Is my browser sending any information about my linux distribution?

Most probably. Click this link to see what your browser sends to the web server. The OS should be listed under the first category called "User Agent"

Another good site that will show you what a website can learn about your system when visiting the site is BrowserSpy.dk; the Electronic Frontier Foundation (EFF) site listed in Keltari's answer uses some of the code from BrowserSpy.dk. If you click on Browser on the menu on the left of the home page, you will see information regarding your browser. When I visit the page using Firefox on a Ubuntu Linux system, I see "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:24.0) Gecko/201001 Firefox/24.0)"

There are several places browser may send info:

• User-Agent HTTP header. It is a part of HTTP specification and each request has one https://en.wikipedia.org/wiki/User_agent .As you can see, there is rendering engine name , browser version and OS name. Use "developer toolbar" or "firebug" to find list of headers your browser sends: https://stackoverflow.com/questions/4423061/view-http-headers-in-google-chrome

• JavaScript: there is Navigator class in JavaScript, it provides os name and browser name: http://www.w3schools.com/js/js_window_navigator.asp . Some sites may use JS to detect this info and send it back to server. You may use simple JS to check alert(navigator.appName); alert(navigator.platform);

• TCP/IP stack fingerprinting: different TCP/IP implementations (WinSock vs Berkley sockets) and different OS versions have slightly different default options. Some heuristic could be used to detect OS family. This is hacker-style approach and used very rarely: https://en.wikipedia.org/wiki/TCP/IP_stack_fingerprinting

• Modern JavaScript can also report your position (like GPS) via Geolocation API, but it always asks you if you want to provide this info to Server or not: http://www.w3schools.com/html/html5_geolocation.asp

By the way, it is not always possible for server to find your real IP. In case of proxy, proxy may send your real IP to server (using X-Real-IP or X-Forwarded-For headers), but it does not have to. And you may even have no public IP if NAT is configured on your router. In this case server will only know your router's public IP and there could the whole office (several hundred people) under one pulic IP.

Your user-agent string is usually sent to the website you are visiting. This UA string is then deciphered and will usually be enough to identify some very basic info about your machine.

Using Chrome, this is how my web browser introduces itself to this site: