Chapter 7 Configuring Networking 205

The ability to network is the primary function of Linux. Installing networking may include basic configuration of network services, access rights, client ser-

vices, Internet services, and remote access. If you know the features of most of the available services, you should be able to prevent major security risks and perform a basic configuration of these services. In fact, you should know that you could make a career out of performing each of these tasks individually. The goal of this chapter is to make you familiar with most of the available services and to show you their basic configurations.

Basic Network Services

3.4 Configure basic network services and settings (e.g., netconfig, linuxconf; settings for TCP/IP, DNS, DHCP)

7.8 Identify basic networking concepts, including how a network works

A number of services are available to the Linux system to provide networking. In general, these services can be used in any environment and can be divided into two types — clients and servers — and can be used in any environment. Clients use their services to connect to servers and servers use their services to provide information to clients. This relationship is true of every networking system in use, including so-called “peer-to-peer” systems in which both systems act as a client and a server. The major networking protocol for Linux is the Transmission Control Protocol and Internet Protocol (TCP/IP) protocol. TCP/IP provides all network services used in the Linux environment. TCP/IP is the primary protocol of the Internet, so interfacing your local Linux network services to the Internet is very straightforward. You must configure these services at the most basic level. You can use these services after you get them running, but keep the following warning in mind: Although client-side services are usually safe, these services have proven to be insecure in some situations. Therefore, always consult network security information or another source before activating one of these services in any environment. In general, you are required to configure the basic network services — even if it is just to connect to the Internet — so you may benefit from some background information before beginning configuration.

TCP/IP Protocol Suite

The TCP/IP suite is used everyday by anyone who accesses the Internetor or uses a local LAN. The strength of TCP/IP comes from its development: It was designed to be robust and includes two sets of systems — connection-oriented (TCP) and connectionless (UDP) protocols — that allow the remote connection to clients or servers. Table 7-1 lists many of the most popular protocols and services that they provide.

206 Part III Configuration

Table 7-1

The TCP/IP Protocol Suite

System	Port (default)	Service provided
FTP	20-21	File Transfer Protocol allows the transfer of
		files from one system to another with
		Transmission Control Protocol (TCP) to ensure
		delivery and directory visibility.
SSH	22	Secure Shell, sometimes known as Secure
		Socket Shell, provides secure access to a
		remote computer.
Telnet	23	Telnet protocol allows communication with
		remote systems, usually to access and use the
		remote system.
SMTP	25	Simple Mail Transfer Protocol used in sending
		and receiving e-mail. Because it is limited in
		its ability to queue messages, it is normally
		used for server-to-server mail transfer.
DNS	53	Domain Name System is used to translate a
		user-friendly name, such as www.linux.org,
		to the related IP address, 198.182.196.56 for
		www.linux.org.
DHCP and BOOTPS	67	Dynamic Host Configuration Protocol is a
		communications protocol that automates the
		assignment of Internet Protocol (IP) addresses
		in a network. Bootstrap Protocol is a protocol
		that automatically configures a network user
		and boots or initiates an operating system.
		BOOTPS is the server-side protocol.
BOOTPC	68	Bootstrap Protocol is a protocol that
		automatically configures a network user and
		boots or initiates an operating system. BOOTP
		is the client-side protocol.
TFTP	69	Trivial File Transfer Protocol is used much like
		FTP; TFTP, however, uses the User Datagram
		Protocol (UDP) and does not ensure delivery
		or provide directory visibility.
Gopher	70	Gopher provides a way to bring text files from
		all over the world to a viewer on your

computer. It has been replaced by the HTTP protocol.

Chapter 7 Configuring Networking 207

System	Port (default)	Service provided
Finger	79	Finger is a program that tells you the name
		associated with an e-mail address.
HTTP	80	The Hypertext Transfer Protocol is the set of
		rules for exchanging files (text, graphic
		images, sound, video, and other multimedia
		files) on the World Wide Web.
POP (3)	110	Post Office Protocol 3 is the most recent
		version of a standard protocol for receiving
		e-mail downloaded to clients from servers.
		POP (3) is an alternative to IMAP.
RPC	111	SUN Remote Procedure Call is a protocol that
		one program can use to request a service
		from another host on the network without
		having to understand network details.
RPC	135	Microsoft RPC.
SMB/CIFS	139	SMB/CIFS/CIFS server.
IMAP	143 and 220	Internet Message Access Protocol is a standard
		protocol for accessing e-mail from your mail
		server; it is not downloaded until instructed to
		do so.
SNMP	161	Simple Network Management Protocol
		governs network management and the
		monitoring of network devices.
rlogin	221	Remote login is a command that allows an
		authorized user to log in to other machines
		(host) on a network and to interact as if the
		user were physically at the host computer. It
		has been replaced by SSH and is less known
		than telnet.
SSL/TSL	443	The Secure Sockets Layer protocol is used for
		the secure transmission of data on the
		Internet. TLS is the successor to the Secure
		Sockets Layer (SSL). It uses the same port but
		provides more security and is backwards-
		compatible with SSL; most browsers now
		use TSL.
SMB/CIFS	445	The Server Message Block Protocol provides a
		method for client applications in a computer
		to read and write to files on — and to request

services from — server programs in a computer network. It is used to connect to the Microsoft Windows Platform.

208 Part III Configuration

Don’t try to memorize these ports; instead, focus on configuring the services and learn what protocol provides what service, such as HTTP for Web pages.

Connection protocols needed

Because the TCP/IP protocol suite is limited to network routing and transportation of information, you may need other protocols to network other systems.

Point-to-Point Protocol

The simplest and most widely used modem-based protocol is the Point-to-Point Protocol (PPP). PPP communicates between two computers by using a serial interface; for example, a personal computer is connected by an analog phone line to a server. Many users access the Internet from home by dialing a modem that connects to a modem pool, which then establishes a point-to-point connection by using PPP. This protocol can also be used for server-to-server connections and router-to- router connections. PPP provides a static connection between two pieces of equipment. It can also be used in many broadband, xDSL or cable, Internet connections with Ethernet.

Ethernet

Ethernet is the most widely used Local Area Network (LAN) access method. It is used to connect most workstations, servers, and routers at hubs that create the LAN.

Ethernet uses the IEEE 802.3 standard to provide connectivity to a network. Ethernet and PPP are the most commonly used access methods, but they aren’t the only ones used.

Serial Line Internet Protocol

SLIP, or Serial Line Internet Protocol, is an alternative to PPP. SLIP is not used very widely because it doesn’t provide for error detection and it doesn’t support synchronous connections.

WAN protocols

Linux can directly support almost any connection, including xDSL, cable, leased lines, and more. To support these connections, Linux employs PPPoE, Frame Relay, and other high-speed interface connection methods. These are not basic configurations of the Linux system, but you should be aware of them.

Other network protocols

Other network protocols include Yellow Pages or NIS (Network Information System) and NFS (Network File System), which use the SUN Remote Procedure Call (part of TCP/IP) to provide networking services. NIS is used to provide access to all systems on a network while only requiring one authentication. NFS is used to provide for file viewing and storage on a remote system.