Selenium ChromeDriver returns 403 errors on build server

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We've created a Selenium test project that starts the (ASP.NET) web application and runs a couple of tests using the ChromeDriver. Locally this all runs fine (in headless and non-headless mode).

But on the build server (using an Azure DevOps agent) this fails without ever starting the tests. It looks like it fails when starting the ChromeDriver: the driver starts, but then it's immediately followed by 403 errors. It never gets to the part where it actually loads a webpage.

Any ideas where to look?

selenium-chromedriver azure-devops selenium c#
1 Answers

Answering my own question to document possible solutions.

After some rigorous investigation (which included using the source code to get to the bottom of things) we found out that the proxy server somehow got in the way. It turned out that the ChromeDriver tries to communicate over a local port (e.g. http://localhost:12345), which was redirected through the proxy server. This failed with a 403 error.

This gave us a lead on possible solutions. First we tried to use the .proxybypass file to exclude localhost addresses. This didn't work -- it turns out that this proxy bypass only works for https requests. And the ChromeDriver control commands are sent over http :-(

We then made sure that no proxy was used in our test code. We did this with the following lines:

var options = new ChromeOptions();
options.AddArgument("--no-sandbox");
options.AddArgument("headless");
options.AddArgument("ignore-certificate-errors");
options.Proxy = new Proxy()
    {
        Kind = ProxyKind.Direct
    };

var driver = new ChromeDriver(options);

In addition to these settings (note that some arguments were added to solve other issues and might not apply to your own situation), we also disabled the proxy for other requests:

WebRequest.DefaultWebProxy = null; 
HttpClient.DefaultProxy = new WebProxy()
{
    BypassProxyOnLocal = true,
};

This allowed our tests to finally run on the build server without the 403 errors. One last remark (which might be obvious) is to always run your tests in non-headless mode if you encounter any issues. This allowed us to see the "invalid certificate error" which would otherwise be hidden.

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