Program Statement

A minimal line-mode telnet: connect to any host and port, then relay stdin↔socket with select — this is the str_cli function from Unit 2:

/* mini_telnet.c — usage: ./mini_telnet host port */
int main(int argc, char **argv) {
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in srv = {0};
    srv.sin_family = AF_INET; srv.sin_port = htons(atoi(argv[2]));
    inet_pton(AF_INET, argv[1], &srv.sin_addr);
    if (connect(fd, (struct sockaddr *)&srv, sizeof(srv)) < 0) {
        perror("connect"); exit(1);
    }
    fd_set rset; char buf[4096]; int n;
    for (;;) {
        FD_ZERO(&rset); FD_SET(0, &rset); FD_SET(fd, &rset);
        select(fd + 1, &rset, NULL, NULL, NULL);
        if (FD_ISSET(fd, &rset)) {                   /* network → screen */
            if ((n = read(fd, buf, sizeof(buf))) <= 0) break;
            write(1, buf, n);
        }
        if (FD_ISSET(0, &rset)) {                    /* keyboard → network */
            if ((n = read(0, buf, sizeof(buf))) <= 0) { shutdown(fd, SHUT_WR); FD_CLR(0,&rset); continue; }
            write(fd, buf, n);
        }
    }
    close(fd); return 0;
}

Testing

Test it against your echo server, or speak HTTP by hand: type GET / HTTP/1.0 followed by a blank line against port 80.

Record Task

Real telnet adds IAC option negotiation (WILL/WONT/DO/DONT) on top of this raw byte relay — describe it in your record. Theory: Unit 4 "Remote Login".