Why do I see Network Traffic without any User Activity?

Question

Why does my computer send any network data, even if I am not doing anything?

Is it possible that this network traffic includes private data (text documents, images,...), to which I did not consent?

Answer 1

I'm going to assume you ask why you see traffic on your machine, even though you don't actively do anything. The reasons for that are that your computer is doing a lot in the background, which is not triggered by any user action. Let me give a few examples:

• Replies to requests: Your computer receives network requests all the time. A good example for that are ARP requests in your local network. Your computer will receive them, and may respond to them. This is normal behavior and required for your PC to work.
• OS and Application Updates: Your OS and your Applications will periodically check to see if updates are available. This is not triggered by any actions of yours, but happens periodically.
• Background Connections: Some applications open various connections to external services as soon as you open them. For example, Chrome and Firefox both try to reach a specific site to see if captive portals are being used, if ISPs behave correctly, etc.

Of course, there are many more reasons why your computer may show traffic, even if you're not "doing anything".

Could these contain private information without my consent?

That depends what you consider private information. For example, Firefox sends requests to fetch http://detectportal.firefox.com/success.txt. An attacker in the same network could listen to this request and detect that you are using Firefox.

As far as your consent goes, it depends on the Terms of Service of each individual application. You would be surprised to what things you consent just by using regular applications.

Could these replies contain personal documents?

Answering a comment, usually no. Of course, if you use a file-sync service such as Google Drive, Dropbox, etc., it is of course possible that personal files will be included in such traffic, but this is by design.

As for other applications, it would be highly unlikely. For example, if you bought a video game from Steam and it would suddenly send copies of all your files to the publisher, such a case would become popular very quickly and would put the developer into hot water (and likely get their game banned from Steam quickly).