net/tsdial: start of new package to unify all outbound dialing complexity

For now this just deletes the net/socks5/tssocks implementation (and
the DNSMap stuff from wgengine/netstack) and moves it into net/tsdial.

Then initialize a Dialer early in tailscaled, currently only use for the
outbound and SOCKS5 proxies. It will be plumbed more later. Notably, it
needs to get down into the DNS forwarder for exit node DNS forwading
in netstack mode. But it will also absorb all the peerapi setsockopt
and netns Dial and tlsdial complexity too.

Updates #1713

Change-Id: Ibc6d56ae21a22655b2fa1002d8fc3f2b2ae8b6df
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tsdial/tsdial.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2021 Tailscale Inc & 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 tsdial provides a Dialer type that can dial out of tailscaled.
+package tsdial
+
+import (
+	"context"
+	"errors"
+	"net"
+	"sync"
+
+	"inet.af/netaddr"
+)
+
+// Dialer dials out of tailscaled, while taking care of details while
+// handling the dozens of edge cases depending on the server mode
+// (TUN, netstack), the OS network sandboxing style (macOS/iOS
+// Extension, none), user-selected route acceptance prefs, etc.
+type Dialer struct {
+	// UseNetstackForIP if non-nil is whether NetstackDialTCP (if
+	// it's non-nil) should be used to dial the provided IP.
+	UseNetstackForIP func(netaddr.IP) bool
+
+	// NetstackDialTCP dials the provided IPPort using netstack.
+	// If nil, it's not used.
+	NetstackDialTCP func(context.Context, netaddr.IPPort) (net.Conn, error)
+
+	mu  sync.Mutex
+	dns DNSMap
+}
+
+func (d *Dialer) SetDNSMap(m DNSMap) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.dns = m
+}
+
+func (d *Dialer) resolve(ctx context.Context, addr string) (netaddr.IPPort, error) {
+	d.mu.Lock()
+	dns := d.dns
+	d.mu.Unlock()
+	return dns.Resolve(ctx, addr)
+}
+
+func (d *Dialer) DialContext(ctx context.Context, network, addr string) (net.Conn, error) {
+	ipp, err := d.resolve(ctx, addr)
+	if err != nil {
+		return nil, err
+	}
+	if d.UseNetstackForIP != nil && d.UseNetstackForIP(ipp.IP()) {
+		if d.NetstackDialTCP == nil {
+			return nil, errors.New("Dialer not initialized correctly")
+		}
+		return d.NetstackDialTCP(ctx, ipp)
+	}
+	// TODO(bradfitz): netns, etc
+	var stdDialer net.Dialer
+	return stdDialer.DialContext(ctx, network, ipp.String())
+}