In this cheat sheet-style guide, we will cover some common ways of connecting with SSH to achieve your objectives. This can be used as a quick reference when you need to know how to do connect to or configure your server in different ways.

In this guide, we will be examining the underlying encryption techniques that SSH employs and the methods it uses to establish secure connections. This information can be useful for understanding the various layers of encryption and the different steps needed to form a connection and authenticate both parties.

SSH tunnels can be created in several ways using different kinds of port forwarding mechanisms. Ports can be forwarded in three ways:

• Local port forwarding 
• Remote port forwarding 
• Dynamic port forwarding

When considering I/O performance from the perspective of applications, file systems can prove to be a better target for analysis than disks. With modern file systems using more DRAM-based cache and performing more asynchronous disk I/O, what the application experiences can vastly differ to what the disks are doing. I’ll demonstrate this by examining I/O performance of a MySQL database at both the disk and file system level.

In kernel developing, sometimes we have to examine some kernel status, or we want to reuse some kernel facilities, we need to access (read, write, execute) kernel symbols. In this article, we will see how the kernel maintains the symbol table, and how we can use the kernel symbols. This article is more of a guide to reading kernel source code and kernel development. So we will work a lot with source code.

A symbol is a name representing an space in the memory, which stores data (variable, for reading and writing) or instructions (function, for executing). To make life easier for cooperation among various kernel function unit, there are thousands of global symbols in Linux kernel. All global symbols are listed in `/proc/kallsyms`

TCP provides an effective abstraction of a reliable network running over an unreliable channel, hiding most of the complexity of network communication from our applications: retransmission of lost data, in-order delivery, congestion control and avoidance, data integrity, and more. When you work with a TCP stream, you are guaranteed that all bytes sent will be identical with bytes received and that they will arrive in the same order to the client. As such, TCP is optimized for accurate delivery, rather than a timely one. This, as it turns out, also creates some challenges when it comes to optimizing for web performance in the browser.

TCP is an adaptive protocol designed to be fair to all network peers and to make the most efficient use of the underlying network. Thus, the best way to optimize TCP is to tune how TCP senses the current network conditions and adapts its behavior based on the type and the requirements of the layers below and above it: wireless networks may need different congestion algorithms, and some applications may need custom quality of service (QoS) semantics to deliver the best experience.

很多年前，当笔者还是少年的时候，就在家里搭建过Windows Domain， Linux与Windows相互共享，交换机和路由器的虚拟环境等。因为读书百遍，其义往往不能自见。唯有亲自动手，解决实验中碰到的问题，才可能真正学会一门技术。尤其是网络协议，很多时候自以为理解了，做过实验才知道是误解。时至今日，笔者早已变成大叔，但偶尔还做做实验，以验证自己对某些概念的理解是否有误。在客串当面试官时，也侧重考察应聘者对基本概念的理解深度，因为这决定了一名工程师的职业高度。比如下面这道面试题，考的都是基础概念，却经常难住应聘者。

问题：两台服务器A和B的网络配置如下，B的子网掩码本应该是255.255.255.0，不小心配成了255.255.255.224。这两台服务器还能正常通信吗？

所有应用软件之中，数据库可能是最复杂的。MySQL的手册有3000多页，PostgreSQL的手册有2000多页，Oracle的手册更是比它们相加还要厚。但是，自己写一个最简单的数据库，做起来并不难。Reddit上面有一个帖子，只用了几百个字，就把原理讲清楚了。下面是我根据这个帖子整理的内容。

If you’re a web developer, you’ve probably had to make a user account system. The most important aspect of a user account system is how user passwords are protected. User account databases are hacked frequently, so you absolutely must do something to protect your users’ passwords if your website is ever breached. The best way to protect passwords is to employ salted password hashing. This page will explain why it’s done the way it is.

DABDA is the five stages of coping with dying were described by Elisabeth Kübler-Ross in her classic book On Death and Dying in 1969. DABDA and stand for: 

• D - Denial 
• A - Anger 
• B - Bargaining 
• D - Depression 
• A - Acceptance 

The five stages of the Kübler-Ross stage model are the best-known description of the emotional and psychological responses that many people experience when faced with a life-threatening illness or life-changing situation.