net: respect hosts file when resolving names for Windows

Fixes #57757.

Change-Id: I896dae8e5905ae98539ab83c9379fd1c9886d44a
Reviewed-on: https://go-review.googlesource.com/c/go/+/467335
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Run-TryBot: Mateusz Poliwczak <mpoliwczak34@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/internal/syscall/windows/security_windows.go

@@ -126,3 +126,9 @@ type UserInfo4 struct {
 }
 
 //sys	NetUserGetLocalGroups(serverName *uint16, userName *uint16, level uint32, flags uint32, buf **byte, prefMaxLen uint32, entriesRead *uint32, totalEntries *uint32) (neterr error) = netapi32.NetUserGetLocalGroups
+
+// GetSystemDirectory retrieves the path to current location of the system
+// directory, which is typically, though not always, `C:\Windows\System32`.
+//
+//go:linkname GetSystemDirectory
+func GetSystemDirectory() string // Implemented in runtime package.

src/net/conf.go

@@ -238,16 +238,7 @@ func (c *conf) lookupOrder(r *Resolver, hostname string) (ret hostLookupOrder, d
 		// Go resolver was explicitly requested
 		// or cgo resolver is not available.
 		// Figure out the order below.
-		switch c.goos {
-		case "windows":
-			// TODO(bradfitz): implement files-based
-			// lookup on Windows too? I guess /etc/hosts
-			// kinda exists on Windows. But for now, only
-			// do DNS.
-			fallbackOrder = hostLookupDNS
-		default:
-			fallbackOrder = hostLookupFilesDNS
-		}
+		fallbackOrder = hostLookupFilesDNS
 		canUseCgo = false
 	} else if c.netCgo {
 		// Cgo resolver was explicitly requested.

src/net/dnsclient_unix_test.go

@@ -619,8 +619,8 @@ func TestGoLookupIPOrderFallbackToFile(t *testing.T) {
 		t.Fatal(err)
 	}
 	// Redirect host file lookups.
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
-	testHookHostsPath = "testdata/hosts"
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
+	hostsFilePath = "testdata/hosts"
 
 	for _, order := range []hostLookupOrder{hostLookupFilesDNS, hostLookupDNSFiles} {
 		name := fmt.Sprintf("order %v", order)
@@ -1966,8 +1966,8 @@ func TestCVE202133195(t *testing.T) {
 	DefaultResolver = &r
 	defer func() { DefaultResolver = originalDefault }()
 	// Redirect host file lookups.
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
-	testHookHostsPath = "testdata/hosts"
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
+	hostsFilePath = "testdata/hosts"
 
 	tests := []struct {
 		name string
@@ -2186,8 +2186,8 @@ func TestRootNS(t *testing.T) {
 }
 
 func TestGoLookupIPCNAMEOrderHostsAliasesFilesOnlyMode(t *testing.T) {
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
-	testHookHostsPath = "testdata/aliases"
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
+	hostsFilePath = "testdata/aliases"
 	mode := hostLookupFiles
 
 	for _, v := range lookupStaticHostAliasesTest {
@@ -2196,8 +2196,8 @@ func TestGoLookupIPCNAMEOrderHostsAliasesFilesOnlyMode(t *testing.T) {
 }
 
 func TestGoLookupIPCNAMEOrderHostsAliasesFilesDNSMode(t *testing.T) {
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
-	testHookHostsPath = "testdata/aliases"
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
+	hostsFilePath = "testdata/aliases"
 	mode := hostLookupFilesDNS
 
 	for _, v := range lookupStaticHostAliasesTest {
@@ -2213,8 +2213,8 @@ var goLookupIPCNAMEOrderDNSFilesModeTests = []struct {
 }
 
 func TestGoLookupIPCNAMEOrderHostsAliasesDNSFilesMode(t *testing.T) {
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
-	testHookHostsPath = "testdata/aliases"
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
+	hostsFilePath = "testdata/aliases"
 	mode := hostLookupDNSFiles
 
 	for _, v := range goLookupIPCNAMEOrderDNSFilesModeTests {
@@ -2541,7 +2541,7 @@ func TestDNSConfigNoReload(t *testing.T) {
 }
 
 func TestLookupOrderFilesNoSuchHost(t *testing.T) {
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
 	if runtime.GOOS != "openbsd" {
 		defer setSystemNSS(getSystemNSS(), 0)
 		setSystemNSS(nssStr(t, "hosts: files"), time.Hour)
@@ -2568,7 +2568,7 @@ func TestLookupOrderFilesNoSuchHost(t *testing.T) {
 	if err := os.WriteFile(tmpFile, []byte{}, 0660); err != nil {
 		t.Fatal(err)
 	}
-	testHookHostsPath = tmpFile
+	hostsFilePath = tmpFile
 
 	const testName = "test.invalid"
 

src/net/hook.go

@@ -13,8 +13,7 @@ var (
 	// if non-nil, overrides dialTCP.
 	testHookDialTCP func(ctx context.Context, net string, laddr, raddr *TCPAddr) (*TCPConn, error)
 
-	testHookHostsPath = "/etc/hosts"
-	testHookLookupIP  = func(
+	testHookLookupIP = func(
 		ctx context.Context,
 		fn func(context.Context, string, string) ([]IPAddr, error),
 		network string,

src/net/hook_plan9.go

@@ -0,0 +1,9 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+var (
+	hostsFilePath = "/etc/hosts"
+)

src/net/hook_unix.go

@@ -11,6 +11,8 @@ import "syscall"
 var (
 	testHookCanceledDial = func() {} // for golang.org/issue/16523
 
+	hostsFilePath = "/etc/hosts"
+
 	// Placeholders for socket system calls.
 	socketFunc        func(int, int, int) (int, error)  = syscall.Socket
 	connectFunc       func(int, syscall.Sockaddr) error = syscall.Connect

src/net/hook_windows.go

@@ -10,6 +10,8 @@ import (
 )
 
 var (
+	hostsFilePath = windows.GetSystemDirectory() + "/Drivers/etc/hosts"
+
 	// Placeholders for socket system calls.
 	wsaSocketFunc func(int32, int32, int32, *syscall.WSAProtocolInfo, uint32, uint32) (syscall.Handle, error) = windows.WSASocket
 	connectFunc   func(syscall.Handle, syscall.Sockaddr) error                                                = syscall.Connect

src/net/hosts.go

@@ -51,7 +51,7 @@ var hosts struct {
 
 func readHosts() {
 	now := time.Now()
-	hp := testHookHostsPath
+	hp := hostsFilePath
 
 	if now.Before(hosts.expire) && hosts.path == hp && len(hosts.byName) > 0 {
 		return

src/net/hosts_test.go

@@ -59,10 +59,10 @@ var lookupStaticHostTests = []struct {
 }
 
 func TestLookupStaticHost(t *testing.T) {
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
 
 	for _, tt := range lookupStaticHostTests {
-		testHookHostsPath = tt.name
+		hostsFilePath = tt.name
 		for _, ent := range tt.ents {
 			testStaticHost(t, tt.name, ent)
 		}
@@ -128,10 +128,10 @@ var lookupStaticAddrTests = []struct {
 }
 
 func TestLookupStaticAddr(t *testing.T) {
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
 
 	for _, tt := range lookupStaticAddrTests {
-		testHookHostsPath = tt.name
+		hostsFilePath = tt.name
 		for _, ent := range tt.ents {
 			testStaticAddr(t, tt.name, ent)
 		}
@@ -151,27 +151,27 @@ func testStaticAddr(t *testing.T, hostsPath string, ent staticHostEntry) {
 func TestHostCacheModification(t *testing.T) {
 	// Ensure that programs can't modify the internals of the host cache.
 	// See https://golang.org/issues/14212.
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
 
-	testHookHostsPath = "testdata/ipv4-hosts"
+	hostsFilePath = "testdata/ipv4-hosts"
 	ent := staticHostEntry{"localhost", []string{"127.0.0.1", "127.0.0.2", "127.0.0.3"}}
-	testStaticHost(t, testHookHostsPath, ent)
+	testStaticHost(t, hostsFilePath, ent)
 	// Modify the addresses return by lookupStaticHost.
 	addrs, _ := lookupStaticHost(ent.in)
 	for i := range addrs {
 		addrs[i] += "junk"
 	}
-	testStaticHost(t, testHookHostsPath, ent)
+	testStaticHost(t, hostsFilePath, ent)
 
-	testHookHostsPath = "testdata/ipv6-hosts"
+	hostsFilePath = "testdata/ipv6-hosts"
 	ent = staticHostEntry{"::1", []string{"localhost"}}
-	testStaticAddr(t, testHookHostsPath, ent)
+	testStaticAddr(t, hostsFilePath, ent)
 	// Modify the hosts return by lookupStaticAddr.
 	hosts := lookupStaticAddr(ent.in)
 	for i := range hosts {
 		hosts[i] += "junk"
 	}
-	testStaticAddr(t, testHookHostsPath, ent)
+	testStaticAddr(t, hostsFilePath, ent)
 }
 
 var lookupStaticHostAliasesTest = []struct {
@@ -195,9 +195,9 @@ var lookupStaticHostAliasesTest = []struct {
 }
 
 func TestLookupStaticHostAliases(t *testing.T) {
-	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	defer func(orig string) { hostsFilePath = orig }(hostsFilePath)
 
-	testHookHostsPath = "testdata/aliases"
+	hostsFilePath = "testdata/aliases"
 	for _, ent := range lookupStaticHostAliasesTest {
 		testLookupStaticHostAliases(t, ent.lookup, absDomainName(ent.res))
 	}

src/runtime/os_windows.go

@@ -243,6 +243,11 @@ func initSysDirectory() {
 	sysDirectoryLen = l + 1
 }
 
+//go:linkname windows_GetSystemDirectory internal/syscall/windows.GetSystemDirectory
+func windows_GetSystemDirectory() string {
+	return unsafe.String(&sysDirectory[0], sysDirectoryLen)
+}
+
 func windowsLoadSystemLib(name []uint16) uintptr {
 	return stdcall3(_LoadLibraryExW, uintptr(unsafe.Pointer(&name[0])), 0, _LOAD_LIBRARY_SEARCH_SYSTEM32)
 }