Start of netcheck package & including network state in Hostinfo.

* adds new packet "netcheck" to do the checking of UDP, IPv6, and
  nearest DERP server, and the Report type for all that (and more
  in the future, probably pulling in danderson's natprobe)
* new tailcfg.NetInfo type
* cmd/tailscale netcheck subcommand (tentative name, likely to
  change/move) to print out the netcheck.Report.

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

ipn/local.go

@@ -5,6 +5,7 @@
 package ipn
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"log"
@@ -14,6 +15,7 @@ import (
 
 	"github.com/tailscale/wireguard-go/wgcfg"
 	"tailscale.com/control/controlclient"
+	"tailscale.com/netcheck"
 	"tailscale.com/portlist"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/empty"
@@ -134,6 +136,7 @@ func (b *LocalBackend) Start(opts Options) error {
 	hi := controlclient.NewHostinfo()
 	hi.BackendLogID = b.backendLogID
 	hi.FrontendLogID = opts.FrontendLogID
+	b.populateNetworkConditions(hi)
 
 	b.mu.Lock()
 
@@ -360,6 +363,7 @@ func (b *LocalBackend) popBrowserAuthNow() {
 	b.interact = 0
 	b.authURL = ""
 	b.mu.Unlock()
+
 	b.logf("popBrowserAuthNow: url=%v\n", url != "")
 
 	b.blockEngineUpdates(true)
@@ -749,3 +753,33 @@ func (b *LocalBackend) assertClientLocked() {
 		panic("LocalBackend.assertClient: b.c == nil")
 	}
 }
+
+// populateNetworkConditions spends up to 2 seconds populating hi's
+// network condition fields.
+//
+// TODO: this is currently just done once at start-up, not regularly on
+// link changes. This will probably need to be moved & rethought. For now
+// we're just gathering some data.
+func (b *LocalBackend) populateNetworkConditions(hi *tailcfg.Hostinfo) {
+	logf := logger.WithPrefix(b.logf, "populateNetworkConditions: ")
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+
+	report, err := netcheck.GetReport(ctx, logf)
+	if err != nil {
+		logf("GetReport: %v", err)
+		return
+	}
+
+	ni := &tailcfg.NetInfo{
+		DERPLatency:           map[string]float64{},
+		MappingVariesByDestIP: report.MappingVariesByDestIP,
+	}
+	for server, d := range report.DERPLatency {
+		ni.DERPLatency[server] = d.Seconds()
+	}
+	ni.WorkingIPv6.Set(report.IPv6)
+	ni.WorkingUDP.Set(report.UDP)
+
+	hi.NetInfo = ni
+}