If you are trying to connect to older ssh servers or Mikrotik routers from Mac OS High Sierra you might encounter some of these error messages:
no matching host key type found. Their offer: ssh-dss
no matching cipher found. Their offer: aes192-cbc,aes128-cbc,aes256-cbc,blowfish-cbc,3des-cbc
DH GEX group out of range
If you can’t upgrade the router firmware (you really should do that first) then you can edit your client ssh config vim .ssh/config
and add these lines:
Host foo.example.com
HostKeyAlgorithms ssh-dss
KexAlgorithms diffie-hellman-group1-sha1
Ciphers +aes192-cbc