fix: keep library scans quiet by default (#916)

`Scanner` no longer prints each open socket to stdout by default, so library users can embed it and use the returned sockets without unsolicited output. The CLI opts in via the new `Scanner::with_open_port_output()` builder. The UDP socket bind error is now logged at debug level instead of printed.

Fixes #706.

src/main.rs

@@ -94,7 +94,8 @@ fn main() {
         opts.accessible,
         opts.exclude_ports.unwrap_or_default(),
         opts.udp,
-    );
+    )
+    .with_open_port_output();
     debug!("Scanner finished building: {scanner:?}");
 
     let mut portscan_bench = NamedTimer::start("Portscan");

src/scanner/mod.rs

@@ -38,6 +38,7 @@ pub struct Scanner {
     accessible: bool,
     exclude_ports: Vec<u16>,
     udp: bool,
+    print_open_ports: bool,
 }
 
 // Allowing too many arguments for clippy.
@@ -64,9 +65,19 @@ impl Scanner {
             accessible,
             exclude_ports,
             udp,
+            print_open_ports: false,
         }
     }
 
+    /// Enables the CLI's incremental open-port output.
+    ///
+    /// Library callers are quiet by default and can inspect the sockets returned by [`Self::run`].
+    #[must_use]
+    pub fn with_open_port_output(mut self) -> Self {
+        self.print_open_ports = true;
+        self
+    }
+
     /// Runs scan_range with chunk sizes
     /// If you want to run RustScan normally, this is the entry point used
     /// Returns all open ports as `Vec<u16>`
@@ -286,7 +297,7 @@ impl Scanner {
                 }
             }
             Err(e) => {
-                println_safe(format_args!("Err E binding sock {e:?}"));
+                debug!("Error binding UDP socket: {e:?}");
                 Err(e)
             }
         }
@@ -294,7 +305,7 @@ impl Scanner {
 
     /// Formats and prints the port status
     fn fmt_ports(&self, socket: SocketAddr) {
-        if !self.greppable {
+        if self.print_open_ports && !self.greppable {
             if self.accessible {
                 println_safe(format_args!("Open {socket}"));
             } else {
@@ -308,8 +319,44 @@ impl Scanner {
 mod tests {
     use super::*;
 
-    // These tests only inspect the payload table generated by build.rs;
-    // they never open sockets.
+    use crate::input::{PortRange, ScanOrder};
+
+    // These tests never open sockets: they only build a `Scanner` or inspect
+    // the payload table generated by build.rs.
+
+    fn test_scanner() -> Scanner {
+        let addrs = vec!["127.0.0.1".parse::<IpAddr>().unwrap()];
+        let strategy = PortStrategy::pick(
+            &Some(PortRange { start: 1, end: 1 }),
+            None,
+            ScanOrder::Serial,
+        );
+        Scanner::new(
+            &addrs,
+            1,
+            Duration::from_millis(100),
+            1,
+            false,
+            strategy,
+            false,
+            Vec::new(),
+            false,
+        )
+    }
+
+    #[test]
+    fn library_scanner_is_quiet_by_default() {
+        let scanner = test_scanner();
+
+        assert!(!scanner.print_open_ports);
+    }
+
+    #[test]
+    fn cli_can_enable_open_port_output() {
+        let scanner = test_scanner().with_open_port_output();
+
+        assert!(scanner.print_open_ports);
+    }
 
     /// Regression test for https://github.com/bee-san/RustScan/issues/933:
     /// the SNMP public-walk probe must be the exact 33-byte BER packet, with