Add new key

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 16126D3A3E5C1192
sudo apt-key del 16126D3A3E5C1192 #delete Key

list all public key with PPA

sudo apt-key finger
apt-get clean # Remove cached packages 
cd /var/lib/apt 
mv lists lists.old # Backup mirror info 
mkdir -p lists/partial # Recreate directory structure 
apt-get clean 
apt-get update # Fetch mirror info

1. get the MX record:

dig MX domainname.com

or http://www.mxtoolbox.com

2. connect to the server with telnet

> telnet ASPMX.L.GOOGLE.COM 25
Trying 74.125.39.27...
Connected to ASPMX.L.GOOGLE.COM.
Escape character is '^]'.
220 mx.google.com ESMTP y26si6167249fag.156
helo mydomain.com
250 mx.google.com at your service
mail from: <me@mydomain.com>
250 2.1.0 OK y26si6167249fag.156
rcpt to: <bogus@pentest.ro>
550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at
550 5.1.1 http://mail.google.com/support/bin/answer.py?answer=6596 y26si6167249fag.156
rcpt to: <somevalidaddress@pentest.ro>
250 2.1.5 OK y26si6167249fag.156
quit
221 2.0.0 closing connection y26si6167249fag.156
Connection closed by foreign host.

The bolded lines are the ones you type, the others are responses from the server.

按照《Unix网络编程》的划分，IO模型可以分为：阻塞IO、非阻塞IO、IO复用、信号驱动IO和异步IO

按照POSIX标准来划分只分为两类：同步IO和异步IO.

如何区分同步IO和异步IO：

首先一个IO操作其实分成了两个步骤：发起IO请求和实际的IO操作

• 同步IO和异步IO的区别就在于第二个步骤是否阻塞，如果实际的IO读写阻塞请求进程，那么就是同步IO，因此阻塞IO、非阻塞IO、IO复用、信号驱动IO都是同步IO
• 如果不阻塞，而是操作系统帮你做完IO操作再将结果返回给你，那么就是异步IO。
• 阻塞IO和非阻塞IO的区别在于第一步，发起IO请求是否会被阻塞，如果阻塞直到完成那么就是传统的阻塞IO，如果不阻塞，那么就是非阻塞IO。

JAVA的传统流：

流是一个抽象的概念。当Java程序需要从数据源读取数据时，会开启一个到数据源的流。数据源可以是文件，内存或者网络等。同样，当程序需要输出数据到目的地时也一样会开启一个流，数据目的地也可以是文件、内存或者网络等。流的创建是为了更方便地处理数据的输入输出。

字节流: InputStream && OutPutStream

字节流的处理类有很多，他们都继承自InputStream或者OutputStream抽象类。

//InputStream:
read();
read(byte[] b) ;
read(byte[] b, int off, int len) ;

//OutputStream:
write(int b);
write(byte[] b);
write(byte[] b, int off, int len)

字符流：Reader && Writer

所有的字符流操作类都继承自Reader或者Writer这两个抽象类。

NIO:

The most important distinction between the original I/O library (found in java.io.*)
and NIO has to do with how data is packaged and transmitted. As previously
mentioned, original I/O deals with data in streams, whereas NIO deals with data in
blocks.

Asynchronous I/O is a method for reading and writing data without blocking.

Asynchronous I/O calls, on the other hand, do not block. Instead, you register your
interest in a particular I/O event — the arrival of readable data, a new socket
connection, and so on — and the system tells you when such an event occurs.

One of the advantages of asynchronous I/O is that it lets you do I/O from a great many inputs and outputs at the same time. Synchronous programs often have to resort to polling, or to the creation of many, many threads, to deal with lots of connections. With asynchronous I/O, you can listen for I/O events on an arbitrary number of channels, without polling and without extra threads.

Selector:

The central object in asynchronous I/O is called the Selector. A Selector is where
you register your intere.

ServerSocketChannel:

In order to receive connections, we need a ServerSocketChannel. In fact, we need
one for each of the ports on which we are going to listen

线程的状态有：new、runnable、running、waiting、timed_waiting、blocked、dead

http://www.iteye.com/topic/1119957

• wait( ) tells the calling thread to give up the monitor and go to sleep until some other
  thread enters the same monitor and calls notify( ).
• notify( ) wakes up the first thread that called wait( ) on the same object.
• notifyAll( ) wakes up all the threads that called wait( ) on the same object. The
  highest priority thread will run first.