Understand and Troubleshoot Tunnel Connections

Introduction to Tunnel Connections


Before starting to plan or trying to troubleshoot Tunnel connections for Per-App or Device Tunnel use cases, it is important to understand how the Workspace ONE Tunnel app connects to a resource. This can help determine the best architecture, understand the traffic flow, network ports, and help in troubleshooting. 

Purpose of This Guide

This guide describes the data communication between Workspace ONE Tunnel Client and Tunnel Service on Unified Access Gateway, considerations when setting up VMware Unified Access Gateway appliances for tunnel use cases behind a load balancer, and troubleshooting best practices.

This guide focuses on the connections between Workspace ONE Tunnel app and Tunnel Service on Unified Access Gateway, and how this understanding can be applied to set up a load balancer and troubleshoot connection issues between both.

Intended Audience

This guide is intended for IT administrators and product evaluators who are familiar with Workspace ONE UEM and Unified Access Gateway. Familiarity with networking, firewall and load balancing configuration is assumed, and hands-on experience deploying and configuring Unified Access Gateway and Workspace ONE UEM for Tunnel use cases is desired.


The core components of Workspace ONE that are used in a Tunnel connection are described in the following table:

Component Description
Workspace ONE Tunnel The Workspace ONE Tunnel app is installed on a client device to access an internal resource (website, applications, etc.) through Tunnel Edge Service on Unified Access Gateway.
Workspace ONE UEM Console Administration console for configuring policies within Workspace ONE UEM to monitor and manage devices and the environment. It allows the administrator to configure and deploy the Workspace ONE Tunnel app to enable Per-App or Device Tunnel.
Unified Access Gateway VMware Unified Access Gateway is a virtual appliance that enables secure remote access from an external network to a variety of internal resources, including Horizon-managed resources.

When providing access to internal resources, Unified Access Gateway can be deployed within the corporate DMZ or internal network, and acts as a proxy host for connections to your company’s resources. Unified Access Gateway directs authenticated requests to the appropriate resource and discards any unauthenticated requests. It can also perform the authentication itself, leveraging an additional layer of authentication when enabled.
Tunnel Service
(aka Tunnel Edge Service)
The Tunnel Service is a service hosted on Unified Access Gateway that provides a secure and effective method for individual applications to access corporate resources hosted in the internal network. The Tunnel Service uses a unique X.509 certificate (delivered to enrolled devices by Workspace ONE) to authenticate and encrypt traffic from applications to the tunnel.

Access to an internal resource can be enabled through:
  • Per-App Tunnel: Enables application-level tunneling for managed applications on iOS, macOS, Android, and Windows devices.
  • Device Tunnel: Enables device-level tunneling of all applications on Windows devices.

TLS and DTLS Channels

DTLS and TLS Connection for UDP and TCP Traffic

Tunnel Service supports TCP and UDP traffic, and the Workspace ONE Tunnel app seamlessly sends the UDP traffic over DTLS and TCP over TLS. After the TLS channel is established, the Workspace ONE Tunnel app establishes a secondary DTLS channel if the UDP port is open on the firewall. Otherwise, UDP traffic is sent over the TCP channel.

This chapter provides in-depth details on the Tunnel communication over DTLS and TLS.

Main Channel (TLS)

When a device connects to the Tunnel Service (aka Tunnel Edge Service) on Unified Access Gateway, the Workspace ONE Tunnel app (Tunnel Client) on the device establishes a single TCP connection (encrypted with TLS 1.2) to the Tunnel Service. 

Authorized (allowed list) applications on the device all share this single TLS connection.  For example, Chrome is added to the Device Traffic Rules (allowed list) when configured for Per-App Tunnel traffic and can start 4 TCP connections to different hosts. In this case, the Workspace ONE Tunnel app establishes flow #1, 2, 3, and 4, and tags each connection with a flow ID. Tunnel Service uses the same flow IDs to identify the connections. This way, data between Workspace ONE Tunnel app and Tunnel Service can be identified and transmitted in both directions using the established TLS channel.

Another allowed listed application, such as Microsoft Remote Desktop Client, can create another 3 connections to hosts. So, flow #5, 6, and 7 will be assigned by the Workspace ONE Tunnel app so there are a total of 7 flows maintained by the Workspace ONE Tunnel app and Tunnel Service.

This also applies to UDP traffic, so both TCP and UDP traffic are tagged with flow IDs and handled similarly.

Figure 1: Device to Tunnel Service communication on Unified Access Gateway (Single Deployment)

Secondary Channel (DTLS)

An optional DTLS channel can be established between the Workspace ONE Tunnel app and Tunnel Service to handle UDP traffic. This allows real-time data such as video or voice to be handled in a more timely fashion, avoiding TCP resend delay between Workspace ONE Tunnel and Tunnel Service. The TCP resend delay can create an echo-like effect for voice and video delays. In this case, opening UDP will switch video traffic when carried by UDP to DTLS to reduce the TCP resend problem. Once a DTLS channel has been established, it sits parallel to the TLS channel and handles only UDP traffic.

In the previous example, if Chrome flow #3 and #4 and Remote Desktop Client #7 are UDP, they will be transmitted through the DTLS channel instead of TLS (see Figure 2 below).

The DTLS channel is optional, so if the Workspace ONE Tunnel app fails to establish the DTLS channel with a Tunnel Service on Unified Access Gateway (such as firewall blocking), UDP traffic can still be transmitted through the TLS channel. On the other hand, if the DTLS channel can be established, UDP traffic will start using the DTLS channel instead of the TLS channel.

Figure 2: Managed Device to Tunnel - Secondary Channel (DTLS)

When using a load balancer to handle DTLS channel, the DTLS channel must be connected to the same Unified Access Gateway's Tunnel Service handling the TLS channel because both channels need to be handled as a pair.  

DTLS channel is encrypted just like TLS and has a TLS session ID, so all persistence rules applied to TLS should also apply to DTLS channel.

Main Channel (TLS) Considerations for Cascade Mode Deployment

When Tunnel Service is configured for Cascade Mode deployment, meaning a Unified Access Gateway (front-end) deployed on the DMZ and another Unified Access Gateway (back-end) on the internal network, it is important to take into consideration the following aspects.

When a device establishes a TLS connection to the front-end, a TLS connection is also established from the front-end to back-end to handle traffic for that device (see Figure 3 below). For each device connection, only one TLS connection will be established between the front-end and back-end and will remain connected for the duration of the Workspace ONE Tunnel app-connected session. Therefore, the duration of this connection is the same as the duration of the TLS connection between the device and the front-end. In other words, if there are 100 devices to front-end connections, there will be 100 front-end to back-end connections.

When the device to front-end connection is disconnected, the front-end to back-end connection will also be disconnected. Similarly, if the front-end to back-end connection is disconnected (for example, due to Unified Access Gateway appliance shutdown), a device to the front-end connection will also be disconnected.

There is no DTLS channel between the front-end and back-end. The reason is due to the closer proximity between front-end and back-end (usually in the same facility) and therefore we expect very little delay and loss in data. Also, the additional complexity to open a UDP port between DMZ and internal network and to maintain two DTLS channels outweigh the insignificant gain in voice or video quality, so it was decided that DTLS is not needed between front-end and back-end.

Figure 3: Communication from Tunnel Service front-end to back-end through TLS Channel only

Load Balancer Requirements


For production deployments, a load balancer is required for any Unified Access Gateway Edge Service. In the case of Tunnel Service, some specific requirements are required to allow the Workspace ONE Tunnel app to establish a TLS and DTLS connection to the Tunnel Service.

The firewall must allow the TCP and UDP traffic in and out of the Unified Access Gateway TCP and UDP listing port. For example, if the Tunnel Service is set up to listen on port 443, the TCP and UDP port 443 must be opened at the firewall to allow all the incoming connections from the devices.

Keep in mind that the Unified Access Gateway HA (supporting on the VIP up to 10,000 concurrent connections) feature can be leveraged to balance Tunnel Service traffic when:

  • Unified Access Gateway is deployed on vSphere or Hyper-V
  • Tunnel Service is configured to use port 443

If both criteria cannot be achieved, an external load balancer is required, such as VMware Advanced NSX Load Balancer or any third-party load balancer.

This chapter provides detailed guidance on the load balancer requirements for Tunnel Service.

Load Balancer Checklist for Tunnel Service

Before diving into the load balancer requirements, the following checklist contains the recommended load balancer settings to properly handle the Tunnel traffic on Unified Access Gateway.

Load Balancer Requirements Tunnel Service on Unified Access Gateway
Port 443 or 8443
Protocol TCP (required)
UDP* (optional and only applicable to Front-End - additional notes below)
SSL Requirement Passthrough
SSL Certificate Details N/A
Persistence Options Supported Source IP (required only for UDP traffic)
Persistence Timeout Disabled
Load Balancing Algorithm Supported Least Connection, Round Robin
Health Check Options HTTPS GET
Health Check URL /favicon.ico
Health Check URL Response HTTP/1.1 200 OK
Health Check Frequency 30 seconds

IMPORTANT notes for UDP requirements:

  • UDP is optional; however, when tunneling UDP traffic, it is highly recommended to open the UDP port on the firewall to enable Tunnel DTLS communication on Front-End only. This allows real-time data such as video or voice to be handled in a more timely fashion, avoiding TCP resend delay. The TCP resend delay can create an echo-like effect for voice and video delays. In this case, opening UDP will switch video traffic when carried by UDP to DTLS to reduce the TCP resend problem.
  • UDP is not required between Tunnel Service Front-End and Back-End.
  • NATed address - When Tunnel Service Front-End is behind a NAT, all clients behind the same NAT device have the same source IP address. Therefore, all DTLS traffic from these clients is routed to the same Front-End, which might not follow the Front-End where the initial TLS connection was established. For DTLS to work properly Tunnel Service Front-End cannot be behind a NAT.

Session Persistence

Some level of persistence should be maintained so the TLS channel can remain intact for the duration of the TLS session, since Tunnel Service maintains a timer and will disconnect the TLS channel once the on-demand timeout has been reached. Timeout settings at load balancers should be set to disabled and should let Tunnel Server determine when to disconnect.

When UDP traffic is allowed on the firewall and the load balancer is able to handle DTLS channel, the DTLS channel must be connected to the same Unified Access Gateway's Tunnel Service handling the TLS channel, because both channels need to be handled as a pair. Session Persistence for the UDP protocol is required.

SSL Offloading and SSL Re-Encryption

SSL Offloading and SSL re-encryption are not supported and must be disabled. This is because Tunnel uses a certificate pinning between the client and server-side, creating an end-to-end encrypted tunnel that does not allow SSL manipulation. The encrypted tunnel between client and server can only be decrypted by the tunnel service on the Unified Access Gateway appliance. All TCP and UDP traffic to the Tunnel Service must be allowed to pass through to the Unified Access Gateway appliance.

Traffic can only be inspected after the Tunnel Service forwards the traffic into the internal network. For cascade mode deployment, this is after the back-end.

Health Check Configuration

In order for the load balancer to properly forward the traffic to the Tunnel Service, the load balancer must check the health of the Unified Access Gateway appliances to determine if it is reachable or not.  

To achieve that, configure the load balancer Health Check URL setting to perform an HTTPS GET on /favicon.ico on each of the Internet IP addresses of the Unified Access Gateways deployed. The response could be:

  1. HTTP/1.1 200 OK when Tunnel Service is up and running and appliance health
  2. HTTP/1.1 503 when Tunnel Service is down or Unified Access Gateway appliance is in Quiesce Mode

Using Quiesce Mode for maintenance and upgrades

Unified Access Gateway can be put into Quiesce Mode, after which it will not respond to the load balancer health monitoring request with an HTTP/1.1 200 OK response. Instead, it will respond with HTTP/1.1 503 to indicate that the Unified Access Gateway service is temporarily unavailable. This setting is often used prior to scheduled maintenance, planned reconfiguration, or planned upgrade of a Unified Access Gateway appliance. In this mode, the load balancer will not direct new sessions to this appliance because it will be marked as unavailable, but can allow existing sessions to continue until the user disconnects or the maximum session time is reached. Consequently, this operation will not disrupt existing user sessions.

The appliance will then be available for maintenance after a maximum of the overall session timer, which is typically 10 hours. This capability can be used to perform a rolling upgrade of a set of Unified Access Gateway appliances in a strategy resulting in zero user downtime for the service.

The administrator can configure Quiesce Mode using the Unified Access Gateway Admin UI under System Configuration or via REST API.

Balancing Traffic Between Front-End and Back-End (Cascade Mode)

When using Tunnel Service in cascade mode, a load balancer mechanism is required between the front-end and back-end. Two methods can be used:

  1. DNS Round Robin
  2. Load Balancer

Method 1 - DNS Round Robin

DNS round-robin can be used by the front-end when a load balancer is not available between the front-end and back-end.

For example, backend.example.com can be set up as the back-end hostname on Workspace ONE UEM Tunnel Configuration to resolve to the following two Unified Access Gateway IP addresses:


During initialization, the front-end determines how many back-end IP addresses can be resolved, and if there are more than one, it puts the addresses in a list. During operation, for each new device connection, the front-end picks the next one in the list in a round-robin fashion.

Figure 4: Load balancing between front-end and back-end through DNS Round Robin

Health Check configuration required between front-end and back-end

When using DNS round-robin, the front-end needs to detect and skip the offline back-end appliance. Failing to detect an unreachable back-end will cause every other device connection to fail in our example.

The cascade_health_check_interval setting must be configured to control the check intervals. The administrator can configure that in Workspace ONE UEM Console under the Tunnel Configuration / Custom Settings. Once set, the front-end will perform TCP Ping to the back-ends. If a back-end is unreachable, it will be marked as down and skipped.

The cascade_health_check_interval value is defined in seconds. When not defined or set to 0, the health check between front-end and back-end is disabled.

Figure 5: Custom Setting configuration for Tunnel in Workspace ONE UEM Console

Method 2 - Load Balancer

When setting up a load balancer between the front-end and back-end, the persistence rules between the front-end and back-end should be similar to the persistence rules between the device and the front-end, because of the similar type of communication (TLS). The timeout interval (default 5 minutes) is controlled by the Tunnel Client's on-demand feature, so the timeout value at the load balancer should be set to disable as well.

Figure 6: Load balancing between front-end and back-end through Load Balancer


Validating Device to Tunnel Service Connectivity

There are multiples ways to validate. If a user is able to access internal/external applications through Workspace ONE Tunnel, the ultimate test consists of enrolling a device and launching the applications that will tunnel traffic to a specific domain defined on the Device Traffic Rules. However, a simple test via the openssl command can help to validate the communication between the Device and Tunnel Service on Unified Access Gateway, depending on the network that the device is connected to. You will also be validating the connection through a load balancer or directly into Unified Access Gateway when on the internal network. Open SSL can be downloaded from here.

To ensure Tunnel Service and Unified Access Gateway are properly configured, it is recommended to perform the openssl test from a device connected as follows:

  1. First, from the internal network without passing through the load balancer. This will help you confirm that any issue on that communication is not related to the load balancer, but with the internal network or Unified Access Gateway configuration.
  2. Second, after successfully testing from the internal network, now try from the external network where the traffic from the openssl test will pass through a load balancer. Any issue in this test most likely is related to load balancing configuration.

Testing connectivity between Device and Tunnel Service

INTERNAL TEST - From an endpoint (Windows, macOS, or others) connected to an internal network, execute the following openssl command replacing the parameters between <> with the respective values:

openssl s_client -connect <UAG-IP/FQDN>:<PORT> -servername <UAG-FQDN>

EXTERNAL TEST - From an endpoint (Windows, macOS, or others) connected to the Internet, execute the following openssl command replacing the parameters between <> with the respective values:

openssl s_client -connect <LB-IP/FQDN>:<PORT> -servername <LB-FQDN>

The expected result is the Tunnel Certificate followed by the message: "Acceptable client certificate CA names".

Output results that indicate a load balancer configuration issue include:

  • The message "No client certificate CA names sent" indicates that the handshake failed and did not hit Tunnel Service at all. In case the test is going through the load balancer, this might indicate that SSL offloading is configured on the load balancer or other mechanism that strips the certificate or inspects the traffic.
  • If the return is only a CONNECTED string and no certificate response, this means a connection with the load balancer was established, but the load balancer did not receive a response back from Tunnel Service on Unified Access Gateway.

Additional device troubleshooting

For additional troubleshooting related to device and Workspace ONE UEM configuration, refer to the Deploying VMware Workspace ONE Tunnel: VMware Workspace ONE Operational Tutorial, which covers end-to-end deploy and configuration of Workspace ONE Tunnel app for all supported device platforms.

Validating Front-End and Back-End Connectivity (Cascade Mode only)

When deploying Unified Access Gateway in Cascade Mode, the device request hits the Tunnel Front-End first and the request gets forwarded to the Back-End. For that reason, both Unified Access Gateway appliances must be able to communicate through the configured port and hostname defined on the Workspace ONE UEM Console.

The following command executed from the Front-End appliance will validate if both appliances are able to communicate, displaying connect as output response:

curl -v telnet://<UAG-BACKEND-FQDN>:<PORT>

Validating Tunnel Service Connectivity to Internal Resource

It is also important to ensure that the Unified Access Gateway appliance can communicate with the internal resource, when the device request hit the Tunnel Service that will be forwarded to the internal resource, such as a internal web application, desktop machine, etc.

A simple curl command from the Unified Access Gateway console can help you to determine if the internal resource is reachable:

curl -lv -k <INTERNAL HTTP(s) URL>:<PORT>

A successful connection returns a connected status and HTML response for the respective website. Otherwise, a "Connection refused" error is raised, as in the following image:

Summary and Additional Resources

Additional Resources

For more information about Workspace ONE Tunnel connections, you can explore the following resources:

About the Authors and Contributors

This tutorial was written by:

  • Andreano Lanusse, End-User-Computing Staff Architect, Technical Marketing, VMware

With significant contributions from:

  • Benson Kwok, End-User Computing Staff Engineer and Workspace ONE Tunnel Lead Architect, VMware

Change Log

The following updates were made to this guide:

Date Description of Changes
2021/03/01 Guide was published.


Your feedback is valuable. 

To comment on this tutorial, contact VMware End-User-Computing Technical Marketing at euc_tech_content_feedback@vmware.com.

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