ssh: handshake failed: knownhosts: key mismatch

[trotterdylan]

This error can happen when performing git remote operations (e.g. a push) on a remote configured with ssh. It seems like this is caused by multiple github.com entries in known_hosts, one of which uses an unsupported key or algorithm: https://github.blog/2021-09-01-improving-git-protocol-security-github/

In this case the Git library (or the underlying SSH library) naively picks the first key, which may not work. Instead of trying other supported keys, the Dial call fails. This has been noted in go-git issue 411 and Golang issue 29286.

A PR that I believe addresses this has been written for go-git, but it hasn't yet been accepted: go-git/go-git#548

I'd rather not fork go-git and instead try to patch the way that we configure SSH auth, but it's not immediately clear how easy this will be.