controlclient, magicsock: add debug knob to request IPv6 endpoints

Add opt-in method to request IPv6 endpoints from the control plane.
For now they should just be skipped. A previous version of this CL was
unconditional and reportedly had problems that I can't reproduce. So
make it a knob until the mystery is solved.

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

control/controlclient/direct.go

@@ -18,6 +18,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -447,8 +448,13 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	allowStream := maxPolls != 1
 	c.logf("PollNetMap: stream=%v :%v %v\n", maxPolls, localPort, ep)
 
+	// TODO(bradfitz): once this is verified to not be problematic, remove this
+	// knob and hard-code true below.
+	includeIPv6, _ := strconv.ParseBool(os.Getenv("DEBUG_INCLUDE_IPV6"))
+
 	request := tailcfg.MapRequest{
 		Version:     4,
+		IncludeIPv6: includeIPv6,
 		KeepAlive:   c.keepAlive,
 		NodeKey:     tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
 		Endpoints:   ep,

wgengine/magicsock/magicsock.go

@@ -1456,6 +1456,10 @@ func (c *Conn) CreateEndpoint(key [32]byte, addrs string) (conn.Endpoint, error)
 			}
 			if ip4 := addr.IP.To4(); ip4 != nil {
 				addr.IP = ip4
+			} else {
+				// TODO(bradfitz): stop skipping IPv6 ones for now.
+				c.logf("magicsock: CreateEndpoint: ignoring IPv6 addr %v for now", addr)
+				continue
 			}
 			a.addrs = append(a.addrs, *addr)
 		}