Generate SSH public key authentication

$ ssh-keygen
You can just press enter to create a rsa key pair;
.ssh/id_rsa is your priviate key stays in this machine
.ssh/id_rsa.pub is the public key you need to copy to the remote machine
I would suggest to use a passphrase, which only need to enter once for each OS boot

$ ssh-copy-id username@remote.machine.com
It copies the public key from local machine to a remote machine, and stores it at .ssh/authorized_key file

Some commands related to networking on Linux

Network Device Up and Down
$ service network restart
$ /etc/init.d/network restart

$ service network start
$ service network stop

$ifdown eth0
$ifup eth0

$ ifconfig

$ dhclient

Name Server
$ vim /ect/resolv.conf
[root@sr1s1 ~]# cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 129.174.97.3
nameserver 129.174.68.227
nameserver 129.174.1.3

HostName
$ echo hostname=$HOSTNAME >> /etc/sysconfig/network
set hostname

Routing Table
$ route
show the routing tables

IP Table
$ iptables --list
view all of the ip table rules

Opening Ports
$lsof -i

Start SSH
$ service sshd restart

Automatically Start SSH on Boot
$ chkconfig --level 5 --del sshd
$ chkconfig --level 5 --add sshd

Query system running service
$ chkconfig

Update bashrc or profile
$ source ~/.profile

Install OpenVZ on CentOS

Installation: I successfully installed OpenVZ on CentOS 6.4, but CentOS 5.5 has booting problem after switching to OpenVZ kernel

http://openvz.org/Quick_installation
https://openvz.org/Quick_Installation_CentOS_6

Create and Run a Container:
http://openvz.org/Basic_operations_in_OpenVZ_environment
http://openvz.org/Quick_installation

Change hostname on CentOS

$vim /etc/sysconfig/network

Change the HOSTNAME to anything you like
e.g.: sr1s5

fix touchpad sensitive issue on ubuntu of Chromebook

http://craigerrington.com/blog/fixing-touchpad-issues-on-arm-chromebook-chrubuntu/

mkdir ~/backup sudo mv /usr/share/X11/xorg.conf.d/* ~/backup/ cd /usr/share/X11/xorg.conf.d/ (you can issue an ls here to make sure the dir is empty if you like) sudo wget http://craigerrington.com/chrome/x_alarm_chrubuntu.zip sudo unzip x_alarm_chrubuntu.zip sudo rm x_alarm_chrubuntu.zip

NOTE: This will set up a UK keyboard, follow these simple steps to change it:

Example:

sudo vi /usr/share/X11/xorg.conf.d/10-keyboard.conf

reboot

Ubuntu file system becomes read only

My Ubuntu on Chromebook has this problem, and it fixed by $fsck -Af

Source Link is:
http://askubuntu.com/questions/197459/read-only-file-system

The filesystem will usually go into read-only while the system is running if there is a filesystem consistency issue. This is specified in fstab as errors=remount-ro and will occur when a FS access fails or an emergency read-only remount is requested via Alt+SysRq+U. You can run:

sudo fsck -Af

to force a check of all filesystems. As the other very good answer states, looking at dmesg is also very helpful.

Capture The Flag notes

Here are some notes for capture the flag:

1. Use backtrack, because it has been installed many attacking tools including Metasploit

2. Use nmap to scan a target host, and identify what services are running

$ nmap -A -T4 www.hostIP.com
 -A: Enable OS detection, version detection, script scanning, and traceroute

 -T4 for faster execution

nmap cheatsheet: https://tiw2013.cse.psu.edu/slides/cheat_sheet.pdf

3. Go to securityfocus website, use the service name and version as the keyword, then find out if there is an vulnerability for that service. 

securityfocus: http://www.securityfocus.com/vulnerabilities

e.g., vsftpd, irc2

4. Open Metasploit to exploit the vulnerability

$ msfconsole

$ use /unix/vsftpd/exploit-path

$ set rhost target-IP

$ exploit

Metasploit set lhost to current IP and payload to reverse-shell as default. 

Additional Tips:

1) try rlogin and rsh commands 

$ rlogin -l username Target-IP

$ rsh -l username Target-IP

2) take a look at port 1524. If it is open, try to login with netcat

$nc TargetIP 1524