From Linux Home Networking
IntroductionNow that you have a firm grasp of many of the most commonly used networking concepts, it is time to apply them to the configuration of your server. Some of these activities are automatically covered during a Linux installation, but you will often find yourself having to know how to modify these initial settings whenever you need to move your server to another network, add a new network interface card or use an alternative means of connecting to the Internet.How to Configure Your NIC's IP AddressYou need to know all the steps needed to configure IP addresses on a NIC card. Web site shopping cart applications frequently need an additional IP address dedicated to them. You also might need to add a secondary NIC interface to your server to handle data backups. Last but not least, you might just want to play around with the server to test your skills.This section shows you how to do the most common server IP activities with the least amount of headaches. Determining Your IP AddressMost modern PCs come with an Ethernet port. When Linux is installed, this device is called eth0. You can determine the IP address of this device with the ifconfig command.[root@bigboy tmp]# ifconfig -a In this example, eth0 has no IP address because this box is using wireless interface wlan0 as its main NIC. Interface wlan0 has an IP address of 192.168.1.100 and a subnet mask of 255.255.255.0 You can see that this command gives good information on the interrupts, or PCI bus ID, used by each card. On very rare occasions you might find that your NIC card doesn't work because it shares both an interrupt and memory access address with some other device. You can look at the contents of the /proc/interrupts file to get a listing of all the interrupt IRQs used by your system. In the example below we can see that there are no conflicts with each IRQ from 0 to 15 having only a single entry. Devices eth0 and eth1 use interrupts 10 and 5, respectively: [root@bigboy tmp]# cat /proc/interruptsIf there are conflicts, you might need to refer to the manual for the offending device to try to determine ways to either use another interrupt or memory I/O location. Changing Your IP AddressIf you wanted, you could give this eth0 interface an IP address using the ifconfig command.[root@bigboy tmp]# ifconfig eth0 10.0.0.1 netmask 255.255.255.0 upThe "up" at the end of the command activates the interface. To make this permanent each time you boot up you'll have to add this command in your /etc/rc.local file which is run at the end of every reboot. Fedora Linux also makes life a little easier with interface configuration files located in the /etc/sysconfig/network-scripts directory. Interface eth0 has a file called ifcfg-eth0, eth1 uses ifcfg-eth1, and so on. You can place your IP address information in these files, which are then used to auto-configure your NICs when Linux boots. See Figure 3-1 for two samples of interface eth0. One assumes the interface has a fixed IP address, and the other assumes it requires an IP address assignment using DHCP. Figure 3-1 - File formats for network-scripts
As you can see eth0 will be activated on booting, because the parameter ONBOOT has the value yes and not no. You can read more about netmasks and DHCP in Chapter 2, "Introduction to Networking", that acts as an introduction to networking. The default RedHat/Fedora installation will include the broadcast and network options in the network-scripts file. These are optional. After you change the values in the configuration files for the NIC you have to deactivate and activate it for the modifications to take effect. The ifdown and ifup commands can be used to do this: [root@bigboy network-scripts]# ifdown eth0Your server will have to have a default gateway for it to be able to communicate with the Internet. This will be covered later in the chapter. How DHCP Affects the DNS Server You UseYour DHCP server not only supplies the IP address your Linux box should use, but also the desired DNS servers. When using DHCP for an interface, make sure your /etc/resolv.conf file has the servers configuration lines commented out to prevent any conflicts.Multiple IP Addresses on a Single NICIn the previous section "Determining Your IP Address" you may have noticed that there were two wireless interfaces: wlan0 and wlan0:0. Interface wlan0:0 is actually a child interface wlan0, a virtual subinterface also known as an IP alias. IP aliasing is one of the most common ways of creating multiple IP addresses associated with a single NIC. Aliases have the name format parent-interface-name:X, where X is the sub-interface number of your choice.The process for creating an IP alias is very similar to the steps outlined for the real interface in the previous section, "Changing Your IP Address":
[root@bigboy tmp]# ifconfig wlan0:0 192.168.1.99 netmask 255.255.255.0 up
DEVICE=wlan0:0The commands to activate and deactivate the alias interface would therefore be: [root@bigboy tmp]# ifup wlan0:0Note: Shutting down the main interface also shuts down all its aliases too. Aliases can be shutdown independently of other interfaces. After completing these four simple steps you should be able to ping the new IP alias from other servers on your network. IP Address Assignment for a Direct DSL ConnectionIf you are using a DSL connection with fixed or static IP addresses, then the configuration steps are the same as those outlined earlier. You plug your Ethernet interface into the DSL modem, configure it with the IP address, subnet mask, broadcast address, and gateway information provided by your ISP and you should have connectivity when you restart your interface. Remember that you might also need to configure your DNS server correctly.If you are using a DSL connection with a DHCP or dynamic IP address assignment, then the process is different. Your ISP will provide you with a PPP authentication over Ethernet (PPPoE) username and password which will allow your computer to login transparently to the Internet each time it boots up. Fedora Linux installs the rp-pppoe RPM software package required to support this. Note: Unless you specifically request static IP addresses, your ISP will provide you with a DHCP based connection. The DHCP IP address assigned to your computer and/or Internet router will often not change for many days and you may be fooled into thinking it is static. Downloading and installing RPMs isn't hard. When searching for the file, remember that the PPPoE RPM's filename usually starts with the word rp-pppoe followed by a version number like this: rp-pppoe-3.5-8.i386.rpm. After installing the RPM, you need to go through a number of steps to complete the connection. The PPPOE configuration will create a software-based virtual interface named ppp0 that will use the physical Internet interface eth0 for connectivity. Here's what you need to do:
[root@bigboy tmp]#
DEVICE=eth0
[root@bigboy network-scripts]# ifdown eth0
[root@bigboy network-scripts]# adsl-setup
Welcome to the ADSL client setup.� First, I will run some checks on
DNS
PASSWORD
USERCTRL
FIREWALLING
Start this connection at boot time
** Summary of what you entered **
Congratulations, it should be all set up!
Some Important Files Created By adsl-setupThe adsl-setup script creates three files that will be of interest to you. The first is the ifcfg-ppp0 file with interface's link layer connection parameters[root@bigboy network-scripts]# more ifcfg-ppp0The others are the duplicate /etc/ppp/pap-secrets and /etc/ppp/chap-secrets files with the username and password needed to login to your ISP: [root@bigboy network-scripts]# more /etc/ppp/pap-secrets Simple TroubleshootingYou can run the adsl-status command to determine the condition of your connection. In this case the package has been installed but the interface hasn't been activated.[root@bigboy tmp]# adsl-statusAfter activation, the interface appears to work correctly. [root@bigboy tmp]# ifup ppp0For further troubleshooting information you can visit the Web site of rp-ppoe at Roaring Penguin (www.roaringpenguin.com). There are some good tips there on how to avoid problems with VPN clients. IP Address Assignment for a Cable Modem ConnectionCable modems use DHCP to get their IP addresses so you can configure your server's Ethernet interface accordingly.How to Activate/Shut Down Your NICThe ifup and ifdown commands can be used respectively to activate and deactivate a NIC interface. You must have an ifcfg file in the /etc/sysconfig/network-scripts directory for these commands to work. Here is an example for interface eth0:[root@bigboy tmp]# ifdown eth0 How to View Your Current Routing TableThe netstat -nr command will provide the contents of the touting table. Networks with a gateway of 0.0.0.0 are usually directly connected to the interface. No gateway is needed to reach your own directly connected interface, so a gateway address of 0.0.0.0 seems appropriate. The route with a destination address of 0.0.0.0 is your default gateway.
[root@bigboy tmp]# netstat -nr
[root@bigboy tmp]# netstat -nr How to Change Your Default GatewayYour server needs to have a single default gateway. DHCP servers will automatically assign a default gateway to DHCP configured NICs, but NICs with configured static IP addresses will need to have a manually configured default gateway. This can be done with a simple command. This example uses a newly installed wireless interface called wlan0, most PCs would be using the standard Ethernet interface eth0.[root@bigboy tmp]# route add default gw 192.168.1.1 wlan0In this case, make sure that the router/firewall with IP address 192.168.1.1 is connected to the same network as interface wlan0! Once done, you'll need to update your /etc/sysconfig/network file to reflect the change. This file is used to configure your default gateway each time Linux boots. NETWORKING=yesNote: In Debian based systems the default gateway is permanently defined in the /etc/network/interfaces file. See the section "Debian / Ubuntu Network Configuration" later in this chapter for more details. Some people don't bother modifying network specific files and just place the route add command in the script file /etc/rc.d/rc.local which is run at the end of each reboot. It is possible to define default gateways in the NIC configuration file in the /etc/sysconfig/network-scripts directory, but you run the risk of inadvertently assigning more than one default gateway when you have more than one NIC. This could cause connectivity problems. If one of the default gateways has no route to the intended destination, every other packet will become lost. Firewalls that are designed to block packets with irregular sequence numbers and unexpected origins could also obstruct your data flow. How to Configure Two GatewaysSome networks may have multiple router/firewalls providing connectivity. Here's a typical scenario:
There are a number of ways to add this new route. Adding Temporary Static RoutesThe route add command can be used to add new routes to your server that will last till the next reboot. It has the advantage of being univeral to all versions of Linux and is well documented in the man pages. In our example the reference to the 10.0.0.0 network has to be preceded with a -net switch and the subnet mask and gateway values also have to be preceded by the netmask and gw switches respectively.[root@bigboy tmp]# route add -net 10.0.0.0 netmask 255.0.0.0 gw 192.168.1.254 wlan0If you wanted to add a route to an individual server, then the "-host" switch would be used with no netmask value. (The route command automatically knows the mask should be 255.255.255.255). Here is an example for a route to host 10.0.0.1. [root@bigboy tmp]# route add -host 10.0.0.1 gw 192.168.1.254 wlan0A universal way of making this change persistent after a reboot would be to place this route add command in the file /etc/rc.d/rc.local, which is always run at the end of the booting process. Adding Permanent Static RoutesIn Fedora Linux, permanent static routes are added on a per interface basis in files located in the/etc/sysconfig/network-scripts directory. The filename format is route-interface-name so the filename for interface wlan0 would be route-wlan0. The format of the file is quite intuitive with the target network coming in the first column followed by the word via and then the gateway's IP address. In our routing example, to set up a route to network 10.0.0.0 with a subnet mask of 255.0.0.0 (a mask with the first 8 bits set to 1) via the 192.168.1.254 gateway, we would have to configure file /etc/sysconfig/network-scripts/route-wlan0 to look like this: #Note: The /etc/sysconfig/network-scripts/route-* filename is very important. Adding the wrong interface extension at the end will result in the routes not being added after the next reboot. There will also be no reported errors on the screen or any of the log files in the /var/log/ directory. You can test the new file by running the /etc/sysconfig/network-scripts/ifup-routes command with the interface name as the sole argument. In the next example we check the routing table to see no routes to the 10.0.0.0 network and execute the ifup-routes command, which then adds the route: [root@bigboy tmp]# netstat -nrNote: In Debian based systems, permanent static routes are configured using the /etc/network/interfaces file. See the section "Debian / Ubuntu Network Configuration" later in this chapter for more details. How to Delete a RouteHere's how to delete the routes added in the previous section.[root@bigboy tmp]# route del -net 10.0.0.0 netmask 255.0.0.0 gw 192.168.1.254 wlan0The file /etc/sysconfig/network-scripts/route-wlan0 will also have to be updated so that when you reboot the server will not reinsert the route. Delete the line that reads: 10.0.0.0/8 via 192.168.1.254 Changing NIC Speed and DuplexThere is no better Linux investment than the purchase of a fully Linux compatible NIC card. Most Linux vendors will have a list of compatible hardware on their Web sites: read this carefully before you start hooking up you machine to the network. If you can't find any of the desired models in your local computer store, then a model in the same family or series should be sufficient. Most cards will work, but only the fully compatible ones will provide you with error-free, consistent throughput.Linux defaults to automatically negotiating the speed and duplex of it's NIC automatically with that of the switch to which it is attached. Configuring a switch port to auto-negotiate the speed and duplex often isn't sufficient because there are frequently differences in the implementation of the protocol standard. Typically, NICs with failed negotiation will work, but this is usually accompanied by many collision type errors being seen on the NIC when using the ifconfig -a command and only marginal performance. Don't limit your troubleshooting of these types of errors to just failed negotiation; the problem could also be due to a bad NIC card, switch port, or cabling. Using mii-toolOne of the original Linux tools for setting the speed and duplex of your NIC card was the mii-tool command. It is destined to be deprecated and replaced by the newer ethtool command, but many older NICs support only mii-tool so you'll need to be aware of it. Issuing the command without any arguments gives a brief status report, as seen in the next example, with unsupported NICs providing an Operation not supported message. NICs that are not compatible with mii-tool often will still work, but you have to refer to the manufacturer's guides to set the speed and duplex to anything but auto-negotiate.[root@bigboy tmp]# mii-toolBy using the verbose mode -v switch you can get much more information. In this case, negotiation was OK, with the NIC selecting 100Mbps, full duplex mode (FD): [root@bigboy tmp]# mii-tool -v Setting Your NIC's Speed Parameters with mii-toolYou can set your NIC to force itself to a particular speed and duplex by using the -F switch with any of the following options: 100baseTx-FD, 100baseTx-HD, 10baseT-FD, or 10baseT-HD. Remember that you could lose all network connectivity to your server if you force your NIC to a particular speed/duplex that doesn't match that of your switch:[root@bigboy tmp]# mii-tool -F 100baseTx-FD eth0Unfortunately there is no way to set this on reboot permanently except by placing it the command in the /etc/rc.local file to let it be run at the very end of the booting process or by creating your own startup script if you need it set earlier. Using ethtoolThe ethtool command is slated to be the replacement for mii-tool in the near future and tends to be supported by newer NIC cards.The command provides the status of the interface you provide as its argument. Here we see interface eth0 not doing autonegotiation and set to a speed of 100 Mbps, full duplex. A list of supported modes is also provided at the top of the output. [root@bigboy tmp]# ethtool eth0 Setting Your NIC's Speed Parameters with ethtoolUnlike mii-tool, ethtool settings can be permanently set as part of the interface's configuration script with the ETHTOOL_OPTS variable. In our next example, the settings will be set to 100 Mbps, full duplex with no chance for auto-negotiation on the next reboot:#You can test the application of these parameters by shutting down the interface and activating it again with the ifup and ifdown commands. These settings can also be changed from the command line using the -s switch followed by the interface name and its desired configuration parameters. [root@bigboy tmp]# ethtool -s eth1 speed 100 duplex full autoneg offThe Linux man pages give more details on other ethtool options, but you can get a quick guide by just entering the ethtool command alone, which provides a quicker summary. [root@bigboy tmp]# ethtool A Note About Duplex SettingsBy default, Linux NICs negotiate their speed and duplex settings with the switch. This is done by exchanging electronic signals called Fast Link Pulses (FLP). When the speed and duplex are forced to a particular setting the FLPs are not sent. When a NIC is in auto-negotiation mode and detects a healthy, viable link but receives no FLPs, it errs on the side of caution and sets its duplex to half-duplex and sometimes it will also set its speed to the lowest configurable value. It is therefore possible to force a switch port to 100 Mbps full duplex, but have the auto-negotiating server NIC set itself to 100Mbps half-duplex which will result in errors. The same is true for the switch if the switch port is set to auto-negotiate and server NIC is set to 100 Mbps full duplex. It is best to either force both the switch port and server NIC to either auto-negotiate or the same forced speed and duplex values.How to Convert Your Linux Server into a Simple RouterRouter/firewall appliances that provide basic Internet connectivity for a small office or home network are becoming more affordable every day, but when budgets are tight you might seriously want to consider modifying an existing Linux server to do the job.Details on how to configure Linux firewall security are covered in Iptables firewall, but you need to understand how to activate routing through the firewall before it can become a functioning networking device. Configuring IP ForwardingFor your Linux server to become a router, you have to enable packet forwarding. In simple terms packet forwarding enables packets to flow through the Linux box from one network to another. The Linux kernel configuration parameter to activate this is namednet.ipv4.ip_forward and can be found in the file /etc/sysctl.conf. Remove the "#" from the line related to packet forwarding. Before:This enables packet forwarding only when you reboot at which time Linux will create a file in one of the subdirectories of the special RAM memory-based /proc filesystem. To activate the feature immediately you have to force Linux to read the /etc/sysctl.conf file with the sysctl command using the -p switch. Here is how it's done: [root@bigboy tmp] sysctl -p Configuring Proxy ARPIf a server needs to send a packet to another device on the same network, it sends out an ARP request to the network asking for the MAC address of the other device.If the same server needs to send a packet to another device on a remote network the process is different. The server first takes a look at its routing table to find out the IP address of the best router on its network that will be able to relay the packet to the destination. The server then sends an ARP request for the MAC address that matches the router's IP address. It then sends the packet to the router using the router's MAC address and a destination IP address of the remote server. If there is no suitable router on its network, the server will then send out an ARP request for the MAC address of the remote server. Some routers can be configured to answer these types of ARP requests for remote networks. This feature is called proxy ARP. There are some disadvantages with this. One of the most common problems occurs if two routers are on the network configured for proxy ARP. In this scenario there is the possibility that either one will answer the local server's ARP request for the MAC address of the remote server. If one of the routers has an incorrect routing table entry for the remote network, then there is the risk that traffic to the remote server will occasionally get lost. In other words you can lose routing control. Note: It is for this and other reasons that it is generally not a good idea to configure proxy ARP on a router. It is also good to always configure a default gateway on your server and use separate routing entries via other routers for all networks your default gateway may not know about. Some types of bridging mode firewalls need to have proxy ARP enabled to operate properly. These devices are typically inserted as part of a daisy chain connecting multiple network switches together on the same LAN while protecting one section of a LAN from traffic originating on another section. The firewall typically isn't configured with an IP address on the LAN and appears to be an intelligent cable capable of selectively blocking packets. If you need to enable proxy ARP on a Linux server the /proc filesystem comes into play again. Proxy ARP is handled by files in the /proc/sys/net/ipv4/conf/ directory. This directory then has subdirectories corresponding to each functioning NIC card on your server. Each subdirectory then has a file called proxy_arp. If the value within this file is 0, then proxy ARP on the interface is disabled; if the value is 1 then it is enabled. You can use the /etc/sysctl.conf file to activate or disable proxy ARP. The next example activates proxy ARP, first for all interfaces and then for interfaces eth0 and wlan0. #You can then activate these settings with the sysctl command. [root@bigboy tmp] sysctl -p Configuring Your /etc/hosts FileThe /etc/hosts file is just a list of IP addresses and their corresponding server names. Your server will typically check this file before referencing DNS. If the name is found with a corresponding IP address then DNS won't be queried at all. Unfortunately, if the IP address for that host changes, you also have to also update the file. This may not be much of a concern for a single server, but can become laborious if it has to be done companywide. For ease of management, it is often easiest to limit entries in this file to just the loopback interface and also the server's own hostname, and use a centralized DNS server to handle most of the rest. Sometimes you might not be the one managing the DNS server, and in such cases it may be easier to add a quick /etc/hosts file entry till the centralized change can be made.192.168.1.101 smallfryIn the example above server smallfry has an IP address of 192.168.1.101. You can access 192.168.1.101 using the ping, telnet or any other network aware program by referring to it as smallfry. Here is an example using the ping command to see whether smallfry is alive and well on the network: [root@bigboy tmp]# ping smallfryYou can also add aliases to the end of the line which enable you to refer to the server using other names. Here we have set it up so that smallfry can also be accessed using the names tiny and littleguy. 192.168.1.101 smallfry tiny littleguyYou should never have an IP address more than once in this file because Linux will use only the values in the first entry it finds. 192.168.1.101 smallfry # (Wrong) The loopback Interface's localhost EntryUsually the first entry in /etc/hosts defines the IP address of the server's virtual loopback interface. This is usually mapped to the name localhost.localdomain (the universal name used when a server refers to itself) and localhost (the shortened alias name). By default, Fedora inserts the hostname of the server between the 127.0.0.1 and the localhost entries like this:127.0.0.1 bigboy localhost.localdomain localhostWhen the server is connected to the Internet this first entry after the 127.0.0.1 needs to be the fully qualified domain name (FQDN) of the server. For example, bigboy.mysite.com, like this: 127.0.0.1 bigboy.my-site.com localhost.localdomain localhostSome programs such as Sendmail are very sensitive to this and if they detect what they feel is an incorrect FQDN they will default to using the name localhost.localdomain when communicating with another server on the network. This can cause confusion, as the other server also feels it is localhost.localdomain. Note: You must always have a localhost and localhost.localdomain entry mapping to 127.0.0.1 for Linux to work properly and securely. Debian / Ubuntu Network ConfigurationMany of the core Fedora / Redhat commands and configuration files covered in this chapter can be used in Debian based operating systems, but there are some key differences.The /etc/network/interfaces FileThe main network configuration file is the /etc/network/interfaces file in which all the network interface parameters are defined. The file is divided into stanzas:The auto StanzaThe auto stanza defines the interfaces that should be automatically initialized when the system boots up.The mapping StanzaThis stanza maps configuration parameters for an interface depending on the output of a script. For example, on booting the script could prompt you as to whether your laptop Linux system is at home or work with the mapping statement using the answer to configure the appropriate IP address.By default the much simpler hotplug system is used which assumes that the interfaces will have only one purpose. Typical hotplug configurations simply assign each physical interface with a matching logical interface name (nick name). mapping hotplugIn this case interface eth0 is specifically given the logical name eth0, while the logical name for eth1 is implied to be the same. The iface StanzaThe iface stanza defines the characteristics of a logical interface. Typically the first line of these stanzas starts with the word iface, followed by the logical name of the interface, the protocol used, and finally the type of addressing scheme to be used, such as DHCP or static. Protocol keywords include inet for regular TCP/IP, inet6 for IPv6, ipx for the older IPX protocol used by Novell, and loopback for loopback addresses.Subsequent lines in the stanza define protocol characteristics such as addresses, subnet masks, and default gateways. In this example, interface eth1 is given the IP address 216.10.119.240/27 while interface eth0 gets its IP address using DHCP. # The primary network interfaceNote: When static IP addresses are used, a default gateway usually needs to be defined. Remember to place the gateway statement in the correct stanza with the appropriate router IP address. Creating Interface AliasesIP aliases can be easily created in the /etc/network/interfaces file once the main interface has already been defined. A modified duplicate of the main interfaces' iface stanza is required. A colon followed by the sub interface number needs to be added to the first line, and only the subnet mask and the new IP address needs to follow as can be seen in this example for interface eth1:1 with the IP address 216.10.119.239.auto eth1:1 Adding Permanent Static RoutesTheup option in the appropriate iface stanza of the /etc/network/interfaces file allows you to selectively run commands once the specified interface becomes activated with the ifup command. This makes it useful when adding permanent static routes. In this example, a route to the 10.0.0.0/8 network via router address 216.10.119.225 has been added. Remember, the up option and the command must reside on the same line of the stanza. # The primary network interface A complete /etc/network/interfaces fileWe can now construct a complete file based on the previous examples we discussed. Just like in Fedora, interfaces can be activated with the ifup and ifdown commands.#For more information on the /etc/network/interfaces file just issue the command man interfaces from the command line. ConclusionAs you can imagine, configuring Linux networking is just a first step in providing Internet access to your server. There always things that can go wrong that may be totally out of your control. Good systems administrators know the tools needed to be able to identify the probable causes of these types of problem which enables them to know the type of help they need to fix it. The next two chapters show you how to confidently test your network and Linux server applications confidently when things appear to go wrong. The skills you develop to identify and rectify these issues could prove to be invaluable to your company and career. |
Monday, 28 May 2012
Linux Networking Practicles
Introduction to Networking
IntroductionInstalling the Linux operating system is only the first step toward creating a fully functional departmental server or Web site. Almost all computers are now networked in some way to other devices therefore a basic understanding of networking and issues related to the topic will be essential to feeling comfortable with Linux servers.This introductory chapter forms the foundation on which the following network configuration and troubleshooting chapters will be built. These chapters will then introduce the remaining chapters that cover Linux troubleshooting, general software installation and the configuration of many of the most popular Linux applications used in corporate departments and Small Office/Home Office (SOHO) environments. Familiarity with the concepts explained in the following sections will help answer many of the daily questions often posed by coworkers, friends, and even yourself. It will help make the road to Linux mastery less perilous, a road that begins with an understanding of the OSI networking model and TCP/IP. The OSI Networking ModelThe Open System Interconnection (OSI) model, developed by the International Organization for Standardization, defines how the various hardware and software components involved in data communication should interact with each other.A good analogy would be a traveler who prepares herself to return home through many dangerous kingdoms by obtaining permits to enter each country at the very beginning of the trip. At each frontier our friend has to hand over a permit to enter the country. Once inside, she asks the border guards for directions to reach the next frontier and displays the permit for that new kingdom as proof that she has a legitimate reason for wanting to go there. In the OSI model each component along the data communications path is assigned a layer of responsibility, in other words, a kingdom over which it rules. Each layer extracts the permit, or header information, it needs from the data and uses this information to correctly forward what's left to the next layer. This layer also strips away its permit and forwards the data to the next layer, and so the cycle continues for seven layers. The very first layer of the OSI model describes the transmission attributes of the cabling or wireless frequencies used at each "link" or step along the way. Layer 2 describes the error correction methodologies to be used on the link; layer 3 ensures that the data can hop from link to link on the way to the final destination described in its header. When the data finally arrives, the layer 4 header is used to determine which locally installed software application should receive it. The application uses the guidelines of layer 5 to keep track of the various communications sessions it has with remote computers and uses layer 6 to verify that the communication or file format is correct. Finally, layer 7 defines what the end user will see in the form of an interface, be it graphical on a screen or otherwise. A description of the functions of each layer in the model can be seen in Table 2-1. Table 2-1: The Seven OSI Layers
An Introduction to TCP/IPTCP/IP is a universal standard suite of protocols used to provide connectivity between networked devices. It is part of the larger OSI model upon which most data communications is based.One component of TCP/IP is the Internet Protocol (IP) which is responsible for ensuring that data is transferred between two addresses without being corrupted. For manageability, the data is usually split into multiple pieces or packets each with its own error detection bytes in the control section or header of the packet. The remote computer then receives the packets and reassembles the data and checks for errors. It then passes the data to the program that expects to receive it. How does the computer know what program needs the data? Each IP packet also contains a piece of information in its header called the type field. This informs the computer receiving the data about the type of layer 4 transportation mechanism being used. The two most popular transportation mechanisms used on the Internet are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). When the type of transport protocol has been determined, the TCP/UDP header is then inspected for the "port" value, which is used to determine which network application on the computer should process the data. This is explained in more detail later. TCP Is a Connection-Oriented ProtocolTCP opens up a virtual connection between the client and server programs running on separate computers so that multiple and/or sporadic streams of data can be sent over an indefinite period of time between them. TCP keeps track of the packets sent by giving each one a sequence number with the remote server sending back acknowledgment packets confirming correct delivery. Programs that use TCP therefore have a means of detecting connection failures and requesting the retransmission of missing packets. TCP is a good example of a connection-oriented protocol.How TCP Establishes A ConnectionAny form of communication requires some form of acknowledgement for it to become meaningful. Someone knocks on the door to a house, the person inside asks "Who is it?", to which the visitor replies, "It's me!" Then the door opens. Both persons knew who was on the other side of the door before it opened and now a conversation can now begin.TCP acts in a similar way. The server initiating the connection sends a segment with the SYN bit set in TCP header. The target replies with a segment with the SYN and ACK bits set, to which the originating server replies with a segment with the ACK bit set. This SYN, SYN-ACK, ACK mechanism is often called the "three-way handshake". The communication then continues with a series of segment exchanges, each with the ACK bit set. When one of the servers needs to end the communication, it sends a segment to the other with the FIN and ACK bits set, to which the other server also replies with a FIN-ACK segment also. The communication terminates with a final ACK from the server that wanted to end the session. This is the equivalent of ending a conversation by saying "I really have to go now, I have to go for lunch", to which the reply is "I think I'm finished here too, see you tomorrow..." The conversation ends with a final "bye" from the hungry person. Here is a modified packet trace obtained from the tethereal program You can clearly see the three way handshake to connect and disconnect the session. hostA -> hostB TCP 1443 > http [SYN] Seq=9766 Ack=0 Win=5840 Len=0In this trace, the sequence number represents the serial number of the first byte of data in the segment. So in the first line, a random value of 9766 was assigned to the first byte and all subsequent bytes for the connection from this host will be sequentially tracked. This makes the second byte in the segment number 9767, the third number 9768 etc. The acknowledgment number or Ack, not to be confused with the ACK bit, is the byte serial number of the next segment it expects to receive from the other end, and the total number of bytes cannot exceed the Win or window value that follows it. If data isn't received correctly, the receiver will re-send the requesting segment asking for the information to be sent again. The TCP code keeps track of all this along with the source and destination ports and IP addresses to ensure that each unique connection is serviced correctly. UDP, TCP's "Connectionless" CousinUDP is a connectionless protocol. Data is sent on a "best effort" basis with the machine that sends the data having no means of verifying whether the data was correctly received by the remote machine. UDP is usually used for applications in which the data sent is not mission-critical. It is also used when data needs to be broadcast to all available servers on a locally attached network where the creation of dozens of TCP connections for a short burst of data is considered resource-hungry.TCP and UDP PortsThe data portion of the IP packet contains a TCP or UDP segment sandwiched inside. Only the TCP segment header contains sequence information, but both the UDP and the TCP segment headers track the port being used. The source/destination port and the source/destination IP addresses of the client & server computers are then combined to uniquely identify each data flow.Certain programs are assigned specific ports that are internationally recognized. For example, port 80 is reserved for HTTP Web traffic, and port 25 is reserved for SMTP e-mail. Ports below 1024 are reserved for privileged system functions, and those above 1024 are generally reserved for non-system third-party applications. Usually when a connection is made from a client computer requesting data to the server that contains the data:
The TCP/IP "Time To Live" FeatureEach IP packet has a Time to Live (TTL) section that keeps track of the number of network devices the packet has passed through to reach its destination. The server sending the packet sets the initial TTL value, and each network device that the packet passes through then reduces this value by 1. If the TTL value reaches 0, the network device will discard the packet.This mechanism helps to ensure that bad routing on the Internet won't cause packets to aimlessly loop around the network without being removed. TTLs therefore help to reduce the clogging of data circuits with unnecessary traffic. The ICMP Protocol and Its Relationship to TCP/IPThere is another commonly used protocol called the Internet Control Message Protocol (ICMP). It is not strictly a TCP/IP protocol, but TCP/IP-based applications use it frequently.ICMP provides a suite of error, control, and informational messages for use by the operating system. For example, IP packets will occasionally arrive at a server with corrupted data due to any number of reasons including a bad connection; electrical interference, or even misconfiguration. The server will usually detect this by examining the packet and correlating the contents to what it finds in the IP header's error control section. It will then issue an ICMP reject message to the original sending machine saying that the data should be re-sent because the original transmission was corrupted. ICMP also includes echo and echo reply messages used by the Linux ping command to confirm network connectivity. ICMP TTL expired messages are also sent by network devices back to the originating server whenever the TTL in a packet is decremented to zero. How IP Addresses Are Used To Access Network DevicesAll TCP/IP enabled devices connected to the Internet have an Internet Protocol (IP) address. Just like a telephone number, it helps to uniquely identify a user of the system. The Internet Assigned Numbers Authority (IANA) is the organization responsible for assigning IP addresses to Internet Service Providers (ISPs) and deciding which ones should be used for the public Internet and which ones should be used on private networks.IP addresses are in reality a string of 32 binary digits or bits. For ease of use, network engineers often divide these 32 bits into four sets of 8 bits (or octets), each representing a number from 0 to 255. Each number is then separated by a period (.) to create the familiar dotted decimal notation. An example of an IP address that follows these rules is 97.65.25.12. Private IP AddressesSome groups of IP addresses are reserved for use only in private networks and are not routed over the Internet. These are called private IP addresses and have the following ranges:10.0.0.0 - 10.255.255.255Home networking equipment/devices usually are configured in the factory with an IP address in the range 192.168.1.1 to 192.168.1.255. You may be wondering how devices using private addresses could ever access the Internet if the use of private addresses on the Internet is illegal. The situation gets even more confusing if you consider the fact that hundreds of thousands of office and home networks use these same addresses. This must cause networking confusion. Don't worry, this problem is overcome by NAT. The localhost IP AddressWhether or not your computer has a network interface card it will have a built-in IP address with which network-aware applications can communicate with one another. This IP address is defined as 127.0.0.1 and is frequently referred to as localhost. This concept is important to understand, and will be revisited in many later chapters.Network Address Translation (NAT) Makes Private IPs PublicYour router/firewall will frequently be configured to give the impression to other devices on the Internet that all the servers on your home/office network have a valid public IP address, and not a "private" IP address. This is called network address translation (NAT) and is often also called IP masquerading in the Linux world. There are many good reasons for this, the two most commonly stated are:
As a general rule, you won't be able to access the public NAT IP addresses from servers on your home network. Basic NAT testing requires you to ask a friend to try to connect to your home network from the Internet. Port Forwarding with NAT Facilitates Home-Based Web sitesIn a simple home network, all servers accessing the Internet will appear to have the single public IP address of the router/firewall because of many to one NAT. Because the router/firewall is located at the border crossing to the Internet, it can easily keep track of all the various outbound connections to the Internet by monitoring:
New connections initiated from the Internet to the public IP address of the router/firewall face a problem. The router/firewall has no way of telling which of the many home PCs behind it should receive the relayed data because the mapping mentioned earlier doesn't exist beforehand. In this case the data is usually discarded. Port forwarding is a method of counteracting this. For example, you can configure your router/firewall to forward TCP port 80 (Web/HTTP) traffic destined to the outside NAT IP to be automatically relayed to a specific server on the inside home network As you may have guessed, port forwarding is one of the most common methods used to host Web sites at home with DHCP DSL. DHCPThe Dynamic Host Configuration Protocol (DHCP) is a protocol that automates the assignment of IP addresses, subnet masks default routers, and other IP parameters.The assignment usually occurs when the DHCP configured machine boots up, or regains connectivity to the network. The DHCP client sends out a query requesting a response from a DHCP server on the locally attached network. The DHCP server then replies to the client PC with its assigned IP address, subnet mask, DNS server and default gateway information. The assignment of the IP address usually expires after a predetermined period of time, at which point the DHCP client and server renegotiate a new IP address from the server's predefined pool of addresses. Configuring firewall rules to accommodate access from machines who receive their IP addresses via DHCP is therefore more difficult because the remote IP address will vary from time to time. You'll probably have to allow access for the entire remote DHCP subnet for a particular TCP/UDP port. Most home router/firewalls are configured in the factory to be DHCP servers for your home network. You can also make your Linux box into a DHCP server, once it has a fixed IP address. The most commonly used form of DSL will also assign the outside interface of your router/firewall with a single DHCP provided IP address. How DNS Links Your IP Address To Your Web DomainThe domain name system (DNS) is a worldwide server network used to help translate easy to remember domain names like www.linuxhomenetworking.com into an IP address that can be used behind the scenes by your computer. Here step by step description of what happens with a DNS lookup.
IP Version 6 (IPv6)Most Internet-capable networking devices use version 4 of the Internet Protocol (IPv4) which I have described here. You should also be aware that there is now a version 6 (IPv6) that has recently been developed as a replacement.With only 32 bits, the allocation of version 4 addresses will soon be exhausted between all the world's ISPs. Version 6, which uses a much larger 128-bit address offers eighty billion, billion, billion times more IP addresses which it is hoped should last for most of the 21st century. IPv6 packets are also labeled to provide quality-of-service information that can be used in prioritizing real-time applications, such as video and voice, over less time-sensitive ones such as regular Web surfing and chat. IPv6 also inherently supports the IPSec protocol suite used in many forms of secured networks, such as virtual private networks (VPNs). Most current operating systems support IPv6 even though it isn't currently being used extensively within corporate or home environments. Expect it to become an increasingly bigger part of your network planning in years to come. How Subnet Masks Group IP Addresses into NetworksSubnet masks are used to tell which part of the IP address represents the network on which the computer is connected (network portion) and the computer's unique identifier on that network (host portion). The term netmasks is often used interchangeably with the term subnet masks, this book will use the latter term for the sake of consistency.A simple analogy would be a phone number, such as (808) 225-2468. The (808) represents the area code, and the 225-2468 represents the telephone within that area code. Subnet masks allow you to specify how long you want the area code to be (network portion) at the expense of the number of telephones in that are in the area code (host portion) Most home networks use a subnet mask of 255.255.255.0. Each 255 means this octet is for the area code (network portion). So if your server has an IP address of 192.168.1.25 and a subnet mask of 255.255.255.0, the network portion would be 192.168.1 and the server or host would be device #25 on that network. In all cases, the first IP address in a network is reserved as the network's base address and the last one is reserved for broadcast traffic that is intended to be received by all devices on the network. In our example, 192.168.1.0 would be the network address and 192.168.1.255 would be used for broadcasts. This means you can then use IP addresses from #1 to #254 on your private network. Calculating The Number of Addresses Assigned to a SubnetMost office and home networks use networks with 255 IP addresses or less in which the subnet mask starts with the numbers 255.255.255. This is not a pure networking text, so I'll not discuss larger networks because that can become complicated, but in cases where less than 255 IP addresses are required a few apply. There are only seven possible values for the last octet of a subnet mask. These are 0, 192, 128, 224, 240, 248 and 252. You can calculate the number of IP addresses for each of these by subtracting the value from 256.In many cases the subnet mask isn't referred to by the dotted decimal notation, but rather by the actual number of bits in the mask. So for example a mask of 255.255.255.0 may be called a /24 (slash 24) mask instead. A list of the most commonly used masks in the office or home environment is presented in Table 2-2. Table 2-2: The "Dotted Decimal" And "Slash" Subnet Mask Notations
Calculating the Range of Addresses on Your NetworkIf someone gives you an IP address of 97.158.253.28 and a subnet mask of 255.255.255.248, how do you determine the network address and the broadcast address, in other words the boundaries, of your network? The following section outlines the steps to do this using both a manual and programmed methodology.Manual CalculationTake out your pencil and paper, manual calculation can be tricky. Here we go!
Let's do this for 192.168.3.56 with a mask of 255.255.255.224:
Let's do this for 10.0.0.75 with a mask of 255.255.255.240
Note: As a rule of thumb, the last octet of your network base address must be divisible by the "256 minus the last octet of your subnet mask" and leave no remainder. If you are sub-netting a large chunk of IP addresses it's always a good idea to lay it out on a spreadsheet to make sure there are no overlapping subnets. Once again, this calculation exercise only works with subnet masks that start with "255.255.255". Calculation Using a ScriptHere is an example of how to use it, just provide the IP address followed by the subnet mask as arguments. It will accept subnet masks in dotted decimal format or /value format[root@bigboy tmp]# ./subnet-calc.sh 216.151.193.92 /28 Subnet Masks for the Typical Business DSL LineIf you purchased a DSL service from your ISP that gives you fixed IP addresses, they will most likely provide you with a subnet mask of 255.255.255.248 that defines 8 IP addresses. For example, if the ISP provides you with a public network address of 97.158.253.24, a subnet mask of 255.255.255.248, and a gateway of 97.158.253.25, then your IP addresses will be:97.158.253.24 - Network base address The Physical and Link LayersTCP/IP can be quite interesting, but a knowledge of the first two layers of the OSI model are important too, because without them, even the most basic communication would be impossible.There are very many standards that define the physical, electrical, and error-control methodologies of data communication. One of the most popular ones in departmental networks is Ethernet, which is available in a variety of cable types and speed capabilities, but the data transmission and error correction strategy is the same in all. Ethernet used to operate primarily in a mode where every computer on a network section shared the same Ethernet cable. Computers would wait until the line was clear before transmitting. They would then send their data while comparing what they wanted to send with what they actually sent on the cable as a means of error detection. If a mathematical comparison, or cyclic redundancy check (CRC), detected any differences between the two, the server would assume that it transmitted data simultaneously with another server on the cable. It would then wait some random time and retransmit at some later stage when the line was clear again. Transmitting data only after first sensing whether the cable, which was strung between multiple devices, had the correct signaling levels is a methodology called carrier sense, multiple access or CSMA. The ability to detect garbling due to simultaneous data transmissions, also known as collisions, is called collision detect or CD. You will frequently see references to Ethernet being a CSMA/CD technology for this reason and similar schemes are now being used in wireless networks. Ethernet devices are now usually connected via a dedicated cable, using more powerful hardware capable of simultaneously transmitting and receiving without interference, thereby making it more reliable and inherently faster than its predecessor versions. The original Ethernet standard has a speed of 10 Mbps; the most recent versions can handle up to 40Gbps! The 802.11 specifications that define many wireless networking technologies are another example of commonly used layer 1 and 2 components of the OSI model. DSL, cable modem standards and, T1 circuits are all parts of these layers. The next few sections describe many physical and link layer concepts and the operation of the devices that use them to connect the computers in our offices and homes. Networking Equipment TerminologyUp to this point you have had only an introduction to the theory of the first two OSI layers. Now we'll cover the hardware used to implement them.Network Interface CardsYour network interface card is also frequently called a NIC. Currently, the most common types of NIC used in the home and office are Ethernet and wireless Ethernet cards.The Meaning of the NIC Link LightThe link light signifies that the NIC card has successfully detected a device on the other end of the cable. This indicates that you are using the correct type of cable and that the duplex has been negotiated correctly between the devices at both ends.Duplex ExplainedFull duplex data paths have the capability of allowing the simultaneous sending and receiving of data. Half duplex data paths can transmit in both directions too, but in only one direction at a time.Full duplex uses separate pairs of wires for transmitting and receiving data so that incoming data flows don't interfere with outgoing data flows. Half duplex uses the same pairs of wires for transmitting and receiving data. Devices that want to transmit information have to wait their turn until the "coast is clear" at which point they send the data. Error-detection and data-retransmission mechanisms ensure that the data reaches the destination correctly and are specifically designed to remedy data corruption caused when multiple devices start transmitting at the same time. A good analogy for full duplex communications is the telephone, in which both parties can speak at the same time. Half duplex on the other hand is more like a walkie-talkie in which both parties have to wait until the other is finished before they can speak. Data transfer speeds will be low and error levels will be high if you have a device at one end of a cable set to full duplex and a device at the other end of the cable set to half duplex. Most modern network cards can autonegotiate duplex with the device on the other end of the wire. It is for this reason that duplex settings aren't usually a problem for Linux servers. The MAC AddressThe media access control (MAC) address can be equated to the serial number of the NIC. Every IP packet is sent out of your NIC wrapped inside an Ethernet frame that uses MAC addresses to direct traffic on your locally attached network.MAC addresses therefore have significance only on the locally attached network. As the packet hops across the Internet, its source/destination IP address stays the same, but the MAC addresses are reassigned by each router on the way using a process called ARP. How ARP Maps the MAC Address to Your IP AddressThe Address Resolution Protocol (ARP) is used to map MAC addresses to network IP addresses. When a server needs to communicate with another server it does the following steps:
Common ARP Problems When Changing A NICYou may experience connectivity problems if you change the MAC address assigned to an IP address. This can happen if you swap a bad NIC card in a server, or replace a bad server but have the new one retain the IP address of the old.Routers typically save learned MAC to IP address map entries in a cache and won't refresh them unless a predefined period of time has elapsed. Changing the NIC, while retaining the IP address can cause problems as the router will continue to send frames onto the network with the correct target IP address but the old target MAC address. The server with the new NIC won't respond as the frame's target MAC doesn't match it's own. This problem can be fixed in one of two ways. You can delete all the ARP entries in the router's cache. The second solution is to log into the server's console and ping it's gateway. The router will detect the MAC to IP address change and it will readjust its ARP table. The Two Broad Types Of Networking EquipmentThere are two main types of networking equipment; Data Communications Equipment (DCE) which is intended to act as the primary communications path, and Data Terminal Equipment (DTE) which acts as the source or destination of the transmitted data.Data Terminal EquipmentDTE devices were originally computer terminals located at remote offices or departments that were directly connected modems. The terminals would have no computing power and only functioned as a screen/keyboard combination for data processing.Nowadays most PCs have their COM and Ethernet ports configured as if they were going to be connected to a modem or other type of purely networking-oriented equipment. Data Communications EquipmentA DCE is also known as Data Circuit-Terminating Equipment and refers to such equipment as modems and other devices designed primarily to provide network access.Using Straight-Through/Crossover Cables to Connect DTEs And DCEsWhen a DCE is connected to a DTE, you will need a straight-through cable. DCEs connected to DCEs or DTEs connected to DTEs require crossover cables. This terminology is generally used with Ethernet cables.The terminology can be different for cables used to connect serial ports together. When connecting a PC's COM port (DTE) to a modem (DCE) the straight-through cable is frequently called a modem cable. When connecting two PCs (DTE) together via their COM ports, the crossover cable is often referred to as a null modem cable. Some manufacturers configure the Ethernet ports of their networking equipment to be either of the DTE or the DCE type, and other manufacturers have designed their equipment to flip automatically between the two types until it gets a good link. As you can see, confusion can arise when selecting a cable. If you fail to get a link light when connecting your Ethernet devices together, try using the other type of cable. A straight-through Ethernet cable is easy to identify. Hold the connectors side by side, pointing in the same direction with the clips facing away from you. The color of the wire in position #1 on connector #1 should be the same as that of position #1 on connector #2. The same would go for positions #2 through #8, that is, the same color for corresponding wires on each end. A crossover cable has them mixed up. Table 2-3 provides some good rules of thumb. Table 2-3: Cabling Rules of Thumb
Connectivity Using HubsA hub is a device into which you can connect all devices on a network so that they can talk together. Hubs physically cross-connect all their ports with one another which causes all traffic sent from a server to the hub to be blurted out to all other servers connected to that hub whether they are the intended recipient or not.Hubs have no, or very little, electronics inside and therefore do not regulate traffic. It is possible for multiple servers to speak at once with all of them receiving garbled messages. When this happens the servers try again, after a random time interval, until the message gets through correctly. It is for these reasons that Ethernet devices that plug into hubs should be set to half duplex. Note: Hubs can add a lot of delays to your network because of the message garbling collisions and retransmissions. A switch is a much more reliable and predictable alternative, and ones made for the home often cost only a few dollars more. Using Switches as a Faster Alternative to HubsA switch is also a device into which you can connect all devices on a home network so that they can talk together. Unlike a hub, traffic sent from Server A to Server B will be received only by Server B. The only exception is broadcast traffic which is blurted out to all the servers simultaneously.Switches regulate traffic, thereby eliminating the possibility of message garbling and providing a more efficient traffic flow. Devices that plug into switches should be set to full duplex to take full advantage of the dedicated bandwidth coming from each switch port. Local Area NetworksA local area network (LAN) is a grouping of ports on a hub, switch or tied to a wireless access point (WAP) that can communicate only with each other.It is possible to connect multiple switches and/or hubs in a chain formation to create a LAN with more ports. This is often called daisy chaining. Switches and hubs provide no access control between servers connected to the same LAN. This is why network administrators group trusted servers having similar roles on the same LAN. Servers use their IP address and subnet mask and the IP address of the remote server to determine whether they are both on the same network. If not, they attempt to communicate with each other via routers that interconnect their LANs. Routers are also capable of filtering traffic passing between the two LANs therefore providing additional security. Larger, more expensive switches can be configured to assign only certain ports to prespecified virtual LANs or (VLANs) chosen by the network administrator. In this case, the switch houses ports on multiple LANs. A router still needs to be connected to each VLAN for internetwork communication. How Routers Interconnect LANsAs stated before, switches and hubs usually have only servers connected to them that have been configured as being part of the same network. By connecting its NIC cards to multiple LANs, a correctly configured router is capable of relaying traffic between networks.Routers can also be configured to deny communication between specific servers on different networks. They can also filter traffic based on the TCP port section of each packet. For example, it is possible to deny communication between two servers on different networks that intend to communicate on TCP port 80, and allow all other traffic between them. Routers therefore direct and regulate traffic between separate networks, much like a traffic policeman. If you intend to route between networks, you must reserve an IP address for a router for each network and make sure that the router is directly connected to the LAN associated with that network. The network engineer responsible for the router will also have to specify which locally connected networks can be advertised to the router's neighbors and whether this information can be relayed to all the routers in an administrative zone, or routing domain. In this example, we can see that the router is aware of many different networks (represented by the slash notation for subnet masks). The routing table also shows the best serial and VLAN type interfaces to use to get to these destinations and the IP address of the neighboring router through which traffic needs to be relayed to get there. router>show ip routeIn home networks, routers usually have only two interfaces that provide connectivity to the Internet via network address translation or NAT. In other words routers act as gateways to the wider world and it won't be surprising to learn that routers are frequently referred to as "gateways". Note: The term gateway specifically refers to a device that routes traffic between dissimilar network protocols (IP to Appletalk) or access methods (Ethernet to DSL). Routers transfer traffic where both the protocols and communications medium are the same. The terms are frequently used interchangeably, especially if only one network protocol is being used. Therefore a home DSL router that provides IP Internet access to an Ethernet network is technically both a gateway and a router. The distinction can be important in complicated networking environments where newer technologies need to talk with older ones using incompatible communications protocols. How Simple Routing WorksIn the broader networking sense, a "route" refers to the path data takes to traverse from its source to its destination. Each router along the way may also be referred to as a hop.Usually when we speak about a route on a Linux box, we are referring to the IP address of the first hop needed to reach the desired destination network. It is assumed that this first hop will know how to automatically relay the packet. As explained previously, routers are designed to exchange routing information dynamically, and can therefore intelligently redirect traffic to bypass failed network links. Home Linux boxes frequently don't run a dynamic routing protocol and therefore rely on "static" routes issued by the system administrator at the command line or in configuration files to determine the next hop to all desired networks. In linux Networking", which covers Linux network topics, shows how to add static routes to your Linux box and also how you can convert it into a simple router. Default Gateways, The Routers Of Last ResortA default gateway is the router that is used when no alternative devices can be found to relay the traffic. They are often called "routers of last resort".Say for example you have two routers R1 and R2. R1 is connected to both your SOHO home network and the Internet. R2 is connected to SOHO home network and is capable of relaying data to other corporate networks with addresses starting with 10.X.X.X via another NIC card. You could put a route on your SOHO servers that states:
For most home networks, your default gateway would be the router/firewall connected to the Internet. Firewalls Help Provide a Secure Routing EnvironmentFirewalls can be viewed as routers with more enhanced abilities to restrict traffic, not just by port and IP address as routers do. Specifically, firewalls can detect malicious attempts to subvert the TCP/IP protocol. A short list of capabilities includes:
Firewalls can often create an encrypted data path between two Private networks across the Internet providing secure communication with a greatly reduced chance of eavesdropping. These communication channels are called Virtual Private Networks or VPNs and are frequently used to connect branch offices to the corporate headquarters and also to allow sales representatives to get access to sensitive pricing information when traveling from town to town. Additional Introductory TopicsThe last few topics of this chapter may not appear to be directly related to networking, but they cover Linux help methods that you'll use extensively and the File Transfer Protocol (FTP) package, which enables you to download all the software you need to get your Linux server operational as quickly as possible.The File Transfer ProtocolFTP is one of the most popular applications used to copy files between computers via a network connection. Knowledge of FTP is especially important and is a primary method of downloading software for Linux systems.There are a number of commercially available GUI based clients you can load on your PC to do this, such as WSFTP and CuteFTP. From the remote user's perspective, there are two types of FTP. The first is regular FTP which is used primarily to allow specific users to download files to their systems. The remote FTP server prompts you for a specific username and password to gain access to the data. The second method, anonymous FTP is used primarily to allow any remote user to download files to their systems. The remote FTP server prompts you for a username, at which point the user types anonymous or ftp with the password being any valid e-mail address. From the systems administrator's perspective, there are another two categories. These are "active" and "passive" It is good to remember that FTP isn't very secure as usernames, passwords and data are sent across the network unencrypted. More secure forms such as SFTP (Secure FTP) and SCP (Secure Copy) are available as a part of the Secure Shell package Linux HelpLinux help files are accessed using the man or manual pages. From the command line you issue the man command followed by the Linux command or file about which you want to get information. If you want to get information on the ssh command, then you'd use the command man ssh.[root@bigboy tmp]# man sshIf you want to search all the man pages for a keyword, then use the man command with the -k switch, for example, man -k ssh which will give a list of all the man pages that contain the word ssh. [root@bigboy tmp]# man -k sshThis book is targeted at proficient Linux beginners and above so I'll be using a wide variety of commands in this book without detailed explanations to help keep the flow brisk. If you need more help on a command, use its man page to get more details on what it does and the syntax it needs. Linux help can sometimes be cryptic, but with a little practice the man pages can become your friend. ConclusionCongratulations! Now that you have an understanding of basic networking, to learn how to configure Linux networking.Feel free to return to this chapter whenever you need to refresh your memory on these foundation concepts. linux-networking-practicles |
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