CHAPTER 24 SCHEDULING TASKS
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In this brief chapter, you looked at how you can schedule tasks under Ubuntu, which essentially means making programs run at certain times. You ve learned to schedule recurring tasks, one-time tasks, and tasks that run on specific time periods.
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C H A P T E R 25
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Accessing Computers Remotely
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One area where Linux particularly excels is in its support for networking, including across the Internet. If you want to learn about how networks operate on a fundamental level, Linux is an ideal choice, because it puts you in direct contact with the technology. The widespread integration and support for networking extends to several useful system tools, which let you access Linux across any kind of network, including the Internet. In fact, it s even possible to access a Linux machine running on a different continent, just as if you were sitting in front of it! This chapter presents the many ways you can access an Ubuntu computer remotely. You might need to do so, for example, if you want to administer your computer from another PC or to help another person who is using Ubuntu in his own computer. In addition, we look at ways that you can use Ubuntu to access almost any other computer, including Windows PCs.
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Using Secure Shell
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The history of UNIX has always featured computers connecting to other computers in some fashion, whether they were dumb terminals connecting to a mainframe computer or UNIX machines acting as nodes on the fledgling Internet. Because of this, a wide variety of techniques and protocols were invented to allow computers to communicate and log in to each other across networks. However, although these still work fine over the modern Internet, we re now faced with threats to the privacy of data. In theory, any data transmitted across the Internet can be picked up by individuals at certain key stages along the route. If the data isn t protected in any way, it can be easily intercepted and read. To counter such an occurrence, the ssh suite of programs was created. Although these programs started as open source, they gradually became proprietary. Therefore, several newer open source versions were created, including the one used on the majority of Linux distributions (including Ubuntu): OpenSSH. The goal of ssh is to create a secure connection between two computers. You can then do just about any task, including initiating a shell session so you can use the remote computer as if you were sitting in front of it, or copying files to and from the remote machine. ssh uses various techniques at both ends of the connection to encrypt not only the data passing between the two machines, but also the username and password.
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CHAPTER 25 ACCESSING COMPUTERS REMOTELY
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Note This chapter refers to remote and local machines. The remote machine is the computer you re connecting to across the network or Internet. The local machine is the one you re sitting in front of. These two terms are widely used in networking documentation.
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Logging In to a Remote Computer
The most basic type of ssh connection is a remote login. This gives you a command prompt on the remote computer, as if you had just sat down in front of it and logged in to a text console. But before you can log in to any machine via ssh, you need to be sure the remote computer is able to accept ssh connections. This means that it needs to be running the ssh server program (called a service or daemon), and also that its firewall has an open port for incoming connections. The two major components of OpenSSH are the client and the server. Some distributions install both packages and run the server component all the time. However, only the client component of OpenSSH is installed under Ubuntu by default. To install the server component, and therefore access your Ubuntu system remotely, you need to open the Ubuntu Software Center (Applications Ubuntu Software Center) and search for openssh-server. An application named secure shell (SSH) server, for secure access from remote machines, will be at the top of the list. Click to install it. Configuration is automatic, although if you re using the Ubuntu firewall (see 8), you will need to configure an incoming rule to open port 22, the standard port for ssh connections. Take into account that for security reasons the SSH port is often changed from this default value to some other, less obvious to an attacker.
Tip If you use Firestarter, as described in 8, you can simply select the default incoming ssh rule. There s no need to manually specify a port number.
Initiating an ssh session with a remote machine is usually achieved by typing something similar to the following at a command prompt on the local machine: ssh <username>@<IP address> In other words, you specify the username you want to log in as, as well as the IP address of the remote machine. If there s a fully qualified domain name (FQDN) for the system you want to access, you could specify that instead of the IP address.
Note An FQDN is the hostname of a system plus its Internet address, such as mycomputer.example.com. Unless you have had this function specifically set up for you by a system administrator, you ll probably have to connect via its IP address. However, if you rent a web server, you might be able to ssh into it by using the domain name of the server.
You ll be prompted for your password, which, obviously, is the password for the account you re trying to log in to on the remote computer. When you log in for the first time, you ll see the following message: