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<chapt> |
<chapt> |
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|
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<heading>TCP/IP</heading> |
<heading>TCP/IP</heading> |
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<p> |
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author = Duncan C Thomsonr |
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<email>duncan@sciuro.demon.co.uk</email></p> |
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<p> |
<p> |
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author = ? |
topics = IP protocol, TCP protocol, IP addresses, IP interfaces, Routing |
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</p> |
</p> |
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<p> |
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topics = ? |
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</p> |
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|
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<sect>Intro |
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<p>TCP/IP, as the name suggests, is a pair of protocols, and what most |
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of the Internet is built on. Although physically the Internet is |
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made up of a wide range of networking technologies, from slow modem |
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links through Ethernet, to high-speed ATM-based switched networks, |
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and a wide range of different applications run over it - the WWW and |
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e-mail to name only two, the protocols which tie everything together |
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are Internet Protocol (IP), and, perhaps almost as great an extent, |
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Transmission Control Protocol. Another protocol, UDP, is used |
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in place of TCP for some applications, especially in LAN |
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environments, but on the Internet the TCP/IP partnership rules.</p> |
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|
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<p>diagram: various physical networks, IP, TCP and UDP, apps</p> |
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|
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<p>This chapter describes firstly the basics of IP networking, and |
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later describes some of the more advanced features of TCP/IP available |
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to the Debian user.</p> |
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</sect> |
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|
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<sect>IP Addresses |
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|
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<p>Every computer connected directly to the Internet (or to any IP-based |
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network) is identified by an IP address. IP addresses are four bytes |
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long, and are usually written as four decimal numbers separated by dots, |
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as in the examples below. |
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<list> |
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<item>10.34.92.111</item> |
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<item>127.0.0.1</item> |
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<item>172.19.220.2</item> |
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<item>192.168.50.109</item> |
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</list> |
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<p>IP addresses typically identify two things. Firstly, they identify the |
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network on which a particular computer is located. Secondly, they |
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identify a particular computer on that network. Both these pieces of |
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information are present in an IP address, and they can be called the |
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<em>network part</em> and the <em>host part</em>. |
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Two special values for the host part |
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should be mentioned here - if the host part is all zeros, the |
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address refers to a network (ie it is a <em>network address</em> as |
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opposed to a <em>host address</em>). If, alternatively, the host part |
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is all ones, the address refers to all hosts on the network (ie it is a |
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<em>broadcast</em> address).</p> |
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|
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<p>In order to identify which part of the IP address is the host part, |
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and which part is the network part, there are two methods we can use. The |
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first (and original) way is the easier by far to understand, so let's start |
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by having a look at how it works. IP addresses are split into a number of |
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<em>classes</em>, and it is this class which tells us how to split an IP |
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address into its network and host parts. |
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<taglist> |
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<tag>Class A</tag> |
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<item>A class A IP address has as its first byte a number between 1 and |
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126. The first byte of a class A IP address identifies the network, and |
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the remaining three bytes identify the host.</item> |
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<tag>Class B</tag> |
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<item>A class B IP address has as its first byte a number between 128 and |
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191. Its first two bytes are the network identifier, and the remaining |
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two bytes are the host identifier on that network.</item> |
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<tag>Class C</tag> |
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<item>A class C IP address has as its first byte a number between 192 and |
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223. Its first three bytes identify the network, and the remaining byte |
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identifies an individual host on that network.</item> |
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</taglist></p> |
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|
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<p>From the above list you might notice that IP addresses beginning with |
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bytes from 224 and upwards are missing. These belong to other classes of |
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IP address, not used for a normal IP host, and are beyond the discussion |
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in this section.</p> |
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|
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<p>You might also notice that IP addresses beginning with 127 are missing. |
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IP addresses beginning with 127 are known as <em>loopback</em> addresses, |
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and can be used for testing TCP/IP without actually having a network |
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connection.</p> |
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|
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<p>This is all very well, but what if we have been assigned a single |
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class C address range, but want to split it among several networks? This |
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is where the second method of specifying the network and host parts can be |
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used. This method specifies, along with an IP address, a <em>netmask</em>, |
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which has its bits set to one in the network part, and set to zero in the |
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host part. So, for example, the default netmask for the various classes |
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of network as as below: |
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<taglist> |
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<tag>Class A</tag> |
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<item>255.0.0.0</item> |
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<tag>Class B</tag> |
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<item>255.255.0.0</item> |
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<tag>Class C</tag> |
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<item>255.255.255.0</item> |
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</taglist></p> |
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|
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<p>These don't give us any new information beyond what the original address |
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classes told us. The power in using netmasks, though, is that we can |
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choose arbitrary splits between the network and host parts - for example, |
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a netmask of 255.255.255.192 would allow us to split a class C network |
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into four parts, each with 62 host addresses. Confused? Let's look at |
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that example in more detail.</p> |
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|
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<p>Let's suppose we've been allocated a class C network with IP addresses |
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beginning with 192.168.50. If we convert the netmask 255.255.255.192 into |
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binary, we can see that in the last byte, the first two bits are one (that |
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is, they are included in the network part of the address) and the last six |
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bits are zeros (that is, they form the host part). So, by using the IP |
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addresses we have been given, along with this netmask, we have split our |
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network into four, with <em>network addresses</em> given by setting these |
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two bits to their four possible combinations (00, 01, 10, 11) while |
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keeping the host part set to zeros (to identify the network): |
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<list> |
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<item>192.168.50.0</item> |
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<item>192.168.50.64</item> |
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<item>192.168.50.128</item> |
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<item>192.168.50.192</item> |
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</list></p> |
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|
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<p>Now we know where our four network addresses come from. What about our |
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host addresses? They come from setting the host part in each <em>subnet</em> |
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to all values from 000001 to 111110 (remember - all ones is a broadcast |
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address). That gives us a total of 62 hosts in each network, with addresses: |
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<list> |
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<item>192.168.50.1 - 192.168.50.62</item> |
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<item>192.168.50.65 - 192.168.50.126</item> |
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<item>192.168.50.129 - 192.168.50.190</item> |
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<item>192.168.50.193 - 192.168.50.254</item> |
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</list></p> |
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|
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<p>In Linux, if we don't mention what netmask we are using, it's usually |
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assumed by the software that we want to use the default netmask for that |
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particular class of IP addresses. You should only need to specify a netmask |
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if you are using one which is not the standard scheme for a particular class |
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of IP addresses. It never does any harm to specify it though.</p> |
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|
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<p>Each IP address should be <em>unique</em> on the Internet, or whichever |
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IP network you are connected to. This means that your cannot assign IP |
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addresses at random to your machines, since most IP addresses are already |
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in use on the Internet. In order to get a legal set of IP addresses for |
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your machines, you will normally go through an Internet Service Provider |
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(ISP). If you have not been assigned such a range of addresses, you |
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should use one of the range of <em>private</em> IP addresses, set aside for |
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internet or testing use. Be aware that you will not be able to connect to |
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the Internet directly from such a network; you will need to use some form |
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of Network Address Translation (NAT) to do this. The ranges of IP addresses |
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set aside for private use are: |
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<list> |
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<item>10.0.0.0: a single class A network</item> |
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<item>172.16.0.0 - 172.31.0.0: 16 class B networks</item> |
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<item>192.168.0.0 - 192.168.255.0: 256 class C networks</item> |
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</list></p> |
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|
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</sect> |
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|
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<sect>IP Interface Configuration |
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|
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<p>Once we know what IP address we wish to use for our machine, we will |
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have to bring up one of our network interfaces, and assign that IP address |
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(possibly along with a netmask) to it. On most Debian systems this is done |
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when installing the system, and you are seldom likely to need to change |
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it on a simple system.</p> |
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|
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<p>The <prgn>ifconfig</prgn> command is used to configure interfaces in |
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order to use IP with them. There are a number of different network |
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interfaces available with the Linux kernel, some of which are summarised |
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below: |
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<taglist> |
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<tag>Loopback</tag> |
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<item>The loopback interface (<tt>lo</tt>), usually configured as IP |
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address 127.0.0.1</item> |
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<tag>Ethernet</tag> |
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<item>Ethernet interfaces (with names like <tt>eth0</tt>, <tt>eth1</tt>, |
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<tt>eth2</tt>) are used to access Ethernet cards</item> |
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<tag>PPP</tag> |
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<item>PPP stands for Point-to-Point Protocol, and is used to run a variety |
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of networking protocols, including IP, over any kind of serial lines |
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(null modem, modem, ISDN). They have names like <tt>ppp0</tt>, |
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<tt>ppp1</tt></item> |
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<tag>Token Ring</tag> |
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<item>Token Ring devices are accessed with device names like <tt>tr0</tt>, |
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<tt>tr1</tt></item> |
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<tag>Dummy</tag> |
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<item>The dummy network drivers are used in systems who have an interface |
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which is not always used, in order to provide a permanent IP interface |
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for the relevant address. The device names are <tt>dummy</tt>, or |
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<tt>dummy0</tt>, <tt>dummy1</tt>, and so on</item> |
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</taglist> |
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There are a wide range of other network devices available, including |
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SLIP, PLIP (serial and parallel line IP), `shaper' devices for controlling |
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the traffic on certain interfaces, the ability to have several IP addresses |
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on a single device, as well as frame relay, AX.25, X.25, ARCnet, LocalTalk |
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and more. Here, though, we'll concentrate on one of the most common - the |
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Ethernet interface.</p> |
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|
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<p>In many cases, if you wish your kernel to automatically load modules |
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for certain device drviers, you may require to make changes to your |
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<prgn>/etc/conf.modules</prgn> or <prgn>/etc/modules</prgn> file. For |
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example, to automatically load the NE2000 driver, you could have the |
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line: |
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<example> |
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alias eth0 ne |
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</example> |
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in your <prgn>/etc/conf.modules</prgn> file.</p> |
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|
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<p>The simplest way to call <prgn>ifconfig</prgn> is to simply type its |
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name: |
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<example> |
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# /sbin/ifconfig |
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lo Link encap:Local Loopback |
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inet addr:127.0.0.1 Bcast:127.255.255.255 Mask:255.0.0.0 |
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UP BROADCAST LOOPBACK RUNNING MTU:3584 Metric:1 |
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RX packets:18584 errors:0 dropped:0 overruns:0 frame:0 |
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TX packets:18584 errors:0 dropped:0 overruns:0 carrier:0 |
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Collisions:0 |
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</example> |
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which simply returns information about the interfaces currently |
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configured. If we wish now to bring up an Ethernet interface with the |
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address 192.168.50.23, we'd specify the interface name and the IP address |
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on <prgn>ifconfig</prgn>'s command line: |
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<example> |
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# /sbin/ifconfig eth0 192.168.50.23 |
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# /sbin/ifconfig |
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lo Link encap:Local Loopback |
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inet addr:127.0.0.1 Bcast:127.255.255.255 Mask:255.0.0.0 |
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UP BROADCAST LOOPBACK RUNNING MTU:3584 Metric:1 |
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RX packets:18584 errors:0 dropped:0 overruns:0 frame:0 |
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TX packets:18584 errors:0 dropped:0 overruns:0 carrier:0 |
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Collisions:0 |
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|
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eth0 Link encap:Ethernet HWaddr 00:00:E8:C5:64:2A |
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inet addr:192.168.50.23 Bcast:192.168.50.255 Mask:255.255.255.0 |
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UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 |
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RX packets:55 errors:0 dropped:0 overruns:0 frame:0 |
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TX packets:5 errors:0 dropped:0 overruns:0 carrier:0 |
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Collisions:7 |
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Interrupt:10 Base address:0x300 |
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</example> |
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As can be seen from the example, the <prgn>ifconfig</prgn> now gives |
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information about the new interface, including the netmask, network |
| 353 |
|
statistics, and information about the network driver itself. The |
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<prgn>ifconfig</prgn> also allows the specification of a netmask when |
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the interface is configured, for example: |
| 356 |
|
<example> |
| 357 |
|
# /sbin/ifconfig eth0 192.168.50.23 netmask 255.255.255.192 |
| 358 |
|
</example> |
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Finally, to bring down an interface, use the following invocation of |
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|
<prgn>ifconfig</prgn>: |
| 361 |
|
<example> |
| 362 |
|
# /sbin/ifconfig eth0 down |
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|
</example> |
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|
Full information on the options available to <prgn>ifconfig</prgn> are |
| 365 |
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available in the manual page - <manref name="ifconfig" section="8">.</p> |
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|
| 367 |
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<p>If you set up the IP addressing on your machine when you installed |
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Debian GNU/Linux, you should find that the <prgn>ifconfig</prgn> command |
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is run automatically on bootup. This is done from the file |
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<prgn>/etc/init.d/network</prgn>. Looking at this file should reveal a |
| 371 |
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number of lines similar to the following: |
| 372 |
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<example> |
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IPADDR=192.168.50.23 |
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NETMASK=255.255.255.0 |
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BROADCAST=192.168.50.255 |
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ifconfig eth0 ${IPADDR} netmask ${NETMASK} broadcast ${BROADCAST} |
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</example> |
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This is where the setting up of the Ethernet interface takes place. Above |
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these lines, you should see a line setting up the loopback interface, and |
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you should see a number of lines which appear to run a command called |
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<prgn>route</prgn>. This program, and its function, is the subject of |
| 382 |
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the next section.</p> |
| 383 |
|
</sect> |
| 384 |
|
|
| 385 |
|
<sect>Basic IP Routing |
| 386 |
|
<p> |
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|
|
| 388 |
|
|
| 389 |
|
</sect> |
| 390 |
</chapt> |
</chapt> |
| 391 |
|
|
| 392 |
<chapt> |
<chapt> |
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