Difficulty: Easy
Correct Answer: You cannot establish an L2TP connection from behind a NAT gateway because the IPsec packets are altered by NAT and fail integrity checks
Explanation:
Introduction / Context:
Windows 2000-era Layer 2 Tunneling Protocol (L2TP) relies on IP Security (IPsec) Encapsulating Security Payload (ESP) for confidentiality and integrity. Traditional Network Address Translation (NAT) modifies IP headers and ports, which breaks ESP integrity because ESP protects the payload and has no ports to translate. NAT traversal (NAT-T) was not generally available in initial Windows 2000 deployments.
Given Data / Assumptions:
Concept / Approach:
IPsec ESP uses protocol number 50 and protects inner headers; classic NAT expects to translate TCP/UDP ports, not ESP. When NAT changes IP addresses, the integrity check value (ICV) fails on the remote end, causing tunnel setup to fail. Without NAT-T encapsulating IPsec in UDP (e.g., UDP/4500), an L2TP/IPsec session through NAT is not feasible in this scenario.
Step-by-Step Solution:
Verification / Alternative check:
Packet traces show IKE negotiation attempts followed by failed ESP packets when passing the NAT boundary. Removing NAT or switching to PPTP allows the connection, confirming the cause.
Why Other Options Are Wrong:
NAT does allow remote networking (e.g., web, PPTP, SSH).
L2TP works with Windows 2000 when not behind NAT.
“Configure NAT to translate IPsec” is not viable without NAT-T; ESP is not translatable in classic NAT.
None: A specific, well-known limitation exists.
Common Pitfalls:
Confusing L2TP with PPTP (which can pass classic NAT). Assuming any VPN type will survive NAT without NAT-T support.
Final Answer:
You cannot establish an L2TP connection from behind a NAT gateway because the IPsec packets are altered by NAT and fail integrity checks
Discussion & Comments