Report closed ports as closed on Windows

CHANGELOG.md

@@ -93,6 +93,11 @@ its heading and collects entries; the date and the link go on with the tag.
   `ff:ff:ff:ff:ff:ff`, and discovery listed it as a host that ignored ping
   (and a sweep then scanned it). The network and broadcast addresses, and broadcast and
   multicast MACs, are no longer reported.
+- **Closed ports showed as filtered on Windows.** Windows waits about two
+  seconds before reporting a refused connection, and the scan stops waiting
+  after half a second, so a port that was plainly closed came back as
+  filtered. The scan now reports it as closed straight away. A scan of 1,024
+  ports on a LAN machine went from 1,022 filtered to 1,022 closed.
 - **The macOS app could be refused as broken on Apple Silicon.** Its only
   signature was the one Apple's linker puts on every arm64 program, which
   claims the app's files are sealed when nothing sealed them. macOS's own

crates/netscli-core/Cargo.toml

@@ -48,7 +48,7 @@ pnet_datalink = "0.35"
 
 [target.'cfg(windows)'.dependencies]
 ipconfig = "0.3"
-windows-sys = { version = "0.61", features = ["Win32_Foundation", "Win32_NetworkManagement_IpHelper", "Win32_NetworkManagement_Ndis", "Win32_Networking_WinSock"] }
+windows-sys = { version = "0.61", features = ["Win32_Foundation", "Win32_NetworkManagement_IpHelper", "Win32_NetworkManagement_Ndis", "Win32_Networking_WinSock", "Win32_System_IO"] }
 
 [features]
 # Lean by default: scan, discover, DNS, ARP, OUI lookup, stats.

crates/netscli-core/src/common.rs

@@ -1,3 +1,4 @@
+mod connect;
 mod constants;
 mod network;
 mod ports;
@@ -8,6 +9,7 @@ mod ports;
 mod system_tools;
 mod terminal;
 
+pub(crate) use connect::connect as tcp_connect;
 pub use constants::{
     DEFAULT_CONCURRENCY, DEFAULT_DNS_TIMEOUT_MS, DEFAULT_PING_TIMEOUT_MS, DEFAULT_PORTS,
     DEFAULT_SCAN_TIMEOUT_MS, DEFAULT_SUBNET, MAX_MDNS_TIMEOUT_MS, MAX_PING_COUNT,

crates/netscli-core/src/common/connect.rs

@@ -0,0 +1,93 @@
+//! TCP connect for the port scanner, which must tell "refused" from "no
+//! answer".
+//!
+//! Windows retries a SYN that was answered with a RST, so a connect to a
+//! closed port only fails with `ConnectionRefused` after about 2 s (measured
+//! 2026-10-02 on Windows 11: 2025-2047 ms to a LAN host, 2043 ms to
+//! loopback). The port scanner gives up at 500 ms by default, so every closed
+//! port read as filtered. Turning SYN retransmissions off for the socket
+//! makes the first RST final: the same connects failed in 0-5 ms.
+//!
+//! With no retransmissions Windows abandons the connect after the initial
+//! retransmission timeout, 1 s by default, whatever timeout the caller asked
+//! for. So the initial timeout is set to the caller's timeout as well.
+
+use std::io;
+use std::net::SocketAddr;
+use std::time::Duration;
+
+use tokio::net::{TcpSocket, TcpStream};
+
+/// Connect to `addr`, giving up after `timeout_ms`. A timeout is reported as
+/// `ErrorKind::TimedOut`, the same kind the OS uses for an unanswered SYN.
+pub(crate) async fn connect(addr: SocketAddr, timeout_ms: u64) -> io::Result<TcpStream> {
+    let socket = if addr.is_ipv4() {
+        TcpSocket::new_v4()?
+    } else {
+        TcpSocket::new_v6()?
+    };
+    #[cfg(windows)]
+    windows::final_rst(&socket, timeout_ms);
+    match tokio::time::timeout(Duration::from_millis(timeout_ms), socket.connect(addr)).await {
+        Ok(result) => result,
+        Err(_) => Err(io::Error::new(io::ErrorKind::TimedOut, "connect timed out")),
+    }
+}
+
+#[cfg(windows)]
+mod windows {
+    use std::os::windows::io::AsRawSocket;
+
+    use tokio::net::TcpSocket;
+    use windows_sys::Win32::Networking::WinSock::{
+        WSAIoctl, SIO_TCP_INITIAL_RTO, SOCKET, TCP_INITIAL_RTO_NO_SYN_RETRANSMISSIONS,
+        TCP_INITIAL_RTO_PARAMETERS,
+    };
+
+    /// Turn off SYN retransmissions and set the initial retransmission
+    /// timeout to `timeout_ms`. Best effort: if the ioctl is refused the
+    /// connect still works, it is only slow to report a closed port.
+    pub(super) fn final_rst(socket: &TcpSocket, timeout_ms: u64) {
+        let params = TCP_INITIAL_RTO_PARAMETERS {
+            Rtt: rtt(timeout_ms),
+            // The C header defines this as (USHORT)-2 and assigns it to the
+            // UCHAR field, which keeps the low byte.
+            MaxSynRetransmissions: TCP_INITIAL_RTO_NO_SYN_RETRANSMISSIONS as u8,
+        };
+        let mut returned = 0u32;
+        // SAFETY: a valid socket handle, an input buffer of the declared size,
+        // no output buffer, and no overlapped I/O.
+        unsafe {
+            WSAIoctl(
+                socket.as_raw_socket() as SOCKET,
+                SIO_TCP_INITIAL_RTO,
+                &params as *const _ as *const core::ffi::c_void,
+                std::mem::size_of::<TCP_INITIAL_RTO_PARAMETERS>() as u32,
+                std::ptr::null_mut(),
+                0,
+                &mut returned,
+                std::ptr::null_mut(),
+                None,
+            );
+        }
+    }
+
+    /// 65535 means "unspecified" to Windows, so the largest usable value is
+    /// 65534 ms. 0 is not a timeout either.
+    pub(super) fn rtt(timeout_ms: u64) -> u16 {
+        timeout_ms.clamp(1, 65_534) as u16
+    }
+
+    #[cfg(test)]
+    mod tests {
+        use super::rtt;
+
+        #[test]
+        fn rtt_stays_inside_the_values_windows_accepts() {
+            assert_eq!(rtt(0), 1);
+            assert_eq!(rtt(500), 500);
+            assert_eq!(rtt(65_535), 65_534);
+            assert_eq!(rtt(u64::MAX), 65_534);
+        }
+    }
+}

crates/netscli-core/src/scan/tcp.rs

@@ -4,10 +4,9 @@ use std::sync::{
     atomic::{AtomicUsize, Ordering},
     Arc,
 };
-use std::time::{Duration, Instant};
+use std::time::Instant;
 use tokio::net::TcpStream;
 use tokio::sync::Semaphore;
-use tokio::time::timeout;
 
 use super::probes::{as_text, ask, first_banner_line, probe_http, probe_tls, read_greeting, Quiet};
 use super::services::{classify_connect_error, guess_service, is_http_port, is_tls_port};
@@ -128,10 +127,8 @@ impl PortScanner {
         let addr = SocketAddr::new(target, port);
         let service = Self::guess_service(port);
         let started = Instant::now();
-        let result = timeout(Duration::from_millis(timeout_ms), TcpStream::connect(addr)).await;
-
-        match result {
-            Ok(Ok(stream)) => {
+        match crate::common::tcp_connect(addr, timeout_ms).await {
+            Ok(stream) => {
                 let latency_ms = started.elapsed().as_millis() as u64;
                 let mut result = PortResult::new(port, PortStatus::Open, service.clone())
                     .with_latency(latency_ms);
@@ -145,7 +142,7 @@ impl PortScanner {
                 }
                 result
             }
-            Ok(Err(e)) => match classify_connect_error(e.kind()) {
+            Err(e) => match classify_connect_error(e.kind()) {
                 PortStatus::Closed => PortResult::new(port, PortStatus::Closed, service)
                     .with_latency(started.elapsed().as_millis() as u64),
                 PortStatus::Filtered => PortResult::new(port, PortStatus::Filtered, service),
@@ -155,7 +152,6 @@ impl PortScanner {
                         .with_error(e.to_string())
                 }
             },
-            Err(_) => PortResult::new(port, PortStatus::Filtered, service),
         }
     }
 

crates/netscli-core/src/scan/tests.rs

@@ -107,16 +107,14 @@ async fn test_scan_closed_local_port_records_latency() {
         .await
         .expect("valid port list");
 
+    // Nothing listens, so the OS answers with a RST and the port is closed,
+    // well inside the timeout. Windows used to retry the SYN for about 2 s
+    // first, so this read as filtered there.
     assert_eq!(results.len(), 1);
     let result = results.remove(0);
     assert!(!result.open);
-    assert!(matches!(
-        result.status,
-        PortStatus::Closed | PortStatus::Filtered | PortStatus::Error
-    ));
-    if matches!(result.status, PortStatus::Closed) {
-        assert!(result.latency_ms.is_some());
-    }
+    assert_eq!(result.status, PortStatus::Closed);
+    assert!(result.latency_ms.is_some_and(|ms| ms < 500));
 }
 
 #[tokio::test]