實驗環境：CentOS7 Minimal安裝，安裝過程及軟體包見http://blog.csdn.net/capricorn90/article/details/52556174
SSH的英文全稱是Secure SHell。通過使用SSH，你可以把所有傳輸的資料進行加密，這樣“中間人”這種攻擊方式就不可能實現了，而且也能夠防止DNS和IP欺騙。還有一個額外的好處就是傳輸的資料是經過壓縮的，所以可以加快傳輸的速度。SSH有很多功能，它既可以代替telnet，又可以為ftp、pop、甚至ppp提供一個安全的“通道”。

SSH在Linux中的服務是sshd，安裝openssh後才可開啟。CentOS 7 安裝後預設情況下是不啟動sshd服務，即無法通過ssh服務遠端連線。
首先檢視系統是否安裝openssh，一般情況想都是預設安裝了，

[[email protected] ~]# 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

如果沒有安裝可以通過yum線上安裝。

[[email protected] ~]# yum install openssh

手動設定啟動ssh服務

簡單的設定就是在命令列中啟動sshd服務。這樣做比較快捷直接，但是隻能對當前狀態有效，一旦重啟系統就丟失了該服務。

[[email protected] ~]# systemctl start sshd
[[email protected] ~]# systemctl status sshd
● sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
Active: active (running) since Fri 2016-09-16 16:18:24 CST; 6h ago
Docs: man:sshd(8)
man:sshd_config(5)
Main PID: 1031 (sshd)
CGroup: /system.slice/sshd.service
└─1031 /usr/sbin/sshd -D
Sep 16 16:18:24 localhost.localdomain systemd[1]: Starting OpenSSH server daemon…
Sep 16 16:18:25 localhost.localdomain sshd[1031]: Server listening on 0.0.0.0 port 22.
Sep 16 16:18:25 localhost.localdomain sshd[1031]: Server listening on :: port 22.
Sep 16 18:18:14 localhost.localdomain systemd[1]: Started OpenSSH server daemon.
Sep 16 18:29:41 localhost.localdomain sshd[11847]: Accepted password for root from 192.168.92.1 port 55149 ssh2
Sep 16 18:37:11 localhost sshd[12969]: Address 192.168.92.1 maps to localhost, but this does not map back to the address - POSSIBLE BREAK-IN ATTEMPT!
Sep 16 18:37:12 localhost sshd[12969]: Accepted password for root from 192.168.92.1 port 55391 ssh2
Sep 16 22:09:59 localhost sshd[15252]: Address 192.168.92.1 maps to localhost, but this does not map back to the address - POSSIBLE BREAK-IN ATTEMPT!
Sep 16 22:10:02 localhost sshd[15252]: Accepted password for root from 192.168.92.1 port 64452 ssh2
Sep 16 22:22:08 localhost systemd[1]: Started OpenSSH server daemon.

設定自動啟動ssh服務

1、systemclt設定自動啟動

通過systemctl命令可以將sshd服務加到開機自啟動列表裡。實現開機自動啟動sshd服務。

[[email protected] ~]# systemctl enable sshd

2、修改ssh監聽埠

在sshd_config檔案中存放了埠、控制策略等資訊。

[[email protected] ~]# 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

首先修改埠，埠設定為自定義埠，即1024之後的埠，這裡設定為8090。

port 8090

禁止空密碼使用者登入。

PermitEmptyPasswords no

開啟密碼登入授權（預設即開啟）

PasswordAuthentication yes

禁止root賬戶使用ssh登入，這種設定通常用於網際網路伺服器，防止提權後用root賬戶登入搞破壞。

PermitRootLogin no

注意其中關於port的提示文字

# 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

修改埠的時候需要新增到防火牆的控制中，否則無法使用ssh連線。

[[email protected] ~]# semanage port -l | grep ssh #檢視當前ssh服務監聽的埠
ssh_port_t tcp 22
[[email protected] ~]# semanage port -a -t ssh_port_t -p tcp 8090 #增加監聽埠8090

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

semanage只是埠工具，修改防火牆只能使用firewall-cmd

[[email protected] ssh]# yum provides firewall-cmd #查詢防火牆工具所在的包
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

[[email protected] ssh]# yum -y install firewalld #安裝防火牆工具

[[email protected] ssh]# systemctl start firewalld #啟動防火牆服務

[[email protected] ssh]# systemctl status firewalld #檢視防火牆狀態
● 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.
[[email protected] ssh]# firewall-cmd –zone=public –add-port=8090/tcp –permanent #防火牆中允許8090埠通過
success

[[email protected] ssh]# semanage port -m -t ssh_port_t -p tcp 8090 #將ssh服務修改為8090埠
[[email protected] ssh]# firewall-cmd –zone=public –remove-port=22/tcp –permanent #刪除22埠
success
[[email protected] ssh]# firewall-cmd –reload #重新載入防火牆服務配置
success