What is a VPN Passthrough? How Does It Work?

In today’s world, VPNs are essential for privacy, secure browsing, and remote work. But if you’ve ever tried to set up a VPN on a home network, you may have read or seen something called VPN Passthrough with an option to turn it on and off.

What is it, why do you need it, and how does it work?

In this article, I’ll explain everything in detail — from protocols to troubleshooting — so you can fully understand VPN passthrough.

What Does “Passthrough” Mean?

A VPN passthrough is a feature on routers and firewalls that allows devices on your local network to initiate VPN connections to external VPN servers.

It doesn’t turn your router into a VPN server; instead, it ensures that VPN traffic can pass through your router without being blocked or corrupted by Network Address Translation (NAT) or firewalls.

Why NAT Breaks Some VPNs?

Most home and small-business routers use NAT to allow multiple devices to share a single public IP address. While NAT works fine for regular internet traffic, it can cause problems for VPNs because:

• VPNs may embed IP headers that NAT alters.
• NAT can break protocol integrity checks.
• Firewalls may drop unrecognized VPN traffic.

Without passthrough, VPN connections like PPTP, L2TP/IPsec, and IPsec may fail to establish or maintain a stable connection.

VPN Protocols and Their Passthrough Needs.

Not all VPNs require passthrough. Here’s a breakdown:

VPN Protocol	Port / Protocol	Passthrough Needed?
PPTP	TCP 1723 + GRE	Yes (router must forward GRE)
L2TP/IPsec	UDP 1701 + ESP + UDP 500	Yes (IPsec passthrough / NAT-T)
IPsec	ESP (50) + IKE (UDP 500)	Yes (NAT-T if NAT exists)
SSTP	TCP 443	Usually no (uses HTTPS)
OpenVPN	TCP/UDP configurable (default UDP 1194)	No (NAT handles UDP/TCP)
WireGuard	UDP configurable	No (NAT handles it)

How VPN Passthrough Works?

Routers use several techniques to allow VPN traffic to traverse NAT:

1. NAT Traversal (NAT-T).

When a router sits between two IPsec peers, it may detect NAT and encapsulate ESP packets inside UDP (UDP 4500). This allows NAT devices to maintain proper packet mapping and ensures the VPN traffic reaches the correct internal client.

2. Application Layer Gateway (ALG).

An ALG inspects VPN control packets and dynamically opens NAT mappings:

• PPTP: ALG monitors TCP 1723 and forwards GRE (protocol 47).
• IPsec: ALG watches IKE (UDP 500) and ESP, enabling NAT-T when needed.

3. Port Forwarding.

For VPN servers hosted inside your LAN, port forwarding is used instead of passthrough. This ensures incoming VPN connections reach the right device.

4. Stateful Firewall Awareness.

Routers maintain connection states for VPN packets. Passthrough ensures the firewall correctly associates returning VPN traffic with the original internal client.

Router vs Client vs Server.

• Client-to-Server (Remote Access): Passthrough allows your device to connect to an external VPN server.
• Site-to-Site (Router-to-Router): Intermediate NAT routers must allow outbound VPN traffic; passthrough is often required.
• VPN Server on Router: Passthrough is not needed. Instead, configure port forwarding and firewall rules.

When You Don’t Need VPN Passthrough?

• Using NAT-friendly VPN protocols like OpenVPN or WireGuard.
• Running a VPN server on your router.
• Using IPv6, which eliminates NAT-related problems.

How to Enable VPN Passthrough?

Most modern routers have passthrough enabled by default. For older routers:

1. Log into your router admin panel (usually at 192.168.0.1 or 192.168.1.1).
2. Navigate to Security, Firewall, or VPN Settings.
3. Enable options like:
   • PPTP Passthrough
   • IPsec Passthrough
   • L2TP Passthrough
4. Save settings and reboot the router.

Tip: Update your router’s firmware to ensure the latest VPN passthrough fixes are applied.

How to Test if VPN Passthrough is Working?

1. Connect a device on your LAN to the VPN.
2. Check VPN client logs for:
   • IKE negotiation success
   • ESP establishment
   • NAT-T usage (UDP 4500)
3. Verify your public IP changes using an online IP check tool.

Common Problems & Troubleshooting.

Issue	Possible Fix
VPN never connects	Enable passthrough, check ISP blocks, avoid double NAT, update firmware
VPN connects but is slow	Lower MTU to 1400, check router CPU usage, try modern VPN protocols
One client works, others don’t	Reboot router, enable ALGs, check conflicting port forwarding
Site-to-site VPN fails	Ensure NAT-T (UDP 4500), static IPs, or proper port forwarding

Security Implications.

• Passthrough is generally secure as it only allows VPN traffic; encrypted data remains private.
• Avoid outdated protocols like PPTP, which are insecure.
• Enable only required passthrough protocols.
• Use router-level VPN or NAT-T capable firewalls for better security.

Alternatives and Best Practices.

• Use OpenVPN or WireGuard for NAT-friendly VPN traffic.
• Configure the VPN on your router to protect all devices.
• Consider IPv6 networks to remove NAT issues.
• Keep firmware updated for security and stability.

FAQs.

Q: Is VPN passthrough secure?

A: Yes, it only forwards VPN traffic. Avoid using PPTP; prefer modern protocols like OpenVPN or WireGuard.

Q: My router doesn’t show passthrough options. What should I do?

A: Update firmware, use OpenVPN/WireGuard, or temporarily set a DMZ for troubleshooting.

Q: Does my ISP need to support passthrough?

A: Usually no. Passthrough is a router function, but some ISPs block GRE or UDP 500/4500 traffic.

Q: Why does my VPN use UDP 4500?

A: That’s NAT-T. IPsec detected NAT and encapsulated ESP inside UDP to traverse it.

Troubleshooting Checklist.

1. Reboot router and client.
2. Enable VPN passthrough in router settings.
3. Avoid double NAT (bridge upstream router if needed).
4. Update router firmware.
5. Disable SPI firewall temporarily for testing.
6. Use OpenVPN/WireGuard on nonstandard ports.
7. Check client logs for IKE, ESP, NAT-T messages.
8. Lower client MTU to 1400.
9. Ensure correct port forwarding for VPN server inside LAN.

Conclusion.

VPN passthrough is a small but critical feature for legacy VPN protocols or NAT-heavy networks. It allows encrypted traffic to traverse routers without interruption. For modern VPN setups, protocols like OpenVPN or WireGuard make passthrough less critical. If you want to protect your entire network in a more controlled and self-managed way, consider running the VPN on your router itself.