Error status 521 returned (Application offline) when accessing web application through ZTNA

Products

• Symantec ZTNA

Issue/Introduction

New ZTNA Web application created referencing internal Web server.

External address defined with appropriate DNS CNAME changes applied.

When users access the application, an error is reported that Application is offline and status code returned in HAR file is 521.

ZTNA connector linked to application is online, and the connector host can ping, resolve web server DNS name and even browse the site via a text base Lynx browser.

Replacing internal address with IP address appears to correct the issue, but application needs to be defined with hostname.

Environment

ZTNA with connector installed as a docker container deployed on Ubuntu 22.04.

Web Application.

Cause

DNS issues with docker container running ZTNA connector.

Troubleshooting testing above showed DNS working but tests performed on the Ubuntu host and not the docker container.

When connecting to the actual docker container, DNS resolution failed.

Resolution

Created a new docker container that pulled the correct DNS settings from the Ubuntu host, and where DNS resolution of internal Web site worked fine. The ZTNA docker container had failed to inherit the DNS settings from the Ubuntu host (not sure why).

Alternatively, docker DNS environment can be changed / corrected manually without the need to recreate the container.