netns_linux: No namespace for localhost.

Connections to a control server or log server on localhost,
used in a number of tests, are working right now because the
calls to SO_MARK in netns fail for non-root but then we ignore
the failure when running in tests.

Unfortunately that failure in SO_MARK also affects container
environments without CAP_NET_ADMIN, breaking Tailscale
connectivity. We're about to fix netns to recognize when SO_MARK
doesn't work and use SO_BINDTODEVICE instead. Doing so makes
tests fail, as their sockets now BINDTODEVICE of the default
route and cannot connect to localhost.

Add support to skip namespacing for localhost connections,
which Darwin and Windows already do. This is not conditional
on running within a test, if you tell tailscaled to connect
to localhost it will automatically use a non-namespaced
socket to do so.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/netns/netns.go

@@ -17,6 +17,8 @@ package netns
 import (
 	"context"
 	"net"
+
+	"inet.af/netaddr"
 )
 
 // Listener returns a new net.Listener with its Control hook func
@@ -66,3 +68,19 @@ type Dialer interface {
 	Dial(network, address string) (net.Conn, error)
 	DialContext(ctx context.Context, network, address string) (net.Conn, error)
 }
+
+func isLocalhost(addr string) bool {
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		// error means the string didn't contain a port number, so use the string directly
+		host = addr
+	}
+
+	// localhost6 == RedHat /etc/hosts for ::1, ip6-loopback & ip6-localhost == Debian /etc/hosts for ::1
+	if host == "localhost" || host == "localhost6" || host == "ip6-loopback" || host == "ip6-localhost" {
+		return true
+	}
+
+	ip, _ := netaddr.ParseIP(host)
+	return ip.IsLoopback()
+}

net/netns/netns_linux.go

@@ -64,6 +64,14 @@ func ignoreErrors() bool {
 // It's intentionally the same signature as net.Dialer.Control
 // and net.ListenConfig.Control.
 func control(network, address string, c syscall.RawConn) error {
+	if hostinfo.GetEnvType() == hostinfo.TestCase {
+		return nil
+	}
+	if IsLocalhost(address) {
+		// Don't bind to an interface for localhost connections.
+		return nil
+	}
+
 	var sockErr error
 	err := c.Control(func(fd uintptr) {
 		if ipRuleAvailable() {

net/netns/netns_test.go

@@ -40,3 +40,40 @@ func TestDial(t *testing.T) {
 	defer c.Close()
 	t.Logf("got addr %v", c.RemoteAddr())
 }
+
+func TestIsLocalhost(t *testing.T) {
+	tests := []struct {
+		name string
+		host string
+		want bool
+	}{
+		{"IPv4 loopback", "127.0.0.1", true},
+		{"IPv4 !loopback", "192.168.0.1", false},
+		{"IPv4 loopback with port", "127.0.0.1:1", true},
+		{"IPv4 !loopback with port", "192.168.0.1:1", false},
+		{"IPv4 unspecified", "0.0.0.0", false},
+		{"IPv4 unspecified with port", "0.0.0.0:1", false},
+		{"IPv6 loopback", "::1", true},
+		{"IPv6 !loopback", "2001:4860:4860::8888", false},
+		{"IPv6 loopback with port", "[::1]:1", true},
+		{"IPv6 !loopback with port", "[2001:4860:4860::8888]:1", false},
+		{"IPv6 unspecified", "::", false},
+		{"IPv6 unspecified with port", "[::]:1", false},
+		{"empty", "", false},
+		{"hostname", "example.com", false},
+		{"localhost", "localhost", true},
+		{"localhost6", "localhost6", true},
+		{"localhost with port", "localhost:1", true},
+		{"localhost6 with port", "localhost6:1", true},
+		{"ip6-localhost", "ip6-localhost", true},
+		{"ip6-localhost with port", "ip6-localhost:1", true},
+		{"ip6-loopback", "ip6-loopback", true},
+		{"ip6-loopback with port", "ip6-loopback:1", true},
+	}
+
+	for _, test := range tests {
+		if got := isLocalhost(test.host); got != test.want {
+			t.Errorf("isLocalhost(%q) = %v, want %v", test.name, got, test.want)
+		}
+	}
+}