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一客户联系说VPS在让其朋友安装Squid后SSH不能登录了，分析OpenSSH未正常启动，然后通过VNC终端使用用户提供的密码也不能登录，当时以为密码有误，不得已进入单用户模式重置密码，然后再次尝试登录依旧，不得已再次进入单用户模式查找原因，验证错误日记如下：

Dec 27 14:36:30 ONOVPS login: pam_securetty(login:auth): /etc/securetty is either world writable or not a normal file
Dec 27 14:36:35 ONOVPS login: FAILED LOGIN 1 FROM (null) FOR root, Authentication failure

可见是/etc/securetty文件或权限有问题，此文件可限制root登录的终端，正常权限600，查看权限：

ls -al /etc/securetty 
-rwxrwxrwx 1 root root 127 Mar  3  2011 /etc/securetty

重置权限正常登入VPS，启动SSH依旧提示权限有误，查看/etc/目录竟然都整成777权限了，那个郁闷呀。。。。

CentOS默认启动6个TTY，在控制台下可用Alt+[F1—F6]键在不同的终端之间切换，通过SSH虚拟终端管理远程服务器是用不到这些的，可减少启动的TTY数量减少系统资源占用。
CentOS5修改配置文件注释禁止启动的TTY即可：

cat /etc/inittab
#2:2345:respawn:/sbin/mingetty tty2
#3:2345:respawn:/sbin/mingetty tty3
#4:2345:respawn:/sbin/mingetty tty4
#5:2345:respawn:/sbin/mingetty tty5
#6:2345:respawn:/sbin/mingetty tty6

CentOS6下修改启动的TTY数量：

sed -i 's/\/dev\/tty\[1-6\]/\/dev\/tty\[1-1\]/g' /etc/init/start-ttys.conf 
sed -i 's/\/dev\/tty\[1-6\]/\/dev\/tty\[1-1\]/g' /etc/sysconfig/init
#修改为启动1个TTY

关闭指定终端：

initctl stop tty TTY=/dev/ttyn

踢出指定终端用户：

pkill -kill -t ttyn

1.创建Github账号，Linux创建SSH密钥：

ssh-keygen

2.将公钥加入到Github账户信息Account Settings，测试验证是否成功。

ssh -T git@github.com
Hi haiyun.me! You've successfully authenticated, but GitHub does not provide shell access.

3.GitHub创建项目。
4.本地配置,新建Git项目并提交到Github。

touch README.md
git init #初始化，建立本地git仓库
git add README.md #添加文件到本地仓库
git status #查看本地代码库状态
git commit -m "first commit" #提交到本地仓库
git remote add origin git@github.com:user/test.git #和远程代码库建立连接
git remote -v #查看连接
git remote rm origin #删除链接
git pull origin master --allow-unrelated-histories #要更新已存的github仓库先拉到本地再提交
git push origin master #将本地仓库文件提交到Github。

5.复制项目到本地：

git clone git://github.com/user/xxx.git

6.删除GitHub文件：

git rm README.md #本地仓库内删除
git commit -m "rm README.md" #提交到本地
git push -u origin master #提交到Github

7.和远程项目连接，注意不要使用https clone：

git clone git@github.com:user/test.git
git add .
git status 
git commit -m "commit"
git push -u origin main

8.删除回滚到最近的commit：

git reflog
git reset --hard commit_id #回滚到的commit id
git push origin master --force

一般使用Xshell通过SSH连接管理Centos/Debina服务器终端为8色，通过设置终端类型可开启256色彩以显示更加鲜艳的色彩。
查看当前终端类型：

echo $TERM 
xterm-color

查看当前服务器终端色彩：

tput colors
8

或使用以下脚本检测并以色彩方式：

#!/usr/bin/env python
#检测当前终端支持色彩
import sys
import os

def echo(msg):
    os.system('echo -n "' + str(msg) + '"')

def out(n):
    os.system("tput setab " + str(n) + "; echo -n " + ("\"% 4d\"" % n))
    os.system("tput setab 0")

# normal colors 1 - 16
os.system("tput setaf 16")
for n in range(8):
    out(n)
echo("\n")
for n in range(8, 16):
    out(n)

echo("\n")
echo("\n")

y=16
while y < 231:
    for z in range(0,6):
        out(y)
        y += 1

    echo("\n")

echo("\n")

for n in range(232, 256):
    out(n)
    if n == 237 or n == 243 or n == 249:
        echo("\n")

echo("\n")

配置Linux终端如果支持就调整为256色终端，添加到.bashrc文件内。

if [ -e /usr/share/terminfo/x/xterm-256color ]; then
#debian在/lib/terminfo/x/xterm-256color
        export TERM='xterm-256color'
else
        export TERM='xterm-color'
fi

如不支持xterm-256color安装：

apt-get install ncurses-base
yum install ncurses

扩展阅读：
Colour colour everywhere! 256 colour-mode for Linux consoles：http://www.robmeerman.co.uk/unix/256colours
256 colors in vim：http://vim.wikia.com/wiki/256_colors_in_vim

1.Cygwin安装OpenSSH，可使用apt-cyg包管理器快速安装。

apt-cyg install openssh

配置OpenSSH服务，以管理员运行Cygwin Terminal。

$ ssh-host-config
*** Info: Generating /etc/ssh_host_key
*** Info: Generating /etc/ssh_host_rsa_key
*** Info: Generating /etc/ssh_host_dsa_key
*** Info: Generating /etc/ssh_host_ecdsa_key
*** Info: Creating default /etc/ssh_config file
*** Info: Creating default /etc/sshd_config file
*** Info: Privilege separation is set to yes by default since OpenSSH 3.3.
*** Info: However, this requires a non-privileged account called 'sshd'.
*** Info: For more info on privilege separation read /usr/share/doc/openssh/README.privsep.
*** Query: Should privilege separation be used? (yes/no) yes
*** Info: Updating /etc/sshd_config file

*** Query: Do you want to install sshd as a service?
*** Query: (Say "no" if it is already installed as a service) (yes/no) yes
*** Query: Enter the value of CYGWIN for the daemon: [] ssh
*** Info: On Windows Server 2003, Windows Vista, and above, the
*** Info: SYSTEM account cannot setuid to other users -- a capability
*** Info: sshd requires.  You need to have or to create a privileged
*** Info: account.  This script will help you do so.

*** Info: You appear to be running Windows XP 64bit, Windows 2003 Server,
*** Info: or later.  On these systems, it's not possible to use the LocalSystem
*** Info: account for services that can change the user id without an
*** Info: explicit password (such as passwordless logins [e.g. public key
*** Info: authentication] via sshd).

*** Info: If you want to enable that functionality, it's required to create
*** Info: a new account with special privileges (unless a similar account
*** Info: already exists). This account is then used to run these special
*** Info: servers.

*** Info: Note that creating a new user requires that the current account
*** Info: have Administrator privileges itself.

*** Info: No privileged account could be found.

*** Info: This script plans to use 'cyg_server'.
*** Info: 'cyg_server' will only be used by registered services.
*** Query: Do you want to use a different name? (yes/no) yes
*** Query: Enter the new user name: ssh
*** Query: Reenter: ssh

*** Query: Create new privileged user account 'ssh'? (yes/no) yes
*** Info: Please enter a password for new user ssh.  Please be sure
*** Info: that this password matches the password rules given on your system.
*** Info: Entering no password will exit the configuration.
*** Query: Please enter the password:
*** Query: Reenter:

*** Info: User 'ssh' has been created with password 'passwd'.
*** Info: If you change the password, please remember also to change the
*** Info: password for the installed services which use (or will soon use)
*** Info: the 'ssh' account.

*** Info: Also keep in mind that the user 'ssh' needs read permissions
*** Info: on all users' relevant files for the services running as 'ssh'.
*** Info: In particular, for the sshd server all users' .ssh/authorized_keys
*** Info: files must have appropriate permissions to allow public key
*** Info: authentication. (Re-)running ssh-user-config for each user will set
*** Info: these permissions correctly. [Similar restrictions apply, for
*** Info: instance, for .rhosts files if the rshd server is running, etc].


*** Info: The sshd service has been installed under the 'ssh'
*** Info: account.  To start the service now, call `net start sshd' or
*** Info: `cygrunsrv -S sshd'.  Otherwise, it will start automatically
*** Info: after the next reboot.

*** Info: Host configuration finished. Have fun!

启动SSH服务：

cygrunsrv -S sshd