net: fixes to dnsReadConfig in dnsconfig_windows.go

- Only search DNS servers for network interfaces with at least one gateway
- Clarify comment on deprecated site local anycast fec0/10 DNS IPv6 addresses
- Minor maintenance: skip not "up" interfaces earlier in outer loop

Change-Id: I98ca7b81d3d51e6aa6bfa4a10dcd651305a843df
GitHub-Last-Rev: 3b358c7
GitHub-Pull-Request: #64441
Reviewed-on: https://go-review.googlesource.com/c/go/+/545775
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>

Author: qdm12

src/internal/syscall/windows/syscall_windows.go

@@ -42,7 +42,10 @@ const (
 	ERROR_NO_UNICODE_TRANSLATION syscall.Errno = 1113
 )
 
-const GAA_FLAG_INCLUDE_PREFIX = 0x00000010
+const (
+	GAA_FLAG_INCLUDE_PREFIX   = 0x00000010
+	GAA_FLAG_INCLUDE_GATEWAYS = 0x0080
+)
 
 const (
 	IF_TYPE_OTHER              = 1
@@ -104,27 +107,45 @@ type IpAdapterPrefix struct {
 	PrefixLength uint32
 }
 
+type IpAdapterWinsServerAddress struct {
+	Length   uint32
+	Reserved uint32
+	Next     *IpAdapterWinsServerAddress
+	Address  SocketAddress
+}
+
+type IpAdapterGatewayAddress struct {
+	Length   uint32
+	Reserved uint32
+	Next     *IpAdapterGatewayAddress
+	Address  SocketAddress
+}
+
 type IpAdapterAddresses struct {
-	Length                uint32
-	IfIndex               uint32
-	Next                  *IpAdapterAddresses
-	AdapterName           *byte
-	FirstUnicastAddress   *IpAdapterUnicastAddress
-	FirstAnycastAddress   *IpAdapterAnycastAddress
-	FirstMulticastAddress *IpAdapterMulticastAddress
-	FirstDnsServerAddress *IpAdapterDnsServerAdapter
-	DnsSuffix             *uint16
-	Description           *uint16
-	FriendlyName          *uint16
-	PhysicalAddress       [syscall.MAX_ADAPTER_ADDRESS_LENGTH]byte
-	PhysicalAddressLength uint32
-	Flags                 uint32
-	Mtu                   uint32
-	IfType                uint32
-	OperStatus            uint32
-	Ipv6IfIndex           uint32
-	ZoneIndices           [16]uint32
-	FirstPrefix           *IpAdapterPrefix
+	Length                 uint32
+	IfIndex                uint32
+	Next                   *IpAdapterAddresses
+	AdapterName            *byte
+	FirstUnicastAddress    *IpAdapterUnicastAddress
+	FirstAnycastAddress    *IpAdapterAnycastAddress
+	FirstMulticastAddress  *IpAdapterMulticastAddress
+	FirstDnsServerAddress  *IpAdapterDnsServerAdapter
+	DnsSuffix              *uint16
+	Description            *uint16
+	FriendlyName           *uint16
+	PhysicalAddress        [syscall.MAX_ADAPTER_ADDRESS_LENGTH]byte
+	PhysicalAddressLength  uint32
+	Flags                  uint32
+	Mtu                    uint32
+	IfType                 uint32
+	OperStatus             uint32
+	Ipv6IfIndex            uint32
+	ZoneIndices            [16]uint32
+	FirstPrefix            *IpAdapterPrefix
+	TransmitLinkSpeed      uint64
+	ReceiveLinkSpeed       uint64
+	FirstWinsServerAddress *IpAdapterWinsServerAddress
+	FirstGatewayAddress    *IpAdapterGatewayAddress
 	/* more fields might be present here. */
 }
 

src/net/dnsconfig_windows.go

@@ -25,16 +25,19 @@ func dnsReadConfig(ignoredFilename string) (conf *dnsConfig) {
 	if err != nil {
 		return
 	}
-	// TODO(bradfitz): this just collects all the DNS servers on all
-	// the interfaces in some random order. It should order it by
-	// default route, or only use the default route(s) instead.
-	// In practice, however, it mostly works.
+
 	for _, aa := range aas {
+		// Only take interfaces whose OperStatus is IfOperStatusUp(0x01) into DNS configs.
+		if aa.OperStatus != windows.IfOperStatusUp {
+			continue
+		}
+
+		// Only take interfaces which have at least one gateway
+		if aa.FirstGatewayAddress == nil {
+			continue
+		}
+
 		for dns := aa.FirstDnsServerAddress; dns != nil; dns = dns.Next {
-			// Only take interfaces whose OperStatus is IfOperStatusUp(0x01) into DNS configs.
-			if aa.OperStatus != windows.IfOperStatusUp {
-				continue
-			}
 			sa, err := dns.Address.Sockaddr.Sockaddr()
 			if err != nil {
 				continue
@@ -47,9 +50,11 @@ func dnsReadConfig(ignoredFilename string) (conf *dnsConfig) {
 				ip = make(IP, IPv6len)
 				copy(ip, sa.Addr[:])
 				if ip[0] == 0xfe && ip[1] == 0xc0 {
-					// Ignore these fec0/10 ones. Windows seems to
-					// populate them as defaults on its misc rando
-					// interfaces.
+					// fec0/10 IPv6 addresses are site local anycast DNS
+					// addresses Microsoft sets by default if no other
+					// IPv6 DNS address is set. Site local anycast is
+					// deprecated since 2004, see
+					// https://datatracker.ietf.org/doc/html/rfc3879
 					continue
 				}
 			default:

src/net/interface_windows.go

@@ -20,7 +20,8 @@ func adapterAddresses() ([]*windows.IpAdapterAddresses, error) {
 	l := uint32(15000) // recommended initial size
 	for {
 		b = make([]byte, l)
-		err := windows.GetAdaptersAddresses(syscall.AF_UNSPEC, windows.GAA_FLAG_INCLUDE_PREFIX, 0, (*windows.IpAdapterAddresses)(unsafe.Pointer(&b[0])), &l)
+		const flags = windows.GAA_FLAG_INCLUDE_PREFIX | windows.GAA_FLAG_INCLUDE_GATEWAYS
+		err := windows.GetAdaptersAddresses(syscall.AF_UNSPEC, flags, 0, (*windows.IpAdapterAddresses)(unsafe.Pointer(&b[0])), &l)
 		if err == nil {
 			if l == 0 {
 				return nil, nil