A VPN passthrough is a router functionality that allows certain VPN traffic to traverse the router’s firewall and Network Address Translation (NAT) system without being blocked. It supports older VPN protocols that aren’t fully compatible with NAT rules on modern routers. This article explains what VPN passthrough is, how it operates, whether it’s still necessary, and how to enable or disable it.
What Is a VPN Passthrough?
A VPN passthrough is a feature built into many routers. Its purpose is to allow VPN connections—especially those that use older protocols—to pass through the router’s firewall and NAT without disruption. Essentially, it identifies specific VPN-related traffic and allows it through, rather than letting the router block or drop it.
It’s important to distinguish between a VPN passthrough and a VPN router:
- VPN Passthrough lets VPN traffic pass through an existing router so the VPN client (on your computer or device) can connect to a VPN server externally.
- VPN Router is a device that builds the VPN connection itself and usually encrypts traffic for all devices connected through it.
How VPN Passthrough Works
To understand how VPN passthrough operates, one must first understand NAT. Routers typically use NAT to map multiple devices on a private network to a single public IP address. NAT needs to track outgoing and incoming traffic to make sure responses go back to the correct device.
Older VPN protocols—such as PPTP, L2TP, or certain implementations of IPsec—don’t always work smoothly with NAT because they might mask or encapsulate information that NAT uses to properly track connections. Without passthrough, such protocols may fail to establish a connection through a router.
Common Protocols That May Require Passthrough
| Protocol | Description | Why Passthrough May Be Needed |
|---|---|---|
| PPTP (Point-to-Point Tunneling Protocol) | One of the oldest VPN protocols. | Has known security vulnerabilities and often doesn’t provide enough NAT‐compatible information. |
| L2TP (Layer 2 Tunneling Protocol, often with IPsec) | Uses dual encapsulation and is more secure than PPTP when paired with encryption. | Extra encapsulation can interfere with NAT unless router is aware of the protocol. |
| IPsec (in older modes) | Secures data at the IP packet level; widely used in corporate and private VPNs. | Certain IPsec modes (e.g. transport mode) may not traverse NAT properly without passthrough or configuration workarounds. |
Modern VPN Protocols That Usually Don’t Require Passthrough
- OpenVPN – Transmits over TCP or UDP and handles NAT well.
- WireGuard – Lightweight, efficient, modern, and designed for compatibility with NAT.
- IKEv2/IPsec (in NAT‐friendly modes) – Newer implementations often include NAT traversal (NAT‐T) or encapsulation to handle NAT smoothly.
How to Enable VPN Passthrough
If you have devices or services that rely on older VPN protocols, you might need to enable passthrough on your router. General steps are:
- Log into your router’s control panel (often via a web interface).
- Locate VPN settings—these might be under sections like “VPN Passthrough”, “Virtual Server”, “Advanced Networking”, or similar.
- Find toggles for PPTP passthrough, L2TP passthrough, and/or IPsec passthrough, and enable them as needed.
- Save settings and reboot the router (if required) so changes take effect.
When You Do (and Don’t) Need VPN Passthrough
- You need it if you use outdated VPN protocols that cannot traverse NAT or firewall rules on modern routers properly.
- You likely don’t need it if you use newer, secure VPN protocols (e.g. OpenVPN, WireGuard, modern IPsec) because they’re built to work with router NAT/firewall setups automatically.
Should You Disable VPN Passthrough?
Disabling passthrough features can be a security-conscious decision if none of your VPN clients rely on older, insecure protocols. Turning off PPTP/L2TP/IPsec passthrough (if not needed) reduces potential attack surface. However, be mindful that devices or applications depending on those protocols will stop working correctly if passthrough is disabled.
Summary
VPN passthrough is a router feature designed to accommodate older VPN protocols that struggle with NAT or firewall configurations. While it was essential years ago, advancements in protocol design (WireGuard, modern implementations of IPsec, OpenVPN) have reduced its significance. Unless your setup requires legacy protocol support, enabling passthrough may be unnecessary—and in some cases, disabling it may improve security.
