Today I learned how to verify if Firefox, or in my case the Tor Browser, is using an advised alt-svc properly. The alt-svc-HTTP-header is used to provide an “alternative service”-address and to transparently upgrade connections to use those. In order to do that you open the about:networking#logging¹ page in Firefox and click on “Start logging” before you open the page for the first time. Then search for AltSvcTransaction² in order to see if Firefox detected the alt-svc-Header properly and for ROUTE-via entries where Firefox actually uses the “alternative service” to connect your page. Additionally, if you are too late and already opened the page, you can still check for AltSvcCache entries, which should provide you with insight about whether or not an alt-svc connection already exists and is used.

# Example to grep for the lines in question
grep -Pe 'ROUTE-via|AltSvcTransaction|AltSvcCache|AltSvcMapping' /tmp/log.txt-main.*

I wanted to verify on the client-side if the “Shivering-Isles Onion Service” was working properly. So far, whenever I wanted to test it, I went on the server side, and used the server-side logs to check if suddenly traffic from the internal internal tor daemon IP was appearing. I’m undecided which method is easier to use, but at least this one works with 3rd-party services.