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Chapter 3: How Clients and Servers Work Together

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Title: 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

3
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.

4
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.

5
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.

7
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.

9
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)

10
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.

11
Ability to Support Virtual Servers and Virtual
Hosts (Continued)
12
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.

13
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.

14
Access Control (Continued)
15
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.

16
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.

17
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.

18
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.

19
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.

20
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.

21
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.

22
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.

23
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.

24
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.

25
Apache Log Files (Continued)
26
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.

27
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.

28
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.

29
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.

30
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.

31
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.

32
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.

33
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

34
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.

35
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.

36
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.

37
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.

38
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.

39
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.

40
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.

41
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.

42
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.

43
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.

44
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.

45
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.

47
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.

48
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.

49
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.

50
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.
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