How to use DNS over TCP to Prevent MiTM Attacks in Android & iOS Apps

Last updated January 17, 2024 by Appdome

Learn to Prevent blocking the transfer of DNS over TCP in Mobile apps, in mobile CI/CD with a Data-Driven DevSecOps™ build system.

Why Permit DNS over TCP?

Usually, DNS resolution is performed over UDP. This protocol enables a fast, stateless query-response model and is used by DNS to achieve rapid resolution and lookups between hostnames and IP addresses.

Nevertheless, in some cases it is preferable to resolve DNS queries over TCP, for the following reasons:

  • Operationally, the UDP payload is limited in size (up to 512b), while TCP has no such restriction. This means that on a single DNS TCP query you can return more hosts to IP resolutions than over UDP. This capability is a clear advantage for sites that have global resolution based on location.
  • From a security standpoint, DNSSEC can only reliably be applied over TCP.

How does Appdome implement DNS Transfer over TCP?

Applying DNS transfer over TCP can be complicated. Moreover, if your infrastructure fails to support TCP, or blocks DNS over TCP at the network layer, this may lead to resolution failure, application-level timeouts, or both.

Appdome’s Permit DNS over TCP feature enables this operation for Android and iOS apps without coding.

Prerequisites for Using Permit DNS over TCP:

To use Appdome’s mobile app security build system to Prevent blocking the transfer of DNS over TCP , you’ll need:

Prevent blocking the transfer of DNS over TCP on Mobile apps using Appdome

On Appdome, follow these 3 simple steps to create self-defending Mobile Apps that Prevent blocking the transfer of DNS over TCP without an SDK or gateway:

  1. Upload the Mobile App to Appdome.

    1. Upload an app to Appdome’s Mobile App Security Build System

    2. Upload Method: Appdome Console or DEV-API
    3. Mobile App Formats: .ipa for iOS, or .apk or .aab for Android
    4. Permit DNS over TCP Compatible With: Obj-C, Java, JS, C#, C++, Swift, Kotlin, Flutter, React Native, Unity, Xamarin, and more
  2. Build the feature: Permit DNS over TCP.

    1. Building Permit DNS over TCP by using Appdome’s DEV-API:

      1. Create and name the Fusion Set (security template) that will contain the Permit DNS over TCP feature as shown below:
      2. fusion set that contains Permit DNS over TCP

        Figure 1: Fusion Set that will contain the Permit DNS over TCP feature
        Note: Naming the Fusion Set to correspond to the protection(s) selected is for illustration purposes only (not required).

      3. Follow the steps in Sections 2.2.1-2.2.2 of this article, Building the Permit DNS over TCP feature via Appdome Console, to add the Permit DNS over TCP feature to this Fusion Set.

      4. Open the Fusion Set Detail Summary by clicking the “...” symbol on the far-right corner of the Fusion Set. Copy the Fusion Set ID from the Fusion Set Detail Summary (as shown below): fusion Set Detail Summary image

        Figure 2: Fusion Set Detail Summary
        Note: Annotating the Fusion Set to identify the protection(s) selected is optional only (not mandatory).

      5. Follow the instructions below to use the Fusion Set ID inside any standard mobile DevOps or CI/CD toolkit like Bitrise, App Center, Jenkins, Travis, Team City, Circle CI or other system:
        1. Build an API for the app – for instructions, see the tasks under Appdome API Reference Guide
        2. Look for sample APIs in Appdome’s GitHub Repository
    2. Building the Permit DNS over TCP feature via Appdome Console

      To build the Permit DNS over TCP protection by using Appdome Console, follow the instructions below.

      1. Where: Inside the Appdome Console, go to Build > Security Tab > Secure Communication section.
      2. How: Check whether is toggled On (enabled), otherwise enable it . The feature Permit DNS over TCP is enabled by default, as shown below. Toggle (turn ON) Permit DNS over TCP, as shown below.
        If needed, Customize the Threat Notification to be displayed to the mobile end-user in a standard OS dialog notification when Appdome Prevents blocking the transfer of DNS over TCP.
        Permit DNS over TCP option

        Figure 3: Prevent blocking the transfer of DNS over TCP option
        Note: The App Compromise Notification contains an easy to follow default remediation path for the mobile app end user. You can customize this message as required to achieve brand specific support, workflow or other messaging.

      3. When you select the Permit DNS over TCP you'll notice that your Fusion Set you created in step 2.1.1 now bears the icon of the protection category that contains Permit DNS over TCP

        Fusion Set applied Permit DNS over TCP

        Figure 4: Fusion Set that displays the newly added Permit DNS over TCP protection

      4. Click Build My App at the bottom of the Build Workflow (shown in Figure 3).
    Congratulations!  The Permit DNS over TCP protection is now added to the mobile app
  3. Certify the Permit DNS over TCP feature in Mobile Apps

    After building Permit DNS over TCP, Appdome generates a Certified Secure™ certificate to guarantee that the Permit DNS over TCP protection has been added and is protecting the app. To verify that the Permit DNS over TCP protection has been added to the mobile app, locate the protection in the Certified Secure™ certificate as shown below: Permit DNS over TCP shown in Certificate secure

    Figure 5: Certified Secure™ certificate

    Each Certified Secure™ certificate provides DevOps and DevSecOps organizations the entire workflow summary, audit trail of each build, and proof of protection that Permit DNS over TCP has been added to each Mobile app. Certified Secure provides instant and in-line DevSecOps compliance certification that Permit DNS over TCP and other mobile app security features are in each build of the mobile app

Using Threat-Events™ for blocking the transfer of DNS over TCP Intelligence and Control in Mobile Apps

Appdome Threat-Events™ provides consumable in-app mobile app attack intelligence and defense control when blocking the transfer of DNS over TCP is detected. To consume and use Threat-Events™ for blocking the transfer of DNS over TCP in Mobile Apps, use AddObserverForName in Notification Center, and the code samples for Threat-Events™ for blocking the transfer of DNS over TCP shown below.

The specifications and options for Threat-Events™ for blocking the transfer of DNS over TCP are:

Threat-Event™ Elements Prevent blocking the transfer of DNS over TCP Method Detail
Appdome Feature Name Permit DNS over TCP
Threat-Event Mode
OFF, IN-APP DEFENSE Appdome detects, defends and notifies user (standard OS dialog) using customizable messaging.
ON, IN-APP DETECTION Appdome detects the attack or threat and passes the event in a standard format to the app for processing (app chooses how and when to enforce).
ON, IN-APP DEFENSE Uses Appdome Enforce mode for any attack or threat and passes the event in a standard format to the app for processing (gather intel on attacks and threats without losing any protection).
Certified Secure™ Threat Event Check x
Visible in ThreatScope™ x
Developer Parameters for Preventing blocking the transfer of DNS over TCP Threat-Event™
Threat-Event NAME
Threat-Event DATA reasonData
Threat-Event CODE reasonCode
Threat-Event REF
Threat-Event SCORE
currentThreatEventScore Current Threat-Event score
threatEventsScore Total Threat-events score
Threat-Event Context Keys
message Message displayed for the user on event
failSafeEnforce Timed enforcement against the identified threat
externalID The external ID of the event which can be listened via Threat Events
osVersion OS version of the current device
deviceModel Current device model
deviceManufacturer The manufacturer of the current device
fusedAppToken The task ID of the Appdome fusion of the currently running app
kernelInfo Info about the kernel: system name, node name, release, version and machine.
carrierPlmn PLMN of the device. Only available for Android devices.
deviceID Current device ID
reasonCode Reason code of the occurred event
buildDate Appdome fusion date of the current application
devicePlatform OS name of the current device
carrierName Carrier name of the current device. Only available for Android.
updatedOSVersion Is the OS version up to date
deviceBrand Brand of the device
deviceBoard Board of the device
buildUser Build user
buildHost Build host
sdkVersion Sdk version
timeZone Time zone
deviceFaceDown Is the device face down
locationLong Location longitude conditioned by location permission
locationLat Location latitude conditioned by location permission
locationState Location state conditioned by location permission
wifiSsid Wifi SSID
wifiSsidPermissionStatus Wifi SSID permission status
threatCode The last six characters of the threat code specify the OS, allowing the Threat Resolution Center to address the attack on the affected device.

With Threat-Events™ enabled (turned ON), Mobile developers can get detailed attack intelligence and granular defense control in Mobile applications and create amazing user experiences for all mobile end users when blocking the transfer of DNS over TCP is detected.


The following is a code sample for native Mobile apps, which uses all values in the specification above for Permit DNS over TCP:


Important! Replace all placeholder instances of <Context Key> with the specific name of your threat event context key across all language examples. This is crucial to ensure your code functions correctly with the intended event data. For example, The <Context Key> could be the message, externalID, OS Version, reason code, etc.



Using Appdome, there are no development or coding prerequisites to build secured Mobile Apps by using Permit DNS over TCP. There is no SDK and no library to code or implement in the app and no gateway to deploy in your network. All protections are built into each app and the resulting app is self-defending and self-protecting.

Releasing and Publishing Mobile Apps with Permit DNS over TCP

After successfully securing your app by using Appdome, there are several available options to complete your project, depending on your app lifecycle or workflow. These include:

Related Articles:

How Do I Learn More?

If you have any questions, please send them our way at support.appdome.com or via the chat window on the Appdome platform.

Thank you!

Thanks for visiting Appdome! Our mission is to secure every app on the planet by making mobile app security easy. We hope we’re living up to the mission with your project.

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