diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index 3064fa7..93abe31 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -13,6 +13,7 @@
     <uses-permission android:name="android.permission.READ_MEDIA_VIDEO"/>
     <uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE"/>
     <uses-permission android:name="android.permission.INTERNET"/>
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
 
     <application
             android:allowBackup="true"
diff --git a/app/src/main/java/org/foxarmy/barcodescannerforemployees/Net.kt b/app/src/main/java/org/foxarmy/barcodescannerforemployees/Net.kt
index 6778609..fe2cf9e 100644
--- a/app/src/main/java/org/foxarmy/barcodescannerforemployees/Net.kt
+++ b/app/src/main/java/org/foxarmy/barcodescannerforemployees/Net.kt
@@ -1,5 +1,6 @@
 package org.foxarmy.barcodescannerforemployees
 
+import android.content.Context
 import okhttp3.*
 import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
@@ -17,6 +18,28 @@ class Net {
     var server = "bsfe.foxarmy.org"
     var token = ""
 
+    fun serverIsAvailable(context: Context): Boolean {
+        if (!isInternetConnectionAvailable(context)) {
+            return false
+        }
+        var flag = false
+        thread {
+            val client = OkHttpClient();
+            val request = Request.Builder()
+                .url("https://$server/status")
+                .get()
+                .build();
+
+            try {
+                val response = client.newCall(request).execute();
+                flag =  response.code == 200;
+            } catch (e: Exception) {
+                flag = false;
+            }
+        }.join()
+        return flag
+    }
+
     fun requestProductFromOnlineDB(barcode: String): String {
         var response = ""
         thread {
diff --git a/app/src/main/java/org/foxarmy/barcodescannerforemployees/Utils.kt b/app/src/main/java/org/foxarmy/barcodescannerforemployees/Utils.kt
index 07dee21..c292df9 100644
--- a/app/src/main/java/org/foxarmy/barcodescannerforemployees/Utils.kt
+++ b/app/src/main/java/org/foxarmy/barcodescannerforemployees/Utils.kt
@@ -7,10 +7,15 @@ import android.graphics.Bitmap
 import android.graphics.BitmapFactory
 import android.graphics.Matrix
 import android.media.ExifInterface
+import android.net.ConnectivityManager
+import android.net.NetworkCapabilities
 import android.net.Uri
 import android.os.Build
 import androidx.annotation.RequiresApi
+import androidx.appcompat.app.AlertDialog
 import androidx.core.content.FileProvider
+import androidx.security.crypto.EncryptedSharedPreferences
+import androidx.security.crypto.MasterKeys
 import com.google.firebase.components.BuildConfig
 import java.io.File
 import java.io.FileInputStream
@@ -18,6 +23,7 @@ import java.io.FileOutputStream
 import java.security.MessageDigest
 import java.text.SimpleDateFormat
 import java.util.*
+import kotlin.system.exitProcess
 
 fun convertToUnixEpochTimestamp(dateString: String): Long {
     val format = SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'")
@@ -27,7 +33,11 @@ fun convertToUnixEpochTimestamp(dateString: String): Long {
 }
 
 fun getImageUri(activity: Activity, imageFile: File): Uri? {
-    return FileProvider.getUriForFile(activity, BuildConfig.APPLICATION_ID + "." + activity.localClassName + ".provider", imageFile)
+    return FileProvider.getUriForFile(
+        activity,
+        BuildConfig.APPLICATION_ID + "." + activity.localClassName + ".provider",
+        imageFile
+    )
 }
 
 @RequiresApi(Build.VERSION_CODES.R)
@@ -47,13 +57,13 @@ fun generateThumbnailForImage(context: Context, imageHash: String) {
     val orientation = exif.getAttributeInt(ExifInterface.TAG_ORIENTATION, ExifInterface.ORIENTATION_NORMAL)
     val matrix = Matrix()
 
-    when(orientation){
+    when (orientation) {
         ExifInterface.ORIENTATION_ROTATE_90 -> matrix.postRotate(90F)
         ExifInterface.ORIENTATION_ROTATE_180 -> matrix.postRotate(180F)
         ExifInterface.ORIENTATION_ROTATE_270 -> matrix.postRotate(270F)
     }
     val rotated = Bitmap.createBitmap(img, 0, 0, img.width, img.height, matrix, true)
-    val scaled = Bitmap.createScaledBitmap(rotated, rotated.width/4, rotated.height/4, true)
+    val scaled = Bitmap.createScaledBitmap(rotated, rotated.width / 4, rotated.height / 4, true)
     scaled.compress(Bitmap.CompressFormat.WEBP_LOSSY, 25, FileOutputStream(thumbnailFile))
 }
 
@@ -64,8 +74,11 @@ fun String.md5(): String {
     return digest.toHexString()
 }
 
-fun stripNetWeight (netWeight: String): Double {
-    return removeSubstringsFromString(netWeight, arrayOf("Л", "л", "мл", "Мл", "г", "Г", "кг", "Кг", "шт", "Шт", ",", " ", ".")).toDouble()
+fun stripNetWeight(netWeight: String): Double {
+    return removeSubstringsFromString(
+        netWeight,
+        arrayOf("Л", "л", "мл", "Мл", "г", "Г", "кг", "Кг", "шт", "Шт", ",", " ", ".")
+    ).toDouble()
 }
 
 fun removeSubstringsFromString(text: String, toRemove: Array<String>): String {
@@ -99,14 +112,31 @@ fun calculateProductFreshness(dateOfProduction: Long, dateOfExpiry: Long): Doubl
     return lifeSpanLeft / productLifeSpan.toDouble()
 }
 
-fun getUnitNameById (context: Context, id: Int): String {
-    return when(id) {
-        0 -> { context.getString(R.string.kilogram) }
-        1 -> { context.getString(R.string.gram) }
-        2 -> { context.getString(R.string.liter) }
-        3 -> { context.getString(R.string.milliliter) }
-        4 -> { context.getString(R.string.pieces) }
-        else -> { "" }
+fun getUnitNameById(context: Context, id: Int): String {
+    return when (id) {
+        0 -> {
+            context.getString(R.string.kilogram)
+        }
+
+        1 -> {
+            context.getString(R.string.gram)
+        }
+
+        2 -> {
+            context.getString(R.string.liter)
+        }
+
+        3 -> {
+            context.getString(R.string.milliliter)
+        }
+
+        4 -> {
+            context.getString(R.string.pieces)
+        }
+
+        else -> {
+            ""
+        }
     }
 }
 
@@ -137,4 +167,47 @@ fun bytesToHex(bytes: ByteArray): String {
         hexString.append(hex)
     }
     return hexString.toString()
+}
+
+fun isInternetConnectionAvailable(context: Context): Boolean {
+    if (context.getSystemService(Context.CONNECTIVITY_SERVICE) == null) return false
+    val connectivityManager =
+        context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
+    if (connectivityManager != null) {
+        val capabilities =
+            connectivityManager.getNetworkCapabilities(connectivityManager.activeNetwork)
+        if (capabilities != null) {
+            if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR)) {
+                return true
+            } else if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_WIFI)) {
+                return true
+            } else if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_ETHERNET)) {
+                return true
+            }
+        }
+    }
+    return false
+}
+
+fun noInternetConnectionAvailableNotification(context: Context) {
+    AlertDialog.Builder(context)
+        .setMessage(context.getString(R.string.no_internet_connection))
+        .setPositiveButton(R.string.quit) { _, _ ->
+            exitProcess(0)
+        }
+        .setNeutralButton(R.string.logout) { _, _ ->
+            val sharedPreferences = EncryptedSharedPreferences.create(
+                "sensitive",
+                MasterKeys.getOrCreate(MasterKeys.AES256_GCM_SPEC),
+                context,
+                EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,
+                EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM
+            )
+
+            sharedPreferences.edit().putString("token", "").apply()
+            sharedPreferences.edit().putString("server", "").apply()
+            sharedPreferences.edit().putStringSet("groups", emptySet()).apply()
+            sharedPreferences.edit().putString("currentGroup", "").apply()
+            exitProcess(0)
+        }.show()
 }
\ No newline at end of file
diff --git a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/GroupActivity.kt b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/GroupActivity.kt
index 39c152a..4dbc9dc 100644
--- a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/GroupActivity.kt
+++ b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/GroupActivity.kt
@@ -8,6 +8,7 @@ import androidx.security.crypto.EncryptedSharedPreferences
 import androidx.security.crypto.MasterKeys
 import org.foxarmy.barcodescannerforemployees.Net
 import org.foxarmy.barcodescannerforemployees.databinding.ActivityGroupBinding
+import org.foxarmy.barcodescannerforemployees.noInternetConnectionAvailableNotification
 
 class GroupActivity : AppCompatActivity() {
 
@@ -31,24 +32,28 @@ class GroupActivity : AppCompatActivity() {
             val groupName = binding.groupNameTextEdit.text.toString()
             val groupPassword = binding.groupPasswordTextEdit.text.toString()
             // group is set to "successful"
-            val n = Net()
-            n.language = sharedPreferences.getString("language", "en-US")!!
-            n.server = sharedPreferences.getString("server", "")!!
-            n.token = sharedPreferences.getString("token", "")!!
+            val net = Net()
+            net.language = sharedPreferences.getString("language", "en-US")!!
+            net.server = sharedPreferences.getString("server", "")!!
+            net.token = sharedPreferences.getString("token", "")!!
 
-            val response = n.createGroup(groupName, groupPassword)
-            val responseText = response.body!!.string()
-
-            if (response.code == 200) {
-                val currentGroups = sharedPreferences.getStringSet("groups", mutableSetOf())
-                currentGroups!!.add(responseText)
-                sharedPreferences.edit().putStringSet("groups", currentGroups).apply()
-                sharedPreferences.edit().putString("currentGroup", responseText).apply()
-                val intent = Intent(this, MainActivity::class.java)
-                startActivity(intent)
-                finish()
+            if (!net.serverIsAvailable(this)) {
+                noInternetConnectionAvailableNotification(this)
             } else {
-                Toast.makeText(this, responseText, Toast.LENGTH_LONG).show()
+                val response = net.createGroup(groupName, groupPassword)
+                val responseText = response.body!!.string()
+
+                if (response.code == 200) {
+                    val currentGroups = sharedPreferences.getStringSet("groups", mutableSetOf())
+                    currentGroups!!.add(responseText)
+                    sharedPreferences.edit().putStringSet("groups", currentGroups).apply()
+                    sharedPreferences.edit().putString("currentGroup", responseText).apply()
+                    val intent = Intent(this, MainActivity::class.java)
+                    startActivity(intent)
+                    finish()
+                } else {
+                    Toast.makeText(this, responseText, Toast.LENGTH_LONG).show()
+                }
             }
         }
 
@@ -56,25 +61,29 @@ class GroupActivity : AppCompatActivity() {
             val groupName = binding.groupNameTextEdit.text.toString()
             val groupPassword = binding.groupPasswordTextEdit.text.toString()
 
-            val n = Net()
-            n.language = sharedPreferences.getString("language", "en-US")!!
-            n.server = sharedPreferences.getString("server", "")!!
-            n.token = sharedPreferences.getString("token", "")!!
+            val net = Net()
+            net.language = sharedPreferences.getString("language", "en-US")!!
+            net.server = sharedPreferences.getString("server", "")!!
+            net.token = sharedPreferences.getString("token", "")!!
 
-            val groupId = n.getGroupId(groupName).toInt()
-            val response = n.joinGroup(groupId, groupPassword)
-            val responseText = response.body!!.string()
-
-            if (response.code == 200) {
-                val currentGroups = sharedPreferences.getStringSet("groups", mutableSetOf())
-                currentGroups!!.add(groupId.toString())
-                sharedPreferences.edit().putStringSet("groups", currentGroups).apply()
-                sharedPreferences.edit().putString("currentGroup", groupId.toString()).apply()
-                val intent = Intent(this, MainActivity::class.java)
-                startActivity(intent)
-                finish()
+            if (!net.serverIsAvailable(this)) {
+                noInternetConnectionAvailableNotification(this)
             } else {
-                Toast.makeText(this, responseText, Toast.LENGTH_LONG).show()
+                val groupId = net.getGroupId(groupName).toInt()
+                val response = net.joinGroup(groupId, groupPassword)
+                val responseText = response.body!!.string()
+
+                if (response.code == 200) {
+                    val currentGroups = sharedPreferences.getStringSet("groups", mutableSetOf())
+                    currentGroups!!.add(groupId.toString())
+                    sharedPreferences.edit().putStringSet("groups", currentGroups).apply()
+                    sharedPreferences.edit().putString("currentGroup", groupId.toString()).apply()
+                    val intent = Intent(this, MainActivity::class.java)
+                    startActivity(intent)
+                    finish()
+                } else {
+                    Toast.makeText(this, responseText, Toast.LENGTH_LONG).show()
+                }
             }
         }
     }
diff --git a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/LoginActivity.kt b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/LoginActivity.kt
index cb94492..aec68d3 100644
--- a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/LoginActivity.kt
+++ b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/LoginActivity.kt
@@ -10,6 +10,7 @@ import androidx.security.crypto.MasterKeys
 import org.foxarmy.barcodescannerforemployees.Net
 import org.foxarmy.barcodescannerforemployees.R
 import org.foxarmy.barcodescannerforemployees.databinding.ActivityLoginBinding
+import org.foxarmy.barcodescannerforemployees.noInternetConnectionAvailableNotification
 import org.foxarmy.barcodescannerforemployees.parseArray
 import org.json.JSONObject
 
@@ -40,31 +41,35 @@ class LoginActivity : AppCompatActivity() {
             val language = resources.getStringArray(R.array.languages)[binding.languageSpinner.selectedItemPosition]
             sharedPreferences.edit().putString("language", language).apply()
 
-            val n = Net()
-            n.language = sharedPreferences.getString("language", "en-US")!!
-            n.server = server
+            val net = Net()
+            net.language = sharedPreferences.getString("language", "en-US")!!
+            net.server = server
 
-            val response = n.login(username, password)
-            val responseText = response.body!!.string()
-            if (response.code != 200) {
-                Toast.makeText(this, responseText, Toast.LENGTH_SHORT).show()
+            if (!net.serverIsAvailable(this)) {
+                noInternetConnectionAvailableNotification(this)
             } else {
-                val json = JSONObject(responseText)
-                sharedPreferences.edit().putString("token", json["token"].toString()).apply()
-                n.token = json["token"].toString()
-                sharedPreferences.edit().putInt("userId", json["id"].toString().toInt()).apply()
-                sharedPreferences.edit().putString("server", server).apply()
+                val response = net.login(username, password)
+                val responseText = response.body!!.string()
+                if (response.code != 200) {
+                    Toast.makeText(this, responseText, Toast.LENGTH_SHORT).show()
+                } else {
+                    val json = JSONObject(responseText)
+                    sharedPreferences.edit().putString("token", json["token"].toString()).apply()
+                    net.token = json["token"].toString()
+                    sharedPreferences.edit().putInt("userId", json["id"].toString().toInt()).apply()
+                    sharedPreferences.edit().putString("server", server).apply()
 
 
-                val r = n.getMyGroups().body!!.string()
-                val myGroups = parseArray(r).map { a -> a.toString()}
+                    val r = net.getMyGroups().body!!.string()
+                    val myGroups = parseArray(r).map { a -> a.toString()}
 
-                sharedPreferences.edit().putStringSet("groups", myGroups.toSet()).apply()
-                sharedPreferences.edit().putString("currentGroup", myGroups[0]).apply()
+                    sharedPreferences.edit().putStringSet("groups", myGroups.toSet()).apply()
+                    sharedPreferences.edit().putString("currentGroup", myGroups[0]).apply()
 
-                val intent = Intent(this, MainActivity::class.java)
-                startActivity(intent)
-                finish()
+                    val intent = Intent(this, MainActivity::class.java)
+                    startActivity(intent)
+                    finish()
+                }
             }
         }
 
@@ -76,22 +81,27 @@ class LoginActivity : AppCompatActivity() {
             sharedPreferences.edit().putString("language", language).apply()
             sharedPreferences.edit().putString("server", server).apply()
 
-            val n = Net()
-            n.language = language
-            n.server = server
-            val response = n.registerAccount(username, password);
-            val responseText = response.body!!.string()
-            if (response.code != 200) {
-                Toast.makeText(this, responseText, Toast.LENGTH_SHORT).show();
-            } else {
-                sharedPreferences.edit().putInt("userId", responseText.toInt()).apply()
-                val token = JSONObject(n.login(username, password).body!!.string())["token"].toString()
-                sharedPreferences.edit().putString("token", token).apply()
+            val net = Net()
+            net.language = language
+            net.server = server
 
-                sharedPreferences.edit().putString("server", server).apply()
-                val intent = Intent(this, GroupActivity::class.java)
-                startActivity(intent)
-                finish()
+            if (!net.serverIsAvailable(this)) {
+                noInternetConnectionAvailableNotification(this)
+            } else {
+                val response = net.registerAccount(username, password);
+                val responseText = response.body!!.string()
+                if (response.code != 200) {
+                    Toast.makeText(this, responseText, Toast.LENGTH_SHORT).show();
+                } else {
+                    sharedPreferences.edit().putInt("userId", responseText.toInt()).apply()
+                    val token = JSONObject(net.login(username, password).body!!.string())["token"].toString()
+                    sharedPreferences.edit().putString("token", token).apply()
+
+                    sharedPreferences.edit().putString("server", server).apply()
+                    val intent = Intent(this, GroupActivity::class.java)
+                    startActivity(intent)
+                    finish()
+                }
             }
         }
     }
diff --git a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/MainActivity.kt b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/MainActivity.kt
index a61a449..3f88bd1 100644
--- a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/MainActivity.kt
+++ b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/MainActivity.kt
@@ -16,10 +16,7 @@ import androidx.security.crypto.EncryptedSharedPreferences
 import androidx.security.crypto.MasterKeys
 import androidx.viewpager.widget.ViewPager
 import com.google.android.material.navigation.NavigationView
-import org.foxarmy.barcodescannerforemployees.Net
-import org.foxarmy.barcodescannerforemployees.R
-import org.foxarmy.barcodescannerforemployees.ViewPagerAdapter
-import org.foxarmy.barcodescannerforemployees.convertToUnixEpochTimestamp
+import org.foxarmy.barcodescannerforemployees.*
 import org.foxarmy.barcodescannerforemployees.database.AbstractProductDAO
 import org.foxarmy.barcodescannerforemployees.database.CategoryDAO
 import org.foxarmy.barcodescannerforemployees.database.DBStorageController
@@ -60,9 +57,16 @@ class MainActivity : AppCompatActivity(), NavigationView.OnNavigationItemSelecte
         actionBarDrawerToggle!!.syncState()
         binding.navView.setNavigationItemSelectedListener(this)
 
-        // to make the Navigation drawer icon always appear on the action bar
         supportActionBar!!.setDisplayHomeAsUpEnabled(true)
 
+        val sharedPreferences = EncryptedSharedPreferences.create(
+            "sensitive",
+            MasterKeys.getOrCreate(MasterKeys.AES256_GCM_SPEC),
+            applicationContext,
+            EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,
+            EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM
+        )
+
         binding.expiryCalendarFab.setOnClickListener { _ ->
             val expiryCalendarIntent = Intent(this, ExpiryCalendarActivity::class.java)
             val extras = Bundle()
@@ -78,8 +82,6 @@ class MainActivity : AppCompatActivity(), NavigationView.OnNavigationItemSelecte
                 "StorageFragment" -> {
                     val addAbstractProductIntent = Intent(this, AddAbstractProductActivity::class.java)
                     val extras = Bundle()
-                    // I reuse the same stuff for editing and adding new product.
-                    // if abstractProduct == null, it means that we need to create new object
                     extras.putParcelable("abstractProduct", null)
                     extras.putString("action", "new")
                     addAbstractProductIntent.putExtras(extras)
@@ -104,7 +106,14 @@ class MainActivity : AppCompatActivity(), NavigationView.OnNavigationItemSelecte
                 }
             }
         }
-        synchronize()
+        val net = Net()
+        net.server = sharedPreferences.getString("server", "")!!
+
+        if (!net.serverIsAvailable(this)) {
+            noInternetConnectionAvailableNotification(this)
+        } else {
+            synchronize()
+        }
     }
 
     private fun synchronize() {
diff --git a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/NavigatorActivity.kt b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/NavigatorActivity.kt
index c3088d1..37324a0 100644
--- a/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/NavigatorActivity.kt
+++ b/app/src/main/java/org/foxarmy/barcodescannerforemployees/activities/NavigatorActivity.kt
@@ -31,8 +31,7 @@ class NavigatorActivity : Activity() {
         if (!isAuthenticated()) {
             intent = Intent(this, LoginActivity::class.java);
         }
-
-
+        
         startActivity(intent);
         finish();
     }
diff --git a/app/src/main/res/values-ru/strings.xml b/app/src/main/res/values-ru/strings.xml
index e045b3d..37c53f9 100644
--- a/app/src/main/res/values-ru/strings.xml
+++ b/app/src/main/res/values-ru/strings.xml
@@ -114,6 +114,9 @@
     <string name="image_compress_factor">Степень сжатия изображения</string>
     <string name="current_group">Текущая группа</string>
     <string name="cancel">Отмена</string>
+    <string name="no_internet_connection">No internet connection available. Please, connect to a network.</string>
+    <string name="ok">Ok</string>
+    <string name="logout">Log out</string>
     <string-array name="languages">
         <item>en-US</item>
         <item>ru-RU</item>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 4eb50aa..37c9170 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -112,6 +112,9 @@
     <string name="image_compress_factor">Image compression factor</string>
     <string name="current_group">Current group</string>
     <string name="cancel">Cancel</string>
+    <string name="no_internet_connection">No internet connection available. Please, connect to a network.</string>
+    <string name="ok">Ok</string>
+    <string name="logout">Log out</string>
     <string-array name="languages">
         <item>en-US</item>
         <item>ru-RU</item>