What is opensnitch?

Opensnitch is an application level firewall, it shows connections made by applications and helps protecting privacy. It is clone of Littesnitch for macOS (paid app, there is a free alternative lulu that has about the same features).

It allows monitoring and blocking unnecessary or suspicious connections by creating firewall rules in the GUI. I use it to monitor firefox and change my configuration.

I don't use chrome in general but according to this article Choose your browser carefully, it also makes unwanted connections.

How to install opensnitch

Download the prebuilt deb packages: daemon and GUI

https://github.com/evilsocket/opensnitch/releases

Then run:

dpkg -i opensnitch_1.4.0.rc-1_amd64.deb
dpkg -i python3-opensnitch-ui_1.4.0.rc-1_all.deb
apt-get -f install

opensnitch-ui

Blocking connections from firefox

Opensnitch gave me a list of domains firefox is connecting to and I went to check how these domains are used and decided whether I want to block them.

  • content-signature-2.cdn.mozilla.net: This is used for securing firefox public services.
  • location.services.mozilla.com: Useless service for me
  • ocsp.digicert.com: Check certificate validity.
  • safebrowsing.googleapis.com: Used to flag malicious sites (disabled with the preference "Block dangerous and deceptive content")
  • push.services.mozilla.com: Push service
  • shavar.services.mozilla.com: is used to update the list of trackers that are getting blocked when tracking protection is turned on.
  • ocsp.pki.goog: Check certificate validity.
  • ocsp.godaddy.com: Check certificate validity

Then I changed my firefox configuration ('about:config') like this:

  • extensions.blocklist.enabled false: I don't need that after I have installed my extensions
  • in Preferences > Privacy & Security > Disable Block dangerous and deceptive content: I browse websites I trust.
  • app.normandy.enabled false: I don't want to test new features.
  • dom.push.enabled false: I don't need that
  • services.sync.prefs.sync.privacy.trackingprotection.cryptomining.enabled false
  • services.sync.prefs.sync.privacy.trackingprotection.enabled false
  • services.sync.prefs.sync.privacy.trackingprotection.fingerprinting.enabled false
  • services.sync.prefs.sync.privacy.trackingprotection.pbmode.enabled false

Mozilla has a page showing how to stop firefox making automatic connections.

I block the OCSP servers because I browse to reliable and trustworthy websites and I think it is privacy invasion. I wish there was an alternative solution with the certificate validity checks done in the client.