net/dns: handle systemd-resolved with DNSStubListener=no

Updates #11342

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I5c368972b97970141ac0d6c96a19c874137c6523

net/dns/direct.go

@@ -495,6 +495,7 @@ type wholeFileFS interface {
 	ReadFile(name string) ([]byte, error)
 	Remove(name string) error
 	Rename(oldName, newName string) error
+	Readlink(name string) (string, error)
 	Stat(name string) (isRegular bool, err error)
 	Truncate(name string) error
 	WriteFile(name string, contents []byte, perm os.FileMode) error
@@ -519,6 +520,10 @@ func (fs directFS) Stat(name string) (isRegular bool, err error) {
 	return fi.Mode().IsRegular(), nil
 }
 
+func (fs directFS) Readlink(name string) (string, error) {
+	return os.Readlink(fs.path(name))
+}
+
 func (fs directFS) Chmod(name string, mode os.FileMode) error {
 	return os.Chmod(fs.path(name), mode)
 }

net/dns/manager_linux.go

@@ -137,16 +137,35 @@ func dnsMode(logf logger.Logf, health *health.Tracker, env newOSConfigEnv) (ret
 	case "systemd-resolved":
 		dbg("rc", "resolved")
 
+		// If systemd-resolved says that the we don't have a stub resolver, but
+		// /etc/resolv.conf is symlinked to /run/systemd/resolve/resolv.conf,
+		// then systemd-resolved is managing DNS but the logic in
+		// resolvedIsActuallyResolver will not detect it.
+		//
+		// Check for this case and return early if we find it. See:
+		//    https://github.com/tailscale/tailscale/issues/11342
+		var isResolvedNoStub bool
+		mode, err := env.dbusReadString("org.freedesktop.resolve1", "/org/freedesktop/resolve1", "org.freedesktop.resolve1.Manager", "DNSStubListener")
+		if err == nil && mode == "no" {
+			target, err := env.fs.Readlink("/etc/resolv.conf")
+			if err == nil && target == "/run/systemd/resolve/resolv.conf" {
+				dbg("resolved", "no-stub")
+				isResolvedNoStub = true
+			}
+		}
+
 		// Some systems, for reasons known only to them, have a
 		// resolv.conf that has the word "systemd-resolved" in its
 		// header, but doesn't actually point to resolved. We mustn't
 		// try to program resolved in that case.
 		// https://github.com/tailscale/tailscale/issues/2136
+		if !isResolvedNoStub {
 			if err := resolvedIsActuallyResolver(logf, env, dbg, bs); err != nil {
 				logf("dns: resolvedIsActuallyResolver error: %v", err)
 				dbg("resolved", "not-in-use")
 				return "direct", nil
 			}
+		}
 		if err := env.dbusPing("org.freedesktop.NetworkManager", "/org/freedesktop/NetworkManager/DnsManager"); err != nil {
 			dbg("nm", "no")
 			return "systemd-resolved", nil

net/dns/manager_linux_test.go

@@ -313,6 +313,17 @@ func (m memFS) Stat(name string) (isRegular bool, err error) {
 	return false, nil
 }
 
+func (m memFS) Readlink(name string) (string, error) {
+	v, ok := m[name]
+	if !ok {
+		return "", fs.ErrNotExist
+	}
+	if s, ok := v.(string); ok {
+		return s, nil
+	}
+	panic("unexpected")
+}
+
 func (m memFS) Chmod(name string, mode os.FileMode) error { panic("TODO") }
 func (m memFS) Rename(oldName, newName string) error      { panic("TODO") }
 func (m memFS) Remove(name string) error                  { panic("TODO") }

net/dns/wsl_windows.go

@@ -159,6 +159,12 @@ func (fs wslFS) Stat(name string) (isRegular bool, err error) {
 	return true, nil
 }
 
+func (fs wslFS) Readlink(name string) (string, error) {
+	// As of 2024-07-01, this function is only used on Linux. We can return
+	// the original path and no error here.
+	return name, nil
+}
+
 func (fs wslFS) Chmod(name string, perm os.FileMode) error {
 	return wslRun(fs.cmd("chmod", "--", fmt.Sprintf("%04o", perm), name))
 }