feat(netxlite): implement hooks for transparent proxy (#561)

See https://github.com/ooni/probe/issues/1803#issuecomment-957323297.

internal/netxlite/dialer.go

@@ -85,12 +85,12 @@ var _ Dialer = &dialerSystem{}
 
 const dialerDefaultTimeout = 15 * time.Second
 
-func (d *dialerSystem) newUnderlyingDialer() *net.Dialer {
+func (d *dialerSystem) newUnderlyingDialer() TProxyDialer {
 	t := d.timeout
 	if t <= 0 {
 		t = dialerDefaultTimeout
 	}
-	return &net.Dialer{Timeout: t}
+	return TProxy.NewTProxyDialer(t)
 }
 
 func (d *dialerSystem) DialContext(ctx context.Context, network, address string) (net.Conn, error) {

internal/netxlite/dialer_test.go

@@ -38,7 +38,7 @@ func TestDialerSystem(t *testing.T) {
 	t.Run("has a default timeout", func(t *testing.T) {
 		d := &dialerSystem{}
 		ud := d.newUnderlyingDialer()
-		if ud.Timeout != dialerDefaultTimeout {
+		if ud.(*net.Dialer).Timeout != dialerDefaultTimeout {
 			t.Fatal("unexpected default timeout")
 		}
 	})
@@ -47,7 +47,7 @@ func TestDialerSystem(t *testing.T) {
 		const smaller = 1 * time.Second
 		d := &dialerSystem{timeout: smaller}
 		ud := d.newUnderlyingDialer()
-		if ud.Timeout != smaller {
+		if ud.(*net.Dialer).Timeout != smaller {
 			t.Fatal("unexpected timeout")
 		}
 	})

internal/netxlite/quic.go

@@ -34,7 +34,7 @@ var _ QUICListener = &quicListenerStdlib{}
 
 // Listen implements QUICListener.Listen.
 func (qls *quicListenerStdlib) Listen(addr *net.UDPAddr) (UDPLikeConn, error) {
-	return net.ListenUDP("udp", addr)
+	return TProxy.ListenUDP("udp", addr)
 }
 
 // QUICDialer dials QUIC sessions.

internal/netxlite/resolver.go

@@ -133,7 +133,7 @@ func (r *resolverSystem) lookupHost() func(ctx context.Context, domain string) (
 	if r.testableLookupHost != nil {
 		return r.testableLookupHost
 	}
-	return net.DefaultResolver.LookupHost
+	return TProxy.LookupHost
 }
 
 func (r *resolverSystem) Network() string {

internal/netxlite/tproxy.go

@@ -0,0 +1,59 @@
+package netxlite
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite/quicx"
+)
+
+// TProxable is the fundamental type used by the netxlite package to perform
+// low-level network operations for which, by default, we use the stdlib.
+//
+// The t stands for transparent. By using this type as the fundamental type,
+// we can transparently intercept connections and implement censorship
+// policies. The implementation of this functionality is not part of netxlite:
+// here we only have the basic mechanism to make this possible.
+type TProxable interface {
+	// ListenUDP creates a new quicx.UDPLikeConn conn.
+	ListenUDP(network string, laddr *net.UDPAddr) (quicx.UDPLikeConn, error)
+
+	// LookupHost lookups a domain using the stdlib resolver.
+	LookupHost(ctx context.Context, domain string) ([]string, error)
+
+	// NewTProxyDialer returns a new TProxyDialer.
+	NewTProxyDialer(timeout time.Duration) TProxyDialer
+}
+
+// TProxyDialer is the dialer type returned by TProxable.NewDialer.
+type TProxyDialer interface {
+	// DialContext behaves like net.Dialer.DialContext.
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// TProxy is the fundamental variable controlling how netxlite creates
+// net.Conn and quicx.UDPLikeConn, as well as how it uses the stdlib
+// resolver. By modifying this variable, you can effectively transparently
+// proxy netxlite (and hence OONI) activities to other services. This is
+// quite convenient when performing quality assurance tests.
+var TProxy TProxable = &TProxyStdlib{}
+
+// TProxyStdlib is the default TProxable implementation that uses
+// the stdlib in the most obvious way for every functionality.
+type TProxyStdlib struct{}
+
+// ListenUDP calls net.ListenUDP.
+func (*TProxyStdlib) ListenUDP(network string, laddr *net.UDPAddr) (quicx.UDPLikeConn, error) {
+	return net.ListenUDP(network, laddr)
+}
+
+// LookupHost calls net.DefaultResolver.LookupHost.
+func (*TProxyStdlib) LookupHost(ctx context.Context, domain string) ([]string, error) {
+	return net.DefaultResolver.LookupHost(ctx, domain)
+}
+
+// NewTProxyDialer returns a &net.Dialer{Timeout: timeout} instance.
+func (*TProxyStdlib) NewTProxyDialer(timeout time.Duration) TProxyDialer {
+	return &net.Dialer{Timeout: timeout}
+}