cmd/tailscale/web: add support for QNAP

Signed-off-by: Maisem Ali <maisem@tailscale.com>

util/groupmember/groupmember_notcgo.go

@@ -0,0 +1,71 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !cgo
+// +build linux darwin
+
+package groupmember
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+
+	"go4.org/mem"
+	"tailscale.com/version/distro"
+)
+
+func isMemberOfGroup(group, name string) (bool, error) {
+	if distro.Get() == distro.Synology {
+		return isMemberOfGroupEtcGroup(group, name)
+	}
+	cmd := exec.Command("/usr/bin/env", "groups", name)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return false, err
+	}
+	groups := strings.Split(strings.TrimSpace(string(out)), " ")
+	for _, g := range groups {
+		if g == group {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+func isMemberOfGroupEtcGroup(group, name string) (bool, error) {
+	f, err := os.Open("/etc/group")
+	if err != nil {
+		return false, err
+	}
+	defer f.Close()
+	s := bufio.NewScanner(f)
+	var agLine string
+	for s.Scan() {
+		if !mem.HasPrefix(mem.B(s.Bytes()), mem.S(fmt.Sprintf("%s:", group))) {
+			continue
+		}
+		agLine = s.Text()
+		break
+	}
+	if err := s.Err(); err != nil {
+		return false, err
+	}
+	if agLine == "" {
+		return false, fmt.Errorf("admin group not defined")
+	}
+	agEntry := strings.Split(agLine, ":")
+	if len(agEntry) < 4 {
+		return false, fmt.Errorf("malformed admin group entry")
+	}
+	agMembers := agEntry[3]
+	for _, m := range strings.Split(agMembers, ",") {
+		if m == name {
+			return true, nil
+		}
+	}
+	return false, nil
+}