diff --git a/control/controlclient/direct.go b/control/controlclient/direct.go
index e5f2c3de7..7798f1480 100644
--- a/control/controlclient/direct.go
+++ b/control/controlclient/direct.go
@@ -18,6 +18,7 @@ import (
 	"io"
 	"io/ioutil"
 	"log"
+	"net"
 	"net/http"
 	"net/url"
 	"os"
@@ -28,6 +29,7 @@ import (
 	"strings"
 	"sync"
 	"sync/atomic"
+	"syscall"
 	"time"
 
 	"github.com/tailscale/wireguard-go/wgcfg"
@@ -574,6 +576,11 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 
 	res, err := c.httpc.Do(req)
 	if err != nil {
+		// Temproary (2020-10-29) issue 839 debugging:
+		if runtime.GOOS == "darwin" && (errors.Is(err, syscall.EADDRNOTAVAIL) || strings.Contains(err.Error(), "can't assign requested address")) {
+			vlogf = c.logf
+			debugAddrNotAvailalable(vlogf)
+		}
 		vlogf("netmap: Do: %v", err)
 		return err
 	}
@@ -1051,3 +1058,35 @@ func TrimWGConfig() opt.Bool {
 	v, _ := controlTrimWGConfig.Load().(opt.Bool)
 	return v
 }
+
+// Temproary (2020-10-29) debugging for https://github.com/tailscale/tailscale/issues/839
+func debugAddrNotAvailalable(logf logger.Logf) {
+	logf("EADDRNOTAVAIL-debug: [unexpected] got EADDRNOTAVAIL on " + runtime.GOOS)
+	var d net.Dialer
+	d.Timeout = 5 * time.Second
+
+	var c [2]net.Conn
+	var a [2]*net.TCPAddr
+	for i := range c {
+		var err error
+		c[i], err = d.Dial("tcp", "login.tailscale.com:80")
+		if err != nil {
+			logf("EADDRNOTAVAIL-debug: failed to debug dial tailscale %d/%d: %v", i+1, len(c), err)
+			return
+		}
+		defer c[i].Close()
+		a[i] = c[i].LocalAddr().(*net.TCPAddr)
+	}
+	if a[0].Port == a[1].Port {
+		logf("EADDRNOTAVAIL-debug: two trial connections had same source port")
+		return
+	}
+	for i := range c {
+		_, err := io.WriteString(c[i], "HEAD / HTTP/1.0\r\n\r\n")
+		if err != nil {
+			logf("EADDRNOTAVAIL-debug: failed to write on connection %d post-dial: %v", i+1, err)
+			return
+		}
+	}
+	logf("EADDRNOTAVAIL-debug: dial+writes normal")
+}