Short answer: VPN passthrough is a router setting that allows VPN traffic using older protocols (IPsec, PPTP, L2TP) to pass through your router's firewall without being blocked. If you're using a modern VPN protocol like WireGuard or OpenVPN, you probably don't need it.
The NAT Problem
Your router uses NAT (Network Address Translation) to let multiple devices share one public IP address. NAT tracks connections using IP addresses and port numbers. When data comes back from the internet, NAT knows which device to send it to based on these identifiers.
Here's the problem: older VPN protocols like PPTP and IPsec don't play nice with NAT. They were designed before NAT became standard, and they either don't use port numbers or encrypt them in ways that confuse NAT.
Without VPN passthrough, your router sees VPN traffic and doesn't know what to do with it. The connection fails.
What VPN Passthrough Actually Does
VPN passthrough is not a VPN connection. It doesn't encrypt anything. It just tells your router: "When you see traffic that looks like VPN protocol X, let it through instead of blocking it."
The actual VPN connection still happens between your device and the VPN server. Passthrough just removes the roadblock.
Types of VPN Passthrough
PPTP Passthrough
PPTP (Point-to-Point Tunneling Protocol) uses TCP port 1723 for control and GRE (Generic Routing Encapsulation) for the actual tunnel. GRE doesn't use port numbers at all, which breaks NAT.
PPTP passthrough assigns a "Call ID" to GRE traffic so your router can track it like a port number.
PPTP is Broken
PPTP's encryption has been cracked since the late 1990s. Microsoft deprecated it years ago. Don't use PPTP for anything requiring actual security. If you need PPTP passthrough, you're probably connecting to legacy infrastructure that should be upgraded.
IPsec Passthrough (NAT-T)
IPsec is a serious encryption protocol still used in enterprise environments. But it also has NAT problems: it encrypts packet headers in a way that breaks NAT tracking.
The solution is NAT-Traversal (NAT-T), which wraps IPsec packets inside UDP packets on port 4500. Your router can handle UDP normally, and the IPsec payload stays encrypted inside.
IPsec passthrough enables NAT-T on your router.
L2TP Passthrough
L2TP (Layer 2 Tunneling Protocol) is usually paired with IPsec for encryption (L2TP/IPsec). It has similar NAT issues and uses a Session ID to help routers track connections.
Do You Need VPN Passthrough?
Probably not. Here's why:
Modern Protocols Don't Need It
WireGuard, OpenVPN, and IKEv2 were designed with NAT in mind. They use standard UDP or TCP ports that routers handle without any special configuration.
| Protocol | Needs Passthrough? | Notes |
|---|---|---|
| WireGuard | No | Uses UDP, NAT-friendly by design |
| OpenVPN | No | Uses UDP or TCP, works through NAT |
| IKEv2 | No | Built-in NAT-T support |
| IPsec | Sometimes | Needs NAT-T, usually enabled by default |
| L2TP/IPsec | Sometimes | Depends on router configuration |
| PPTP | Yes | Don't use PPTP |
Most Routers Have It Enabled by Default
Unless you're using very old hardware, VPN passthrough is probably already enabled on your router. It's been a standard feature for over a decade.
Consumer VPN Services Work Without It
If you're using Mullvad, ProtonVPN, NordVPN, or any other consumer VPN service, they use modern protocols that don't need passthrough. Just install the app and connect.
When You Might Need It
The main scenario where VPN passthrough matters:
- Connecting to legacy corporate VPNs: Some older enterprise VPN systems still use pure IPsec without NAT-T, or even PPTP (god help you)
- Hosting an IPsec VPN server behind NAT: If you're running your own VPN endpoint and clients connect via IPsec
- Very old router firmware: Some ancient routers shipped with passthrough disabled
Security Implications
VPN passthrough itself isn't a major security risk, but there are considerations:
The Good
- Passthrough doesn't weaken your router's firewall for normal traffic
- VPN traffic is still encrypted end-to-end
- Modern routers handle passthrough safely
The Concerns
- Opens ports that could be exploited: PPTP passthrough keeps TCP 1723 available for GRE traffic
- Enables insecure protocols: If passthrough is enabled, someone on your network could use PPTP (which is broken)
- No deep packet inspection: Your router can't inspect what's inside the encrypted VPN tunnel
Best Practice
If you don't need passthrough for legacy protocols, disable it. Specifically:
- PPTP Passthrough: Disable unless you have a specific legacy requirement
- IPsec Passthrough: Keep enabled if you use any IPsec-based VPN
- L2TP Passthrough: Keep enabled if you use L2TP/IPsec
How to Find VPN Passthrough Settings
VPN passthrough settings are usually buried in your router's firewall or security section:
- Log into your router's admin panel (usually 192.168.1.1 or 192.168.0.1)
- Look for: Security, Firewall, VPN, or Advanced Settings
- Find "VPN Passthrough" or individual protocol toggles (PPTP, IPsec, L2TP)
Common router brands and where to find it:
- Linksys: Security → VPN Passthrough
- Netgear: Advanced → WAN Setup → NAT Filtering
- ASUS: VPN → VPN Support
- TP-Link: Advanced → NAT Forwarding → VPN Passthrough
Troubleshooting VPN Connections
If your VPN won't connect, passthrough is rarely the problem. Check these first:
- Use a modern protocol: Switch to WireGuard or OpenVPN instead of IPsec/PPTP
- Check if VPN ports are blocked: Some ISPs or networks block common VPN ports
- Try TCP instead of UDP: OpenVPN over TCP on port 443 looks like HTTPS and is rarely blocked
- Update router firmware: Old firmware may have VPN compatibility bugs
- Check double NAT: If you have a router behind another router (like an ISP modem), NAT issues multiply
If you've exhausted other options and you're using IPsec, then check if passthrough is enabled.
The Bottom Line
VPN passthrough is a legacy feature that solved problems from the pre-NAT-T era. Modern VPN protocols don't need it. If you're using a consumer VPN service with WireGuard or OpenVPN, you can ignore passthrough entirely.
The only people who need to care about this setting are:
- IT admins managing legacy enterprise VPNs
- People hosting their own IPsec VPN servers
- Anyone troubleshooting connections to very old VPN infrastructure
For everyone else: use WireGuard, forget passthrough exists.
Related Guides
- VPN Strategy Guide: Choosing the Right VPN
- Build Your Own VPN for Free
- What is a VPN Concentrator?
- Encrypted DNS Comparison
References
- NordVPN - What is a VPN passthrough?
- TechTarget - How do I disable VPN passthrough?
- Check Point - VPN Passthrough and IPsec Passthrough
- Surfshark - VPN passthrough and IPSec passthrough
Last updated: December 2025