Beginning Red Hat Linux 9 phần 7 potx

The TCP/IP networks can also be divided by public and private IP addresses: Your public IP address is assigned to you by your Internet service provider (ISP). In fact, this IP address is allocated to the ISP by the Internet Assigned Numbers Authority (IANA − http://www.iana.org). It's your public IP address that identifies you on the Internet. • Private IP addresses are those (like the addresses used in the diagram above) that identify a device within a private or non−Internet connected network. There are three TCP/IP network address ranges reserved for use in private networks: 10.0.0.0−10.255.255.255, 172.16.0.0−172.31.255.255, and 192.168.0.0−192.168.255.255. • A netmask is a 32−bit string that hides the network part of an IP address, so that only the host (computer) part of the address remains. For example, the netmask 255.255.255.0 looks like an IP address, but in fact hides the first 24 bits of the IP address so that only the last 8 bits remain (recall that 255 is the decimal representation of the binary 11111111). Hence, the netmask 255.255.255.0 is commonly used for Class C IP addresses to reveal the specific host computer address publicly. There are a few more definitions that will be useful here: A gateway is a host that is connected to two or more physical networks, and hence allows messages to switch between them. • The term hostname refers to the unique name of the machine, so that it can identify itself on the network. • Finally, the domain name system (or DNS) is a distributed database that translates domain names into IP addresses and vice versa. For example, DNS translates the domain name www.linux4biz.net to the IP address 212.69.200.83. • With all that in mind, let's return to those two machines on our network − the Red Hat Linux 9 server machine and the Windows 2000 client laptop machine. Here are the network configuration settings we used for those two machines in this chapter. The Red Hat Linux 9 server machine is set up as a firewall and router, and has Internet connectivity. Over the course of the chapter we'll show how it can be configured as a file server, printer server, DHCP server, FTP server, web server, or mail server: Server Value Operating system Red Hat Linux 9 IP address 192.168.0.99 Netmask 255.255.255.0 Hostname linux4biz The laptop is a simple client machine, designed for an end−user and (potentially) sharing resources with other similar machines on the network: Setting Value Purpose A Client laptop system to be used to share resources configured on Linux server Operating system Windows 2000 Professional IP address 192.168.0.132 Netmask 255.255.255.0 Assumptions for Network Setup 269 Hostname Win As we've said, your network topology, and the network configuration of your machine(s), may be different; but many of the principles in this chapter remain the same, and you should be able to get any of these services going regardless of your own environment. We will assume that you have root access to the Red Hat Linux server. Throughout the chapter, we'll use the terms Linux server and client to refer to the Red Hat Linux 9 server machine and the Microsoft Windows 2000 client machine respectively. Setting up a Web Server When we want to publish web pages on the Internet (or on an intranet), we use a web server. In essence, a web server is an application that does two things: It listens for page requests.• When it receives a page request, it examines the request and responds with the page that was requested. • For example, when you use a web browser to browse http://www.wrox.com, the browser turns this into a request message and sends it across the Internet to Wrox's own web server. When the web server receives this request, it processes it, works out what page you requested, puts that page together from whatever resources are necessary, and sends the page back to your browser, in the form of a response message. Of course, there are many different web browsers in existence (including Mozilla, Opera, Internet Explorer, and others), and there are also a great many types of web server software. To enable a browser to request pages from a web server, they communicate using Hypertext Transfer Protocol (HTTP) − this is the standard protocol for the Internet. The request and response messages are composed using HTTP, and this is what allows any browser to request web pages from any type of web server. Note By default, all web servers listen for HTTP requests on port 80. Web servers also use port 443 to listen for requests made through secure HTTP connections, over SSL (secure sockets layer), through a protocol called HTTPS. So, if you want to publish your own web site, you'll need a machine with some web server software. However, the chances are that if you build your own web site, you probably won't want to expose it to the Internet from your own machine. There are security and maintenance issues to manage, and you'd need to buy enough hardware and bandwidth to handle all the page requests. More likely, you'd choose an Internet service provider (ISP), and use their web servers to host your web site for you. So, why would you want to install a web server on your Red Hat Linux machine? Well, here are two scenarios: First, if you're building a web site, then you'll need a web server so that you can test your site as you're developing it • Second, although you might not host an Internet site from your own machine, you might host an intranet site − a private web site available only to other machines inside your private network. The demand for intranet pages is much more predictable than for Internet pages, and the security risks are not so significant. • Setting up a Web Server 270 So, in this section, we'll show you how to set up a web server on your machine, configure it, and publish pages on it. From there, you'll soon be developing your own sites. The Apache Web Server As we've mentioned, there are a number of commercial and freely available web servers in existence. Among the commercial web servers are offerings from Netscape, IPlanet, SunONE, Microsoft (the Internet Information Server, or IIS), and Zeus. Among the open source web servers, we can list Apache, thttpd, and Redhat TUX. Of all these, Apache is most widely used. At the time of writing, 66% of all web sites are hosted on Apache web servers (according to the Netcraft Web Server Survey, http://www.netcraft.com/survey) − the vast majority of them running on Linux or Unix operating systems. Apache's popularity is due not only of its open source pedigree, but also to its highly competitive levels of performance, functionality, stability, flexibility, and security: Apache's flexibility comes from the fact that it is a modular web server. That means that you can meet your requirements by plugging any number of external modules into the core httpd daemon. Of course, being open source software, you also have access to Apache's source code, which you can customize to fit your needs. Apache is also very scalable. You can run Apache on high−end hardware, and it's possible to increase the capacity of Apache web servers by sharing the load across any number of servers. It's also very portable, being available for a number of operating systems. • Apache's security is very good in comparison to other web servers. Moreover, the Apache Foundation is extremely active in the continued defense of Apache from security problems − particularly in the form of announcements and patches. • Apache performs very well − it boasts a highly optimized daemon for serving static content which dramatically outperforms its nearest rivals. Moreover, it rarely crashes and achieves extremely long up−times. • Apache comes with detailed documentation, which helps to make the setup and configuration easy. And of course, because it's so popular there's a wide network of support for Apache, in the form of mailing lists, newsgroups, and commercial vendors like Red Hat. • Apache development is active. The Apache Foundation is actively involved in development of new modules; new versions of Apache to make it reliable stable and secure. • Since you've already got Red Hat Linux 9, there's another good reason for choosing Apache as your web server software − it's included as part of the Red Hat Linux 9 distribution! At the time of writing, the latest version of Apache is 2.0.44. Installing Apache As we've alredy noted, Apache is a modular server − the core server provides the basic functionality, with extended features available in various modules. This makes it very flexible and easy to configure, becuase you need to configure only the modules you need. So, it's worth looking at how to control the installation and removal of these modules. In fact, the different modules are contained in the different packages that we can install via the Red Hat Package Manager (RPM). Recall that to start RPM's graphical interface, you can select Main Menu | System Settings | Add/Remove Applications, or type the following command at the command line: The Apache Web Server 271 $ redhat−config−packages The Apache web server packages are listed in the Web Server package group, which is found under the Applications category in the RPM. If you look at the details of this package group now, you'll see that there are 17 packages listed: Only one of these packages in the group is a Standard Package − that's the httpd package that contains the base Apache web server functionality. The remaining 16 packages are all optional packages (the RPM calls them Extra Packages). The important packages are explained below: Package Description httpd−manual Contains the documentation for the Apache web server. After installation, you can access this documentation from the command line by typing man httpd hwcrypto Provides support for hardware SSL acceleration cards. This package should be installed if you have hardware SSL acceleration cards like Ncipher Nforce on your server mod_ssl Provides an SSL interface to the HTTPS web server, and hence enables the Apache web server to support SSL. This package should be installed if you want to provide secure connections to your clients php Provides the PHP module for Apache, which enables the web server to serve PHP web pages. This package is required if you if you want to host web sites which contain pages written with the PHP scripting language webalizer Provides programs for web server log file analysis. This package enables you to generate HTML usage reports for your website Note You can find out more about other Apache modules at the Apache web site, http://httpd.apache.org/docs−2.0/mod. You can choose whichever extra packages fit your requirements; it's definitely worth installing the httpd−manual package, and then you should choose whatever other packages fit your own requirements. Remember that you can re−launch the RPM at any time, and add or remove packages, as you need them. The Apache Web Server 272 When you've selected the packages you need, click the Close button to dismiss the Web Server Package Details window, and then click the Update button on the on main Package Management window to begin the installation. During the installation, you'll be prompted to insert the distribution discs, as they're required. The RPM will also inform you of any conflict between package dependencies. The Apache Configuration Files Apache's configuration information is contained in a select bunch of configuration files: The /etc/httpd/httpd.conf file is Apache's main configuration file.• The /etc/httpd/conf.d directory contains configuration files for any installed modules (such as PHP, SSL, and so on). • The /etc/httpd/logs directory is a symbolic link to /var/log/httpd directory, which contains all the Apache log files. • The /etc/httpd/modules directory is a symbolic link to /usr/lib/httpd/modules directory, which contains all the Apache modules configured as dynamic shared objects. Note Dynamic shared objects (or DSOs) are modules that are compiled separately from the Apache httpd binary. They are so−called because they can be loaded on demand. There's more information on DSOs at http://httpd.apache.org/docs−2.0/mod. • The /etc/httpd/run directory is a symbolic link to /var/run, which contains the process ID file (httpd.pid) of the httpd process. • /etc/rc.d/init.d/httpd is a shell script, used for starting and stopping the Apache web server.• Starting Apache for the First Time You can start Apache using the Red Hat Service Configuration GUI dialog, or via the command line. Here's how to start Apache from the Service Configuration dialog: First, you need to fire up the GUI. To do this, click on Main Menu | System Settings | Server Settings | Services or type the following at the command line: $ redhat−config−services It will ask for root password if you started it as a normal user. When you do this, you get the Service Configuration dialog: 1. The Apache Web Server 273 This dialog contains a list of all the services that are supported on your computer. The checkboxes indicate which of these services are started automatically when the machine boots up (as part of the cron). There are also Start, Stop, and Restart buttons so that we can start and stop services manually. Scroll down the list on the left of the dialog, and select the entry for httpd. As we said, the httpd service is the Apache web server service. Check the status of the httpd service, in the bottom−right of the dialog; if it is stopped (as above), then click the Start button to start it. If you want it to start automatically next time you reboot your machine, check the checkbox too. Then select File | Save Changes to save the new settings. That's it; you can close the Service Configuration dialog by selecting File | Quit. 2. To control the Apache web server from the command line, we can use the service command to fire the httpd script. Here's how we use it to start the web server: # service httpd start Starting httpd: [ OK ] If there are difficulties in starting the web server, then you'll find out about it here. For example, if you attempt to do this without root privileges, then you'll get a message telling you that permission is denied. And here's another example: # service httpd start Starting httpd: httpd: Could not determine the server's fully qualified domain name, using 192.168.0.99 for ServerName [ OK ] Here, the message explains that the server's fully qualified domain name (FQDN) couldn't be determined, and that it's using the IP address (192.168.0.99) instead. If you get this message, it's because you have neither setup a FQDN for your Linux server, nor configured the ServerName configuration directive in the httpd.conf configuration file. Note An FQDN is a unique name, consisting of hostname and domain name, which can be resolved to an IP address. For example, www.linux4biz.net is a fully qualified domain name. Returning to the httpd script itself, there are similar switches for stopping or restarting the service: # service httpd restart Stopping httpd: [ OK ] Starting httpd: [ OK ] # service httpd stop Stopping httpd: [ OK ] There are a few other options you can use with the httpd script. If you run the script without an option, the resulting usage message reveals all the available options: # service httpd Usage: httpd {start|stop|restart|condrestart|reload|status|fullstatus|graceful| help|configtest} The configtest option is a particularly useful one. The configuration file is quite complex, and so it's useful to The Apache Web Server 274 check it for errors after you make changes to it. To do this, you use the configtest option: # service httpd configtest Syntax OK Testing the Apache Web Server Once you've started the Apache web server, you should test it to see if it's working properly. To do that, we'll use a web browser to request a web page from our server! There's a page provided by default for this purpose, and you can request it via the URL http://localhost. So, launch a web browser (Main Menu | Internet | Mozilla Web Browser), and type this URL into the address box: Configuring your Web Server It's a good idea to configure your web server. Red Hat does provide a GUI tool, apacheconf, for configuring Apache, but it does not format the configuration file very well. Webmin (http://www.webmin.com − see Chapter 13) is an alternative web−based software that can be used to manage most of the Linux services, including the Apache web server. However, in this section we'll show you how to do some basic configuration by editing the httpd.conf Apache configuration file directly. Try it Out: Configuring Your Web Server We're going to configure the web server by adjusting a couple of the settings to suit our needs. Launch the gedit text editor (by selecting Main Menu | Accessories | Text Editor), or your favorite text editor. Use it to open the file /etc/httpd/conf/httpd.conf. 1. Select Search | Find and use the resulting dialog to find the word ServerAdmin in the file. The first occurrence should be the ServerAdmin directive, which looks like this: # ServerAdmin: Your server address, where problems with the server should be # e−mailed. This address appears on some server−generated pages, such # as error documents. e.g. admin@your−domain.com ServerAdmin root@localhost 2. As you can see, the configuration file describes this directive very well. Change the email address to your own email address, or an address that you may have set up for web site administration. For example: The Apache Web Server 275 # ServerAdmin: Your server address, where problems with the server should be # e−mailed. This address appears on some server−generated pages, such # as error documents. e.g. admin@your−domain.com ServerAdmin webmaster@linux4biz.net Now use the same technique to find the ServerName directive (you'll probably find that it's right after the ServerAdmin directive). Change this directive first by removing the leading # character. Then, if you have a registered DNS name, add it here (in place of the DNS name test.linux4biz.net, which I'm using here to demonstrate): # ServerName gives the name and port that the server uses to identify itself. # ServerName test.linux4biz.net The server name you specify should be a FQDN (because it will need to be resolved to an IP address by DNS). If you haven't setup a DNS, then you can enter the IP address allocated to the Linux server instead, like this: ServerName 192.168.0.99 3. Save the httpd.conf file, and close it.4. Restart the httpd daemon to reflect the changes. To do this, use Red Hat Services Configuration GUI, or the httpd command line script as described earlier in this chapter: # service httpd restart Stopping httpd: [ OK ] Starting httpd: [ OK ] Now browse to http://localhost again, to check that the web server is still serving web pages. You should see the Apache Test Page that we saw earlier in this section. 5. There are many configuration settings that you can control via the httpd.conf configuration file. For more information, refer to the Apache web server documentation at http://httpd.apache.org/docs−2.0/. Setting up Your First Web Site In the remainder of this section, we'll set up a simple web site, which will be accessible on the network. This will involve creating a simple HTML web page, and saving it to a location on the hard disk that is used by the web server to store published web pages. Then, when a user requests the page, the web server will be able to respond by retrieving it from this location and sending it to the requestor. Try it Out: Publishing Your First Web Page Launch an editor (you can use a dedicated web page editor such as Mozilla Composer if you have it installed; alternatively, gedit will do). Create a new file − call it index.html and save it in the directory /var/www/html. 1. Type some HTML into the file. For example: <html> 2. The Apache Web Server 276 <head> <title>Beginning Red Hat Linux − Apache Test Page</title> </head> <body> <center> <h1>Beginning Red Hat Linux − Apache Test Page</h1> This is the first page of your web site! </center> </body> </html> Now open a browser and browse to the page using your server's FQDN address or IP address. (For example, I'm using the URL http://test.linux4biz.net/index.html below to match my DNS name, but you might use something like http://192.168.0.99/index.html.) Your browser will send this request to the web server listening for connections on port 80 by default. Web server will serve the default web site pages. The following screenshot shows the output you should see: 3. How it works In this example, we set up a simple web page under Apache web server's default root directory, /var/www/html, and tested it on an internal network. You can add more pages and directories under the /var/www/html directory as explained in the previous example. If you want to make your web site available on the Internet, then you'd need to connect your web server to the Internet and assign it a public IP address, so that external users send page requests to it. You can also register a DNS so that your website as a memorable name (so users don't need to refer to your web site by its IP address!). By default, your Apache web server hosts just one web site (whose root is at /var/www/html). However, you can configure your Apache web server to host as many web sites as you like, and specify the root directory of each site to be whatever folder you wish. If you want more information about acquiring and using Apache, try http://httpd.apache.org/, or Professional Apache 2.0 and Professional Apache Security. Setting up an FTP Server If you want to enable other users to download files from a location on your server's hard disk, and/or to upload files to that location, then one solution is to install an FTP server. You can think of an FTP server essentially as an area of disk space that is used for storing files, plus the software and configuration required to allow Setting up an FTP Server 277 other users to upload and download files. When users want to upload or download from your FTP server, they use a program called an FTP client. These communications between FTP server and FTP client take place using the File Transfer Protocol (FTP). FTP is a TCP protocol that is designed specifically for the transfer of files over a network, and it's one of the oldest Internet protocols still in widespread use. The availability of so many different FTP client programs, and the fact that many operating systems come with FTP software pre−installed, are indications of how relevant FTP still is today. FTP is not considered a secure protocol, because communication between the FTP client and server are unencrypted. Consequently, Secure FTP (SFTP) is also becoming popular (and, indeed, is part of the openssh package that comes with Red Hat Linux 9), and it's also possible to configure your FTP server in other ways, for example by forcing users to log in, or by using access control lists (ACLs) to allow different rights to different groups of users. Note You need to take careful security precautions if you plan to expose your FTP server on the Internet. We'll talk about how to minimize the security implications of supporting an FTP server in Chapter 12. There's also information on this subject, and on the FTP protocol in general, at RFCs 959 and 2577 (ftp://ftp.rfc−editor.org/in−notes/rfc959.txt and ftp://ftp.rfc−editor.org/in−notes/rfc2577.txt). In fact, many FTP servers still allow anonymous FTP access, which means that the FTP server allows any user to access its disk space and download its files. Anonymous FTP access is used mostly to enable users to access freely available documents and files via the Internet without access control. In spite of the security issues, FTP remains popular − it's fast and easy to use, and it is the Internet standard protocol for file transfer. FTP Servers in the Red Hat Linux Distribution There are a number of FTP servers available for Red Hat Linux 9 For example: vsftpd is a simplified FTP server implementation. It is designed to be a very secure FTP server, and can also be configured to allow anonymous access. We'll explore vsftpd in this section. • TUX is a kernel−based, threaded, extremely high performance HTTP server, which also has FTP capabilities. TUX is perhaps the best in terms of performance, but offers less functionality than other FTP server software. TUX is installed by default with Red Hat Linux 9. • wu−ftpd is a highly configurable and full−featured FTP daemon, which was popular in earlier versions of Red Hat Linux but has since given way to the more security−conscious vsftpd. • gssftpd is a kerberized FTP daemon, which means that it is suitable for use with the Kerberos authentication system. • All of these FTP servers ship as part of the Red Hat Linux 9 distribution. However, we will consider only the first of these, vsftpd, here. FTP Servers in the Red Hat Linux Distribution 278 [...]... role of the client to quickly test that the service is working From a command line, issue the ftp command to start an FTP session, naming your FTP server as the server that you want to connect to: $ ftp 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ) 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): In the above, recall that 192 .168.0 .99 is the IP address of the Linux server You should get a Name login... ftp to indicate that you want anonymous access: 281 Using Your FTP Server $ ftp 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ) 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): anonymous 331 Please specify the password Password: 230 Login successful Have fun Remote system type is UNIX Using binary mode to transfer files 2 Now, we can start to examine the contents of the FTP site that are available... vsftpd restart That's it Now we're ready to test this configuration Using an FTP Client to Test Anonymous Write Access So, let's test our configuration with another simple session on our FTP client: 1 Connect to the client and log in (using the username anonymous or ftp) as you did before: $ ftp 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ) 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): anonymous... refused: $ ftp 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ) 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): eziodm 530 This FTP server is anonymous only Login failed Blocking FTP Access for User Accounts Alternatively, we can deny FTP access to individual users (rather than to all users) We control this using the file /etc/vsftpd.ftpusers file, which contains the list of users that are not allowed... 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ) 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): eziodm 331 Please specify the password Password: 530 Login incorrect Login failed Configuring an FTP Greeting Banner Once you've installed the vsftpd FTP server, there are all sorts of ways you can configure it to behave as you want Here's a simple example We can change the default greeting message that... ftp>ls 2 27 Entering Passive Mode ( 192 ,168,0 ,99 , 69, 34) 150 Here comes the directory listing drwxr−r−x 2 0 0 4 096 Feb 28 13:40 pub 226 Directory send OK This shows that the root directory contains just one subdirectory, called pub Now we'll use cd to change to this directory, and we'll list its contents: ftp> cd pub 250 Directory successfully changed ftp> ls −al 2 27 Entering Passive Mode ( 192 , 168, 0, 99 ,... directory listing drwxr−xr−−x 2 0 0 4 096 Feb 28 13:40 drwxr−xr−x 3 0 0 4 096 Feb 27 19: 15 −rw−r−−r−− 1 0 0 22 Feb 28 13:30 test.txt 226 Directory send OK 3 Now, we'll attempt to download the test.txt file we've just located To do this, we'll use the get command: ftp> get test.txt local: test.txt remote: test.txt 2 27 Entering Passive Mode ( 192 , 168, 0, 99 , 98 , 2 07) 150 Opening BINARY mode data connection... the login banner string: ftpd_banner=Welcome to the Wrox Press FTP service Then restart the vsftpd service, so that the change takes effect Now use your FTP client to log in to the FTP server, and you'll be greeted with this new message: $ ftp 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ) 220 Welcome to the Wrox Press FTP service For more information about the vsftpd FTP server, and all the configuration... network printer browsing 286 Print Servers on Redhat Linux 9 The Red Hat Linux 9 installation process should install the cups package by default To check, launch the RPM (for example, using Main Menu | System Settings | Add/Remove Applications), locate the Printing Support package group, which is under the System category, click its Details button, and ensure that the checkbox (next to the cups option)... transfer files 2 Change directory to the pub/upload directory Try to list its contents − you'll find that you can't, because that's the way we configured the permissions on the upload directory: ftp> cd /pub/upload 250 Directory successfully changed ftp> ls 2 27 Entering Passive Mode ( 192 , 168, 0, 99 , 95 , 148) 150 Here comes the directory listing 226 Transfer done (but failed to open directory) 3 However, . 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ). 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): In the above, recall that 192 .168.0 .99 is the IP address of the Linux server. You should get. ftp to indicate that you want anonymous access: 1. Using Your FTP Server 281 $ ftp 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ). 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): anonymous 331. username anonymous or ftp) as you did before: $ ftp 192 .168.0 .99 Connected to 192 .168.0 .99 ( 192 .168.0 .99 ). 220 (vsFTPd 1.1.3) Name ( 192 .168.0 .99 :none): anonymous 1. Using Your FTP Server 283 331