netprobe: accept multiple comma-separated addresses (#3358)

This allows probing both an IPv4 and an IPv6 address, so that network
detection works on IPv4-only as well as IPv6-only networks.

Author: jedisct1

dnscrypt-proxy/example-dnscrypt-proxy.toml

@@ -381,6 +381,9 @@ netprobe_timeout = 60
 ## when the system starts.
 ## On other operating systems, the connection will be initialized
 ## but nothing will be sent at all.
+## Multiple comma-separated addresses can be given, for example to support
+## both IPv4-only and IPv6-only networks. Connectivity is detected as soon
+## as any of them works: '9.9.9.9:53,[2620:fe::fe]:53'
 
 netprobe_address = '9.9.9.9:53'
 

dnscrypt-proxy/netprobe.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"errors"
+	"net"
+	"strings"
+)
+
+// resolveNetprobeAddresses parses a netprobe address specification.
+// Multiple addresses (for example an IPv4 and an IPv6 address) can be
+// separated by commas; connectivity is detected as soon as any of them works.
+func resolveNetprobeAddresses(address string) ([]*net.UDPAddr, error) {
+	var addrs []*net.UDPAddr
+	for _, part := range strings.Split(address, ",") {
+		part = strings.TrimSpace(part)
+		if len(part) == 0 {
+			continue
+		}
+		addr, err := net.ResolveUDPAddr("udp", part)
+		if err != nil {
+			return nil, err
+		}
+		addrs = append(addrs, addr)
+	}
+	if len(addrs) == 0 {
+		return nil, errors.New("No valid netprobe address in [" + address + "]")
+	}
+	return addrs, nil
+}

dnscrypt-proxy/netprobe_others.go

@@ -20,7 +20,7 @@ func NetProbe(proxy *Proxy, address string, timeout int) error {
 	} else {
 		dlog.Critical(err)
 	}
-	remoteUDPAddr, err := net.ResolveUDPAddr("udp", address)
+	remoteUDPAddrs, err := resolveNetprobeAddresses(address)
 	if err != nil {
 		return err
 	}
@@ -31,7 +31,15 @@ func NetProbe(proxy *Proxy, address string, timeout int) error {
 		timeout = Min(MaxTimeout, timeout)
 	}
 	for tries := timeout; tries > 0; tries-- {
-		pc, err := net.DialTimeout("udp", remoteUDPAddr.String(), proxy.timeout)
+		var err error
+		for _, remoteUDPAddr := range remoteUDPAddrs {
+			var pc net.Conn
+			pc, err = net.DialTimeout("udp", remoteUDPAddr.String(), proxy.timeout)
+			if err == nil {
+				pc.Close()
+				break
+			}
+		}
 		if err != nil {
 			if !retried {
 				retried = true
@@ -41,7 +49,6 @@ func NetProbe(proxy *Proxy, address string, timeout int) error {
 			time.Sleep(1 * time.Second)
 			continue
 		}
-		pc.Close()
 		dlog.Notice("Network connectivity detected")
 		return nil
 	}

dnscrypt-proxy/netprobe_test.go

@@ -0,0 +1,19 @@
+package main
+
+import "testing"
+
+func TestResolveNetprobeAddresses(t *testing.T) {
+	addrs, err := resolveNetprobeAddresses("9.9.9.9:53")
+	if err != nil || len(addrs) != 1 || addrs[0].String() != "9.9.9.9:53" {
+		t.Fatalf("single address: %v %v", addrs, err)
+	}
+	addrs, err = resolveNetprobeAddresses("9.9.9.9:53, [2620:fe::fe]:53")
+	if err != nil || len(addrs) != 2 || addrs[1].String() != "[2620:fe::fe]:53" {
+		t.Fatalf("multiple addresses: %v %v", addrs, err)
+	}
+	for _, bad := range []string{"9.9.9.9", "9.9.9.9:53,bogus", " , "} {
+		if _, err := resolveNetprobeAddresses(bad); err == nil {
+			t.Fatalf("expected error for %q", bad)
+		}
+	}
+}

dnscrypt-proxy/netprobe_windows.go

@@ -18,7 +18,7 @@ func NetProbe(proxy *Proxy, address string, timeout int) error {
 	} else {
 		dlog.Critical(err)
 	}
-	remoteUDPAddr, err := net.ResolveUDPAddr("udp", address)
+	remoteUDPAddrs, err := resolveNetprobeAddresses(address)
 	if err != nil {
 		return err
 	}
@@ -29,15 +29,21 @@ func NetProbe(proxy *Proxy, address string, timeout int) error {
 		timeout = Min(MaxTimeout, timeout)
 	}
 	for tries := timeout; tries > 0; tries-- {
-		pc, err := net.DialTimeout("udp", remoteUDPAddr.String(), proxy.timeout)
-		if err == nil {
+		var err error
+		for _, remoteUDPAddr := range remoteUDPAddrs {
+			var pc net.Conn
+			pc, err = net.DialTimeout("udp", remoteUDPAddr.String(), proxy.timeout)
+			if err != nil {
+				continue
+			}
 			// Write at least 1 byte. This ensures that sockets are ready to use for writing.
 			// Windows specific: during the system startup, sockets can be created but the underlying buffers may not be
 			// set up yet. If this is the case Write fails with WSAENOBUFS: "An operation on a socket could not be
 			// performed because the system lacked sufficient buffer space or because a queue was full"
 			_, err = pc.Write([]byte{0})
-			if err != nil {
-				pc.Close()
+			pc.Close()
+			if err == nil {
+				break
 			}
 		}
 		if err != nil {
@@ -49,7 +55,6 @@ func NetProbe(proxy *Proxy, address string, timeout int) error {
 			time.Sleep(1 * time.Second)
 			continue
 		}
-		pc.Close()
 		dlog.Notice("Network connectivity detected")
 		return nil
 	}