Make options easier to use

The local port is now optional, in which case a random free port will be used
(and reported in the output). The `--host` argument is now `--remote-host`.

src/main.rs

@@ -9,8 +9,8 @@ static DEBUG: AtomicBool = AtomicBool::new(false);
 
 fn print_usage(program: &str, opts: Options) {
     let program_path = std::path::PathBuf::from(program);
-    let program_name = program_path.file_stem().unwrap().to_str().unwrap();
-    let brief = format!("Usage: {} [-b BIND_ADDR] -l LOCAL_PORT -h REMOTE_ADDR -r REMOTE_PORT",
+    let program_name = program_path.file_stem().unwrap().to_string_lossy();
+    let brief = format!("Usage: {} [-b BIND_ADDR] -h REMOTE_HOST -r REMOTE_PORT [-l LOCAL_PORT]",
                         program_name);
     print!("{}", opts.usage(&brief));
 }
@@ -21,34 +21,36 @@ async fn main() -> Result<(), BoxedError> {
     let program = args[0].clone();
 
     let mut opts = Options::new();
-    opts.reqopt("l",
-                "local-port",
-                "The local port to which tcpproxy should bind to",
-                "LOCAL_PORT");
+    opts.reqopt("h",
+                "remote-host",
+                "The remote host (ip or host name) to which packets will be forwarded",
+                "REMOTE_HOST");
     opts.reqopt("r",
                 "remote-port",
                 "The remote port to which TCP packets should be forwarded",
                 "REMOTE_PORT");
-    opts.reqopt("h",
-                "host",
-                "The remote address to which packets will be forwarded",
-                "REMOTE_ADDR");
     opts.optopt("b",
                 "bind",
-                "The address on which to listen for incoming requests",
+                "The address on which to listen for incoming requests, defaulting to localhost",
                 "BIND_ADDR");
+    opts.optopt("l",
+                "local-port",
+                "The local port to which tcpproxy should bind to, randomly chosen otherwise",
+                "LOCAL_PORT");
     opts.optflag("d", "debug", "Enable debug mode");
 
     let matches = match opts.parse(&args[1..]) {
         Ok(opts) => opts,
-        Err(_) => {
+        Err(e) => {
+            eprintln!("{}", e);
             print_usage(&program, opts);
             std::process::exit(-1);
         }
     };
 
     DEBUG.store(matches.opt_present("d"), Ordering::Relaxed);
-    let local_port: i32 = matches.opt_str("l").unwrap().parse()?;
+    // let local_port: i32 = matches.opt_str("l").unwrap_or("0".to_string()).parse()?;
+    let local_port: i32 = matches.opt_str("l").map(|s| s.parse()).unwrap_or(Ok(0))?;
     let remote_port: i32 = matches.opt_str("r").unwrap().parse()?;
     let remote_host = matches.opt_str("h").unwrap();
     let bind_addr = match matches.opt_str("b") {