SSH/SCP connection error: Couldn't agree a key exchange algorithm on ESXi or vCenter

Products

• VMware vSphere ESX 8.x
• VMware vCenter Server 8.0
• VMware vCenter Server

Issue/Introduction

The following error message appears when attempting to connect:

Couldn't agree a key exchange algorithm (available: ecdh-sha2-n###256,ecdh-sha2-n###384,ecdh-sha2-n###521, diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,)

Environment

• VMware vSphere ESX 8.x
• VMware vCenter Server 8.x

Cause

This is expected behavior when using older versions of PuTTY, WinSCP, or other SSH clients that do not support the high-strength key exchange algorithms required by the ESXi or vCenter Server security configuration.

Resolution

To resolve this issue, perform the following steps:

1. Upgrade Client Software: Download and install the latest versions of your connection tools:
   • PuTTY Download Page
   • WinSCP Download Page
2. Test Alternate Hardware: Attempt the connection from a different computer or laptop to rule out local configuration issues.
3. Address SFTP Packet Size Errors: If you receive a "Received too large SFTP packet" error after upgrading WinSCP, refer to KB 326317.