net/netns: move SOCKS dialing to netns for now

This lets control & logs also use SOCKS dials.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netns/netns.go

@@ -9,9 +9,15 @@
 //
 // Despite the name netns, the exact mechanism used differs by
 // operating system, and perhaps even by version of the OS.
+//
+// The netns package also handles connecting via SOCKS proxies when
+// configured by the environment.
 package netns
 
-import "net"
+import (
+	"context"
+	"net"
+)
 
 // Listener returns a new net.Listener with its Control hook func
 // initialized as necessary to run in logical network namespace that
@@ -20,9 +26,43 @@ func Listener() *net.ListenConfig {
 	return &net.ListenConfig{Control: control}
 }
 
-// Dialer returns a new net.Dialer with its Control hook func
-// initialized as necessary to run in a logical network namespace that
-// doesn't route back into Tailscale.
-func Dialer() *net.Dialer {
-	return &net.Dialer{Control: control}
+// NewDialer returns a new Dialer using a net.Dialer with its Control
+// hook func initialized as necessary to run in a logical network
+// namespace that doesn't route back into Tailscale. It also handles
+// using a SOCKS if configured in the environment with ALL_PROXY.
+func NewDialer() Dialer {
+	return FromDialer(new(net.Dialer))
+}
+
+// FromDialer returns sets d.Control as necessary to run in a logical
+// network namespace that doesn't route back into Tailscale. It also
+// handles using a SOCKS if configured in the environment with
+// ALL_PROXY.
+func FromDialer(d *net.Dialer) Dialer {
+	d.Control = control
+	if wrapDialer != nil {
+		return wrapDialer(d)
+	}
+	return d
+}
+
+// IsSOCKSDialer reports whether d is SOCKS-proxying dialer as returned by
+// NewDialer or FromDialer.
+func IsSOCKSDialer(d Dialer) bool {
+	if d == nil {
+		return false
+	}
+	_, ok := d.(*net.Dialer)
+	return !ok
+}
+
+// wrapDialer, if non-nil, specifies a function to wrap a dialer in a
+// SOCKS-using dialer. It's set conditionally by socks.go.
+var wrapDialer func(Dialer) Dialer
+
+// Dialer is the interface for a dialer that can dial with or without a context.
+// It's the type implemented both by net.Dialer and the Go SOCKS dialer.
+type Dialer interface {
+	Dial(network, address string) (net.Conn, error)
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
 }