
        printf("USAGE: %s COOKIE [ADDRESS]\n", argv[0]);
        return 1;
    }
}

Blog post for this code: here.