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Sierra (macOS 10.12) uses OpenSSH v7.2 (El Capitan used OpenSSH v6.9) which no longer supports some of the older, less secure algorithms by default. If an SSH connection is refused with one of the following errors, the fix is to re-enable them in ssh_config.
Unable to negotiate with x.x.x.x port 22: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1Unable to negotiate with x.x.x.x port 22: no matching host key type found. Their offer: ssh-dss
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