Sometimes I work with very old IT server infrastructure, but this should not be the topic of today. We focus on the client side today, where it happens from time to time that companies and people from various fields, such as old client software based on Java 5, 6, 7, or 8, are bound to this kind of legacy for other reasons.
As an operator of services such as SFTP, this really is not ideal. You either have to exclude such clients to maintain an up-to-date server infrastructure, or include them and provide some form of backwards compatibility.
To test that compatibility, you would need to either have all clients available that your peers use or at least be able to simulate them. I happened to have issues with clients that require, by nowadays’ standards, very short Diffie-Hellman parameters to be supplied by the server. Java 5, 6, 7, 8 are limited to 1024 bit of length.
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