Support disabled resolved stub server mode (#1493)

In the case of disabled stub listeren the list of name servers is unordered. The solution is to configure the resolv.conf file directly instead of dbus API.
Because third-party services also can manipulate the DNS settings the agent watch the resolv.conf file and keep it up to date.

- apply file type DNS manager if in the name server list does not exist the 127.0.0.53 address
- watching the resolv.conf file with inotify service and overwrite all the time if the configuration has changed and it invalid
- fix resolv.conf generation algorithm

client/internal/dns/file_parser_linux_test.go

@@ -0,0 +1,149 @@
+//go:build !android
+
+package dns
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func Test_parseResolvConf(t *testing.T) {
+	testCases := []struct {
+		input          string
+		expectedSearch []string
+		expectedNS     []string
+		expectedOther  []string
+	}{
+		{
+			input: `domain chello.hu
+search chello.hu
+nameserver 192.168.0.1
+`,
+			expectedSearch: []string{"chello.hu"},
+			expectedNS:     []string{"192.168.0.1"},
+			expectedOther:  []string{},
+		},
+		{
+			input: `# This is /run/systemd/resolve/resolv.conf managed by man:systemd-resolved(8).
+# Do not edit.
+#
+# This file might be symlinked as /etc/resolv.conf. If you're looking at
+# /etc/resolv.conf and seeing this text, you have followed the symlink.
+#
+# This is a dynamic resolv.conf file for connecting local clients directly to
+# all known uplink DNS servers. This file lists all configured search domains.
+#
+# Third party programs should typically not access this file directly, but only
+# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
+# different way, replace this symlink by a static file or a different symlink.
+#
+# See man:systemd-resolved.service(8) for details about the supported modes of
+# operation for /etc/resolv.conf.
+
+nameserver 192.168.2.1
+nameserver 100.81.99.197
+search netbird.cloud
+`,
+			expectedSearch: []string{"netbird.cloud"},
+			expectedNS:     []string{"192.168.2.1", "100.81.99.197"},
+			expectedOther:  []string{},
+		},
+		{
+			input: `# This is /run/systemd/resolve/resolv.conf managed by man:systemd-resolved(8).
+# Do not edit.
+#
+# This file might be symlinked as /etc/resolv.conf. If you're looking at
+# /etc/resolv.conf and seeing this text, you have followed the symlink.
+#
+# This is a dynamic resolv.conf file for connecting local clients directly to
+# all known uplink DNS servers. This file lists all configured search domains.
+#
+# Third party programs should typically not access this file directly, but only
+# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
+# different way, replace this symlink by a static file or a different symlink.
+#
+# See man:systemd-resolved.service(8) for details about the supported modes of
+# operation for /etc/resolv.conf.
+
+nameserver 192.168.2.1
+nameserver 100.81.99.197
+search netbird.cloud
+options debug
+`,
+			expectedSearch: []string{"netbird.cloud"},
+			expectedNS:     []string{"192.168.2.1", "100.81.99.197"},
+			expectedOther:  []string{"options debug"},
+		},
+		{
+			input: `# This is /run/systemd/resolve/resolv.conf managed by man:systemd-resolved(8).
+# Do not edit.
+#
+# This file might be symlinked as /etc/resolv.conf. If you're looking at
+# /etc/resolv.conf and seeing this text, you have followed the symlink.
+#
+# This is a dynamic resolv.conf file for connecting local clients directly to
+# all known uplink DNS servers. This file lists all configured search domains.
+#
+# Third party programs should typically not access this file directly, but only
+# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
+# different way, replace this symlink by a static file or a different symlink.
+#
+# See man:systemd-resolved.service(8) for details about the supported modes of
+# operation for /etc/resolv.conf.
+
+nameserver 192.168.2.1
+nameserver 100.81.99.197
+search netbird.cloud
+options debug
+options edns0 trust-ad
+`,
+			expectedSearch: []string{"netbird.cloud"},
+			expectedNS:     []string{"192.168.2.1", "100.81.99.197"},
+			expectedOther:  []string{"options debug", "options edns0 trust-ad"},
+		},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+		t.Run("test", func(t *testing.T) {
+			t.Parallel()
+			tmpResolvConf := fmt.Sprintf("%s/%s", t.TempDir(), "resolv.conf")
+			err := os.WriteFile(tmpResolvConf, []byte(testCase.input), 0644)
+			if err != nil {
+				t.Fatal(err)
+			}
+			cfg, err := parseResolvConfFile(tmpResolvConf)
+			if err != nil {
+				t.Fatal(err)
+			}
+			ok := compareLists(cfg.searchDomains, testCase.expectedSearch)
+			if !ok {
+				t.Errorf("invalid parse result for search domains, expected: %v, got: %v", testCase.expectedSearch, cfg.searchDomains)
+			}
+
+			ok = compareLists(cfg.nameServers, testCase.expectedNS)
+			if !ok {
+				t.Errorf("invalid parse result for ns domains, expected: %v, got: %v", testCase.expectedNS, cfg.nameServers)
+			}
+
+			ok = compareLists(cfg.others, testCase.expectedOther)
+			if !ok {
+				t.Errorf("invalid parse result for others, expected: %v, got: %v", testCase.expectedOther, cfg.others)
+			}
+		})
+	}
+
+}
+
+func compareLists(search []string, search2 []string) bool {
+	if len(search) != len(search2) {
+		return false
+	}
+	for i, v := range search {
+		if v != search2[i] {
+			return false
+		}
+	}
+	return true
+}