Network Administration

Network Administration

The Debian GNU/Linux install program lets you specify a network configuration that's used when your system is first booted. If your network configuration changes, you can re-install Linux. However, you can spare yourself much inconvenience by learning how Linux stores its network configuration. As you'll see, by using a text editor to revise some files, you can alter your system's network configuration without going through the pain of re-installing Linux.

10.2.1 Network Hardware Configuration

If you replace your network adapter card with a different model card, you must run the modconf program, which lets you specify the driver that operates your card. To do so, simply login as root and type the command:

modconf

You're already familiar with the modconf program. It's the same program you used to specify drivers when you originally installed Linux. If you have difficulty using modconf, refer to the section titled Section 3.1.2.13, "Configuring device driver modules" in Chapter 3, Installing Linux. You must reboot your system before changes made by modconf take effect.

10.2.2 Basic Host Information

When you installed Linux, you specified a hostname for your system. If you want to change the hostname associated with your system, you can edit the file /etc/hostname by using ae or another editor of your choosing. Because the file - like most configuration files - has restrictive permissions, you must login as root in order to modify it.

The format of the /etc/hostname file is simple. The file contains a single line, which contains the hostname of your system; for example, debian. If you change the hostname, be sure to specify only the hostname itself; do not specify a fully qualified hostname that includes the domain name (for example, debian.ora.com).

10.2.3 Name Server Specification

When you installed Linux, you may have specified one or more nameservers. Your system accesses a nameserver when it needs to determine the network address that corresponds to a hostname. If your network configuration changes, you may need to specify a new nameserver or servers. Your ISP should provide you with the proper IP address or addresses.

The network addresses of your system's name servers are specified in the file /etc/resolv.conf, which you can edit by using ae or another editor while logged in as root. The format of the file is simple, though not as simple as that of the /etc/hostname file. To specify a name server, include a line of the form:

nameserver 
xxx.xxx.xxx.xxx

where xxx.xxx.xxx.xxx denotes the network address (IP number) of the name server; for example, 192.168.1.1. You can include as many as three such lines; when your system needs to determine a network address, it will attempt to contact the name server specified by the first such line. If that server is unavailable, your system will attempt to contact the name server specified in the second such line, if any. If that werver is unavailable, your system will contact the name server specified in the third such line, if any.

10.2.4 Routing and Gateways

If your computer is part of a local area network attached to the Internet, your computer doesn't generally send data packets directly to Internet hosts. Instead, it sends data packets to a designated computer - called the gateway - on the local area network. The gateway forwards data packets to the Internet on behalf of your system. It also performs the complementary service, forwarding data packets from Internet hosts to your system.

NOTE: If your system connects to the Internet via PPP, the PPP system establishes a network configuation dynamically. You'll learn how this works in the next chapter.

The information that describes your local area network is contained in the file /etc/init.d/network, which you can easily edit. Here's a typical /etc/init.d/network file:

#! /bin/sh
inconfig lo 127.0.0.1
route add -net 127.0.0.0
IPADDR=192.168.1.10
NETMASK=255.255.255.0
BROADCAST=192.168.1.255
GATEWAY=192.168.1.1
ifconfig eth0 ${IPADDR} netmask ${NETMASK} broadcast ${BROADCAST}
route add default qw ${GATEWAY} metric 1

The lines you're concerned with are the lines four through eight, each of which has the following form:

variable=
ipnumber

The lines associate a name, given by variable, with a network address, given by ipnumber. The variables are referenced by the following three lines, which are commands that configure networking.

To change your network configuration, you need merely to associate the proper IP number with each variable. You can do so by logging in as root and modifying the /etc/init.d/network file by using a text editor. Table 10.1 describes each variable. Your network administrator should be able to provide you with the proper values.

Table 10.1: Network Configuration Variables

Variable	Meaning
IPADDR	Specifies the network address of your system.
NETMASK	Specifies the network address of your network, by indicating which bits of the 32-bit network address of your system pertain to the network and which pertain to the system. Many local area networks are so-called Class C networks, which require a netmask of 255.255.255.0.
BROADCAST	Specifies the address used to send a message to every system on the local area network. Often you can determine the broadcast address of a local area network from the address of a system on the local area network: simply replace the last of the four components of the network address of the host by 255.
GATEWAY	Specifies the network address of the gateway used by your system.

10.2.5 Hostname Search Path

Your Linux system can use as many as three methods to determine the IP address that corresponds to a hostname. Your system can:

• Query a DNS server (you configured your system's DNS client earlier)
• Read the contents of the file /etc/hosts, known as the hosts file, which you'll learn about in the next subsection
• Query an NIS (Network Information System) server

However, unless your system is part of a sophisticated local area network, it's unlikely that an NIS server is available. Therefore, most systems can query a DNS server and, failing to obtain an answer, read the /etc/hosts file. Alternatively, most systems can read the etc/hosts file and, failing to obtain an answer, query a DNS server. The second alternative is generally better, because reading the /etc/hosts file takes less time than querying a DNS server.

The /etc/host.conf/ file specifies which of these three operations are performed, and the order in which they're attempted. You can edit this file by logging in as root. Here's a typical file:

order hosts,bind
multi on

The order line specifies that the system should first consult the /etc/hosts file and then query a DNS server, referred to as bind because of the Berkeley Internet Name Daemon, an early DNS server.

The multi line specifies that your system will attempt to locate all possible names for a host when it reads the /etc/hosts file. Unless that file is very large (hundreds or thousands of lines), you should include the multi line.

10.2.6 Miscellaneous Network Configuration Options

The hosts file, /etc/hosts, lets your system determine the network address number that corresponds to a hostname, without querying a DNS server. Besides being faster than querying a DNS server, the /etc/hosts file is always available.

Entries in the file have two parts:

• an IP address
• a hostname, or a list of hostnames separated by spaces

By default, the hosts file contains an entry that associates the hostname localhost with the IP address 127.0.0.1. It's not necessary that you include any other entries in the /etc/hosts file. However, most system administrators include at least a second line, which associates the local hostname with its network address. Here's a typical file:

127.0.0.1      localhost
192.168.1.10   debian.mccarty.org debian

Notice that the second line gives both the fully qualified hostname, consisting of the hostname and domain name, as well as the hostname alone.

The /etc/networks file, known as the networks file, performs a function similar to that of the hosts file; whereas the hosts file associates hostnames with network addresses, the networks file associates networks' names with network addresses. By default, the networks file contains a single line associating the network address of the local area network with the name localnet:

localnet 192.168.1.0

Generally, it's not necessary that you add other entries to the networks file. However, by doing so, you can access frequently used networks by name even if your DNS server is unavailable.

Setting Up a Linux-Based LAN

This chapter explains how to set up a local-area network (LAN) that includes a Linux Samba server, which lets Microsoft Windows and UNIX systems access shared files and printers hosted by your Linux system. The chapter explains how to administer a simple LAN and describes how to install, configure, and administer Samba servers and clients. Integrating your Linux system with an existing LAN is no more complicated than setting up your own LAN; the chapter also explains how to connect to an existing network. The chapter also explains how to use Linux backup and recovery utilities so that client systems can create and use backups stored on the server.

10.1 Introduction

One of the great strengths of Linux is its powerful and robust networking capabilities. The good news is that everything about Linux's networking setup is open to inspection and completely configurable. Nothing is hidden from the user, and no parameters are forced on you. The challenge is to get the most out of this setup for your needs.

Basic networking principles don't differ much between Windows and Linux, and indeed the principles aren't unfamiliar. This chapter starts with an overview of networking, and then looks in more detail at Linux networking on a Local Area Network (LAN). In the next two chapters, you'll learn about making a dialup Internet connection, and setting up Wide Area Network (WAN) services.

Most computers today handle network traffic much as the post office handles mail. Think, for example, of the steps involved in sending and receiving a letter. Your postal carrier must know where to drop off and where to pick up mail. So your home must have some kind of recognizable interface; we call this a mailbox. And whereas your postal carrier may know your neighborhood quite well, delivery in other areas will require other carriers. Mail is passed to these other carriers through a gateway; we call this the Post Office. Although you can think of the whole postal system as one big network, it's easier to understand if you think of it as a hierarchy of subnetworks (or subnets): the postal system is divided into states, states are divided by zip code, zip codes contain a number of streets, and each street contains unique addresses.

Computer networking mirrors this model. Let's trace an email message from you to a coworker. You compose the message and press Send. Your computer passes the message to a network interface. This interface may be a modem by which you dial up an ISP, or it may be an Ethernet card that connects you to a LAN. Either way, on the other side of the interface is a gateway machine. The gateway knows how to look at the address of the recipient on the email message, and interpret that message in terms of networks and subnets. Using this information, your gateway passes the message to other gateways until the message reaches the gateway for the destination machine. That gateway in turn delivers the message via a recognizable interface (such as modem or Ethernet card) to the recipient's inbox.

If you review this story, you can easily see what parts of networking you'll need to configure on your Linux system. You'll need to know the address of your machine. Just as the town name Menlo Park and the zip code 94025 are two different names for the same location, you may have both a name, called a hostname, and a number, called an IP number, that serve as the address for your machine.

To translate between these two notations, you may need to know the address of a Domain Name Server. This is a machine that matches IP numbers with hostnames. You'll also need to know the address of a gateway machine through which network traffic will be routed. Finally, you'll need to be able to bring up an interface on your system for networking, and you'll need to assign a route from that interface to the gateway.

While all of this can seem complex, it really isn't any more complex than the postal system, and functions in much the same way. Fortunately, Linux comes with tools to help you automate network configuration. In this chapter you'll look at networking on a LAN, and we'll start by looking at how to set up LAN networking.

How to Recover Deleted Items Using Microsoft Outlook

Note: these procedures only apply if you have installed Outlook and you are connected to an Exchange Server.

When items are deleted from a folder in a mailbox, those items are first transferred to the deleted items folder of that mailbox. The items can then be deleted from the deleted items folder. This provides a measure of protection in case of accidental deletion. After the items are deleted from the deleted items folder, the items can still be recovered for up to 30 days by using the Recover Deleted Items funtion.

To Recover Deleted Items

To recover deleted items, follow these steps:

1. On the folder list, click to select the Deleted Items folder.
2. On the Tools menu, click Recover Deleted Items.
3. Click the item you wish to recover. If you want to recover multiple items, hold the Shift key down and click to select contiguous items or hold the CTRL key down and click to select noncontiguous items. If you want to recover the entire list, click select all.
4. Click the Recover Selected Items button. The items will be returned to your deleted items folder, and then you can move them to other folders. if the menu item is not visible, hover the mouse pointer over the chevrons at the bottom of the menu, and then click recover selected items.

To Purge Deleted Items

1. On the folder list, click to select the Deleted Items folder.
2. On the Tools menu, click Recover Deleted Items.
3. In the Recover Deleted Items window, click the item you wish to purge. If you want to purge multiple items, press and hold Shift and click to select contiguous items or press and hold CTRL and click to select noncontiguous items. If you want to purge the entire list, click Select All.
4. click the purge selected items button, and then close the recover deleted items window. if the menu item is not visible, move the mouse pointer over the chevrons at the end of the menu, and then click recover selected items.

The items are now permanently removed from the server and will not be recoverable.

How to create a really invisible folder

This is a trick to make a folder invisible. We are not going to try to hide the file anywhere. In fact the folder would be right before your eyes but you can’t see it. Sounds intriguing? Actually it is very simple – just 2 steps.
Here’s how:
You can either create a new folder or choose an existing folder. While naming or renaming the folder hold [Alt] Key and enter “0160” (without the quotes). Release the [Alt] Key. You can notice that the name part of the folder is just a blank. What we have done is we have inserted a ‘Non-Breaking Space‘ as the name of the folder virtually making it appear as if the folder has no name. You can’t do this just by typing a space using the space-bar. That doesn’t work. Try it for yourself.
Now right-click on the folder and select “Properties” and choose the “Customize” tab. Click on the “Change Icon” button. Scroll along and you should find a few blank spaces. Click on any one of them and click Ok. That’s it. You can’t see the folder anymore.
Of course, if you select the folder (click on the folder), it will get highlighted