Merge 3451d9ea4c into 955ad12489

cmd/tailscale/cli/netcheck.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"runtime"
 	"sort"
 	"strings"
 	"time"
@@ -18,6 +19,7 @@ import (
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn"
+	"tailscale.com/net/dns"
 	"tailscale.com/net/netcheck"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/portmapper"
@@ -147,6 +149,13 @@ func printReport(dm *tailcfg.DERPMap, report *netcheck.Report) error {
 		printf("\t* CaptivePortal: %v\n", report.CaptivePortal)
 	}
 
+	if runtime.GOOS == "linux" {
+		mode := dns.GetGlobalDnsMode()
+		if mode != "" {
+			printf("\t* Current DNS Mode: %v\n", mode)
+		}
+	}
+
 	// When DERP latency checking failed,
 	// magicsock will try to pick the DERP server that
 	// most of your other nodes are also using

net/dns/manager_linux.go

@@ -31,6 +31,11 @@ func (kv kv) String() string {
 
 var publishOnce sync.Once
 
+var (
+	globalDnsMode string
+	dnsModeMutex  sync.RWMutex
+)
+
 func NewOSConfigurator(logf logger.Logf, interfaceName string) (ret OSConfigurator, err error) {
 	env := newOSConfigEnv{
 		fs:                directFS{},
@@ -44,6 +49,8 @@ func NewOSConfigurator(logf logger.Logf, interfaceName string) (ret OSConfigurat
 	if err != nil {
 		return nil, err
 	}
+
+	setGlobalDnsMode(mode)
 	publishOnce.Do(func() {
 		sanitizedMode := strings.ReplaceAll(mode, "-", "_")
 		m := clientmetric.NewGauge(fmt.Sprintf("dns_manager_linux_mode_%s", sanitizedMode))
@@ -425,3 +432,17 @@ func dbusReadString(name, objectPath, iface, member string) (string, error) {
 	}
 	return result.String(), nil
 }
+
+// setGlobalDnsMode safely sets the global DNS mode variable
+func setGlobalDnsMode(mode string) {
+	dnsModeMutex.Lock()
+	defer dnsModeMutex.Unlock()
+	globalDnsMode = mode
+}
+
+// GetGlobalDnsMode safely returns the global DNS mode variable
+func GetGlobalDnsMode() string {
+	dnsModeMutex.RLock()
+	defer dnsModeMutex.RUnlock()
+	return globalDnsMode
+}

net/dns/manager_notlinux.go

@@ -0,0 +1,9 @@
+//go:build !linux
+
+package dns
+
+// GetGlobalDnsMode exists to make the build happy on non-linux platforms.
+// The actual implementation is in manager_linux.go & in there it safely returns the current resolv.conf mode
+func GetGlobalDnsMode() string {
+	return ""
+}