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I have a firewall (pfsense) acting as a gateway for my network users. All user traffic is going through this firewall.

I need to block HotspotShield VPN traffic on this firewall. As known, this VPN is using TCP port 443 and changing its server IPs every day. I can't block port 443 due to https traffic and can't block IPs.

What kind of solution do I have?

By the way, MiTM is not possible. We cannot make everyone trust our own certificate.

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  • Do you want to allow other VPNs or all VPNs?
    – schroeder
    Apr 6, 2020 at 10:51
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    I'm not sure I see the difference between letting something in your network talk to random websites on port 443 but not to a vpn on port 443. What's the difference?
    – Luc
    Apr 6, 2020 at 13:20
  • I was to block all VPNs (particularly HotSpotShield VPN) that are using port 443.
    – tevfik
    Apr 7, 2020 at 14:23

2 Answers 2

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I can't block port 443 due to https traffic.

MiTM is not possible. We cannot make everyone trust our own certificate.

That's not completely true, even when you can't terminate the HTTPS connections on the firewall. In general, it's possible to filter traffic based on the (plain text) server name indication (SNI) in ClientHello.

Then, if a VPN is using non-standard traffic on port 443, you could block everything on port 443 that doesn't meet the TLS standards. That won't break HTTPS, but may affect other non-standard services, requiring exceptions for their IP addresses.

The only problem with these measures is that they are not compatible with TLS 1.3, because it has the ability to encrypt the SNI. For TLS 1.3 you can only block connections based on DNS names (requiring additional and inaccurate DNS lookups), and/or block DNS requests to certain domains (not affecting solutions using hard-coded IP addresses). However, this approach might also work for blocking VPN solutions that rely on a working DNS for their domain(s).

I have a firewall (pfsense) acting as a gateway

The documentation on pfSense's transparent proxy features are quite limited, but it does tell pfSense is using Squid to implement these features, and Squid can do a lot of things. The SNI inspection should be possible with the current pfSense's Squid versions (since 0.4.35).

The Squid feature is called SslBump Peek and Splice. With this feature you can perform actions based on the SNI, and there are two actions that can either block or allow the connection without intercepting it:

  • splice. Become a TCP tunnel without decoding the connection. The client and the server exchange data as if there is no proxy in between.

  • terminate. Close client and server connections.

As the documention comes with clear configuration examples, the rest is left as homework. It's also likely that the pfSense's web interface doesn't have these features, but they should be accessible through configuration files. (Some commercial solutions are more intuitive with this, but using open source is a decision that usually comes with the requirement of learning more.)

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  • Very good point. I made some quick tests with wireshark. Bad news is HotSpot Shield VPN is setting SNI names to a common web site (such as twitter.com). Looks like SNI is not an option.
    – tevfik
    Apr 7, 2020 at 14:28
  • Does it look for a DNS name before it makes a connection? Could that solve your problem? Or could it be possible to block connection, if SNI doesn't match an A record? Just general ideas, don't know in detail what pfSense is capable of. Apr 7, 2020 at 14:34
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It seems to me Deep Packet Inspection (DPI) is what you want. Don't think Pfsense will do this out of the box. Maybe in conjunction with Snort you can have something that will segregate traffic regardless of destination port.

I would research the layer 7 classification tools currently available on the market. I understand SolarWinds does that kind of stuff and there are other commercial software for this purpose. One open-source alternative that looks promising is ndpi. I have not evaluated any of these solutions personally.

Traffic shaping is a related concept. I would have a look at what Pfsense can do in this aspect. Even if you cannot easily block the traffic you can perhaps throttle it by allocating ridiculously low bandwidth and ruin the user experience (the Dilbert character comes to mind).

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