SSH的英文全称是Secure SHell。通过使用SSH，你可以把所有传输的数据进行加密，这样“中间人”这种攻击方式就不可能实现了，而且也能够防止DNS和IP欺骗。还有一个额外的好处就是传输的数据是经过压缩的，所以可以加快传输的速度。SSH有很多功能，它既可以代替telnet，又可以为ftp、pop、甚至ppp提供一个安全的“通道”。

SSH在Linux中的服务是sshd，安装openssh后才可开启。CentOS 7 安装后默认情况下是不启动sshd服务，即无法通过ssh服务远程连接。
首先查看系统是否安装openssh，一般情况想都是默认安装了，

[ ~]# rpm -qa | grep ssh
libssh2-1.4.3-10.el7.x86_64
openssh-server-6.6.1p1-22.el7.x86_64
openssh-clients-6.6.1p1-22.el7.x86_64
openssh-6.6.1p1-22.el7.x86_64

如果没有安装可以通过yum在线安装。

[ ~]# yum install openssh

手动设置启动ssh服务

1、先检查确认有没有安装ssh-server服务器，输入命令：ps –e|grep ssh
2、在CentOS命令区输入：yum install openssh-server
3、启动、关闭、重启命令：
开启ssh服务：service sshd start

关闭ssh服务：service sshd stop

重启ssh服务：servcie sshd restart
4、开机自启动相关设置：
SSH服务开机自动启动：chkconfigsshd on

取消开机自启动:chkconfig sshd off

开启服务后，检查服务状态：service sshd status

[ ~]# vi /etc/ssh/sshd_config

# $OpenBSD: sshd_config,v 1.93 2014/01/10 05:59:19 djm Exp $

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/local/bin:/usr/bin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

# If you want to change the port on a SELinux system, you have to tell
# SELinux about this change.
# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER
#
#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

# The default requires explicit activation of protocol 1
#Protocol 2

# HostKey for protocol version 1
#HostKey /etc/ssh/ssh_host_key
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

# Lifetime and size of ephemeral version 1 server key
#KeyRegenerationInterval 1h
#ServerKeyBits 1024

# Ciphers and keying
#RekeyLimit default none

# Logging
# obsoletes QuietMode and FascistLogging
#SyslogFacility AUTH
SyslogFacility AUTHPRIV
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
#PermitRootLogin yes
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

#RSAAuthentication yes
#PubkeyAuthentication yes

# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2
# but this is overridden so installations will only check .ssh/authorized_keys
AuthorizedKeysFile      .ssh/authorized_keys

#AuthorizedPrincipalsFile none

#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#RhostsRSAAuthentication no
# similar for protocol version 2
#HostbasedAuthentication no
# Change to yes if you don‘t trust ~/.ssh/known_hosts for
# RhostsRSAAuthentication and 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
PasswordAuthentication yes

# Change to no to disable s/key passwords
#ChallengeResponseAuthentication yes
ChallengeResponseAuthentication no

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
#KerberosUseKuserok yes

# GSSAPI options
GSSAPIAuthentication yes
GSSAPICleanupCredentials no
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no
#GSSAPIEnablek5users no

# Set this to ‘yes‘ to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to ‘no‘.
# WARNING: ‘UsePAM no‘ is not supported in Red Hat Enterprise Linux and may cause several
# problems.
UsePAM yes

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
#PrintMotd yes
#PrintLastLog yes
#TCPKeepAlive yes
#UseLogin no
UsePrivilegeSeparation sandbox          # Default for new installations.
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#ShowPatchLevel no
#UseDNS yes
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none

# no default banner path
#Banner none

# Accept locale-related environment variables
AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
AcceptEnv XMODIFIERS

# override default of no subsystems
Subsystem       sftp    /usr/libexec/openssh/sftp-server

# Example of overriding settings on a per-user basis
#Match User anoncvs
#       X11Forwarding no
#       AllowTcpForwarding no
#       PermitTTY no
#       ForceCommand cvs server

修改端口的时候需要添加到防火墙的控制中，否则无法使用ssh连接。

[ ~]# semanage port -l | grep ssh #查看当前ssh服务监听的端口
ssh_port_t tcp 22
[ ~]# semanage port -a -t ssh_port_t -p tcp 8090 #增加监听端口8090

[ ~]# semanage port -l | grep ssh
ssh_port_t tcp 8090,22

semanage只是端口工具，修改防火墙只能使用firewall-cmd

[ ssh]# yum provides firewall-cmd #查找防火墙工具所在的包
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile

• base: mirror.bit.edu.cn
• extras: mirrors.btte.net
• updates: mirrors.btte.net
  firewalld-0.3.9-14.el7.noarch : A firewall daemon with D-BUS interface providing a dynamic firewall
  Repo : base
  Matched from:
  Filename : /usr/bin/firewall-cmd

[ ssh]# yum -y install firewalld #安装防火墙工具

[ ssh]# systemctl start firewalld #启动防火墙服务

[ ssh]# systemctl status firewalld #查看防火墙状态
● firewalld.service - firewalld - dynamic firewall daemon
Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled)
Active: active (running) since Sat 2016-09-17 04:22:15 CST; 15s ago
Main PID: 16979 (firewalld)
CGroup: /system.slice/firewalld.service
└─16979 /usr/bin/python -Es /usr/sbin/firewalld –nofork –nopid
Sep 17 04:22:14 localhost systemd[1]: Starting firewalld - dynamic firewall daemon…
Sep 17 04:22:15 localhost systemd[1]: Started firewalld - dynamic firewall daemon.
[ ssh]# firewall-cmd –zone=public –add-port=8090/tcp –permanent #防火墙中允许8090端口通过
success

[ ssh]# semanage port -m -t ssh_port_t -p tcp 8090 #将ssh服务修改为8090端口
[ ssh]# firewall-cmd –zone=public –remove-port=22/tcp –permanent #删除22端口
success
[ ssh]# firewall-cmd –reload #重新加载防火墙服务配置
success