Add a shorter timeout and close connections earlier

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

README.md

@@ -43,6 +43,10 @@ To make the upstream address listen on all interfaces, use `0.0.0.0` instead of
 
 The port for the from and to addresses do not need to match.
 
+See also:
+* `-t` - specify the dial timeout for an upstream host in the "to" field of the config file.
+* `-v` - verbose logging - set to false to turn off logs of connections established and closed.
+
 ## License
 
 This software is licensed MIT.

main.go

@@ -28,45 +28,62 @@ type Rule struct {
 
 func main() {
 	var (
-		file string
+		file        string
+		verbose     bool
+		dialTimeout time.Duration
 	)
 
 	flag.StringVar(&file, "f", "", "Job to run or leave blank for job.yaml in current directory")
-
+	flag.BoolVar(&verbose, "v", true, "Verbose output for opened and closed connections")
+	flag.DurationVar(&dialTimeout, "t", time.Millisecond*1500, "Dial timeout")
 	flag.Parse()
 
+	if len(file) == 0 {
+		fmt.Fprintf(os.Stderr, "usage: mixctl -f rules.yaml\n")
+		os.Exit(1)
+	}
+
 	set := ForwardingSet{}
 	data, err := os.ReadFile(file)
 	if err != nil {
-		log.Fatalf("error reading file %s %s", file, err.Error())
+		fmt.Fprintf(os.Stderr, "error reading file %s %s", file, err.Error())
+		os.Exit(1)
 	}
 	if err = yaml.Unmarshal(data, &set); err != nil {
-		log.Fatalf("error parsing file %s %s", file, err.Error())
+		fmt.Fprintf(os.Stderr, "error parsing file %s %s", file, err.Error())
+		os.Exit(1)
+	}
+
+	if len(set.Rules) == 0 {
+		fmt.Fprintf(os.Stderr, "no rules found in file %s", file)
+		os.Exit(1)
 	}
 
-	fmt.Printf("mixctl by inlets..\n")
+	fmt.Printf("Starting mixctl by https://inlets.dev/\n\n")
 
 	wg := sync.WaitGroup{}
 	wg.Add(len(set.Rules))
-	for _, f := range set.Rules {
-
-		r := f
-		go func(rule *Rule) {
-			fmt.Printf("Forward (%s) from: %s to: %s\n", rule.Name, rule.From, rule.To)
+	for _, rule := range set.Rules {
+		fmt.Printf("Forward (%s) from: %s to: %s\n", rule.Name, rule.From, rule.To)
+	}
+	fmt.Println()
 
-			if err := forward(rule.Name, rule.From, rule.To); err != nil {
+	for _, rule := range set.Rules {
+		// Copy the value to avoid the loop variable being reused
+		r := rule
+		go func() {
+			if err := forward(r.Name, r.From, r.To, verbose, dialTimeout); err != nil {
 				log.Printf("error forwarding %s", err.Error())
 				os.Exit(1)
 			}
-
 			defer wg.Done()
-		}(&r)
+		}()
 	}
-	wg.Wait()
 
+	wg.Wait()
 }
 
-func forward(name, from string, to []string) error {
+func forward(name, from string, to []string, verbose bool, dialTimeout time.Duration) error {
 	seed := time.Now().UnixNano()
 	rand.Seed(seed)
 
@@ -76,42 +93,92 @@ func forward(name, from string, to []string) error {
 		return fmt.Errorf("error listening on %s %s", from, err.Error())
 	}
 
+	defer l.Close()
+
 	for {
-		conn, err := l.Accept()
+		// accept a connection on the local port of the load balancer
+		local, err := l.Accept()
 		if err != nil {
 			return fmt.Errorf("error accepting connection %s", err.Error())
 		}
 
+		// pick randomly from the list of upstream servers
+		// available
 		index := rand.Intn(len(to))
+		upstream := to[index]
 
-		remote, err := net.Dial("tcp", to[index])
-		if err != nil {
-			return fmt.Errorf("error dialing %s %s", to[index], err.Error())
-		}
+		// A separate Goroutine means the loop can accept another
+		// incoming connection on the local address
+		go connect(local, upstream, from, verbose, dialTimeout)
+	}
+}
 
-		go func() {
-			log.Printf("[%s] %s => %s",
-				from,
-				conn.RemoteAddr().String(),
-				remote.RemoteAddr().String())
-			if err := forwardConnection(conn, remote); err != nil && err.Error() != "done" {
-				log.Printf("error forwarding connection %s", err.Error())
-			}
-		}()
+// connect dials the upstream address, then copies data
+// between it and connection accepted on a local port
+func connect(local net.Conn, upstreamAddr, from string, verbose bool, dialTimeout time.Duration) {
+	defer local.Close()
+
+	// If Dial is used on its own, then the timeout can be as long
+	// as 2 minutes on MacOS for an unreachable host
+	upstream, err := net.DialTimeout("tcp", upstreamAddr, dialTimeout)
+	if err != nil {
+		log.Printf("error dialing %s %s", upstreamAddr, err.Error())
+		return
+	}
+	defer upstream.Close()
+
+	if verbose {
+		log.Printf("Connected %s => %s (%s)",
+			from,
+			upstream.RemoteAddr().String(),
+			local.RemoteAddr().String())
+	}
+
+	ctx := context.Background()
+	if err := copy(ctx, local, upstream); err != nil && err.Error() != "done" {
+		log.Printf("error forwarding connection %s", err.Error())
+	}
+
+	if verbose {
+		log.Printf("Closed %s => %s (%s)",
+			from,
+			upstream.RemoteAddr().String(),
+			local.RemoteAddr().String())
 	}
 }
 
-func forwardConnection(from, to net.Conn) error {
-	errgrp, _ := errgroup.WithContext(context.Background())
+// copy copies data between two connections using io.Copy
+// and will exit when either connection is closed or runs
+// into an error
+func copy(ctx context.Context, from, to net.Conn) error {
+
+	ctx, cancel := context.WithCancel(ctx)
+	errgrp, _ := errgroup.WithContext(ctx)
 	errgrp.Go(func() error {
 		io.Copy(from, to)
+		cancel()
 
 		return fmt.Errorf("done")
 	})
 	errgrp.Go(func() error {
 		io.Copy(to, from)
+		cancel()
+
 		return fmt.Errorf("done")
 	})
+	errgrp.Go(func() error {
+		<-ctx.Done()
+
+		// This closes both ends of the connection as
+		// soon as possible.
+		from.Close()
+		to.Close()
+		return fmt.Errorf("done")
+	})
+
+	if err := errgrp.Wait(); err != nil {
+		return err
+	}
 
-	return errgrp.Wait()
+	return nil
 }

rules.example.yaml

@@ -14,3 +14,8 @@ rules:
     - 192.168.1.19:22
     - 192.168.1.21:22
     - 192.168.1.20:22
+
+- name: remap-local-ssh-port
+  from: 127.0.0.1:2222
+  to:
+    - 127.0.0.1:22