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Domain names On the Internet, computer names are organized in a hierarchy of domain names and hostnames. If you want to have and maintain your own Internet domain, you need to be assigned one that fits into one of the top-level domains (domains such as .com, .org, .net, .edu, .us, and so on). Hostnames If a domain name is assigned to your organization, you are free to create your own hostnames within that domain. This is a way of associating a name (hostname) with an address (IP address). When you use the Internet, you use a fully qualified domain name to identify a host computer. For example, in the domain handsonhistory.com, a host computer named baskets would have a fully qualified domain name of baskets.handsonhistory.com. Within an organization, you should choose a host-naming scheme that makes sense to you. For example, for handsonhistory.com, you could have hostnames dedicated to different crafts (baskets, decoys, weaving, and so on). Routers If you have a LAN or other type of network in your home or organization that you want to connect to the Internet, you can share an Internet connection. You do this by setting up a router. The router connects to both your network and the Internet, providing a route for data to pass between your network and the Internet. Firewalls and IP masquerading To keep your private network somewhat secure, yet still allow some data to pass between it and the Internet, you can set up a firewall. The firewall restricts the kind of data packets or services that can pass through the boundary between the private and public networks. If your network uses private addresses, or if you just want to protect the addresses of computers behind your firewall, you can use a technique called IP masquerading. Note Though you can set up a firewall to filter packets on any computer on your private network, firewalls are typically configured on the machine that routes packets between the public and private networks. In this way, intruders can be stopped before they get on your private network and security can be relaxed somewhat between your computers behind the firewall. Proxies You can bypass some of the configuration required to allow the computers on your LAN to communicate directly with the Internet by configuring a proxy server. With a proxy server, a computer on your LAN can run Internet applications (such as a Web browser) and have them appear to the Internet as if they are actually running on the proxy server. Cross-Reference You can read about firewalls in Chapter 14. IP masquerading is described later in this chapter. Internet domains You can t read a magazine, watch a TV commercial, or open a cereal box these days without hitting a something.com. When a company, organization, or person wants you to connect to them on the Internet, it relies on the uniqueness of its particular domain name. However, within that domain name, the company or organization to which it has been assigned can arrange its content however it chooses. Internet domains are organized in a structure called the domain name system (DNS). At the top of that structure is a set of top-level domains (or TLDs). Some of the top-level domains are used commonly in the United States, although they are available for worldwide use. TLDs such as edu (for colleges and universities), gov (for United States government), and mil (for United States military sites) were among the most used TLDs in the early Internet. In more recent years, com (for commercial sites) has experienced the most growth. The us domain was added to include U.S. institutions, such as local governments and
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Chapter 16: Connecting to the Internet Overview This chapter demonstrates how to connect Red Hat Linux to any TCP/IP-based network, such as the Internet, a private intranet, or a company extranet. The differences in how you connect have more to do with the network medium you use (that is, telephone lines, LAN router, and so on) than they do with whether you are connecting to the public Internet or a company s private network. Connections to the Internet described in this chapter include a simple dial-up connection from your own Red Hat Linux system. The most popular protocols for making dial-up connections to the Internet are Point-to-Point Protocol (PPP) and Serial Line Internet Protocol (SLIP). This chapter focuses on PPP (it is more widely used than SLIP). It also builds on the procedures in Chapter 15 for creating your own Local Area Network (LAN) by teaching you how to connect your LAN to the Internet. This chapter first provides an overview of the structure of the Internet, including descriptions of domains, routing, and proxy service. It then discusses how to connect your Red Hat Linux system to the Internet using PPP dial-up connections. For those who want to connect a LAN to the Internet, it describes how to use Red Hat Linux as a router and set it up to do IP masquerading (to protect your private LAN addresses). Finally, it describes how to configure Red Hat Linux as a proxy server, including how to configure client proxy applications such as Netscape and Microsoft Internet Explorer. Understanding How the Internet Is Structured In order to operate, the Internet relies on maintaining a unique set of names and numbers. The names are domain names and hostnames, which enable the computers connected to the Internet to be identified in a hierarchy. The numbers are Internet Protocol (IP) addresses and port numbers, which enable computers to be grouped together into interconnected sets of subnetworks, yet remain uniquely addressable by the Internet. An Internet Service Provider (ISP) will give you the information you need to set up a connection to the Internet. You plug that information into the programs used to create that connection, such as scripts to create a Point-to-Point Protocol (PPP) connection over telephone lines. See the section later in this chapter on outgoing dial-up connections for descriptions of the information needed from your ISP and the procedures for configuring PPP to connect to the Internet. The following list describes basic Internet structure in more detail: IP addresses These are the numbers that uniquely define each computer known to the Internet. Internet authorities assign pools of IP addresses (along with network masks, or netmasks) so that network administrators can assign addresses to each individual computer that they control. An alternative to assigned addresses is to use a reserved set of private IP addresses. Cross-Reference See Chapter 15 for a description of IP addresses. Port numbers Port numbers provide access points to particular services. A server computer will listen on the network for packets that are addressed to its IP address, along with one or more port numbers. For example, a Web server listens to port 80 to respond to requests for HTTP content.
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Summary Red Hat Linux is at its best when it is connected to a network. Configuring a LAN enables you to share resources with other computers in your home or organization. These resources can include files, printers, CD-ROM drives, and backup media. This chapter describes how to create a LAN with a Red Hat Linux system being used on one of the computers on that LAN. It helps you determine the kind of equipment you need to obtain, and the layout (topology) of the network. On the Red Hat Linux side, you learned about choosing and installing Ethernet cards (also called NICs). You also learned to configure TCP/IP so that you can later employ a variety of TCP/IP tools to use the network. If something isn t working with your Red Hat Linux interface to the LAN, you can use utilities such as ifconfig to check that your Ethernet interface is configured and running properly. You can also check that Linux found and installed the proper driver for your Ethernet card. After an Ethernet interface is working, you can use the Ethereal window to monitor the packets coming and going across the interface between your computer and the network.
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You can filter for certain special activities on the network, using such things as the gateway, broadcast, or multicast primitives. Entering gateway host lets you find packets sent to a gateway host that is neither a Source nor Destination for the packet (which is determined because the Ethernet address doesn’t match either of those IP addresses). Enter ether broadcast to monitor broadcast packets on your Ethernet network, such as announcements from name servers announcing availability. Likewise, you could filter for multicast packets on ether or ip protocols (ether multicast). Interpreting captured Ethernet data With the captured data displayed in your Ethereal window, you can get a detailed view of the network traffic that your computer is exposed to. The Ethereal window is divided into three parts. The top part contains a scrollable list of packets. The protocol tree for the current packet appears in the middle part of the display. A hexadecimal dump of the entire contents of the packet appears in the bottom part. You can sort data in different ways from the top part of the window by clicking on the column headings. To see more details relating to different items in the protocol tree for the current packet, you can click the plus sign next to the protocol information that interests you. The following are some tips that will help you interpret what the data means: The Source and Destination columns show where each packet came from and where it went. If the Enable name resolution option is on (which is recommended), the host name associated with IP packets is displayed. This makes it much easier to see which computer is communicating with you. To see all activity associated with a particular location, click the Source or Destination column. Packets will be sorted alphabetically, making it easier for you to scroll through activity for the location that interests you. If you are trying to debug a particular feature, you may want to click the Protocol column to gather activities based on protocol. For example, if you were trying to get Samba to work (for Windows file or printer sharing), sorting by protocol would enable you to see all NetBIOS and NBNS (NetBIOS name server) requests that came to your computer. To mark a packet of interest to you, click the middle mouse button on it. This will highlight the packet, making it easier to find later. (If you only have a two-button mouse, and you indicated during installation that it should emulate a three-button mouse, you can click both mouse buttons together to emulate the middle mouse button.) The Info column gives you details about the intention of the packet. For example, you can see the type of service that was requested (such as http for Web service or FTP for file transfer). You can see what information is being broadcast and determine when attempts to find particular host computers are failing. If you believe someone is using your network improperly, you can see which sites they are visiting and the services they are requesting. Another handy option is one that lets you follow the stream of TCP information. Click Tools Follow TCP Stream. The “Contents of TCP stream” window that appears lets you see the total output of the HTTP, SMTP, or other protocol being used.
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The Ethereal Capture window displays information on how many incoming and outgoing packets have crossed the interface since the capture began. The number of packets that are associated with each protocol Ethereal monitors is displayed, along with the percentage of total packets associated with each protocol. For this example, I opened a Web page (resulting in TCP packets) and ran the ping command (resulting in ICMP packets). Figure 15-11: Ethernet activity is displayed by protocol as packets are captured. 6. Click Stop. The snapshot of data you just took will appear on the Ethereal window. Packets are displayed in the order in which they traversed the interface. 7. If you did not already ask to save the capture data to a file, you may do so now by choosing File Save As. At this point, you can start interpreting the data. Using Ethereal Filters If you are monitoring a busy server or a busy network, Ethereal can gather so much data that it can become almost unusable. If you know what you are looking for, however, you can use Ethereal to filter what packets are captured based on values you enter. Filters in Ethereal are implemented using the pcap library (type man pcap to read about it). The filter expressions you can use with Ethereal are described on the tcpdump man page. Here are some examples of filters that you could enter into the Filter box when you capture Ethernet data with Ethereal: host 10.0.0.15 The host primitive lets you only capture packets that are either to or from a particular host computer (by IP address or host name). By preceding host with src or des, you can indicate that you only want packages sent from a particular source or to a particular destination host. tcp port 80 You can enter a protocol name (such as tcp, ether, udp, or ip) to limit captured packets to those that are assigned to that protocol. As shown in the previous example, with tcp you could also indicate a port number (such as 80, to monitor traffic to and from your Web server).
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lo, you can see requests from local users for local TCP/IP services). 4. Choose other options relating to what data is captured: Count: By default, Ethereal will capture data from the moment you click OK on the Capture Preferences until you click Stop (with this value set to 0). Optionally, you can type any number into the Count field to capture only that number of packets. Filter: This optional field lets you enter a filter that can be used to filter capture data. You can type in filters individually or click the Filter button to use a filter you have stored earlier. Cross-Reference Filtering is one of the most powerful features of Ethereal. See the sidebar “Using Ethereal Filters” for further information on how to enter filters into the Filter field. File: Enter the name of a file in which you want to capture the data gathered. If you don’t enter a file name, the information is displayed on the Ethereal window without being saved to a file. Capture length: Enter the maximum number of bytes of data that can be displayed for each packet. Capture packets in promiscuous mode: Any computer on a LAN can see all packets that traverse the LAN, except for those packets intended for switched portions of the LAN. With this option on, all packets that are seen by your network interface are captured. With this mode off, only packets intended specifically for your network interface (including multicast and broadcast packets) are captured. In other words, turn on promiscuous mode to monitor the whole LAN and turn it off to monitor only your interface. Update list of packets in real time: Select this option to have packet information appear in the Ethereal window as each packet crosses the interface. With this option off, the information is displayed after you stop capturing it. Automatic scrolling in live capture: If you are updating packets in real time, select this option to have packet information scroll up after the screen fills. With this option off, you will just see the first screen of packets and have to scroll down manually to see the rest. Enable name resolution: With this option on, Source and Destination IP addresses are displayed as host names (if they can be resolved from /etc/hosts or DNS). With this off, IP addresses appear for the Source and Destination columns. 5. Click OK. Ethereal will begin gathering data on the packets encountered by the selected interface. Figure 15-11 shows an example of the Ethereal capture window that appears.
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Figure 15-9: Configure your Ethernet card for TCP/IP during installation. The primary function of Ethereal is to take a snapshot of the packets coming across your network interfaces and display that data in the Ethereal window. You can filter the data based on a variety of filter primitives. When the capture is done, you can step through and sort the data based on the values in different columns. Optionally, you can save the captured data to a file to study the data at a later time. Tip If you can’t use Ethereal because you don’t have a GUI available, you can use the tcpdump command from the shell. It is not as friendly as Ethereal, but it supports the same filtering syntax. Because tcpdump can produce a lot of output, you will probably want to use some form of filtering and/or direct the output of the command to a file. (Type man tcpdump for information on filter options.) Capturing Ethernet data With the Ethereal window displayed, you can capture data relating to packet activities on any of your Ethernet network interfaces by doing the following: 1. Click Capture. 2. Click Start. An Ethereal Capture Preferences window appears, as shown in Figure 15-10. Figure 15-10: Choose preferences for capturing Ethernet data from the Ethereal window. 3. Click the down arrow next to the Interface box, to see what interfaces are available, and select one. If you have an Ethernet card installed, select eth0 to choose to capture data for packets being sent across that card. You can also choose to monitor the lo interface, to watch the loopback driver. (By choosing
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# /etc/init.d/network restart Another way to see statistics for your Ethernet driver is to list the contents of the process pseudo file system for network devices. To do that, type the following: # cat /proc/net/dev The output should look similar to the following: Inter-| Receive | Transmit face |bytes packets errs drop fifo frame compressed multicast|bytes packets errs drop fifo colls carrier compressed lo: 5362 64 0 0 0 0 0 0 5362 64 0 0 0 0 0 0 eth0: 3083 35 0 0 0 0 0 0 3876 31 0 0 0 0 0 0 The output is a bit hard to read (our book isn t wide enough to show it without wrapping around). With this output, you can see Receive and Transmit statistics for each interface. This output also shows you how many Receive and Transmit errors occurred during communication. For a more detailed look at your network, you can use the Ethereal window. Ethereal is described in the following section. Watching LAN traffic with Ethereal If you really want to understand the coming and going of information on your LAN, you need a tool that analyzes network traffic. Ethereal is a graphical tool for capturing and displaying the packets being sent across your network interfaces. Using filters to select particular hosts, protocols, or direction of data, you can specifically monitor activities and track down problems on your network. In addition to reading Ethernet packet data gathered by Ethereal, the Ethereal window can be used to display captured files from LanAlyzer, Sniffer, Microsoft Network Monitor, Snoop, and a variety of other tools. These files can be read from their native formats or after being compressed with gzip (.gz). Ethereal can track more than 100 packet types (representing different protocols). It can also display specific fields related to each protocol, such as various data sizes, source and destination addresses, port numbers and other values. Starting Ethereal To start Ethereal, choose Programs Internet ethereal, or type the following (as root user) from a Terminal window: # ethereal & The Ethereal window appears, as shown in Figure 15-9. (If the ethereal command is not found, the package is probably not installed. You can install the ethereal package from the Red Hat Linux installation CD-2.)
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round-trip min/avg/max = 0.5/0.5/0.6 ms A line of output is printed each time a packet is sent and received in return. It shows how much data was sent and how long it took for each package to be received. After you have watched this for a while, type Ctrl+C to stop ping. At that point, it will show you statistics of how many packets were transmitted, received, and lost. If you don t see output that shows packets have been received, it means that you are not contacting the other computer. Try to verify that the names and addresses of the computers that you want to reach are in your /etc/hosts file or that your DNS server is accessible. Next, confirm that the names and IP addresses you have for the other computers that you are trying to reach are correct (the IP addresses are the most critical). Is your Ethernet connection up? Using the ifconfig command, you can determine whether your Ethernet (and other network interfaces) are up and running. Type the following command: # ifconfig The output that appears will be similar to the following: eth Link encap:Ethernet HWaddr 00:90:27:4E:67:35 inet addr:10.0.0.11 Bcast:10.255.255.255 Mask:255.0.0.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:156 errors:0 dropped:0 overruns:0 frame:0 TX packets:104 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 RX bytes:20179 (19.7 Kb) TX bytes:19960 (19.4 Kb) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:3924 Metric:1 RX packets:56 errors:0 dropped:0 overruns:0 frame:0 TX packets:56 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 RX bytes:3148 (3.0 Kb) TX bytes:3148 (3.0Kb) In this example, there are currently two network interfaces up on the current computer. The first section shows your Ethernet interface (eth0) and its hardware address, Ethernet hardware address, IP address (inet addr), broadcast address, and network mask. The next lines provide information on packets that have been sent, along with the number of errors and collisions that have occurred. Note The lo entry is for loopback. This enables you to run TCP/IP commands on your local system without having a physical network up and running. I describe this and other network interfaces in more detail in Chapter 16. If your eth0 interface does not appear, it may still be configured properly, but not running at the moment. Try to start the eth0 interface by typing the following: # ifconfig eth0 up After this, type ifconfig again to see if eth0 is now running. If it is, it may be that eth0 is simply not configured to start automatically at boot time. You can change it so Ethernet starts at boot-time (which I recommend), using the Network Configuration window (described earlier in this chapter). Tip If your network interfaces are not running at all, you can try to start them from the network initialization script. This interface reads parameters and basically runs ifconfig for all network interfaces on your computer. Type the following to restart your network:
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of this book describe how to configure these services.) Checking Your Ethernet Connection After your LAN has been set up, your Ethernet cards installed, and host names and addresses added, there are several methods you can use to check that everything is up and working. For example, you can check your boot messages to make sure that your board was detected and that you can use the ping command to make sure you can connect to other computers. Did Linux find your Ethernet driver at boot-time? Type the following to check that Linux found your card and installed the Ethernet interface properly: dmesg | grep eth The dmesg command lists all the messages that were output by Linux at boot-time. The grep eth command causes only those lines that contain the word eth to be printed. The first message shown below appeared on my laptop computer with the Netgear card. The second example is from my computer with the EtherExpress Pro/100 card: eth0: NE2000 Compatible: port 0×300, irq3, hw_addr 00:80:C8:8C:8E:49 eth0: OEM i82557/i82558 10/100 Ethernet at 0xccc0, 00:90:27:4E:67:35, IRQ 17. The message in the first example shows that a card was found at IRQ3 with a port address of 0×300 and an Ethernet hardware address of 00:80:C8:8C:8E:49. In the second example, the card is at IRQ 17, the port address is 0xccc0, and the Ethernet address is 00:90:27:4E:67:35. If the eth0 interface is not found, but you know that you have a supported Ethernet card, check that your Ethernet card is properly seated in its slot. Can you reach another computer on the LAN? Try communicating with another computer on the LAN. The ping command can be used to send a packet to another computer and to ask for a packet in return. You could give ping either a host name (pine) or an IP address (10.0.0.10). For example, to ping a computer on the network called pine, type the following command: # ping pine If the computer can be reached, the output will look similar to the following: PING pine.trees (10.0.0.10): 56 data bytes 64 bytes from 10.0.0.10: icmp_seq=0 ttl=255 time=0.6 ms 64 bytes from 10.0.0.10: icmp_seq=1 ttl=255 time=0.5 ms 64 bytes from 10.0.0.10: icmp_seq=2 ttl=255 time=0.5 ms 64 bytes from 10.0.0.10: icmp_seq=3 ttl=255 time=0.5 ms 64 bytes from 10.0.0.10: icmp_seq=4 ttl=255 time=0.5 ms 64 bytes from 10.0.0.10: icmp_seq=5 ttl=255 time=0.5 ms 64 bytes from 10.0.0.10: icmp_seq=6 ttl=255 time=0.5 ms 64 bytes from 10.0.0.10: icmp_seq=7 ttl=255 time=0.6 ms 64 bytes from 10.0.0.10: icmp_seq=8 ttl=255 time=0.5 ms 64 bytes from 10.0.0.10: icmp_seq=9 ttl=255 time=0.5 ms — pine.trees ping statistics — 10 packets transmitted, 10 packets received, 0% packet loss
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