[banister]

Mobile is an exception (but they already state android is immune), let's stick to desktop for the sake of discussion, the 3 major desktop platforms: mac, win, linux :)

On mac - just implement a block everything rule with pf and then just allow traffic on the tunnel and whitelist the VPN endpoint. Boom, a kill switch that defends against this exploit. And there's no racey nordvpn-style "control channel" (if nord really works like this i have an even lower opinion of them than i do currently).

On linux - iptables (for example) - just implement a general DROP policy then override with a specific ALLOW on the tunnel interface.

On Windows - Use WFP to implement a block everything rule, then provide a higher priority rule to allow on the tunnel interface.

All three of these techniques are the recommended way to implement a kill switch and it's used heavily in the VPN industry by anyone sensible. It completely defends against this TunnelVision exploit too.

The way that you suggest kill switch is implemented (reactive and monitoring the connection?) is very fragile, racey and prone to leak, i absolutely would not trust it and it shouldn't even be called a kill switch. It's an embarrassment. :)

[jcrawfordor]

I'm not saying how it should be implemented, I'm saying how it is implemented by a number of popular VPN services. Take up the argument with them. There are VPN providers that do it right on at least some platforms, but unfortunately the way most document it it's very hard to tell without experimenting with the client to verify.

As far as I know, use of the term "kill switch" closely correlates with an untrustworthy implementation. Consider the case of Mullvad who handle this a lot better and also decline to call it a "kill switch" for that reason. And that's not to say that Mullvad is perfect, easy to find forum threads by people who had traffic leakage for various reasons. I wouldn't trust anything you didn't set up yourself.

[Borealid]

You wrote three different ways to end up with all traffic dropped and a broken VPN connection.

Traffic sent to the VPN interface gets encapsulated by the VPN client software and then routed to the Internet. If your firewall rule is dropping all traffic not destined for the VPN interface, it will drop the encapsulated traffic.

You need two (sets of) rules: one allowing traffic on the VPN interface and one allowing traffic which is already cloaked by the VPN software (or not cloaked, but used to establish/maintain the tunnel itself). That second category is a bit complicated, because you need to be able to route to the VPN server regardless of which network you're connected to - and the DHCP server tells you how to do that.

[banister]

Yes. I just provided simplified firewall rules in my answer. You also need to whitelist either the VPN endpoint itself (and add a route to that endpoint) or you need to whitelist the process (such as wireguard or openvpn) that hits that endpoint.

Not sure how a DHCP server is relevant in the slightest here except for the initial host network config of course. But the host network should already be configured before the VPN comes up.

Source: i've implemented this dozens of times (and you probably have too, it sounds like) so let's not quibble over the details ;)

[Borealid]

If you're connected to a random network, whose configuration you don't know in advance, how do you route packets to your VPN server?

The usual answer is that the network's router tells you how to do that, by supplying DHCP options.

The point I'm making here is that you can't just configure a firewall rule and have it work properly. What actually needs to happen is that the VPN client software is using one routing table - let's call it "host routing" - and everything else on the system is using a second routing table - let's call that "VPN routing".

The DHCP server inserts rules into the host routing table, and the only software using those rules is the VPN client for its management and tunnel traffic.

Otherwise, what if the network to which you connect says "the next hop for all internet traffic is 10.10.10.10"? You need to respect that rule when sending traffic to your VPN server, and ignore it for applications whose traffic will be tunneled.

[banister]

Let's walk through this step by step because there's a lot of confusion on your end.

* Step one - You connect your computer to a network - yes you'll get a DHCP lease, and you'll get an ip address, and a default gateway. This default route will be added to your routing table.

* Step two - If the TunnelVision exploit (DHCP option 121) is at play you'll also get a few MORE SPECIFIC routes than the default gateway. These also get added to your routing table

* Step three - You connect your VPN. The VPN will bring up a firewall. It will also bring up `128/1` and `0/1` routes that point at the VPN tunnel. The VPN tunnel now takes over the default route. This firewall will block all traffic that's not on the tun device (the VPN interface). Further, it will whitelist the VPN endpoint IP and create a route for it (it can do this since it already received the default gateway from the DHCP server)

* Step four - Your host starts sending traffic - either this traffic will go through the VPN tunnel (the default route) OR it will attempt to go through the more specific option 121 pushed malicious routes added by the compromised DHCP server (depending on the destination ip of the outbound packets).

* Step five - All traffic that would go down the malicious option 121 routes are BLOCKED by the firewall rule. Hence nullifying the TunnelVision exploit.

That's all. Done. Where's the complexity in that? As i said before i've done this dozens of times. I'm talking from experience. I know this works.

Further you say:

> The point I'm making here is that you can't just configure a firewall rule and have it work properly. What actually needs to happen is that the VPN client software is using one routing table - let's call it "host routing" - and everything else on the system is using a second routing table - let's call that "VPN routing".

You are aware we're talking about consumer VPNs right? The majority of users are on Windows and Mac. Neither of those OSes support multiple routing tables. Only Linux supports multiple routing tables.

You're also just plain wrong - as i demonstrated above - you CAN just configure a firewall rule and it WILL just work properly. Again, i'm talking from experience.

[pests]

In your step 4, what happens when the VPN traffic gets routed over option 121 pushed routes?

Don't you block it - thus blocking your entire VPN?

> OR it will attempt to go through the more specific option 121 pushed malicious routes added by the compromised DHCP server (depending on the destination ip of the outbound packets).

This right here... we don't want our VPN-secued traffic going out over routes broadcast by the malicious DHCP server, so you block it... right?

How does that traffic leave the local network and reach the VPN server?

[banister]

Read my reply to the other poster, i answer exactly this. Actually test it yourself. Stop theorizing. I tested it. It works exactly as I said.

I think i know where you're confused. There is a firewall whitelist on the VPN endpoint route. Also it's impossible for the DHCP server to push a route more specific than this since it's a /32 route, so it's unaffected (together with the firewall rule allowing it) by anything the DHCP server attempts to do.

[Brian_K_White]

The only problem with this persons comments is saying "you're wrong" "you're confused" so much.

The actual content is 100%.

Get over the "you're wrong" tone and ingest the tech message.

It's really a misnomer to call the firewall a kill switch since it isn't reacting, it's already in effect, already blocking the bad traffic before the bad traffic happens. No switch is thrown.

Any vpns that DO work that way are silly and should not be used. If this is most popular commercial vpns today, oh well so be it.

The articles going around saying "affects all vpns and nothing can stop it" are also just silly and wrong. But it is probably true that most convenient vpns are currently leaking.

[Borealid]

I can see how you can write rules that block "bad traffic", but I can't see how you write them so they don't also block some "good traffic" when the network assigns a routing rule.

I think the person here might be glossing over writing overzealous rules that cause the VPN connection to go down when an Option 121 route is assigned, when the ideal solution leaves the VPN functional (and causes tunneled traffic to ignore the route).

[Borealid]

I think you missed a step in your progression.

Step one, you connect to the network and get routes.

Step two, you connect your VPN.

Step three, your host starts sending traffic. At this point, your firewall rules are now active and dropping any traffic you've told them to.

Step four, you renew your DHCP lease and get new routes via option 121. Those routes might be malicious, or they might not.

One of three things is true at step four. Either:

A. Your firewall rules will block all traffic over the new routes

B. Your firewal rules will not block any traffic over the new routes

C. Your firewall rules will block some subset of traffic over the new routes

If A is true, then your VPN tunnel goes down (undesirable), as the VPN server can no longer be contacted.

If B is true, then you are vulnerable to the TunnelVision exploit.

If C is true, and the subset of traffic blocked is exactly the subset intended to route over the VPN but maliciously diverted, then the VPN tunnel goes down because the firewall rules are blocking its traffic.

If C is true, and somehow the firewall rule is rewriting the traffic that's pointed not-over-the-VPN to be instead routed over the VPN (by using NAT?), then the VPN tunnel stays up and there is no problem.

I'd be interested in seeing the set of firewall rules that will let the VPN tunnel stay up, with management traffic going over the added-after-the-tunnel-was-brought-up next-hop, and tunneled traffic continuing to flow ignoring the new route. I haven't seen those rules in the past so if you have experience writing them, please show me.

Personally I've only used Linux multiple routing tables to plug this leak.

EDIT: formatting

[banister]

No, you're wrong again. I just tested this (simulating routes added by a DHCP option 121) and it works exactly as I said.

C is what happens. But it doesn't happen the way you say at all.

Only the traffic heading to the new 121 routes are blocked - why is it blocked? because the routes are on the physical interface, and the firewall rules blocks all off-VPN traffic (except traffic to the VPN endpoint itself)

The tunnel stays up because the tunnel connection is over the physical interface. The VPN endpoint has a physical route from the host to the VPN endpoint which is whitelisted in the firewall. So new physical routes (which option 121 would push) don't impact anything as VPN endpoint route is physical anyway. Also it's impossible for the DHCP server to push a route MORE specific than the endpoint route (which is a /32) that already exists, so it can't be overridden (and it wouldn't matter anyway since it would still be a physical route, which is what is desired here).

Can you stop just talking and actually TRY it? it's all theoretical for you since you're not actually testing it and your theory is completely wrong.

I am actually testing it, so i know I am correct.