NSE7 Troubleshooting and Diagnostics Practice Question
A FortiGate admin runs 'diagnose debug application sslvpn -1' and sees repeated messages: 'SSL VPN tunnel establishment failed: no response from client.' The remote user reports that the FortiClient VPN connects but no traffic passes. What is the MOST likely cause?
⚠ Common exam trap
Watch out — candidates often assume 'connects' means the entire VPN is working, when in reality FortiClient can show a connected status after the HTTPS phase completes even if the IPsec data tunnel fails due to UDP port blocking.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall on the remote user's network is blocking UDP port 4500 or TCP port 443
The 'no response from client' message indicates the FortiGate is not receiving the expected reply from the client during SSL VPN tunnel establishment. This typically occurs when a firewall on the remote user's network blocks UDP port 4500 (IPsec NAT traversal) or TCP port 443 (HTTPS control channel), preventing the FortiClient from completing the tunnel negotiation. The fact that the client 'connects' but no traffic passes suggests the initial SSL handshake succeeds, but the subsequent tunnel setup fails due to packet loss or filtering.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The SSL VPN realm is misconfigured
Why it's wrong here
Realm misconfiguration would usually cause authentication failure, not tunnel establishment failure after authentication.
- ✗
The SSL VPN certificate has expired
Why it's wrong here
An expired certificate would show a different error, such as certificate verification failure.
- ✗
The remote user's FortiClient version is incompatible
Why it's wrong here
Version incompatibility typically causes connection rejection, not silent failure after connection.
- ✓
A firewall on the remote user's network is blocking UDP port 4500 or TCP port 443
Why this is correct
SSL VPN tunnel establishment requires UDP 4500 (for NAT traversal) or TCP 443 for initial handshake. If blocked, the client cannot respond, leading to the 'no response from client' message.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This NSE7 practice question is part of Courseiva's free Fortinet certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NSE7 exam.