///usage:
//
// $ gcc counter.c -o counter
// $ ./counter --interface eth0 --host localhost

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <getopt.h>
#include <signal.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/types.h>

#define CMDLEN 1024
#define TCPDUMPLEN 4096


int show_stats;
long counter;

void alarm_handler(int signum)
{
    show_stats = 1;
}

void install_handler(void)
{
    struct sigaction sa;

    memset(&sa, 0, sizeof(sa));

    sigemptyset(&sa.sa_mask);
    sa.sa_handler = &alarm_handler;
    if (sigaction(SIGALRM, &sa, NULL) == -1) {
        perror("Can't install alarm handler");
        exit(1);
    }
}

int count_lines(char *p, int bytes)
{
    int counter = 0;
    char *i;
    for (i=p; i < p+bytes ; i++) {
        if (*i == '\n')
            counter++;
    }
    return counter;
}

int spawn_tcpdump(char *host, char *interface)
{
    int fd[2];
    pid_t child;

    if (pipe(fd) == -1) {
        perror("Can't create pipes");
        exit(1);
    }

    child = fork();

    if (child == -1) {
        perror("Can't fork(2) for tcpdump");
        exit(1);
    }

    if (child == 0) {
        int null;
        int len;
        char paramss[CMDLEN];

        len = snprintf(paramss, CMDLEN, "tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420 and dst %s", host);

        if (len > CMDLEN) {
            perror("host argument too long");
            exit(1);
        }

        /* child writes into pipe */
        close(fd[0]);
        dup2(fd[1], STDOUT_FILENO);

        /* throw away first two lines of tcpdump output */
        null = open("/dev/null", O_WRONLY);

        if (null == -1) {
            perror("Can't open /dev/null");
            exit(1);
        }

        dup2(null, STDERR_FILENO);

        execl("/usr/sbin/tcpdump", "tcpdump", "-A", "-n", "-s 0", "-i", interface, paramss, (char *) NULL);
        /* can't reach */
        perror("Cannot execute tcpdump");
        exit(1);
    } else {
        /* parent reads from pipe */
        close(fd[1]);
        return fd[0];
    }
}

int main(int argc, char *argv[])
{
    int tcpdump;
    char *host;
    char *interface;
    long interval;


    while (1) {
        int option_index;
        int c;

        static struct option opts[] = {
            {"host", required_argument, NULL, 'h'},
            {"interface", required_argument, NULL, 'i'},
            {"interval", required_argument, NULL, 'n'},
            {0, 0, 0, 0},
        };

        c = getopt_long(argc, argv, "", opts, &option_index);

        if (c == -1)
            break;

        switch (c) {
            case 'h':
                host = strdup(optarg);
                break;
            case 'i':
                interface = strdup(optarg);
                break;
            case 'n': {
                    char *endptr;
                    interval = strtol(optarg, &endptr, 10);
                    if (!(optarg[0] != '\0' && endptr[0] == '\0')) {
                        fprintf(stderr, "Expected integer; invalid"
                                " input '%s'\n", optarg);
                        exit(1);
                    }
                }
                break;
            default:
                fprintf(stderr, "Option parsing error\n");
                exit(1);
        }

    }

    if (optind < argc) {
        fprintf(stderr, "unexpected arguments: ");
        while (optind < argc) {
            fprintf(stderr, "%s ", argv[optind++]);
        }
        fprintf(stderr, "\n");
    }

    tcpdump = spawn_tcpdump(host, interface);

    install_handler();
    alarm(interval);

    while(1) {
        if (show_stats) {
            printf("%ld requests in %ld seconds; average %2.2f req/seq\n",
                    counter, interval, (double)counter / (double)interval);
            counter = 0;
            show_stats = 0;
            alarm(interval);
        } else {
            char buffer[TCPDUMPLEN];
            int ret;

            memset(buffer, 0, TCPDUMPLEN);
            ret = read(tcpdump, buffer, TCPDUMPLEN);
            if (ret == -1 && errno == EINTR) {
                /* nop */
            } else if (ret == -1) {
                perror("read");
                exit(1);
            } else {
                counter += count_lines(buffer, ret);
            }
        }
    }

    exit(0);
}