# ssh yum.example.com
root@yum.example.com's password:
[root@yum ~]# exit
logout
Connection to yum.example.com closed.

# ssh remoteuser@yum.example.com
remoteuser@yum.example.com's password:

# ssh root@yum.example.com hostname
root@yum.example.com's password:
yum.example.com

# w -f
 17:22:20 up 19:04,  2 users,  load average: 0.00, 0.01, 0.05
USER     TTY        LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0     17:21    4.00s  0.04s  0.00s w -f
kylin    pts/1     17:21   36.00s  0.04s  0.04s -bash

# ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:S3l+TPKZdysNqHjnlESimBw87Cf3iyd9UNGEdnSt+hg root@registry.example.com
The key's randomart image is:
+---[RSA 2048]----+
|            =o ..|
|    o      + o. .|
|     =   ...o  . |
|    o = ..o.  .  |
|     * +S +.o.   |
|      +..=.*E+   |
|        +.+o*++ .|
|       o.=o+.o.o.|
|       .+.+.  .. |
+----[SHA256]-----+

# ssh-copy-id -i .ssh/id_rsa.pub yum.example.com
/bin/ssh-copy-id: INFO: Source of key(s) to be installed: ".ssh/id_rsa.pub"
/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
root@yum.example.com's password:

Number of key(s) added: 1

Now try logging into the machine, with:   "ssh 'yum.example.com'"
and check to make sure that only the key(s) you wanted were added.

# systemctl restart sshd
# systemctl status sshd