nettest: add SupportsRawSocket

Also updates API docs.

Change-Id: I0c744a286bf637270156101bfdee1e87e306fa5e
Reviewed-on: https://go-review.googlesource.com/c/net/+/169539
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

nettest/nettest.go

diff --git a/nettest/nettest.go b/nettest/nettest.go
index c75facf..717bbb0 100644
--- a/nettest/nettest.go
+++ b/nettest/nettest.go
@@ -20,10 +20,11 @@
 )
 
 var (
-	stackOnce   sync.Once
-	ipv4Enabled bool
-	ipv6Enabled bool
-	aixTechLvl  int
+	stackOnce     sync.Once
+	ipv4Enabled   bool
+	ipv6Enabled   bool
+	rawSocketSess bool
+	aixTechLvl    int
 
 	aLongTimeAgo = time.Unix(233431200, 0)
 	neverTimeout = time.Time{}
@@ -41,12 +42,12 @@
 		ln.Close()
 		ipv6Enabled = true
 	}
+	rawSocketSess = supportsRawSocket()
 	if runtime.GOOS == "aix" {
 		out, err := exec.Command("oslevel", "-s").Output()
-		if err != nil {
-			return
+		if err == nil {
+			aixTechLvl, _ = strconv.Atoi(string(out[5:7]))
 		}
-		aixTechLvl, _ = strconv.Atoi(string(out[5:7]))
 	}
 }
 
@@ -69,8 +70,17 @@
 	return ipv6Enabled
 }
 
+// SupportsRawSocket reports whether the current session is available
+// to use raw sockets.
+func SupportsRawSocket() bool {
+	stackOnce.Do(probeStack)
+	return rawSocketSess
+}
+
 // TestableNetwork reports whether network is testable on the current
 // platform configuration.
+//
+// See func Dial of the standard library for the supported networks.
 func TestableNetwork(network string) bool {
 	ss := strings.Split(network, ":")
 	switch ss[0] {
@@ -248,7 +258,6 @@
 
 // LoopbackInterface returns an available logical network interface
 // for loopback test.
-// It returns nil if no suitable interface is found.
 func LoopbackInterface() (*net.Interface, error) {
 	ift, err := net.Interfaces()
 	if err != nil {
@@ -264,8 +273,6 @@
 
 // RoutedInterface returns a network interface that can route IP
 // traffic and satisfies flags.
-// It returns nil when an appropriate network interface is not
-// found.
 //
 // The provided network must be "ip", "ip4" or "ip6".
 func RoutedInterface(network string, flags net.Flags) (*net.Interface, error) {

nettest/nettest_stub.go

diff --git a/nettest/nettest_stub.go b/nettest/nettest_stub.go
new file mode 100644
index 0000000..2bb8c05
--- /dev/null
+++ b/nettest/nettest_stub.go
@@ -0,0 +1,11 @@
+// Copyright 2019 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.
+
+// +build !aix,!darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris,!windows
+
+package nettest
+
+func supportsRawSocket() bool {
+	return false
+}

nettest/nettest_unix.go

diff --git a/nettest/nettest_unix.go b/nettest/nettest_unix.go
new file mode 100644
index 0000000..afff744
--- /dev/null
+++ b/nettest/nettest_unix.go
@@ -0,0 +1,21 @@
+// Copyright 2019 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.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package nettest
+
+import "syscall"
+
+func supportsRawSocket() bool {
+	for _, af := range []int{syscall.AF_INET, syscall.AF_INET6} {
+		s, err := syscall.Socket(af, syscall.SOCK_RAW, 0)
+		if err != nil {
+			continue
+		}
+		syscall.Close(s)
+		return true
+	}
+	return false
+}

nettest/nettest_windows.go

diff --git a/nettest/nettest_windows.go b/nettest/nettest_windows.go
new file mode 100644
index 0000000..4939964
--- /dev/null
+++ b/nettest/nettest_windows.go
@@ -0,0 +1,26 @@
+// Copyright 2019 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 nettest
+
+import "syscall"
+
+func supportsRawSocket() bool {
+	// From http://msdn.microsoft.com/en-us/library/windows/desktop/ms740548.aspx:
+	// Note: To use a socket of type SOCK_RAW requires administrative privileges.
+	// Users running Winsock applications that use raw sockets must be a member of
+	// the Administrators group on the local computer, otherwise raw socket calls
+	// will fail with an error code of WSAEACCES. On Windows Vista and later, access
+	// for raw sockets is enforced at socket creation. In earlier versions of Windows,
+	// access for raw sockets is enforced during other socket operations.
+	for _, af := range []int{syscall.AF_INET, syscall.AF_INET6} {
+		s, err := syscall.Socket(af, syscall.SOCK_RAW, 0)
+		if err != nil {
+			continue
+		}
+		syscall.Closesocket(s)
+		return true
+	}
+	return false
+}