Chapter 3: How Clients and Servers Work Together

1
Chapter 3 How Clients and Servers Work Together

• i-Net Guide to the Internet
• Third Edition

2
Objectives

• Learn about the interaction of clients and
  servers
• Explore the features and functions of Web servers
• Examine how e-mail server and client software
  work
• Use FTP to transfer files
• Initiate and use a Telnet session
• Subscribe to and use newsgroups
• Learn about gophers and gopher space

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Understanding How Clients and Servers Interact

• A server can be software that provides a service
  to other software or the computer on which the
  server software is running.
• Consider a server as a software application that
  must be installed and managed by someone with the
  title of system administrator, network
  administrator, or, for Web servers, Webmaster.

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User Interfaces GUI Versus Command-Driven

• When you use a command-driven interface, you type
  commands into a user interface to perform a task
  and achieve a desired result.
• In contrast to a command-driven interface, a
  graphical user interface (GUI) has icons or menus
  that you can select to perform a function or run
  a program.

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User Interfaces GUI Versus Command-Driven
(Continued)

• Applications that run on computers and servers
  can also be controlled by configuration or
  initialization files.
• An initialization file (INI file) is an ASCII
  text file with a .ini file extension.

6
Client/Server Sessions

• As shown in Figure 3-9, TCP (Transmission Control
  Protocol) manages the three-way handshake that
  establishes a session to be used by application
  protocols, such as HTTP or FTP.

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Tracking Information in Log Files

• Servers and client applications often track
  information about their activities in log files.
• These are text files that administrators can use
  to troubleshoot problems with the software, to
  track activities to analyze traffic patterns or
  user preferences, or to look for clues indicating
  that hackers are using the server.

8
All-in-One Clients

• As the number of applications that use the
  Internet grows, the need for client software on
  computers to use these applications also grows.
• One solution to the growing number of Internet
  applications is an all-in-one client or universal
  client.
• These clients can handle several applications and
  adjust appropriately, changing buttons and
  functions to accommodate each application.
• Microsoft Internet Explorer is a good example of
  an all-in-one client.

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Exploring the Features and Functions of Web
Servers

• A wide variety of Web servers are available, many
  of which you can download for free.
• Some popular Web servers include the following
• Apache Web Server
• Microsoft Internet Information Services (IIS)
• Border Manager
• Netscape Enterprise Server (NES)

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Ability to Support Virtual Servers and Virtual
Hosts

• A Web server should be able to support virtual
  servers and virtual hosting.
• Most virtual hosts handle multiple domain names
  on the same server by having the Uniform Resource
  Locator (URL) serve as a path to a file.
• Figure 3-10 illustrates the difference between
  virtual servers and virtual hosting.

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Ability to Support Virtual Servers and Virtual
Hosts (Continued)
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Protocols Supported

• All Web servers support HTTP, and some also
  support FTP so developers can send files to the
  site from remote locations.
• If you want to use your Web server for e-mail,
  the server must support e-mail protocols.

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Access Control

• Based on the users IP address or user ID, access
  control allows the Web server to limit to which
  files a user can read or write. User Ids are
  associated with passwords to verify a users
  identity.
• Another method of access control is changing the
  port at which a server is listening.
• Port 80 is the default port for Web servers.
• Apache Web Server controls access to its
  resources via a process known as authentication,
  which requires a user to enter a valid user ID
  and password to access a Web site.

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Access Control (Continued)
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Encrypting Protocols

• A secure protocol used by Web servers is SSL
  (Secure Sockets Layer).
• When you see a URL with https at the beginning
  instead of http, you know that this Web server
  is using the SSL protocol for security.

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Chroot Mode

• Chroot mode restricts the portion of the file
  system that the server occupies.
• Running in chroot mode offers security because
  all private files can be kept outside of the
  server area.

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Server Side Scripting

• A script is a short list of instructions that
  certain software can perform.
• The instructions must be written in a format
  called a scripting language.
• Popular scripting languages include Perl,
  VBScript, and JavaScript.

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Standard CGI-Based Scripts

• Common Gateway Interface (CGI) is the set of
  specifications that defines how a Web server
  passes a Web users input to an application
  program running on the server, receives a
  response, and passes data back to the user.
• One advantage of CGI scripts is that they are
  consistent among operating systems.

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Server-Side Include (SSI)

• Server-Side Include (SSI) is a simple form of
  scripting that allows you to include variable
  values in HTML code before it is sent to the
  browser.
• SSI scripts insert a line in the HTML file that
  indicates a variable value needs to be entered
  before the file is sent to the browser.

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Database Interfaces

• Before selecting a Web server or a virtual
  hosting service, find out what databases the
  server supports and what tools can exchange
  information with the database.
• Popular databases are MS Access, MySQL, Oracle,
  and SQL Server.

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Ability to Monitor Performance

• Microsoft IIS uses Performance Monitor, a program
  that comes with Windows Server 2003 and Windows
  2000 Server, to monitor performance.
• Other utility programs are Microsoft Web Capacity
  Analysis Tool (WCAT) and Server Check Pro by
  NetMechanic.

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Web Server Protocols

• HTTP and TCP/IP are the two main protocols used
  with Web servers.
• HTTP methods used for browser requests are GET,
  POST, HEAD, PUT, and DELETE.
• The most frequently used method is GET, which
  requests files from the Web server.
• A dialog is a series of commands from the sender
  to the receiver and replies from the receiver to
  the sender.

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Starting and Using Apache

• You can start Apache from the Windows Start menu
  or from a Command Prompt window.
• If you start Apache from a Command Prompt window,
  you can see error messages if they arise.
• The home page of your Apache Web Server can be
  accessed from another computer on your local area
  network.
• Follow the instructions to start and use Apache
  shown on pages 145 and 146.

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Apache Log Files

• You can look at the log files that Apache
  generates to help troubleshoot problems with the
  Web site.
• The two log files provided by Apache are the
  error log file and the access log file
  (access.log).
• Figure 3-17 shows a snapshot of the access log,
  which includes many GET methods where clients
  have requested Web pages.

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Apache Log Files (Continued)
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Examining E-Mail Client and Server Software

• Figure 3-18 shows the journey made by an e-mail
  message and the protocols that control the
  different parts of the journey.
• The senders computer and e-mail server both use
  SMTP (Simple Mail Transfer Protocol) to send an
  e-mail message to its destination.
• After the message arrives at the destination
  e-mail server, it remains there until the
  recipient requests delivery.

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Examining E-Mail Client and Server Software
(Continued)

• The recipients e-mail server uses one of two
  protocols to deliver the message POP (Post
  Office Protocol) or IMAP4 (Internet Message
  Access Protocol, version 4).
• Internet protocols are described and defined in
  an RFC (Request for Comments) sent to the
  Internet development community.
• When e-mail experts speak of error messages
  created during e-mail transactions, they
  sometimes call these messages 822 messages.

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Managing Your E-Mail Clients

• E-mail client software communicates with an
  e-mail server when it sends and receives e-mail.
• Some of the most common e-mail clients are
  Eudora, Outlook Express, Outlook, and Pegasus
  Mail.
• When you configure your e-mail client software
  for the first time, you need to enter the
  addresses of your e-mail servers.

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How E-Mail Clients and Servers Interact

• The three e-mail protocols are SMTP, POP, and
  IMAP.
• The goal is to prepare you to respond to e-mail
  errors by understanding the source of the error
  and what you must do to resolve it.

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SMTP

• SMTP is the protocol used to send e-mail over the
  Internet.
• SMTP is typical of many client/server protocols
  in the TCP/IP protocol suite in that
  character-based commands are issued from the
  client and the server replies with numeric codes.
• An SMTP transaction begins when an e-mail client
  program sends an e-mail message to a recipient.
• Table 3-3 shows one example of an SMTP dialog
  between servers.

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SMTP (Continued)

• SMTP is considered a stateful protocol because it
  can recognize and interpret the nature of the
  material being sent, such as commands or data.
• In contrast, TCP is considered a stateless
  protocol because it is not concerned with what is
  being sent.
• TCP establishes the session but does not
  interpret the transmissions that occur during the
  session.

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POP

• POP is used when a client downloads its e-mail
  messages from a server.
• First, the client sends the user ID and password
  to the server.
• The server verifies that the user has an e-mail
  account with the server.
• Then a session is established between the client
  and the server.

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POP (Continued)

• Next, transactions occur as the client requests
  the mail, and then the session is closed.
• This process contains three states
• Authentication
• Transaction
• Update

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IMAP

• IMAP is expected to replace POP because it offers
  these additional functions
• Messages can be archived in folders on the
  server.
• Mailboxes can be shared, so multiple users can
  access the same mail.
• Users can easily access multiple mail servers.
• Users can choose to read only the header
  information about an attached file without
  opening the file.
• Attached files need not be downloaded with every
  message.

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E-Mail Server Software

• An ISP or large business using the Internet or
  having an intranet is responsible for providing
  an e-mail server for its subscribers or
  employees.
• E-mail servers most likely are installed on UNIX,
  Linux, Windows 2000 Server, or Windows Server
  2003.

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Microsoft Exchange Server

• Protocols supported by Exchange Server include
  HTTP, MAPI, POP3, IMAP4, and NNTP protocols.
• NNTP (Network News Transfer Protocol) is the
  protocol used by newsgroups.
• MAPI (Messaging Application Programming
  Interface) is a specification that allows an
  application to interact with an e-mail client to
  send and receive e-mail.
• Microsoft Exchange Server supports hot backups
  and dynamic rerouting.

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IBM Lotus Domino

• Lotus Domino by IBM is designed for large
  companies and ISPs.
• It can be run on a variety of server platforms,
  such as Windows NT Server, Windows 2000 Server,
  Windows Server 2003, Sun Solaris, IBM OS/400, and
  several flavors of Linux.
• Domino provides a useful feature that allows
  administrators to remotely manage the e-mail
  server from the Web browser on any computer in
  their network.

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Novell GroupWise

• This product is designed for medium to large
  companies running NetWare or Windows Server
  platforms.
• To run GroupWise, you must set up Novell
  Directory Services.

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E-Mail Client Support for HTML

• For most of the time e-mail has existed, e-mail
  messages have consisted of text only.
• Recently, HTML e-mail has become very popular.
• Eudora, Outlook Express, and Outlook clients now
  support HTML in the body of e-mail messages.

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Using FTP to Transfer Files

• Web servers (using HTTP) and e-mail software
  (using SMTP) must encode data so it appears as
  text when it travels over the Internet.
• FTP (File Transfer Protocol) offers an
  alternative.
• FTP can transfer binary files over the Internet
  without the encoding and decoding overhead,
  making it a popular protocol for moving files
  over the Internet.
• An FTP site is a computer running an FTP server
  application.

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Using FTP to Transfer Files (Continued)

• Large organizations might maintain several FTP
  sites in different parts of the world to speed up
  download time across the globe.
• These are called mirror sites.
• A mirror site is a server that contains the same
  set of files as a heavily used server to off-load
  some of the burden of providing the files to the
  community using them.
• Mirror sites also serve as a backup for the main
  server in case the main server fails.

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How FTP Works

• An FTP server identifies users on an FTP site by
  their user IDs.
• FTP client and server software create a session
  after you are logged on.
• The FTP client has access to the file system on
  the server.
• The local computer (the client) issues
  character-like commands, and the remote computer
  (the server) replies with numbers that are
  interpreted by the local computer.

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FTP Via a Web Browser

• Have you ever attempted to download software from
  a Web site and clicked a hyperlink that says
  Click here to download now or a similar
  message?
• If you carefully note the URL after you click to
  download the software, you will see that the
  protocol changes from http// to ftp// in the
  Web browsers Address box.

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FTP from a Command Prompt

• Most operating systems, including Windows 9x,
  Windows NT, Windows 2000, and Windows XP, offer
  FTP client software that runs from a command
  prompt.
• A batch file is a file with a .bat file extension
  that contains a list of DOS-like commands that
  can be executed as a group.

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FTP from a Command Prompt (Continued)

• Another protocol similar to FTP is TFTP (Trivial
  FTP).
• TFTP has fewer commands than FTP and can be used
  only to send and receive files.
• It can be used for multicasting in which a file
  is sent to more than one client at the same time
  using the UDP (User Datagram Protocol).

46
Initiating and Using Telnet Sessions

• A Telnet window on a computer is a command window
  to a remote computer in which any command can be
  executed just as though the user were sitting at
  the computer console.
• Telnet is a protocol used to pass commands and
  replies between the client the the UNIX computer.
• All UNIX systems support some form of Telnet.

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Subscribing to and Using Newsgroups

• A newsgroup is a service on the Internet or
  private network where a group of people can post
  articles and responses to those articles so
  information can be shared among the members of
  the group.
• A newsgroup uses NNTP.
• This protocol works much like SMTP, whereby
  commands are issued from the client or requesting
  server as character-based words followed by
  arguments, and replies come from the news server
  in the form of numeric codes followed by
  descriptive text.

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Understanding Gophers and Gopher Space

• A gopher is a distribution service for text files
  on the Internet that runs on a UNIX computer
  using the Gopher protocol.
• A gopher service runs on a UNIX computer,
  tracking the documents available on the server in
  the form of a hierarchical site menu called
  gopher space.
• When you access the service, you can browse the
  gopher space by searching these top-down lists.

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Summary

• A server can be software that provides a service
  to other software or the computer on which the
  server software is running.
• TCP creates sessions that application protocols,
  such as HTTP and FTP, can use.
• Apache Web Server by the Apache Software
  Foundation is a common Web server that runs on a
  UNIX or Windows platform.
• Protocol dialogs can be used to troubleshoot
  problems with servers.

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Summary (Continued)

• E-mail uses SMTP to send text messages over the
  Internet and uses POP or IMAP when an e-mail
  client requests messages stored on its e-mail
  server.
• FTP sites can be accessed by client software,
  such as Web browsers, operating system command
  utilities, or GUI software dedicated to FTP, such
  as WS_FTP Pro.
• FTP also can be accessed from a command prompt.
• News servers work by getting news articles from
  other news servers.