[b4b71892] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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[ff769b8c] | 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
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[b4b71892] | 4 | <!ENTITY % general-entities SYSTEM "../../general.ent">
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| 5 | %general-entities;
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| 6 | ]>
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| 7 |
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[dd362e5] | 8 | <sect1 id="fw-firewall" xreflabel="Firewalling">
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[8920dfa] | 9 | <?dbhtml filename="firewall.html"?>
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| 10 |
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| 11 | <sect1info>
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| 12 | <othername>$LastChangedBy$</othername>
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| 13 | <date>$Date$</date>
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| 14 | </sect1info>
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| 15 |
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| 16 | <title>Setting Up a Network Firewall</title>
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| 17 |
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| 18 | <para>Before you read this part of the chapter, you should have
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| 19 | already installed iptables as described in the previous section.</para>
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| 20 |
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| 21 | <sect2 id="fw-intro" xreflabel="Firewalling Introduction">
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| 22 | <title>Introduction to Firewall Creation</title>
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| 23 |
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| 24 | <para>The general purpose of a firewall is to protect a computer or
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| 25 | a network against malicious access.</para>
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| 26 |
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| 27 | <para>In a perfect world, every daemon or service on every machine
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| 28 | is perfectly configured and immune to flaws such as buffer overflows
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| 29 | or other problems regarding its security. Furthermore, you trust
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| 30 | every user accessing your services. In this world, you do not need
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| 31 | to have a firewall.</para>
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| 32 |
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| 33 | <para>In the real world however, daemons may be misconfigured and
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| 34 | exploits against essential services are freely available. You may
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| 35 | wish to choose which services are accessible by certain machines or
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| 36 | you may wish to limit which machines or applications are allowed
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| 37 | external access. Alternatively, you may simply not trust some of
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| 38 | your applications or users. You are probably connected to the
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| 39 | Internet. In this world, a firewall is essential.</para>
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| 40 |
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| 41 | <para>Don't assume however, that having a firewall makes careful
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| 42 | configuration redundant, or that it makes any negligent
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| 43 | misconfiguration harmless. It doesn't prevent anyone from exploiting
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| 44 | a service you intentionally offer but haven't recently updated or
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| 45 | patched after an exploit went public. Despite having a firewall, you
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| 46 | need to keep applications and daemons on your system properly
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| 47 | configured and up to date. A firewall is not a cure all, but should
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| 48 | be an essential part of your overall security startegy.</para>
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| 49 |
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| 50 | </sect2>
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| 51 |
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| 52 | <sect2>
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| 53 | <title>Meaning of the Word "Firewall"</title>
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| 54 |
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| 55 | <para>The word firewall can have several different meanings.</para>
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| 56 |
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| 57 | <sect3>
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| 58 | <title><xref linkend="fw-persFw"/></title>
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| 59 |
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| 60 | <para>This is a hardware device or software program commercially
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| 61 | sold by companies such as Symantec which claims that it secures
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| 62 | a home or desktop computer with Internet access. This type of
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| 63 | firewall is highly relevant for users who do not know how their
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| 64 | computers might be accessed via the Internet or how to disable
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| 65 | that access, especially if they are always online and connected
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| 66 | via broadband links.</para>
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| 67 |
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| 68 | </sect3>
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| 69 |
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| 70 | <sect3>
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| 71 | <title><xref linkend="fw-masqRouter"/></title>
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| 72 |
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| 73 | <para>This is a system placed between the Internet and an intranet.
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| 74 | To minimize the risk of compromising the firewall itself, it should
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| 75 | generally have only one role—that of protecting the intranet.
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| 76 | Although not completely risk free, the tasks of doing the routing and
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| 77 | IP masquerading (rewriting IP headers of the packets it routes from
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| 78 | clients with private IP addresses onto the Internet so that they seem
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| 79 | to come from the firewall itself) are commonly considered relatively
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| 80 | secure.</para>
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| 81 |
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| 82 | </sect3>
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| 83 |
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| 84 | <sect3>
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| 85 | <title><xref linkend="fw-busybox"/></title>
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| 86 |
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| 87 | <para>This is often an old computer you may have retired and nearly
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| 88 | forgotten, performing masquerading or routing functions, but offering
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| 89 | non-firewall services such as a web-cache or mail. This may be used
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| 90 | for home networks, but is not be considered as secure as a firewall
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| 91 | only machine because the combination of server and router/firewall on
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| 92 | one machine raises the complexity of the setup.</para>
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| 93 |
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| 94 | </sect3>
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| 95 |
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| 96 | <sect3>
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| 97 | <title>Firewall with a Demilitarized Zone [Not Further
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| 98 | Described Here]</title>
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| 99 |
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| 100 | <para>This box performs masquerading or routing, but grants public
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| 101 | access to some branch of your network which, because of public IP's
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| 102 | and a physically separated structure, is essentially a separate
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| 103 | network with direct Internet access. The servers on this network are
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| 104 | those which must be easily accessible from both the Internet and
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| 105 | intranet. The firewall protects both networks. This type of firewall
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| 106 | has a minimum of three network interfaces.</para>
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| 107 |
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| 108 | </sect3>
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| 109 |
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| 110 | <sect3>
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| 111 | <title>Packetfilter</title>
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| 112 |
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| 113 | <para>This type of firewall does routing or masquerading, but does
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| 114 | not maintain a state table of ongoing communication streams. It is
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| 115 | fast, but quite limited in its ability to block inappropriate packets
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| 116 | without blocking desired packets.</para>
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| 117 |
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| 118 | </sect3>
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| 119 |
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| 120 | </sect2>
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| 121 |
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| 122 | <sect2 id="fw-writing" xreflabel="writing the firewalling-setup-scripts">
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| 123 | <title>Now You Can Start to Build your Firewall</title>
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| 124 |
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| 125 | <caution>
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| 126 | <para>This introduction on how to setup a firewall is not a
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| 127 | complete guide to securing systems. Firewalling is a complex
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| 128 | issue that requires careful configuration. The scripts quoted
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| 129 | here are simply intended to give examples of how a firewall
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| 130 | works. They are not intended to fit into any particular
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| 131 | configuration and may not provide complete protection from
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| 132 | an attack.</para>
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| 133 |
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| 134 | <para>Customization of these scripts for your specific situation
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| 135 | will be necessary for an optimal configuration, but you should
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| 136 | make a serious study of the iptables documentation and creating
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| 137 | firewalls in general before hacking away. Have a look at the
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| 138 | list of <xref linkend="fw-library"/> at the end of this section for
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| 139 | more details. There you will find a list of URLs that contain quite
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| 140 | comprehensive information about building your own firewall.</para>
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| 141 | </caution>
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| 142 |
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| 143 | <para>The firewall configuration script installed in the last section
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| 144 | differs from the standard configuration script. It only has two of
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| 145 | the standard targets: start and status. The other targets are clear
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| 146 | and lock. For instance when you run:</para>
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| 147 |
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| 148 | <screen role="root"><userinput>/etc/rc.d/init.d/iptables start</userinput></screen>
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| 149 |
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| 150 | <para>the firewall will be restarted just as it is upon system startup.
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| 151 | The status target will present a list of all currently implemented
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| 152 | rules. The clear target turns off all firewall rules and the lock
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| 153 | target will block all packets in and out of the computer with the
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| 154 | exception of the loopback interface.</para>
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| 155 |
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| 156 | <para>The main startup firewall is located in the file
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| 157 | <filename>/etc/rc.d/rc.iptables</filename>. The sections below provide
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| 158 | three different approaches that can be used for a system.</para>
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| 159 |
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| 160 | <note>
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| 161 | <para>You should always run your firewall rules from a script.
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| 162 | This ensures consistency and a record of what was done. It also
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| 163 | allows retention of comments that are essential for understanding
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| 164 | the rules long after they were written.</para>
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| 165 | </note>
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| 166 |
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| 167 | <sect3 id="fw-persFw" xreflabel="Personal Firewall">
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| 168 | <title>Personal Firewall</title>
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| 169 |
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| 170 | <para>A Personal Firewall is designed to let you access all the
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| 171 | services offered on the Internet, but keep your box secure and
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| 172 | your data private.</para>
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| 173 |
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| 174 | <para>Below is a slightly modified version of Rusty Russell's
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| 175 | recommendation from the <ulink
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| 176 | url="http://www.netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html">
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| 177 | Linux 2.4 Packet Filtering HOWTO</ulink>. It is still applicable
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| 178 | to the Linux 2.6 kernels.</para>
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| 179 |
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| 180 | <screen role="root"><userinput>cat > /etc/rc.d/rc.iptables << "EOF"
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| 181 | <literal>#!/bin/sh
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[b4b71892] | 182 |
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[0e3848e3] | 183 | # Begin $rc_base/rc.iptables
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[b4b71892] | 184 |
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[dd362e5] | 185 | # Insert connection-tracking modules
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| 186 | # (not needed if built into the kernel)
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[b4b71892] | 187 | modprobe ip_tables
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| 188 | modprobe iptable_filter
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| 189 | modprobe ip_conntrack
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| 190 | modprobe ip_conntrack_ftp
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| 191 | modprobe ipt_state
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| 192 | modprobe ipt_LOG
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| 193 |
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[0e3848e3] | 194 | # Enable broadcast echo Protection
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| 195 | echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
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[dd362e5] | 196 |
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[0e3848e3] | 197 | # Disable Source Routed Packets
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| 198 | echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route
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[b4b71892] | 199 |
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[0e3848e3] | 200 | # Enable TCP SYN Cookie Protection
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| 201 | echo 1 > /proc/sys/net/ipv4/tcp_syncookies
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[b4b71892] | 202 |
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[0e3848e3] | 203 | # Disable ICMP Redirect Acceptance
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| 204 | echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects
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| 205 |
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| 206 | # Don¹t send Redirect Messages
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| 207 | echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
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| 208 |
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| 209 | # Drop Spoofed Packets coming in on an interface, where responses
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| 210 | # would result in the reply going out a different interface.
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| 211 | echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
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[b4b71892] | 212 |
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[0e3848e3] | 213 | # Log packets with impossible addresses.
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| 214 | echo 1 > /proc/sys/net/ipv4/conf/all/log_martians
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[b4b71892] | 215 |
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[dd362e5] | 216 | # be verbose on dynamic ip-addresses (not needed in case of static IP)
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[b4b71892] | 217 | echo 2 > /proc/sys/net/ipv4/ip_dynaddr
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| 218 |
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[0e3848e3] | 219 | # disable Explicit Congestion Notification
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[dd362e5] | 220 | # too many routers are still ignorant
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[b4b71892] | 221 | echo 0 > /proc/sys/net/ipv4/tcp_ecn
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| 222 |
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[0e3848e3] | 223 | # Set a known state
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| 224 | iptables -P INPUT DROP
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| 225 | iptables -P FORWARD DROP
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| 226 | iptables -P OUTPUT DROP
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| 227 |
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| 228 | # These lines are here in case rules are already in place and the
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| 229 | # script is ever rerun on the fly. We want to remove all rules and
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| 230 | # pre-exisiting user defined chains before we implement new rules.
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| 231 | iptables -F
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| 232 | iptables -X
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| 233 | iptables -Z
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| 234 |
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| 235 | iptables -t nat -F
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| 236 |
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| 237 | # Allow local-only connections
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| 238 | iptables -A INPUT -i lo -j ACCEPT
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| 239 |
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| 240 | # Free output on any interface to any ip for any service
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| 241 | # (equal to -P ACCEPT)
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| 242 | iptables -A OUTPUT -j ACCEPT
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| 243 |
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| 244 | # Permit answers on already established connections
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| 245 | # and permit new connections related to established ones
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| 246 | # (e.g. port mode ftp)
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| 247 | iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
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| 248 |
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| 249 | # Log everything else. What's Windows' latest exploitable vulnerability?
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| 250 | iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
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| 251 |
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[8920dfa] | 252 | # End $rc_base/rc.iptables</literal>
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| 253 | EOF</userinput></screen>
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[b4b71892] | 254 |
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[8920dfa] | 255 | <para>This script is quite simple, it drops all traffic coming
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| 256 | in into your computer that wasn't initiated from your box, but
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| 257 | as long as you are simply surfing the Internet you are unlikely
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| 258 | to exceed its limits.</para>
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[b4b71892] | 259 |
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[8920dfa] | 260 | <para>If you frequently encounter certain delays at accessing
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| 261 | ftp-servers, take a look at <xref linkend="fw-BB-4"/>.</para>
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[b4b71892] | 262 |
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[8920dfa] | 263 | <para>Even if you have daemons or services running on your system,
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| 264 | these will be inaccessible everywhere but from your computer itself.
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| 265 | If you want to allow access to services on your machine, such as
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| 266 | <command>ssh</command> or <command>ping</command>, take a look at
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| 267 | <xref linkend="fw-busybox"/>.</para>
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[b4b71892] | 268 |
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[8920dfa] | 269 | </sect3>
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[b4b71892] | 270 |
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[8920dfa] | 271 | <sect3 id="fw-masqRouter" xreflabel="Masquerading Router">
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| 272 | <title>Masquerading Router</title>
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[b4b71892] | 273 |
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[8920dfa] | 274 | <para>A true Firewall has two interfaces, one connected to an
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| 275 | intranet, in this example <emphasis role="strong">eth0</emphasis>,
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| 276 | and one connected to the Internet, here <emphasis
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| 277 | role="strong">ppp0</emphasis>. To provide the maximum security
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| 278 | for the firewall itself, make sure that there are no unnecessary
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| 279 | servers running on it such as <application>X11</application> et
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| 280 | al. As a general principle, the firewall itself should not access
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| 281 | any untrusted service (Think of a remote server giving answers that
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| 282 | makes a daemon on your system crash, or, even worse, that implements
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| 283 | a worm via a buffer-overflow).</para>
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[0e3848e3] | 284 |
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[8920dfa] | 285 | <screen role="root"><userinput>cat > /etc/rc.d/rc.iptables << "EOF"
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| 286 | <literal>#!/bin/sh
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[b4b71892] | 287 |
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[0e3848e3] | 288 | # Begin $rc_base/rc.iptables
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[b4b71892] | 289 |
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| 290 | echo
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[0e3848e3] | 291 | echo "You're using the example configuration for a setup of a firewall"
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| 292 | echo "from Beyond Linux From Scratch."
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[b4b71892] | 293 | echo "This example is far from being complete, it is only meant"
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| 294 | echo "to be a reference."
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| 295 | echo "Firewall security is a complex issue, that exceeds the scope"
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[0e3848e3] | 296 | echo "of the configuration rules below."
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| 297 | echo "You can find additional information"
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[b4b71892] | 298 | echo "about firewalls in Chapter 4 of the BLFS book."
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| 299 | echo "http://www.linuxfromscratch.org/blfs"
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| 300 | echo
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| 301 |
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| 302 | # Insert iptables modules (not needed if built into the kernel).
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| 303 |
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| 304 | modprobe ip_tables
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| 305 | modprobe iptable_filter
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| 306 | modprobe ip_conntrack
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| 307 | modprobe ip_conntrack_ftp
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| 308 | modprobe ipt_state
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| 309 | modprobe iptable_nat
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| 310 | modprobe ip_nat_ftp
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| 311 | modprobe ipt_MASQUERADE
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| 312 | modprobe ipt_LOG
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| 313 | modprobe ipt_REJECT
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| 314 |
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[0e3848e3] | 315 | # Enable broadcast echo Protection
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| 316 | echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
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| 317 |
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| 318 | # Disable Source Routed Packets
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| 319 | echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route
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| 320 |
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| 321 | # Enable TCP SYN Cookie Protection
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| 322 | echo 1 > /proc/sys/net/ipv4/tcp_syncookies
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| 323 |
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| 324 | # Disable ICMP Redirect Acceptance
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| 325 | echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects
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| 326 |
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| 327 | # Don¹t send Redirect Messages
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| 328 | echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
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| 329 |
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| 330 | # Drop Spoofed Packets coming in on an interface where responses
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| 331 | # would result in the reply going out a different interface.
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| 332 | echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
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| 333 |
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| 334 | # Log packets with impossible addresses.
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| 335 | echo 1 > /proc/sys/net/ipv4/conf/all/log_martians
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| 336 |
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| 337 | # Be verbose on dynamic ip-addresses (not needed in case of static IP)
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| 338 | echo 2 > /proc/sys/net/ipv4/ip_dynaddr
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| 339 |
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| 340 | # Disable Explicit Congestion Notification
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| 341 | # Too many routers are still ignorant
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| 342 | echo 0 > /proc/sys/net/ipv4/tcp_ecn
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| 343 |
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| 344 | # Set a known state
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| 345 | iptables -P INPUT DROP
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| 346 | iptables -P FORWARD DROP
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| 347 | iptables -P OUTPUT DROP
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| 348 |
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| 349 | # These lines are here in case rules are already in place and the
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| 350 | # script is ever rerun on the fly. We want to remove all rules and
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| 351 | # pre-exisiting user defined chains before we implement new rules.
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| 352 | iptables -F
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| 353 | iptables -X
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| 354 | iptables -Z
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| 355 |
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| 356 | iptables -t nat -F
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| 357 |
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| 358 | # Allow local connections
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[b4b71892] | 359 | iptables -A INPUT -i lo -j ACCEPT
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| 360 | iptables -A OUTPUT -o lo -j ACCEPT
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| 361 |
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[0e3848e3] | 362 | # Allow forwarding if the initiated on the intranet
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[b4b71892] | 363 | iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
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[0e3848e3] | 364 | iptables -A FORWARD -i ! ppp+ -m state --state NEW -j ACCEPT
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[b4b71892] | 365 |
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[0e3848e3] | 366 | # Do masquerading
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[dd362e5] | 367 | # (not needed if intranet is not using private ip-addresses)
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[b4b71892] | 368 | iptables -t nat -A POSTROUTING -o ppp+ -j MASQUERADE
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| 369 |
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[dd362e5] | 370 | # Log everything for debugging
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[0e3848e3] | 371 | # (last of all rules, but before policy rules)
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[b4b71892] | 372 | iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
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| 373 | iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD"
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| 374 | iptables -A OUTPUT -j LOG --log-prefix "FIREWALL:OUTPUT "
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| 375 |
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[0e3848e3] | 376 | # Enable IP Forwarding
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[8920dfa] | 377 | echo 1 > /proc/sys/net/ipv4/ip_forward</literal>
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| 378 | EOF</userinput></screen>
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| 379 |
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| 380 | <para>With this script your intranet should be reasonably secure
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| 381 | against external attacks. No one should be able to setup a new
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| 382 | connection to any internal service and, if it's masqueraded,
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| 383 | makes your intranet invisible to the Internet. Furthermore, your
|
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| 384 | firewall should be relatively safe because there are no services
|
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| 385 | running that a cracker could attack.</para>
|
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| 386 |
|
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| 387 | <note>
|
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| 388 | <para>If the interface you're connecting to the Internet
|
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| 389 | doesn't connect via ppp, you will need to change
|
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| 390 | <replaceable>ppp+</replaceable> to the name of the interface,
|
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| 391 | e.g. <emphasis role="strong">eth1</emphasis>, which you are
|
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| 392 | using.</para>
|
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| 393 | </note>
|
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| 394 |
|
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| 395 | </sect3>
|
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| 396 |
|
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| 397 | <sect3 id="fw-busybox" xreflabel="BusyBox">
|
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| 398 | <title>BusyBox</title>
|
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| 399 |
|
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| 400 | <para>This scenario isn't too different from the <xref
|
---|
| 401 | linkend="fw-masqRouter"/>, but additionally offers some
|
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| 402 | services to your intranet. Examples of this can be when
|
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| 403 | you want to administer your firewall from another host on
|
---|
| 404 | your intranet or use it as a proxy or a name server.</para>
|
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| 405 |
|
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| 406 | <note>
|
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| 407 | <para>Outlining a true concept of how to protect a server that
|
---|
| 408 | offers services on the Internet goes far beyond the scope of
|
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| 409 | this document. See the references at the end of this section
|
---|
| 410 | for more information.</para>
|
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| 411 | </note>
|
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| 412 |
|
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| 413 | <para>Be cautious. Every service you have enabled makes your
|
---|
| 414 | setup more complex and your firewall less secure. You are
|
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| 415 | exposed to the risks of misconfigured services or running
|
---|
| 416 | a service with an exploitable bug. A firewall should generally
|
---|
| 417 | not run any extra services. See the introduction to the
|
---|
| 418 | <xref linkend="fw-masqRouter"/> for some more details.</para>
|
---|
| 419 |
|
---|
| 420 | <para>If you want to add services such as internal samba or
|
---|
| 421 | name servers that do not need to access the Internet themselves,
|
---|
| 422 | the additional statements are quite simple and should still be
|
---|
| 423 | acceptable from a security standpoint. Just add the following lines
|
---|
| 424 | into the script <emphasis>before</emphasis> the logging rules.</para>
|
---|
| 425 |
|
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| 426 | <screen><literal>iptables -A INPUT -i ! ppp+ -j ACCEPT
|
---|
| 427 | iptables -A OUTPUT -o ! ppp+ -j ACCEPT</literal></screen>
|
---|
| 428 |
|
---|
| 429 | <para>If daemons, such as squid, have to access the Internet
|
---|
| 430 | themselves, you could open OUTPUT generally and restrict
|
---|
| 431 | INPUT.</para>
|
---|
| 432 |
|
---|
| 433 | <screen><literal>iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
|
---|
| 434 | iptables -A OUTPUT -j ACCEPT</literal></screen>
|
---|
| 435 |
|
---|
| 436 | <para>However, it is generally not advisable to leave OUTPUT
|
---|
| 437 | unrestricted. You lose any control over trojans who would like
|
---|
| 438 | to "call home", and a bit of redundancy in case you've
|
---|
| 439 | (mis-)configured a service so that it broadcasts its existence
|
---|
| 440 | to the world.</para>
|
---|
| 441 |
|
---|
| 442 | <para>To accomplish this, you should restrict INPUT and OUTPUT
|
---|
| 443 | on all ports except those that it's absolutely necessary to have
|
---|
| 444 | open. Which ports you have to open depends on your needs: mostly
|
---|
| 445 | you will find them by looking for failed accesses in your log
|
---|
| 446 | files.</para>
|
---|
| 447 |
|
---|
| 448 | <itemizedlist spacing="compact" role='iptables'>
|
---|
| 449 | <title>Have a Look at the Following Examples:</title>
|
---|
| 450 | <listitem>
|
---|
| 451 | <para>Squid is caching the web:</para>
|
---|
| 452 |
|
---|
| 453 | <screen><literal>iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
|
---|
[0e3848e3] | 454 | iptables -A INPUT -p tcp --sport 80 -m state --state ESTABLISHED \
|
---|
[8920dfa] | 455 | -j ACCEPT</literal></screen>
|
---|
| 456 |
|
---|
| 457 | </listitem>
|
---|
| 458 | <listitem>
|
---|
| 459 | <para>Your caching name server (e.g., named) does its
|
---|
| 460 | lookups via udp:</para>
|
---|
| 461 |
|
---|
| 462 | <screen><literal>iptables -A OUTPUT -p udp --dport 53 -j ACCEPT</literal></screen>
|
---|
| 463 |
|
---|
| 464 | </listitem>
|
---|
| 465 | <listitem>
|
---|
| 466 | <para>You want to be able to ping your box to
|
---|
| 467 | ensure it's still alive:</para>
|
---|
| 468 |
|
---|
| 469 | <screen><literal>iptables -A INPUT -p icmp -m icmp --icmp-type echo-request -j ACCEPT
|
---|
| 470 | iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply -j ACCEPT</literal></screen>
|
---|
| 471 |
|
---|
| 472 | </listitem>
|
---|
| 473 | <listitem>
|
---|
| 474 | <para><anchor id='fw-BB-4' xreflabel="BusyBox example number 4"/>If
|
---|
| 475 | you are frequently accessing ftp servers or enjoy chatting, you might
|
---|
| 476 | notice certain delays because some implementations of these daemons
|
---|
| 477 | have the feature of querying an identd on your system to obtain
|
---|
| 478 | usernames. Although there's really little harm in this, having an
|
---|
| 479 | identd running is not recommended because many security experts feel
|
---|
| 480 | the service gives out too much additional information.</para>
|
---|
| 481 |
|
---|
| 482 | <para>To avoid these delays you could reject the requests
|
---|
| 483 | with a 'tcp-reset':</para>
|
---|
| 484 |
|
---|
| 485 | <screen><literal>iptables -A INPUT -p tcp --dport 113 -j REJECT --reject-with tcp-reset</literal></screen>
|
---|
| 486 |
|
---|
| 487 | </listitem>
|
---|
| 488 | <listitem>
|
---|
| 489 | <para>To log and drop invalid packets (packets
|
---|
| 490 | that came in after netfilter's timeout or some types of
|
---|
| 491 | network scans):</para>
|
---|
| 492 |
|
---|
| 493 | <screen><literal>iptables -I INPUT -p tcp -m state --state INVALID \
|
---|
[0e3848e3] | 494 | -j LOG --log-prefix "FIREWALL:INVALID"
|
---|
[8920dfa] | 495 | iptables -I INPUT -p tcp -m state --state INVALID -j DROP</literal></screen>
|
---|
[b4b71892] | 496 |
|
---|
[8920dfa] | 497 | </listitem>
|
---|
| 498 | <listitem>
|
---|
| 499 | <para>Anything coming from the outside should not have a
|
---|
| 500 | private address, this is a common attack called IP-spoofing:</para>
|
---|
[0e3848e3] | 501 |
|
---|
[8920dfa] | 502 | <screen><literal>iptables -A INPUT -i ppp+ -s 10.0.0.0/8 -j DROP
|
---|
[0e3848e3] | 503 | iptables -A INPUT -i ppp+ -s 172.16.0.0/12 -j DROP
|
---|
[8920dfa] | 504 | iptables -A INPUT -i ppp+ -s 192.168.0.0/16 -j DROP</literal></screen>
|
---|
[0e3848e3] | 505 |
|
---|
[8920dfa] | 506 | <para>There are other addresses that you may also want to
|
---|
| 507 | drop: 0.0.0.0/8, 127.0.0.0/8, 224.0.0.0/3 (multicast and
|
---|
| 508 | experimental), 169.254.0.0/16 (Link Local Networks), and
|
---|
| 509 | 192.0.2.0/24 (IANA defined test network).</para>
|
---|
| 510 | </listitem>
|
---|
| 511 | <listitem>
|
---|
| 512 | <para>If your firewall is a DHCP client, you need to allow
|
---|
| 513 | those packets:</para>
|
---|
[0e3848e3] | 514 |
|
---|
[8920dfa] | 515 | <screen><literal>iptables -A INPUT -i ppp0 -p udp -s 0.0.0.0 --sport 67 \
|
---|
| 516 | -d 255.255.255.255 --dport 68 -j ACCEPT</literal></screen>
|
---|
[b4b71892] | 517 |
|
---|
[8920dfa] | 518 | </listitem>
|
---|
| 519 | <listitem>
|
---|
| 520 | <para>To simplify debugging and be fair to anyone who'd like
|
---|
| 521 | to access a service you have disabled, purposely or by mistake,
|
---|
| 522 | you could REJECT those packets that are dropped.</para>
|
---|
[b4b71892] | 523 |
|
---|
[8920dfa] | 524 | <para>Obviously this must be done directly after logging as the very
|
---|
| 525 | last lines before the packets are dropped by policy:</para>
|
---|
[b4b71892] | 526 |
|
---|
[8920dfa] | 527 | <screen><literal>iptables -A INPUT -j REJECT</literal></screen>
|
---|
[b4b71892] | 528 |
|
---|
[8920dfa] | 529 | </listitem>
|
---|
| 530 | </itemizedlist>
|
---|
[b4b71892] | 531 |
|
---|
[8920dfa] | 532 | <para>These are only examples to show you some of the capabilities
|
---|
| 533 | of the firewall code in Linux. Have a look at the man page of iptables.
|
---|
| 534 | There you will find much more information. The port numbers needed for
|
---|
| 535 | this can be found in <filename>/etc/services</filename>, in case you
|
---|
| 536 | didn't find them by trial and error in your log file.</para>
|
---|
[0e3848e3] | 537 |
|
---|
[8920dfa] | 538 | </sect3>
|
---|
[b4b71892] | 539 |
|
---|
[8920dfa] | 540 | </sect2>
|
---|
[b4b71892] | 541 |
|
---|
[8920dfa] | 542 | <sect2 id="fw-finale" xreflabel="Conclusion">
|
---|
| 543 | <title>Conclusion</title>
|
---|
[b4b71892] | 544 |
|
---|
[8920dfa] | 545 | <para>Finally, there is one fact you must not forget: The effort spent
|
---|
| 546 | attacking a system corresponds to the value the cracker expects to gain
|
---|
| 547 | from it. If you are responsible for valuable information, you need to
|
---|
| 548 | spend the time to protect it properly.</para>
|
---|
[b4b71892] | 549 |
|
---|
[8920dfa] | 550 | </sect2>
|
---|
[b4b71892] | 551 |
|
---|
[8920dfa] | 552 | <sect2 id="postlfs-security-fw-extra" xreflabel="Extra Information">
|
---|
| 553 | <title>Extra Information</title>
|
---|
[b4b71892] | 554 |
|
---|
[8920dfa] | 555 | <sect3 id="fw-library" xreflabel="Links for further reading">
|
---|
| 556 | <title>Where to Start with Further Reading on Firewalls.</title>
|
---|
| 557 |
|
---|
| 558 | <blockquote>
|
---|
| 559 | <literallayout>
|
---|
[b4b71892] | 560 | <ulink url="http://www.netfilter.org/">www.netfilter.org - Homepage of the netfilter/iptables project</ulink>
|
---|
| 561 | <ulink url="http://www.netfilter.org/documentation/FAQ/netfilter-faq.html">Netfilter related FAQ</ulink>
|
---|
| 562 | <ulink url="http://www.netfilter.org/documentation/index.html#HOWTO">Netfilter related HOWTO's</ulink>
|
---|
| 563 | <ulink url="http://en.tldp.org/LDP/nag2/x-087-2-firewall.html">en.tldp.org/LDP/nag2/x-087-2-firewall.html</ulink>
|
---|
| 564 | <ulink url="http://en.tldp.org/HOWTO/Security-HOWTO.html">en.tldp.org/HOWTO/Security-HOWTO.html</ulink>
|
---|
| 565 | <ulink url="http://en.tldp.org/HOWTO/Firewall-HOWTO.html">en.tldp.org/HOWTO/Firewall-HOWTO.html</ulink>
|
---|
| 566 | <ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire.html">www.ibm.com/developerworks/security/library/s-fire.html</ulink>
|
---|
| 567 | <ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire2.html">www.ibm.com/developerworks/security/library/s-fire2.html</ulink>
|
---|
| 568 | <ulink url="http://www.interhack.net/pubs/fw-faq/">www.interhack.net/pubs/fw-faq/</ulink>
|
---|
| 569 | <ulink url="http://www.linuxsecurity.com/docs/">www.linuxsecurity.com/docs/</ulink>
|
---|
| 570 | <ulink url="http://www.little-idiot.de/firewall">www.little-idiot.de/firewall (German & outdated, but very comprehensive)</ulink>
|
---|
| 571 | <ulink url="http://www.linuxgazette.com/issue65/stumpel.html">www.linuxgazette.com/issue65/stumpel.html</ulink>
|
---|
| 572 | <ulink url="http://linux.oreillynet.com/pub/a/linux/2000/03/10/netadmin/ddos.html">linux.oreillynet.com/pub/a/linux/2000/03/10/netadmin/ddos.html</ulink>
|
---|
| 573 | <ulink url="http://staff.washington.edu/dittrich/misc/ddos">staff.washington.edu/dittrich/misc/ddos</ulink>
|
---|
| 574 | <ulink url="http://www.e-infomax.com/ipmasq">www.e-infomax.com/ipmasq</ulink>
|
---|
| 575 | <ulink url="http://www.circlemud.org/~jelson/writings/security/index.htm">www.circlemud.org/~jelson/writings/security/index.htm</ulink>
|
---|
| 576 | <ulink url="http://www.securityfocus.com">www.securityfocus.com</ulink>
|
---|
| 577 | <ulink url="http://www.cert.org/tech_tips/">www.cert.org - tech_tips</ulink>
|
---|
| 578 | <ulink url="http://security.ittoolbox.com/">security.ittoolbox.com</ulink>
|
---|
| 579 | <ulink url="http://www.linux-firewall-tools.com/linux/">www.linux-firewall-tools.com/linux/</ulink>
|
---|
| 580 | <ulink url="http://logi.cc/linux/athome-firewall.php3">logi.cc/linux/athome-firewall.php3</ulink>
|
---|
| 581 | <ulink url="http://www.insecure.org/reading.html">www.insecure.org/reading.html</ulink>
|
---|
| 582 | <ulink url="http://www.robertgraham.com/pubs/firewall-seen.html">www.robertgraham.com/pubs/firewall-seen.html</ulink>
|
---|
[8920dfa] | 583 | </literallayout>
|
---|
| 584 | </blockquote>
|
---|
| 585 |
|
---|
| 586 | </sect3>
|
---|
| 587 |
|
---|
| 588 | </sect2>
|
---|
[b4b71892] | 589 |
|
---|
[f45b1953] | 590 | </sect1>
|
---|
| 591 |
|
---|