knock.c

/*
 * knock.c
 * Make a knock packet
 * By J. Stuart McMurray
 * Created 20160317
 * Last Modified 20160317
 */

#include "plainshell.h"

int make_knock(const char *address, const char *port) {
        unsigned char a[4];
        uint16_t p;
        int ret;

        /* Convert the address */
        if (0 == (ret = inet_pton(AF_INET, address, (void*)a))) {
                errx(1, "invalid address");
        } else if (-1 == ret) {
                err(2, "address parse error");
        }


        /* Convert the port */
        if (1 != sscanf(port, "%hu", &p)) {
                err(3, "port parse error");
        }
        p = htons(p);

        /* Write the bytes to stdout */
        if (-1 == write(STDOUT_FILENO, a, sizeof(a))) {
                err(4, "write");
        }
        if (-1 == write(STDOUT_FILENO, &p, sizeof(p))) {
                err(5, "write");
        }

        return 0;
}