#################sshd services two computers can login without password ssh, which is a safer way.
sshd connection:
Host Connect Virtual Machine

[kiosk@foundation71 ~]$ ssh root@172.25.254.100 -X
root@172.25.254.100's password: 
Last login: Sun Aug 18 15:57:46 2019
[root@desktop ~]# 

########## The first link to an unfamiliar host is to establish an authentication file, so I will ask if it is established. I need to enter yes tree. When I link to this host again, because it has been generated.
#### The method of not requiring a key after loading ssh connection
server200:

[root@server .ssh]# ssh-keygen                               ##Commands to generate keys
Generating public/private rsa key pair.                      
Enter file in which to save the key (/root/.ssh/id_rsa):       ##Specify a file that holds encrypted characters (using default)
Enter passphrase (empty for no passphrase):                     ##Set a password (using an empty password)
Enter same passphrase again:                                    ##Confirm password
Your identification has been saved in /root/.ssh/id_rsa.       ##Private key (key)
Your public key has been saved in /root/.ssh/id_rsa.pub.        
The key fingerprint is:
17:a7:d2:68:ed:a4:80:a8:29:95:74:27:84:fc:4a:33 root@server
The key's randomart image is:
+--[ RSA 2048]----+
| . ..            |
|  o.             |
|  ..o .   . .    |
| .E+.+   + +     |
| .++. . S *      |
| +.    o *       |
|+       . .      |
|.                |
|                 |
+-----------------+
[root@server .ssh]# ssh-copy-id -i /root/.ssh/id_rsa.pub root@172.25.254.200            ##Encrypted sshd service
The authenticity of host '172.25.254.200 (172.25.254.200)' can't be established.
ECDSA key fingerprint is eb:24:0e:07:96:26:b1:04:c2:37:0c:78:2d:bc:b0:08.
Are you sure you want to continue connecting (yes/no)? yes
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/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@172.25.254.200's password:                          ##Native password Number of key(s) added: 1Now try logging into the machine, with:   "ssh 'root@172.25.254.200'"             ###Attempt to connect to the local machine with ssh
and check to make sure that only the key(s) you wanted were added.              ##Give you the key you want.[root@server .ssh]# ls                                                         ##There are two keys in it.
authorized_keys  id_rsa  id_rsa.pub  known_hosts

### Key issuance

[root@server .ssh]# scp /root/.ssh/id_rsa root@172.25.254.100:/root/.ssh/       ##Give the private key to 100 clients
The authenticity of host '172.25.254.100 (172.25.254.100)' can't be established.
ECDSA key fingerprint is eb:24:0e:07:96:26:b1:04:c2:37:0c:78:2d:bc:b0:08.
Are you sure you want to continue connecting (yes/no)? yes                       ##Input consent
Warning: Permanently added '172.25.254.100' (ECDSA) to the list of known hosts.
root@172.25.254.100's password:                                           ##Enter client password
id_rsa 

client100:

[root@desktop .ssh]# ssh root@172.25.254.200                  ##On the client side, we saw that the connection was asked once, but no password was needed.
The authenticity of host '172.25.254.200 (172.25.254.200)' can't be established.
ECDSA key fingerprint is eb:24:0e:07:96:26:b1:04:c2:37:0c:78:2d:bc:b0:08.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '172.25.254.200' (ECDSA) to the list of known hosts.
Last failed login: Sun Aug 18 17:56:36 EDT 2019 from 172.25.254.100 on ssh:notty
There were 2 failed login attempts since the last successful login.
Last login: Sun Aug 18 17:56:01 2019 from 172.25.254.100
[root@server ~]# 
[root@server ~]# exit
//Logout
Connection to 172.25.254.200 closed.
[root@desktop .ssh]# ls                                      ###In client. There is a secret key in ssh
id_rsa  known_hosts

############### ssh is so convenient that we can set up relevant settings in the configuration file to ensure the security of ssh links.
server200

[root@server .ssh]# vim /etc/ssh/sshd_config    ##In the configuration file, we can add the relevant functions we want.
48 PermitRootLogin yes|no	 ##Are root users allowed to authenticate through sshd services
52 Allowusers student westos	 ##Set a user whitelist, whitelist appears default no longer list users can not use sshd
53 Denyusers	westos		 ##Set user blacklist, blacklist appears default no longer list users can use sshd
78 PasswordAuthentication yes|no ##Is the user allowed to authenticate sshd with the password of the login system

########## In addition to setting information, we also want to set the information displayed when sshd is connected.
server200:

[root@server .ssh]# vim /etc/motd                   ##File entry
hello                                              ##Write down information
client100                                           ##Client
[root@desktop .ssh]# ssh root@172.25.254.200        ##ssh Connect Server
Last login: Sun Aug 18 18:01:34 2019 from 172.25.254.100
hello                                                 ##Displayed information
[root@server ~]# 

########## View ssh connections locally
1.w # View Users Using the Current System
- f # # View the Source of Use
- i # Display IP
/var/run/utmp
2.last # View User Information Used and Exit
/var/log/wtmp
3.lastb # Users who tried to log in but failed
/var/log/btmp