A safe localhost-only scanner

Lesson

Local-only scanners hard-code their target as 127.0.0.1 and refuse anything else. The defensive structure:

int scan(const char *host, int lo, int hi) {
    if (strcmp(host, "127.0.0.1") != 0) return -1;  // refuse anything else
    /* …try connect() on each port… */
}

The hard-coded check is a guardrail: even if a future maintainer wires user input into host, the scanner can't be redirected to a third-party. Never soften this check on a production tool.