hss_try_connect

Source: hss-probe.h:18

Attempts a non-blocking TCP connect to a host:port and reports whether the connection succeeded within a timeout.

Signature

static int hss_try_connect(const char *host, int port);

Returns 1 if a listener is detected, 0 otherwise.

Why non-blocking?

A blocking connect() to an unresponsive host can hang for minutes (TCP retransmission timeout). Since hss probes many candidate ports, a single hung connect would stall the entire scan. Non-blocking mode returns immediately with EINPROGRESS, and select() lets us wait with a bounded timeout.

The pattern

int sock = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
fcntl(sock, F_SETFL, fcntl(sock, F_GETFL, 0) | O_NONBLOCK);

Create a TCP socket.
Set O_NONBLOCK via fcntl. From this point, connect() returns immediately instead of blocking.

if (connect(sock, res->ai_addr, res->ai_addrlen) == 0) {
    status = 1;  // connected immediately (localhost common case)
}
else if (errno == EINPROGRESS) {
    fd_set fdset; FD_ZERO(&fdset); FD_SET(sock, &fdset);
    struct timeval tv = {TIMEOUT_SEC, 0};
    if (select(sock + 1, NULL, &fdset, NULL, &tv) > 0) {
        int err = 0; socklen_t l = sizeof(err);
        getsockopt(sock, SOL_SOCKET, SO_ERROR, &err, &l);
        status = !err;
    }
}

Two outcomes:

connect() returns 0 — connection succeeded immediately. Common for localhost listeners where the handshake completes before the kernel returns to userspace.
connect() returns -1 with EINPROGRESS — the handshake is in-flight. We select() on the socket for writeability with a 2-second timeout. When the socket becomes writable, we check SO_ERROR via getsockopt: 0 means the handshake succeeded; anything else means the connection was refused or timed out.

close(sock);
freeaddrinfo(res);
return status;

The socket is always closed and the addrinfo always freed, regardless of outcome.

POSIX portability

O_NONBLOCK, fcntl, select, getsockopt, and connect are all POSIX-standard interfaces. They are not Linux-specific. This is the reason hss compiles and runs on GNU/Hurd — the entire network probing layer uses only portable APIs. The Hurd-specific code is isolated to hss-proc.h (Mach IPC) and hss.c (the getproc() call).

Timeout

TIMEOUT_SEC is defined as 2 (hss-probe.h:16). Two seconds is long enough for remote hosts on a LAN and short enough that scanning 200+ ports completes in reasonable time. The timeout is per-port, not global.

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