BrowserUpProxy.addRequestFilter is not working on localhost when system under test is also on localhost

[kroegr]

I want to start the automated tests on localhost against my local system to check that the tests still green/functional after working on it.
The Webapplication is a Single-Page-Application and for security-reasons are HTTP-Header injected by a thirdparty-proxy for authentication. For the automated tests I use browserup-proxy to emulate that thirdparty-proxy and inject the HTTP-Header with the needed values for testing.

When I run the test locally against the DEV- or TEST-Stage, the proxy on localhost is working as expected - the header wil be injected.
When I run the test locally against my local environment, the proxy on localhost is not working as expected - there will be no headers set.

Here is the code for the configuration of the proxy:

    BrowserUpProxy proxy = new BrowserUpProxyServer();
        proxy.start();
        Proxy seleniumProxy = ClientUtil.createSeleniumProxy(proxy);

        // put our custom header to each request
        proxy.addRequestFilter((request, contents, messageInfo) -> {
            if (Host.isApiUrl(messageInfo.getUrl())) {
                request.headers().add("x-protect-authorization-user", "any_user");
            }
            System.err.println(messageInfo.getUrl());
            System.out.println(request.headers().entries().toString());
            return null;
        });

        DesiredCapabilities capabilities = new DesiredCapabilities();
        capabilities.setCapability(CapabilityType.PROXY, seleniumProxy);

        String proxyOption = "--proxy-server=" + seleniumProxy.getHttpProxy();
        options.addArguments(proxyOption);

        options.setCapability(CapabilityType.PROXY, seleniumProxy);

On localhost against localhost the method proxy.addRequestFilter is not called, only on urls != localhost.

Versions that I am using:
com.browserup.browserup-proxy-core 2.1.0
org.seleniumhq.selenium.selenium-java 3.141.59
geckodriver 0.26.0

If more informations are needed or missing, please let me know.

What I have asked me: Is it even possible to run a proxy and the application under test on the same host or is this conflicting with the behavior of a proxy?

[kroegr]

Good Morning!

I have found the solution, it's not a problem or bug with the proxy or anything else.

Firefox/geckodriver is per default configured to not allow any proxy hijack anything on localhost. It's possible to allow that.

        FirefoxOptions options = new FirefoxOptions();
        options.setHeadless(true);
        FirefoxProfile p = new FirefoxProfile();
        p.setPreference("network.proxy.allow_hijacking_localhost", true);

        options.setProfile(p);
        options.setLogLevel(FirefoxDriverLogLevel.INFO);
        driver = new FirefoxDriver(options);