Utilities/nc: Add an option to test a TCP-listening service

Similarly to OpenBSD nc, an option to just connect without transmitting
any actual data is added.

However, we don't allow UDP-mode when testing a remote service, as it
will always succeed and has no technical meaning for the user if they're
not able to view the traffic on the remote machine.

Base/usr/share/man/man1/nc.md

@@ -21,6 +21,7 @@ Network cat: Connect to network sockets as if it were a file.
 * `-u`, `--udp`: UDP mode
 * `-p port`: Local port for remote connections
 * `-v`, `--verbose`: Log everything that's happening
+* `-z`, `--test-listening-service`: Test a TCP-listening service
 
 ## Arguments
 

Userland/Utilities/nc.cpp

@@ -51,6 +51,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     bool should_close = false;
     bool udp_mode = false;
     bool numeric_mode = false;
+    bool test_listening_daemons = false;
     ByteString target;
     u16 port = 0;
     u16 local_port = 0;
@@ -60,6 +61,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     args_parser.set_general_help("Network cat: Connect to network sockets as if it were a file.");
     args_parser.add_option(maximum_tcp_receive_buffer_size_input, "Set maximum tcp receive buffer size", "length", 'I', nullptr);
     args_parser.add_option(should_listen, "Listen instead of connecting", "listen", 'l');
+    args_parser.add_option(test_listening_daemons, "Test a TCP-listening service", "test-listening-service", 'z');
     args_parser.add_option(should_close, "Close connection after reading stdin to the end", nullptr, 'N');
     args_parser.add_option(numeric_mode, "Suppress name resolution", nullptr, 'n');
     args_parser.add_option(udp_mode, "UDP mode", "udp", 'u');
@@ -69,6 +71,11 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     args_parser.add_positional_argument(port, "Port to connect to or listen on", "port", Core::ArgsParser::Required::No);
     args_parser.parse(arguments);
 
+    if (test_listening_daemons && (udp_mode || should_listen)) {
+        warnln("listening/udp-mode while testing TCP services is not supported");
+        return 1;
+    }
+
     if (udp_mode) {
         if (should_listen) {
             warnln("listening on UDP not yet supported");
@@ -171,6 +178,17 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
             warnln("connecting to {}:{}", inet_ntop(dst_addr.sin_family, &dst_addr.sin_addr, addr_str, sizeof(addr_str)), ntohs(dst_addr.sin_port));
         }
 
+        if (test_listening_daemons) {
+            auto connect_or_error = Core::System::connect(fd, (struct sockaddr*)&dst_addr, sizeof(dst_addr));
+            char addr_str[INET_ADDRSTRLEN];
+            if (connect_or_error.is_error()) {
+                warnln("connection to {}:{} has failed due to {}", inet_ntop(dst_addr.sin_family, &dst_addr.sin_addr, addr_str, sizeof(addr_str)), ntohs(dst_addr.sin_port), strerror(connect_or_error.error().code()));
+                return 1;
+            }
+            warnln("connection to {}:{} has succeeded", inet_ntop(dst_addr.sin_family, &dst_addr.sin_addr, addr_str, sizeof(addr_str)), ntohs(dst_addr.sin_port));
+            return 0;
+        }
+
         TRY(Core::System::connect(fd, (struct sockaddr*)&dst_addr, sizeof(dst_addr)));
         if (verbose)
             warnln("connected!");