LPIC-2 Advanced Networking Configuration Practice Question
A Linux administrator is setting up an IPsec VPN between two sites using strongSwan. The VPN established successfully, but traffic between the sites is not being encrypted. What is the most probable cause?
⚠ Common exam trap
A common mix-up: candidates assume a successful IKE/SA establishment guarantees encryption for all traffic, but LPIC-2 tests the understanding that IPsec policies (SPD selectors) independently control which packets are encrypted, and a mismatch leaves traffic unencrypted.
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
✓
The IPsec policies are not configured to match the traffic.
The most probable cause for traffic not being encrypted after a successful IPsec tunnel establishment is that the IPsec policies (e.g., strongSwan's `ipsec.conf` `conn` section with `leftsubnet`/`rightsubnet`) do not match the actual traffic flows. Even if the IKE/SA is up, strongSwan only applies encryption to packets that match the configured selectors; mismatched subnets or protocols result in cleartext traffic bypassing the tunnel.
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 IPsec policies are not configured to match the traffic.
Why this is correct
The SPD defines which traffic should be encrypted; without matching policies, traffic flows in plain text.
- ✗
The firewall is blocking UDP port 500.
Why it's wrong here
UDP 500 is needed for IKE; if it were blocked, the VPN would not establish.
- ✗
The kernel does not support IPsec (net.ipv4.ip_forward=0).
Why it's wrong here
ip_forward is not related to IPsec; it controls routing, not encryption.
- ✗
The IPsec daemon is not started.
Why it's wrong here
If the daemon were not started, the VPN would not establish at all.
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 LPIC-2 practice question is part of Courseiva's free LPI 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 LPIC-2 exam.