Explore common reasons for Phase 1 handshake failures in site-to-site VPN setups, including mismatched IKE settings, cipher suites, and protocol incompatibilities. Learn how to troubleshoot and resolve these issues for secure remote connections.
Table of Contents
Question
A security technician is trying to connect a remote site to the central office over a site-to-site VPN. The technician has verified the source and destination IP addresses are correct, but the technician is unable to get the remote site to connect. The following error message keeps repeating:
An error has occurred during Phase 1 handshake. Deleting keys and retrying…
Which of the following is most likely the reason the connection is failing?
A. The IKE hashing algorithm uses different key lengths on each VPN device.
B. The IPSec settings allow more than one cipher suite on both devices.
C. The Diffie-Hellman group on both sides matches but is a legacy group.
D. The remote VPN is attempting to connect with a protocol other than SSL/TLS.
Answer
The most likely reason for the connection failure in this scenario is:
A. The IKE hashing algorithm uses different key lengths on each VPN device.
Explanation
The error message “An error has occurred during Phase 1 handshake. Deleting keys and retrying…” indicates a problem during the initial negotiation phase of the VPN connection, specifically in Phase 1 of the Internet Key Exchange (IKE) protocol. This phase is responsible for establishing a secure channel between the two VPN endpoints before the actual VPN tunnel is created.
The IKE protocol uses various parameters to establish this secure channel, including encryption algorithms, hashing algorithms, and authentication methods. When these parameters don’t match on both sides of the connection, the Phase 1 handshake fails.
In this case, the most likely cause of the failure is that the IKE hashing algorithm is using different key lengths on each VPN device. The hashing algorithm is a critical component of the IKE negotiation, used for integrity checking and authentication. If the key lengths don’t match, the devices cannot successfully complete the handshake.
Let’s examine why the other options are less likely:
B. The IPSec settings allow more than one cipher suite on both devices.
This is not likely to cause a Phase 1 handshake failure. Having multiple cipher suites available can actually increase the chances of finding a compatible set of parameters.
C. The Diffie-Hellman group on both sides matches but is a legacy group.
While using a legacy Diffie-Hellman group might be a security concern, it wouldn’t typically cause a handshake failure if both sides are using the same group.
D. The remote VPN is attempting to connect with a protocol other than SSL/TLS.
This is incorrect because site-to-site VPNs typically use IPSec, not SSL/TLS. The error message specifically mentions Phase 1, which is part of the IKE protocol used in IPSec VPNs.
To resolve this issue, the security technician should:
- Check the IKE configuration on both VPN devices, particularly the hashing algorithm settings.
- Ensure that the key lengths for the hashing algorithm match on both sides.
- Verify other IKE parameters such as encryption algorithms and authentication methods are consistent across both devices.
- If necessary, consult the documentation for the specific VPN devices or software being used to confirm the correct configuration for IKE Phase 1 parameters.
By addressing the mismatch in the IKE hashing algorithm key lengths, the technician should be able to resolve the Phase 1 handshake error and successfully establish the site-to-site VPN connection.
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