Fixing IPSec VPN NAT Issue Once and For All

In this comprehensive guide, we'll walk you through the challenges and solutions for setting up an IPSec VPN when it's located behind a Network Address Translation (NAT) device.
We start by explaining why IPSec VPNs face issues when behind NAT, including the intricacies of IP address translation and how it affects VPN tunnels.Discover the concept of NAT Traversal and how it helps IPSec VPNs to work seamlessly behind NAT. We'll discuss how NAT-T encapsulates IPSec packets in UDP to bypass the translation issues.
By the end of this video, you'll have a solid understanding of how to configure and troubleshoot IPSec VPN in environments where NAT is present. Whether you're a network administrator or an enthusiast looking to secure your home network, this tutorial has you covered.

[@staticroute]

Hey everyone, this has been the second video on the VPN topic, I value your feedback, let me know your thoughts...!

[@pouyasaberi3359]

Thanks. You have explained so simple

[@bph3649]

Not many people can explain clearly like this, good job!

[@staticroute]

Thank you!

[@Rejo-ni3hz]

You're better than my teachers

[@staticroute]

Thank you @Rejo-ni3hz, I try to be rooted in theory but apply practical application so that anyone can easily understand, I’m glad the content is achieving that…😀 thank you for being part of this community..

[@jayanvv-oi8hp]

Could you please share packet flow in fortigate firewall

[@staticroute]

Yea I’m definitely doing a video on that soon…

[@mrmendes4ever]

i have a challenge. A tunnel has failed to come up between Fortigate and Linux server running strongSwan. The Fortigate has NAT-T enabled and they are translating their external IP from Private to Public. Can you assist.

[@staticroute]

Hi @mrmendes4ever, I assume you have NAT-T enabled on the StrongSwan as well?
From Fortigate try to run the following and observe output:
1. get vpn ipsec tunnel summary
we are interested in status: selectors(total,up)..
2. diagnose sniffer packet any 'host x.x.x.x' 4
we want to see bidirectional IKE exchange, be sure to use the public address of the StrongSwan.
3. diagnose vpn ike gateway list name "tunnel-name" or simply diagnose vpn ike gateway list if there's only 1 tunnel
The idea is to see what status phase 1 tunnel is in: connecting or Established.
Then we can take it from there..

[@staticroute]

Assuming the 2 devices are in fact correctly exchanging IKE and UDP/500 UDP/4500 and ESP are not blocked anywhere, try this to see what the peers are disagreeing on:
- diagnose debug application ike -1
observe the output and hopefully this leads us to the root cause.
Best of luck!