Workaround for broken vagrant up ssh "unsupported encryption type"

Vagrant is still not playing nicely with SSH certificates loaded into ssh-agent. In my case this seemed to only be a problem during provisioning (i.e. "vagrant up"), using "vagrant ssh" after the box was up worked fine. The error is:

The private key you're attempting to use with this Vagrant box uses
an unsupported encryption type. The SSH library Vagrant uses does not support
this key type. Please use `ssh-rsa` or `ssh-dss` instead. Note that
sometimes keys in your ssh-agent can interfere with this as well,
so verify the keys are valid there in addition to standard
file paths.

You can try clearing out some keys from ssh agent with:

$ ssh-add -D
All identities removed.

Except ssh is probably lying if you're running goobuntu, the keys are still there. There's all sorts of confusion about this behaviour, which seems to be the fault of gnome-keyring, which allegedly only allows you to delete manually added keys. If your SSH certs are automatically loaded it seems like you're out of luck.

By far the easiest workaround is to simply temporarily disable the ssh agent:

SSH_AUTH_SOCK="" vagrant up