VPN Kill Switches Explained (and What They Miss)

What a VPN Kill Switch Actually Does

A kill switch is not magic. It’s a feature—sometimes built into a VPN app, sometimes part of your OS or firewall—that stops your device from sending data over the internet if the encrypted tunnel to your VPN server goes down. Its job is simple: prevent accidental exposure of your real IP address and location when the connection fails. Without it, your traffic could spill out through your regular ISP connection the moment the VPN drops, potentially exposing your identity, location, or activity.

But here’s the reality: a kill switch doesn’t make you anonymous. It doesn’t encrypt your data beyond what the VPN already does. It doesn’t stop tracking scripts, ads, or malware. It’s a fail-safe, not a privacy shield. If your threat model includes someone monitoring your network—like an ISP, employer, or government—then a working kill switch reduces the risk of exposure during brief disconnections. But if you’re relying on it as your only defense, you’re missing the bigger picture.

There are two main ways kill switches work: through firewall rules or by manipulating routing tables. The firewall method actively blocks traffic using system-level packet filtering. The routing method removes or alters the default route so that no traffic can leave except through the VPN interface. Both have trade-offs in reliability, speed, and compatibility.

How Kill Switches Work: Firewall vs. Routing

Firewall-based kill switches, like those used by IVPN [2], rely on the operating system’s built-in firewall (such as pf on macOS, nftables or iptables on Linux, or Windows Firewall) to block all non-VPN traffic. When the kill switch is enabled, the firewall is configured to allow traffic only through the encrypted tunnel (e.g., the tun0 interface). If the tunnel goes down, the firewall rules remain active, blocking everything else. This method is generally more reliable because it doesn’t depend on the routing stack staying intact—if the rules are applied correctly, traffic simply cannot leave.

Routing-based kill switches, on the other hand, work by changing the system’s routing table. They remove the default gateway (the route that tells your device where to send traffic when no specific route exists) and replace it with a route that points only to the VPN server. If the connection drops, the default route isn’t restored, so the system has nowhere to send traffic. This method is faster to implement in software but can be fragile. A background process or network manager might override the routing changes, especially on mobile or multi-network environments.

Neither method is foolproof. Firewall rules can be bypassed by misconfiguration or privilege escalation. Routing changes can be undone by network managers like NetworkManager on Linux or automatic reconnection tools. And both can fail silently—meaning you think you’re protected when you’re not. That’s why testing matters.

Blocking All Traffic vs. Only VPN Traffic

Some kill switches are “smart” enough to only block traffic that isn’t going through the VPN, while others cut off all internet access. The difference sounds minor, but it has real implications.

A partial block allows local network traffic (like printing or file sharing) while blocking internet-bound packets that aren’t routed through the tunnel. This is convenient if you work from home and need access to shared drives or smart devices. But it increases the attack surface. If your local network is compromised, an attacker could intercept traffic or use your device as a pivot point.

A full block disables all network activity when the VPN drops—no internet, no local network. This is more secure, especially in public Wi-Fi environments where rogue devices are common. But it’s also disruptive. If the VPN disconnects for a few seconds, your entire connection dies, and you have to manually reconnect.

Most commercial VPNs default to full blocking, but not all. Some apps let you choose. If privacy is your goal, go with full blocking. Convenience should never outweigh exposure risk when your real IP is on the line.

What Kill Switches Miss: IPv6, DNS, and WebRTC

Even a perfectly implemented kill switch can’t protect you from everything. Three major leak vectors often go unaddressed: IPv6, DNS, and WebRTC.

IPv6 leaks happen when your ISP assigns you an IPv6 address, but your VPN only routes IPv4 traffic. If the kill switch doesn’t account for IPv6, your device might still communicate over IPv6 outside the tunnel. Since IPv6 adoption is growing, this is a real risk. According to Wikipedia, IPv6 uses 128-bit addresses and was designed to replace IPv4 due to address exhaustion [3]. Many kill switches still focus only on IPv4, leaving IPv6 traffic exposed.

DNS leaks occur when your system sends DNS queries (which translate domain names like google.com into IP addresses) outside the encrypted tunnel. This can reveal your browsing history even if the rest of your traffic is encrypted. A good kill switch should block all DNS traffic not routed through the VPN, but not all do. Some apps rely on the OS’s DNS settings, which can be overridden by network managers or malicious software.

WebRTC leaks are browser-specific. WebRTC (Web Real-Time Communication) is a protocol that allows browsers to establish direct peer-to-peer connections—for video calls, file sharing, etc. But it can also expose your real IP address, even when you’re using a VPN. This isn’t a failure of the kill switch; it’s a failure of the browser. The kill switch operates at the system level, but WebRTC operates within the browser sandbox. To stop WebRTC leaks, you need browser extensions or settings that disable or block the feature—like turning off media.peerconnection.enabled in Firefox.

None of these are fixed by a kill switch alone. You need layered defenses: disable IPv6 if your VPN doesn’t support it, use a VPN with DNS leak protection, and harden your browser.

How to Test Your Kill Switch

Don’t trust the marketing. Test it yourself.

Start by visiting a leak test tool like our leak checker. It checks for IPv4, IPv6, DNS, and WebRTC leaks. Run it while connected to your VPN to confirm everything is hidden. Then, simulate a disconnect.

Here’s how:

1. Connect to your VPN.
2. Open the leak test in a browser tab.
3. Manually disconnect the VPN (don’t close the app—disconnect).
4. Refresh the page.

If your real IP appears, your kill switch failed. If the page won’t load at all, the kill switch is working. But that’s not enough.

Next, test reconnection behavior. After disconnecting, reconnect the VPN. Does the leak test still show your real IP? Some kill switches fail to restore firewall rules or routing tables properly after a reconnect, leaving you exposed even when the app says you’re protected.

Also, test background behavior. Leave the VPN connected overnight. Use a script or tool to monitor your IP at intervals. A kill switch should remain active even after system sleep, Wi-Fi switching, or app updates.

Finally, test on multiple networks—home, mobile hotspot, public Wi-Fi. Network managers behave differently in each environment, and your kill switch might not survive the transition.

Why “No Logs” Isn’t Enough

A kill switch protects against connection drops. But it doesn’t protect you from the VPN provider itself. If the company keeps logs—connection timestamps, IP addresses, bandwidth usage—then your privacy depends on their honesty and security.

“No logs” is a claim, not a fact. Until it’s audited or tested in court, treat it as marketing. Some providers, like Mullvad and IVPN, have undergone independent audits or faced legal challenges without producing user data [2]. Others make the claim but offer no proof.

Even with a perfect kill switch, you’re trusting the provider with your traffic. If they’re compromised, subpoenaed, or malicious, your data could still be exposed. That’s why jurisdiction matters. A provider based in a Five Eyes country may be legally required to cooperate with surveillance requests. Check where the company is incorporated, what laws apply, and whether they’ve published transparency reports.

And remember: a kill switch doesn’t stop malware, phishing, or social engineering. It doesn’t hide your behavior from websites. It’s one piece of a larger privacy strategy—not the whole thing.