mirror of https://github.com/tLDP/LDP
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<row><entry>Application layer</entry><entry>HTTP, DNS, SMTP, POP, ...</entry></row>
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<row><entry>Transport layer</entry><entry>TCP, UDP</entry></row>
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<row><entry>Network layer</entry><entry>IP, IPv6</entry></row>
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<row>Network access layer</entry><entry>PPP, PPPoE, Ethernet</entry></row>
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<row><entry>Network access layer</entry><entry>PPP, PPPoE, Ethernet</entry></row>
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</tbody>
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</tgroup>
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</table>
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@ -155,7 +155,7 @@ hosts: files dns
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<sect2 id="sect_10_02_03"><title>Network configuration commands</title>
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<sect3 id="sect_10_02_03_01"><title>The ip command</title>
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<para>The distribution-specific scripts and graphical tools are front-ends to <command>ip<indexterm><primary>ip</primary></indexterm></command> (or <command>ifconfig<indexterm><primary>ifconfig</primary></indexterm></command> and <command>route<indexterm><primary>route</primary><secondary></command> on older systems) to display and configure the kernel's networking configuration.</para>
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<para>The distribution-specific scripts and graphical tools are front-ends to <command>ip<indexterm><primary>ip</primary></indexterm></command> (or <command>ifconfig<indexterm><primary>ifconfig</primary></indexterm></command> and <command>route<indexterm><primary>route</primary></indexterm></command> on older systems) to display and configure the kernel's networking configuration.</para>
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<para>The <command>ip</command> command is used for assigning IP addresses to interfaces, for setting up routes to the Internet and to other networks, for displaying TCP/IP configurations etcetera.</para>
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<para>The following commands show IP address<indexterm><primary>networking</primary><secondary>display IP address</secondary></indexterm> and routing<indexterm><primary>networking</primary><secondary>display routes</secondary></indexterm> information:</para>
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@ -252,7 +252,7 @@ Destination Gateway Genmask Flags MSS Window irtt Iface
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<para>This is a typical client machine in an IP network. It only has one network device, <emphasis>eth0</emphasis>. The <emphasis>lo</emphasis> interface is the local loop.</para>
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<note><title>The modern way</title>
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<para>The novel way to get this info from your system is by using the <command>ip</command> command:</para>
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<cmdsynopsis><command>ip <parameter>route show</parameter></cmdsynopsis>
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<cmdsynopsis><command>ip <parameter>route show</parameter></command></cmdsynopsis>
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</note>
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<para>When this machine tries to contact a host that is on another network than its own, indicated by the line starting with 0.0.0.0, it will send the connection requests to the machine (router) with IP address 192.168.42.1, and it will use its primary interface, eth0, to do this.</para>
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<para>Hosts that are on the same network, the line starting with 192.168.42.0, will also be contacted through the primary network interface, but no router is necessary, the data are just put on the network.</para>
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@ -450,7 +450,7 @@ localhost.localdomain: anonymous/bob@his.server.com: IDLE
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<para>Most Linux distributions include <command>fetchmail<indexterm><primary>fetchmail</primary></indexterm></command>, a mail-retrieval and forwarding utility. It fetches mail from remote mail servers (POP, IMAP and some others) and forwards it to your local delivery system. You can then handle the retrieved mail using normal mail clients. It can be run in daemon mode to repeatedly poll one or more systems at a specified interval. Information and usage examples can be found in the Info pages; the directory <filename>/usr/share/doc/fetchmail[-<version>]</filename> contains a full list of features and a FAQ for beginners.</para>
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<para>The <command>procmail<indexterm><primary>procmail</primary></indexterm></command> filter can be used for filtering incoming mail, to create mailing lists, to pre-process mail, to selectively forward mail and more. The accompanying <command>formail<indexterm><primary>formail</primary></indexterm></command> program, among others, enables generation of auto-replies and splitting up mailboxes. Procmail has been around for years on UNIX and Linux machines and is a very robust system, designed to work even in the worst circumstances. More information may be found in the <filename>/usr/share/doc/procmail[-<version>];</filename> directory and in the man pages.</para>
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<para>The <command>procmail<indexterm><primary>procmail</primary></indexterm></command> filter can be used for filtering incoming mail, to create mailing lists, to pre-process mail, to selectively forward mail and more. The accompanying <command>formail<indexterm><primary>formail</primary></indexterm></command> program, among others, enables generation of auto-replies and splitting up mailboxes. Procmail has been around for years on UNIX and Linux machines and is a very robust system, designed to work even in the worst circumstances. More information may be found in the <filename>/usr/share/doc/procmail[-<version>]</filename> directory and in the man pages.</para>
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</sect3>
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@ -475,7 +475,7 @@ localhost.localdomain: anonymous/bob@his.server.com: IDLE
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<para>For instance:</para>
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<cmdsynopsis><command>export <varname>http_proxy</varname>=<parameter>http://willy:Appelsi3ntj3@proxy:80</parameter></command></cmdsynopsis>
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<para>If you do not need to give a username and password, simply leave out everything before the <quote>@</quote> sign, this sign included.</para>
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</sect4>
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</sect3>
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</sect2>
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<sect2 id="sect_10_04_04"><title>File Transfer Protocol</title>
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@ -874,7 +874,7 @@ tcp 0 0 ::1:x11-ssh-offset *:* LISTEN
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<sect3 id="sect_10_06_04_02"><title>Packet filters</title>
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<para>The first line of defense is a <emphasis>packet filter<indexterm><primary>security</primary><secondary>packet filters</secondary></indexterm></emphasis>, which can look inside IP packets and make decisions based on the content. Most common is the <application>Netfilter</application> package, providing the <command>iptables</command> command, a next generation packet filter for Linux.</para>
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<para>One of the most noteworthy enhancements in the newer kernels is the <emphasis>stateful inspection</emphasis> feature, which not only tells what is inside a packet, but also detects if a packet belongs or is related to a new or existing connection.</para>
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<para>The <application>Shoreline Firewall</application> or <application>Shorewall</application> for short is a front-end for the
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<para>The <application>Shoreline Firewall</application> or <application>Shorewall</application> for short is a front-end for the standard firewall functionality in Linux.</para>
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<para>More information can be found at <ulink url="http://www.netfilter.org/">the Netfilter/iptables project page</ulink>.</para>
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</sect3>
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<sect3 id="sect_10_06_04_03"><title>TCP wrappers</title>
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@ -903,7 +903,7 @@ tcp 0 0 ::1:x11-ssh-offset *:* LISTEN
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<para>Some general things<indexterm><primary>security</primary><secondary>tips</secondary></indexterm> you should keep in mind:</para>
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<itemizedlist>
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<listitem><para>Do not allow root logins. UNIX developers came up with the <command>su</command> over two decades ago for extra security.</para></listitem>
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<listitem><para>Direct root access is always dangerous and susceptible to human errors, be it by allowing root login or by using the <command>su <option>-</option></command> command. Rather than using <command>su</command>, it is even better to use <command>sudo</commmand> to only execute the command that you need extra permissions for, and to return afterwards to your own environment.</para></listitem>
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<listitem><para>Direct root access is always dangerous and susceptible to human errors, be it by allowing root login or by using the <command>su <option>-</option></command> command. Rather than using <command>su</command>, it is even better to use <command>sudo</command> to only execute the command that you need extra permissions for, and to return afterwards to your own environment.</para></listitem>
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<listitem><para>Take passwords seriously. Use shadow passwords. Change your passwords regularly.</para></listitem>
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<listitem><para>Try to always use SSH or SSL. Avoid <command>telnet</command>, FTP and E-mail clients and other client programs which send unencrypted passwords over the network. Security is not only about securing your computer, it is also about securing your passwords.</para></listitem>
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<listitem><para>Limit resources using <command>quota</command> and/or <command>ulimit</command>.</para></listitem>
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@ -1090,7 +1090,7 @@ sent-mail: On Mon, 24 Dec 2001, Arno.Hintjens@celeb.com wrote:
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<sect2 id="sect_03_03_04"><title>More ways to view file content</title>
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<sect3 id="sect_03_03_04_01"><title>General</title>
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<para>Apart from <command>cat</command>, which really doesn't do much more than sending files to the standard output, there are other tools to view file content.</para>
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<para>The easiest way of course would be to use graphical tools instead of command line tools. In the introduction we already saw a glimpse of an office application, <application>OpenOffice.org</application>. Other examples are the <application>GIMP</application> (start up with <command>gimp<indexterm><primary>gimp</primary></indexterm></command> from the command line), the GNU Image Manipulation Program; <command>xpdf<indexterm><primary>xpdf</primary></indexterm></command> to view Portable Document Format files (PDF); <application>GhostView</application> (<command>gv<indexterm><primary>gv</primary></indexterm></command>) for viewing PostScript files; <application>Mozilla/FireFox</application>, <command>links</command> (a text mode browser), <application>Konqueror<application>, <application>Opera</application> and many others for web content; XMMS, <application>CDplay</application> and others for multimedia file content; <application>AbiWord</application>, <application>Gnumeric</application>, <application>KOffice</application> etc. for all kinds of office applications and so on. There are thousands of Linux applications; to list them all would take days.</para>
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<para>The easiest way of course would be to use graphical tools instead of command line tools. In the introduction we already saw a glimpse of an office application, <application>OpenOffice.org</application>. Other examples are the <application>GIMP</application> (start up with <command>gimp<indexterm><primary>gimp</primary></indexterm></command> from the command line), the GNU Image Manipulation Program; <command>xpdf<indexterm><primary>xpdf</primary></indexterm></command> to view Portable Document Format files (PDF); <application>GhostView</application> (<command>gv<indexterm><primary>gv</primary></indexterm></command>) for viewing PostScript files; <application>Mozilla/FireFox</application>, <command>links</command> (a text mode browser), <application>Konqueror</application>, <application>Opera</application> and many others for web content; XMMS, <application>CDplay</application> and others for multimedia file content; <application>AbiWord</application>, <application>Gnumeric</application>, <application>KOffice</application> etc. for all kinds of office applications and so on. There are thousands of Linux applications; to list them all would take days.</para>
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<para>Instead we keep concentrating on shell- or text-mode applications, which form the basics for all other applications. These commands work best in a text environment on files containing text. When in doubt, check first using the <command>file<indexterm><primary>file</primary></indexterm></command> command.</para>
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<para>So let's see what text tools we have that are useful to look inside files.</para>
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<note><title>Font problems</title>
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@ -108,6 +108,7 @@ printer blob now printing blob-253. enabled since Jan 01 18:01
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<para>The man<indexterm><primary>man pages</primary><secondary>printing</secondary></indexterm> pages contain pre-formatted <command>troff<indexterm><primary>troff</primary></indexterm></command> data which has to be formatted before it can roll out of your printer. Printing<indexterm><primary>printing</primary><secondary>man pages</secondary></indexterm> is done using the <option>-t</option> option to the <command>man</command> command:</para>
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<cmdsynopsis><command>man <option>-t</option> <parameter>command</parameter> > <filename>man-command.ps</filename></command></cmdsynopsis>
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<para>Then print the PostScript file. If a default print destination is configured for your system/account, you can just issue the command <command>man <option>-t</option> <parameter>command</parameter></command> to send the formatted page to the printer directly.</para>
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</tip>
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</sect3>
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<sect3 id="sect_08_01_02_02"><title>Previewing formatted files</title>
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<para>Anything that you can send to the printer can normally be sent to the screen as well. Depending on the file format, you can use one of these commands<indexterm><primary>printing</primary><secondary>preview</secondary></indexterm>:</para>
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<para>Apart from that, encryption can be applied to E-mails as well: normally, mail is not encrypted and it is often sent in the open over the netwerk or the Internet. If your message contains sensitive information, better encrypt it.</para>
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</sect3>
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<sect3 id="sect_09_04_01_02"><title>GNU Privacy Guard</title>
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<para>On Linux systems you will find <application>GnuPG</application>, the GNU Privacy Guard<indexterm><primary>encryption</primary><secondary>GnuPG</secondary></indexterm></application>, which is a suite of programs that are compatible with the PGP (<application>Pretty Good Privacy</application>) tools that are commercially available.</para>
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<para>On Linux systems you will find <application>GnuPG</application>, the <application>GNU Privacy Guard<indexterm><primary>encryption</primary><secondary>GnuPG</secondary></indexterm></application>, which is a suite of programs that are compatible with the PGP (<application>Pretty Good Privacy</application>) tools that are commercially available.</para>
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<para>In this guide we will only discuss the very simple usage of the encryption tools and show what you will need in order to generate an encryption key and use it to encrypt data for yourself, which you can then safely store in a public place. More advanced usage directions can be found in the man pages of the various commands.</para>
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</sect3>
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</sect2>
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<sect2 id="sect_09_04_02"><title>Generate a key</title>
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<para>Before you can start encrypting your data, you need to create<indexterm><primary>encryption</primary><secondary>generate keys</secondary></indexterm> a pair of keys. The pair consists of a private and a public key. You can send the public key to correspondents, who can use it to encrypt data for you, which you decrypt with your private key. You always keep the private key, never share it with somebody else, or they will be able to decrypt data that is only destined for you. Just to make sure that no accidents happen, the private key is protected with a password. The key pair is created using this command<indexterm><primary>gpg</primary></indexterm>:</para>
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<screen>
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