Author: Shawn

On the local machine, type the BOLD part. The non-bold part is what you might see as output or prompt.

• Step 1:
  % ssh-keygen -t dsa
  Generating public/private dsa key pair.
  Enter file in which to save the key (~/.ssh/id_dsa): (just type return)
  Enter passphrase (empty for no passphrase): (just type return)
  Enter same passphrase again: (just type return)
  Your identification has been saved in ~/.ssh/id_dsa
  Your public key has been saved in ~/.ssh/id_dsa.pub
  The key fingerprint is:
  Some really long string
  %
• Step 2:
  Then, paste the content of the local ~/.ssh/id_dsa.pub file into the file ~/.ssh/authorized_keys on the remote host.
• RSA instead of DSA
  • If you want something strong, you could try
    % ssh-keygen -t rsa -b 4096
  • Instead of the names id_dsa and id_dsa.pub, it will be id_rsa and id_rsa.pub , etc.
  • The rest of the steps are identical.

That’s it!

FAQ:

• Q: I follow the exact steps, but ssh still ask me for my password!
• A: Check your remote .ssh directory. It should have only your own read/write/access permission (octal 700)
  % chmod 700 ~/.ssh

Source: http://www.ece.uci.edu/~chou/ssh-key.html

今天花了一晚上倒腾VPN服务器，因为是用的CENTOS，没法直接apt-get install pptpd，虽然网上有RPM包，但最后还是选择了源码安装，过程中碰到很多问题，在此总结一下：

1. make install后默认没有安装服务，需要自行创建/etc/rc.d/init.d/pptpd,写入代码如下：

#!/bin/sh
#
# Startup script for pptpd
#
# chkconfig: 345 85 15
# description: PPTP server
# processname: pptpd
# config: /etc/pptpd.conf

# Source function library.
. /etc/rc.d/init.d/functions
# See how we were called.
case "$1" in
start)
echo -n "Starting pptpd: "
if [ -f /var/lock/subsys/pptpd ] ; then
echo
exit 1
fi

daemon /usr/local/sbin/pptpd
echo
touch /var/lock/subsys/pptpd
;;
stop)
echo -n "Shutting down pptpd: "
killproc pptpd
echo
rm -f /var/lock/subsys/pptpd
;;
status)
status pptpd
;;
restart)
$0 stop
$0 start
;;
*)
echo "Usage: $0 {start|stop|restart|status}"
exit 1
esac

exit 0

2. 将pptpd安装为服务并随机启动:

chkconfig --add pptpd
chkconfig pptpd on
service pptpd start

3. 客户端链接的时候很可能提示错误，这是因为通过yum install ppp安装的组件版本与pptpd所支持的版本不一致，解决办法，修改/etc/pptpd.options,注释掉logwtmp即可。这个问题折磨我好久！=-=!

4. 还有一个很折腾人的问题，就是连上VPN后无法访问外网只能访问VPN服务器，原因是要通过iptables转发数据包才行，代码如下：

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
//add it to /etc/rc.d/rc.local for auto execution when rebooted.

5. 附上具体安装步骤：

//install ppp 
yum install ppp

//install pptpd, download source code and
./configure
make
make install

// update pptpd configurations in file /etc/pptpd.conf:
localip     192.168.9.1
remoteip    192.168.9.11-30

// /etc/ppp/options.pptpd:
ms-dns    8.8.8.8
ms-dns    8.8.4.4

// /etc/ppp/chap-secrets. Each line in the file has the format:
<username> pptpd <passwd> *

// /etc/sysctl.conf, use the following config:
net.ipv4.ip_forward = 1