ubuntu-to-ubuntu ssh connection

ubuntu has pre-installed ssh client but not server.

ref:  https://ubuntu.com/server/docs/service-openssh

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# ENABLE SSH #

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To have the REMOTE ubuntu accepting ssh connection, install ssh server:

    $ sudo apt update

    $ sudo apt upgrade 

    $ sudo apt install openssh-server

The SSH service is automatically started.

In case ssh config changes are needed, for example, using port 5000 instead of default port 22, modify the config file:

    $ sudo nano /etc/ssh/ssh_config

then restart the server

    $ systemctl restart ssh.service

or

    $ systemctl restart sshd

or 

    $ sudo service sshd restart

LOCAL ubuntu for the first time ssh'ing to a new server, command prompts to confirm fingerprint.  

For comparison, show the fingerprint on the SSH server side (REMOTE ubuntu):

    $ ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key.pub

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# SKIP PASSWORD #

###################

If frequently SSH to a server, one could set up SSH key to avoid typing password for connection.

1. LOCAL, generate a key pair - private key and public key

    $ ssh-keygen -f /path/to/myserver.ppk

2. copy public key from LOCAL to REMOTE

    $ scp -i /path/to/myserver.ppk.pub  user@REMOTE

    # above command create or append to REMOTE   /home/user/.ssh/authorized_keys

3. connect with no password

    $ ssh -i /path/to/myserver.ppk  user@REMOTE

4. (optionally) disable password login

    With the ssh key set up properly, now we can disable regular password login.

    a. change in /etc/ssh/sshd_config, from 

        #PasswordAuthentication yes

        to

        PasswordAuthentication no

    b. restart service

        service ssh restart

#############################

# OTHER settings in sshd_config #

#############################

https://askubuntu.com/questions/869945/how-to-disable-password-and-root-ssh

• Deny all root login
• Deny all password logins for all users
• Allow other users with other authentication methods (publickey)

This is achieved using below configuration options:

    PermitRootLogin no
    PasswordAuthentication no
    ChallengeResponseAuthentication no

Then restart service

        service ssh restart

#################

# X11 Forwarding  #

#################

    1. REMOTE side to enable X11 Forwarding in ssh_config

    2. LOCAL side

            ssh -X -i /path/to/private_key  user@IP  "gvim ~/dummy.txt"

    or to remotely run local script with parameters:

        ssh user@REMOTE 'bash -s' < test.sh true true true