Can your ISP tell if you're using a VPN

[javajolt]

A VPN can be an essential tool in your cybersecurity toolbox, and is useful for protecting your identity and privacy online. VPNs offer a way to encrypt your internet traffic, routing it down an encrypted tunnel, making it more difficult for prying eyes to monitor your online behavior.

One common question that often arises, as with all internet anonymization tools, is whether your internet service provider can tell if you're using a VPN. There's some complexity in answering this, which relies on an understanding of how VPNs work. So let's dive into this for a moment.

How does a VPN work?

Understanding how a VPN works is fundamental



As we've covered previously at XDA, most VPNs work by creating a secure and encrypted tunnel between your device and a VPN server, which is typically located somewhere else in the world. When you connect to a VPN, all your internet traffic is directed to this server via an encrypted channel, before being forwarded on to its destination.

Redirecting and encrypting your traffic in this way makes it appear as though your internet traffic is coming from the VPN server rather than from your actual location. Additionally, while most internet traffic now uses http (i.e. it is encrypted), not all traffic originating from your computer is. These fragments of unencrypted traffic, like initial website requests or DNS requests, can be used to build a comprehensive profile of your internet use by an observer. The added encryption offered by a VPN protects against this snooping - yes, including by your ISP.

If you're looking for a more in-depth read on how a VPN works, check out our previous content to understand more about how VPNs work, and escape some of the marketing mumbo-jumbo around them. But let's get on to the question at hand.



Does my ISP monitor my traffic?

ISPs are often making an easy dollar on the side from your traffic



ISPs have been known to monitor internet traffic for a number of reasons, some more nefarious than others. These reasons might include targeted advertising, anti-piracy measures, data sales, or information may even be collected on behalf of the authorities or law enforcement.

It might be easy to think "I use http everywhere in my browser, what data could possibly be collected?" - but it's been demonstrated repeatedly that through advanced machine learning and inference, as well as scraping all possible data (including things like the timestamps, duration, and quantity of your usage), that a reasonably comprehensive profile of the likely uses of your internet usage can be built up. Your ISP can also work to identify the owners of the IP addresses you're connecting to (a fairly trivial process for larger sites) in order to make assertions like "You spend more time on average between 6-7pm on Facebook during the winter months," which might be a valuable data point they can sell later.

A VPN protects against some of this, but not all, by ensuring that your ISP cannot see the destination of your traffic, encrypted or not. Instead, all they can see is that you're sending encrypted traffic to a specific server. They won't know what you're doing within that encrypted tunnel, but they will know that you're using a VPN.

Can my ISP tell if I'm using a VPN?


Source: Lenovo

Yes, your ISP can tell if you're using a VPN. While it's not a trivial process, there are some signs that an ISP can look out for to identify if you're using a VPN. Some VPN providers make more effort than others to hide these signs, but it's safe to assume that if you are using a VPN, your ISP (and potentially the government) will be able to tell when one is in use. The common signs of VPN usage could include some of the signals below.

Common port usage

Depending on the VPN provider, configuration, and protocol in use, traffic on a specific port can indicate the use of a VPN. Some providers hide this, routing VPN traffic over a common port like port 53 (this can also be useful to evade firewalls or network policies that disallow VPNs), but this isn't foolproof. Regardless of the port in use, any deeper inspection of your traffic over a port might provide an indication that you're using a VPN.

IP address tracing

The server your VPN runs on has a given IP address, and all of your encrypted traffic will appear to your ISP as if it's going to that IP address. The addresses of these servers aren't easy to change on an ongoing basis, and companies or your ISP can maintain lists of known VPN servers and flag traffic originating from or going to them. This is how Netflix detects VPN usage, and why some companies respond by offering VPN configurations designed for streaming that have 'rolling' (or constantly changing / new) IP addresses that are less likely to be blocked.

Traffic pattern inspection

This one is a bit more involved on the ISPs front, but by tracking the flow and volume of traffic from an address they can potentially identify hallmarks associated with VPN use, such as consistent and stable traffic to a single IP. ISPs can also inspect individual packets themselves for hallmarks of VPN use, like the encryption protocol in use. Again, there are ways to nullify these, but it depends on your VPN provider.

Assume your ISP can tell you're using a VPN

While we've highlighted some ways your ISP can tell if a VPN is in use, and mentioned some mitigations, we'd advise that you assume your ISP can tell if you're using a VPN. Unless you're extremely technically able, and confident in your configurations and mitigations, this is likely to be a safe assumption. Some other solutions do exist, like using Tor, multiple VPNs, self-hosting a VPN, or Tor over VPN, but these also come with their own issues and potential ways of being identified.



This is especially difficult if you're in a country where VPN use is restricted or banned, as there are often more effective identification techniques in place to detect when someone is ignoring the law. If you face legal consequences for using a VPN, we can't condone it, and you should consider carefully the risks involved.

In summary, yes, it's likely that your ISP can tell you're using a VPN, even if it cannot easily identify what you're doing over it.

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