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I am a domestic internet user on my Mac through a cable modem with a wifi router provided by a very large cable TV & broadband provider. I have a 7 Mbps connectivity with a download limit of 25GB. I have been using this for the last 18 months and my usage used to be below 12 GB per month. The last two months I have been getting huge bills from the ISP claiming that my usage has gone above 40 to 50 GB. There is no change in the usage pattern or even the software deployed in the last many months but the billed usage currently is unexplainable. The router does not have wifi range out side my home where one Mac and two iPhones are only used. The internet usage on the iPhone is also limited to some emails and simple browsing which we have been doing for 18 months.

Now, sometimes the usage statistics from the tool provided by the ISP shows 700 to 800 MB usage in 20 to 30 minutes even when all my systems are idle.

How is it possible for me to plug this hole?

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    You are sure the wifi is not hijacked? You might want to change the password anyway. – CousinCocaine May 8 '15 at 16:53
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    You can install a program like 'LittleSnitch' to find out what is using your bandwith. Another method can be to manually enable and disable your wifi from your device. See if that makes a difference. Can you get some info/log files from your host as well? Or just check activity monitor. – CousinCocaine May 8 '15 at 16:55
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The easiest (and cheapest) way is to check the data usage through Activity Monitor. Just type in the name in Spotlight to launch it, and navigate to the Network tab. You can sort by bytes sent/received, though I think you'll have to keep it running to get historical data by app (otherwise shows only total data sent/received).

For a complete solution, where you would get prompts for all apps trying to access the internet, and the ability to create filters etc., you can check out Little Snitch 3 (https://www.obdev.at/products/littlesnitch/index.html).

  • Little snitch would be my solution as well. – CousinCocaine May 8 '15 at 16:56
  • Yes, saw your comment just as I posted the answer :) However, it is 30€, and the OP might solve his problem for free. – Marko Nikolovski May 8 '15 at 17:00
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    Neither Little Snitch nor Activity Monitor will cover the wifi being possibly hijacked - you'd need something plugged at router-level to spot that [which would depend on what the router was, of course, & whether it's interrogable in that way] ...which in no way makes this answer bad, just not complete. – Tetsujin May 8 '15 at 20:42
  • Little Snitch is free for 30 days. This should be enough to pinpoint the problem. And sometimes it is good to pay developers a little money for their hard work :) especially when it saves you on the bills. – CousinCocaine May 9 '15 at 5:38
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    @Tetsujin You're right, of course... I didn't think to include that, thanks for pointing out that possibility. – Marko Nikolovski May 10 '15 at 14:10
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There are some absolutly free tools you can use to look what is going on.

Use the Terminal app located in your Utility folder.

Copy/Paste following:

sudo lsof -i -n -P | grep TCP 

It will ask for your Password, and show you which apps are using your Internet.

If you want to monitor it in real time use

Netttop in Terminal. It will automatically periodically update the bites in / bites out information so you can see who is using your bandwidth and how much.

Use up and down arrows to see more, or use (q) to quit it.

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