On Monday, 11 March 2019 05:41:19 GMT Philip Webb wrote:

[snip ...]
> However, I've tried to insert an instruction in config files,
> but nothing changes after a reboot.
> I've tried adding to  ~/.ssh/config  &  /etc/ssh/ssh_config :
> 
>   Host 128.100.160.1
>     KexAlgorithms +diffie-hellman-group1-sha1
> 
> That is what seems to be required by 'man 5 ssh_config'.
> 
> Can anyone suggest what + where to tell Ssh to do it every time ?

You probably have more than one User and identity file and you could define 
them both in .ssh/config to make sure the correct user is invoked, without 
having to add it to the CLI:

Host 128.100.160.1
User my_remote_ssh_user
IdentityFile /home/<username>/.ssh/id_rsa
KexAlgorithms +diffie-hellman-group1-sha1

-- 
Regards,
Mick