net/dns: fix freebsd DNS manager selection.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/direct.go

@@ -92,6 +92,36 @@ func readResolvConf() (OSConfig, error) {
 	return readResolvFile(resolvConf)
 }
 
+// resolvOwner returns the apparent owner of the resolv.conf
+// configuration in bs - one of "resolvconf", "systemd-resolved" or
+// "NetworkManager", or "" if no known owner was found.
+func resolvOwner(bs []byte) string {
+	b := bytes.NewBuffer(bs)
+	for {
+		line, err := b.ReadString('\n')
+		if err != nil {
+			return ""
+		}
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		if line[0] != '#' {
+			// First non-empty, non-comment line. Assume the owner
+			// isn't hiding further down.
+			return ""
+		}
+
+		if strings.Contains(line, "systemd-resolved") {
+			return "systemd-resolved"
+		} else if strings.Contains(line, "NetworkManager") {
+			return "NetworkManager"
+		} else if strings.Contains(line, "resolvconf") {
+			return "resolvconf"
+		}
+	}
+}
+
 // isResolvedRunning reports whether systemd-resolved is running on the system,
 // even if it is not managing the system DNS settings.
 func isResolvedRunning() bool {

net/dns/manager_freebsd.go

@@ -4,16 +4,25 @@
 
 package dns
 
-import "tailscale.com/types/logger"
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
 
-func isResolvconfActive() bool {
-	// TODO(danderson): implement somewhere.
-	return false
-}
+	"tailscale.com/types/logger"
+)
 
 func NewOSConfigurator(logf logger.Logf, _ string) (OSConfigurator, error) {
-	switch {
-	case isResolvconfActive():
+	bs, err := ioutil.ReadFile("/etc/resolv.conf")
+	if os.IsNotExist(err) {
+		return newDirectManager()
+	}
+	if err != nil {
+		return nil, fmt.Errorf("reading /etc/resolv.conf: %w", err)
+	}
+
+	switch resolvOwner(bs) {
+	case "resolvconf":
 		return newResolvconfManager(logf)
 	default:
 		return newDirectManager()

net/dns/manager_linux.go

@@ -12,7 +12,6 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
-	"strings"
 	"time"
 
 	"github.com/godbus/dbus/v5"
@@ -132,33 +131,3 @@ func dbusPing(name, objectPath string) error {
 	call := obj.CallWithContext(ctx, "org.freedesktop.DBus.Peer.Ping", 0)
 	return call.Err
 }
-
-// resolvOwner returns the apparent owner of the resolv.conf
-// configuration in bs - one of "resolvconf", "systemd-resolved" or
-// "NetworkManager", or "" if no known owner was found.
-func resolvOwner(bs []byte) string {
-	b := bytes.NewBuffer(bs)
-	for {
-		line, err := b.ReadString('\n')
-		if err != nil {
-			return ""
-		}
-		line = strings.TrimSpace(line)
-		if line == "" {
-			continue
-		}
-		if line[0] != '#' {
-			// First non-empty, non-comment line. Assume the owner
-			// isn't hiding further down.
-			return ""
-		}
-
-		if strings.Contains(line, "systemd-resolved") {
-			return "systemd-resolved"
-		} else if strings.Contains(line, "NetworkManager") {
-			return "NetworkManager"
-		} else if strings.Contains(line, "resolvconf") {
-			return "resolvconf"
-		}
-	}
-}