第2步：配置openssh服务器

默认配置文件：

/etc/ssh/sshd_config

打开配置文件进行更改：

sudo nano /etc/ssh/sshd_config

以#开头的行表示注释：

#       $OpenBSD: sshd_config,v 1.100 2014/08/15 12:32:04 naddy Exp $
# This is the sshd server system-wide configuration file.  See
# default value.
Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::
# Authentication:
#LoginGraceTime 2m
#PermitRootLogin prohibit-password
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10
#PubkeyAuthentication yes
# Expect .ssh/authorized_keys2 to be disregarded by default in future.
#AuthorizedKeysFile     .ssh/authorized_keys .ssh/authorized_keys2
#AuthorizedPrincipalsFile none
#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody
# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes
# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no
# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no
# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes
#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
PrintMotd no
#PrintLastLog yes
#TCPKeepAlive yes
#UseLogin no
#UsePrivilegeSeparation sandbox
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#UseDNS no
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none
# no default banner path
#Banner none
# Allow client to pass locale environment variables
AcceptEnv LANG LC_*
# override default of no subsystems
Subsystem       sftp    /usr/lib/openssh/sftp-server
# Example of overriding settings on a per-user basis
#Match User anoncvs
#       X11Forwarding no
#       AllowTcpForwarding no
#       PermitTTY no
#       ForceCommand cvs server

第3步：进行基本的修改

OpenSSH默认设置允许密码身份验证。
要禁用，请将行更改为：

PasswordAuthentication  no

要完全禁用root用户，请将行更改为：

PermitRootLogin  no

默认情况下，root用户没有密码设置，它会自动禁用该帐户。
但是当在root帐户上设置密码时，如果PermitRootLogin没有禁用，那么root将可以通过ssh登录。

修改后，需要重新启动OpenSSH服务器使设置生效。

sudo systemctl restart ssh

第1步：安装OpenSSH服务器

通过运行下面的命令，可以在Ubuntu上安装OpenSSH服务器。

sudo apt-get update
sudo apt-get install openssh-server openssh-client

上面的命令将安装服务器并配置其默认设置，其中有帐户访问的人可以使用有效密码登录系统。
这是作为密码身份验证。

除root用户外，任何人都可以使用有效的帐户名称和密码登录。