Sometimes we need to share our ~/.ssh/config with other host (or even a docker container) that might not be understanding all the options. The usual case is with UseKeychain on MacOS that won’t work on Linux. To fix that we add IgnoreUnknown UseKeychain and move on.

If you have multiple Host entries in the config, it might happen you will need for multiple entries. But adding it per host will just confuse SSH and you will end up with errors even if IgnoreUnknown is used for each of that.

Solution to this is to move IgnoreUnknown to the top of the config file, outside any host scope. So instead of

Host alpha
  IgnoreUnknown UseKeychain
  User alchemyx
Host beta
  IgnoreUnknown UseKeychain
  User root

just use

IgnoreUnknown UseKeychain
Host alpha
  User alchemyx
Host beta
  User root