add Proxy.AddSNIRouteFunc to do lookups by SNI dynamically

Author: bradfitz

Diff for: sni.go

@@ -57,7 +57,16 @@ func (p *Proxy) AddSNIMatchRoute(ipPort string, matcher Matcher, dest Target) {
 		cfg.acmeTargets = append(cfg.acmeTargets, dest)
 	}
 
-	p.addRoute(ipPort, sniMatch{matcher, dest})
+	p.addRoute(ipPort, sniMatch{matcher: matcher, target: dest})
+}
+
+// SNITargetFunc is the func callback used by Proxy.AddSNIRouteFunc.
+type SNITargetFunc func(ctx context.Context, sniName string) (t Target, ok bool)
+
+// AddSNIRouteFunc adds a route to ipPort that matches an SNI request and calls
+// fn to map its nap to a target.
+func (p *Proxy) AddSNIRouteFunc(ipPort string, fn SNITargetFunc) {
+	p.addRoute(ipPort, sniMatch{targetFunc: fn})
 }
 
 // AddStopACMESearch prevents ACME probing of subsequent SNI routes.
@@ -71,10 +80,22 @@ func (p *Proxy) AddStopACMESearch(ipPort string) {
 type sniMatch struct {
 	matcher Matcher
 	target  Target
+
+	// Alternatively, if targetFunc is non-nil, it's used instead:
+	targetFunc SNITargetFunc
 }
 
 func (m sniMatch) match(br *bufio.Reader) (Target, string) {
 	sni := clientHelloServerName(br)
+	if sni == "" {
+		return nil, ""
+	}
+	if m.targetFunc != nil {
+		if t, ok := m.targetFunc(context.TODO(), sni); ok {
+			return t, sni
+		}
+		return nil, ""
+	}
 	if m.matcher(context.TODO(), sni) {
 		return m.target, sni
 	}

Diff for: tcpproxy_test.go

@@ -17,6 +17,7 @@ package tcpproxy
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"crypto/rand"
 	"crypto/rsa"
 	"crypto/tls"
@@ -287,6 +288,49 @@ func TestProxySNI(t *testing.T) {
 	}
 }
 
+func TestAddSNIRouteFunc(t *testing.T) {
+	front := newLocalListener(t)
+	defer front.Close()
+
+	backFoo := newLocalListener(t)
+	defer backFoo.Close()
+	backBar := newLocalListener(t)
+	defer backBar.Close()
+
+	p := testProxy(t, front)
+	p.AddSNIRouteFunc(testFrontAddr, func(ctx context.Context, sniName string) (_ Target, ok bool) {
+		if sniName == "bar.com" {
+			return To(backBar.Addr().String()), true
+		}
+		t.Fatalf("failed to match %q", sniName)
+		return nil, false
+	})
+	if err := p.Start(); err != nil {
+		t.Fatal(err)
+	}
+
+	toFront, err := net.Dial("tcp", front.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer toFront.Close()
+
+	msg := clientHelloRecord(t, "bar.com")
+	io.WriteString(toFront, msg)
+
+	fromProxy, err := backBar.Accept()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	buf := make([]byte, len(msg))
+	if _, err := io.ReadFull(fromProxy, buf); err != nil {
+		t.Fatal(err)
+	}
+	if string(buf) != msg {
+		t.Fatalf("got %q; want %q", buf, msg)
+	}
+}
 func TestProxyPROXYOut(t *testing.T) {
 	front := newLocalListener(t)
 	defer front.Close()
@@ -362,7 +406,7 @@ func (t *tlsServer) Close() {
 // cert creates a well-formed, but completely insecure self-signed
 // cert for domain.
 func cert(t *testing.T, domain string) tls.Certificate {
-	private, err := rsa.GenerateKey(rand.Reader, 512)
+	private, err := rsa.GenerateKey(rand.Reader, 1024)
 	if err != nil {
 		t.Fatal(err)
 	}