[b4b71892] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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| 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
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| 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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[f45b1953] | 8 | <sect1 id="postlfs-security-fw-firewall" xreflabel="Firewalling">
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[a0f03b0] | 9 | <sect1info>
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[5cd0959d] | 10 | <othername>$LastChangedBy$</othername>
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| 11 | <date>$Date$</date>
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[a0f03b0] | 12 | </sect1info>
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[bae6e15] | 13 | <?dbhtml filename="firewall.html"?>
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[f45b1953] | 14 | <title>Setting up a network firewall</title>
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| 15 |
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[36ae4612] | 16 | <para>Before you read this part of the chapter, note that we assume that you
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| 17 | have already installed iptables as described in the previous section.</para>
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[f45b1953] | 18 |
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| 19 |
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[b4b71892] | 20 | <sect2 id="postlfs-security-fw-intro" xreflabel="Firewalling Introduction">
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| 21 | <title>Introduction to Firewall Creation</title>
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| 22 |
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| 23 | <para>The general purpose of a firewall is to protect a network
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| 24 | against malicious access by using a single machine as a firewall.
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| 25 | This does imply that the firewall is to be considered a single point
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| 26 | of failure, but it can make the administrator's life a lot easier.</para>
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| 27 |
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| 28 | <para>In a perfect world where you knew that every daemon or service
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| 29 | on every machine was perfectly configured and was immune to, e.g.,
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| 30 | buffer-overflows and any other imaginable problem regarding its
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| 31 | security, and where you trusted every user accessing your services
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| 32 | to aim no harm, you wouldn't need to have a firewall!
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| 33 | In the real world however, daemons may be misconfigured,
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| 34 | exploits against essential services are freely available, you
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| 35 | may wish to choose which services are accessible by certain machines,
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| 36 | you may wish to limit which machines or applications are allowed
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| 37 | to have Internet access, or you may simply not trust some of your
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| 38 | apps or users.
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| 39 | In these situations you might benefit by using a firewall.</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 also doesn't prevent anyone from exploiting a
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| 44 | service you intentionally offer but haven't recently updated or patched
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| 45 | after an exploit went public. Despite having a firewall, you need to
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| 46 | keep applications and daemons on your system well-configured and
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| 47 | up-to-date; a firewall is not a cure-all!</para>
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| 48 |
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| 49 | </sect2>
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| 50 |
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| 51 | <sect2>
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| 52 | <title>Meaning of the word firewall.</title>
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| 53 |
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| 54 | <para>The word firewall can have several different meanings.</para>
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| 55 |
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| 56 | <sect3><title><xref linkend="postlfs-security-fw-persFw"/></title>
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| 57 |
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| 58 | <para>This is a setup or program, for Windows commercially sold by
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| 59 | companies such as Symantec, of which they claim or pretend that it
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| 60 | secures a home or desktop-pc with Internet access. This topic is
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| 61 | highly relevant for users who do not know the methods their computers
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| 62 | might be accessed via the Internet or how to disable them,
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| 63 | especially if they are always online and connected via
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| 64 | broadband links.</para></sect3>
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| 65 |
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| 66 | <sect3><title><xref linkend="postlfs-security-fw-masqRouter"/></title>
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| 67 | <para>This is a box placed between the Internet and an intranet.
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| 68 | To minimize the risk of compromising the firewall itself it
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| 69 | should generally have only one role, that of protecting the intranet.
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| 70 | Although not completely risk free, the tasks of doing the routing
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| 71 | and eventually IP masquerading (rewriting IP-headers
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| 72 | of the packets it routes from clients with private IP-addresses onto
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| 73 | the Internet so that they seem to come from the firewall
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| 74 | itself) are commonly considered harmless.</para></sect3>
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| 75 |
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| 76 | <sect3><title><xref linkend="postlfs-security-fw-busybox"/></title>
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| 77 | <para>This is often an old box you may have retired and nearly forgotten,
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| 78 | performing masquerading or routing functions, but offering a bunch of
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| 79 | services, e.g., web-cache, mail, etc. This may be very commonly used
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| 80 | for home networks, but can definitely not be considered as secure
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| 81 | anymore because the combining of server and router on one machine raises
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| 82 | the complexity of the setup.</para></sect3>
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| 83 |
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| 84 | <sect3><title>Firewall with a demilitarized zone [not further described
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| 85 | here]</title>
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| 86 | <para>This box performs masquerading or routing, but grants public access to
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| 87 | some branch of your network which, because of public IP's and a physically
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| 88 | separated structure, is neither considered to be part of the inter- nor
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| 89 | intranet. These servers are those which must be easily accessible
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| 90 | from both the inter- and intranet. The firewall protects
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| 91 | them all.</para></sect3>
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| 92 |
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| 93 | <sect3><title>Packetfilter / partly accessible net [partly described
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| 94 | here, see <xref linkend="postlfs-security-fw-busybox"/>]</title>
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| 95 | <para>Doing routing or masquerading, but permitting only selected
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| 96 | services to be accessible, sometimes only by selected internal users or boxes;
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| 97 | mostly used in highly secure business contexts, sometimes by distrusting
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| 98 | employers. This was the common configuration of a firewall at the time of
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| 99 | the Linux 2.2 kernel. It's still possible to configure a firewall this way,
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| 100 | but it makes the rules quite complex and lengthy.</para></sect3>
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| 101 |
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| 102 | </sect2>
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| 103 |
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| 104 | <sect2 id="postlfs-security-fw-disclaimer" xreflabel="Disclaimer">
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| 105 | <title>Disclaimer</title>
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| 106 |
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| 107 | <!-- <para><emphasis>NEITHER THE AUTHOR NOR ANY OF THE LINUXFROMSCRATCH TEAM
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| 108 | ARE RESPONSIBLE FOR ANY DAMAGES INCURRED DUE TO ACTIONS TAKEN BASED ON THIS
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| 109 | DOCUMENT.</emphasis></para> -->
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| 110 |
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| 111 | <para>This document is meant as an introduction to how to setup a firewall. It
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| 112 | is not a complete guide to securing systems. Firewalling is a complex issue
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| 113 | that requires careful configuration. The scripts quoted here are simply
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| 114 | intended to give examples as to how a firewall works, they are not intended to
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| 115 | fit into any imaginable configuration and may not prevent any imaginable
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| 116 | attack.</para>
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| 117 |
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| 118 | <para>The purpose of this text is simply to give you a hint on how to get
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| 119 | started with a firewall.</para>
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| 120 |
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| 121 | <para>Customization of these scripts for your specific situation will
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| 122 | be necessary for an optimal configuration, but you should make a serious
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| 123 | study of the iptables documentation and creating firewalls in general before hacking
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| 124 | away. Have a look at the list of <xref linkend="postlfs-security-fw-library"/> at the end
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| 125 | of this section for more details. Here you will find a list of URLs that
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| 126 | contain quite comprehensive information about building your own firewall.</para>
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| 127 |
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| 128 | </sect2>
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| 129 |
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| 130 |
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| 131 | <sect2 id="postlfs-security-fw-kernel" xreflabel="getting a firewalling-enabled Kernel">
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| 132 | <title>Getting a firewall enabled Kernel</title>
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| 133 |
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| 134 | <para>If you want your Linux-Box to have a firewall, you must first ensure
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| 135 | that your kernel has been compiled with the relevant options turned on.
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| 136 | <!-- <footnote><para>If you needed assistance how to configure, compile and install
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| 137 | a new kernel, refer back to chapter VIII of the LinuxFromScratch book,
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| 138 | <ulink url="http://www.linuxfromscratch.org/view/3.1/chapter08/kernel.html">Installing a kernel</ulink>
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| 139 | and eventually
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| 140 | <ulink url="http://www.linuxfromscratch.org/view/3.1/chapter08/lilo.html">Making the LFS system bootable</ulink>
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| 141 | ; note, that you'll need to reboot
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| 142 | to actually run your new kernel.</para></footnote>-->
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| 143 | </para>
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| 144 |
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| 145 | <para>How to configure your kernel, with enabling the options to be
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| 146 | either compiled into the kernel or as modules, depends on your personal
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| 147 | preferences and experience. Note, that for the quoted scripts it is assumed
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| 148 | that the modules need to be loaded at first.</para>
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| 149 |
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| 150 | <screen>Network options menu
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| 151 | Network packet filtering: Y
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| 152 | Unix domain sockets: Y or M
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| 153 | TCP/IP networking: Y
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| 154 | IP: advanced router: Y
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| 155 | IP: verbose route monitoring: Y
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| 156 | IP: TCP Explicit Congestion Notification support: Y
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| 157 | IP: TCP syncookie support: Y
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| 158 | IP: Netfilter Configuration menu
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| 159 | Every option except: Y or M
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| 160 | ipchains (2.2-style) support N
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| 161 | ipfwadm (2.0-style) support N
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| 162 | Fast switching: N</screen>
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| 163 |
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| 164 | <!--
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| 165 | <table frame='none'>
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| 166 | <title>Essential config-options for a firewall enabled Kernel</title>
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| 167 |
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| 168 | <tgroup cols='5'>
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| 169 | <colspec colnum='1' colwidth='8*' align='center'/>
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| 170 | <colspec colnum='2' colwidth='19*' align='left'/>
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| 171 | <colspec colnum='3' colwidth='11*' align='center'/>
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| 172 | <colspec colnum='4' colwidth='1*' align='center'/>
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| 173 | <colspec colnum='5' colwidth='14*' align='left'/>
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| 174 |
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| 175 | <tbody>
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| 176 |
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| 177 | <row>
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| 178 | <entry><emphasis><userinput>Networking options:</userinput></emphasis></entry>
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| 179 | <entry><userinput>Network packet filtering</userinput></entry>
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| 180 | <entry></entry>
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| 181 | <entry>=</entry>
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| 182 | <entry>CONFIG_NETFILTER</entry>
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| 183 | </row>
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| 184 |
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| 185 | <row>
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| 186 | <entry></entry>
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| 187 | <entry><userinput>Unix domain sockets</userinput></entry>
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| 188 | <entry></entry>
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| 189 | <entry>=</entry>
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| 190 | <entry>CONFIG_UNIX</entry>
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| 191 | </row>
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| 192 |
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| 193 | <row>
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| 194 | <entry></entry>
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| 195 | <entry><userinput>IP: TCP/IP networking</userinput></entry>
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| 196 | <entry></entry>
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| 197 | <entry>=</entry>
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| 198 | <entry>CONFIG_INET</entry>
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| 199 | </row>
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| 200 |
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| 201 | <row>
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| 202 | <entry></entry>
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| 203 | <entry><userinput>IP: advanced router</userinput></entry>
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| 204 | <entry></entry>
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| 205 | <entry>=</entry>
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| 206 | <entry>CONFIG_IP_ADVANCED_ROUTER</entry>
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| 207 | </row>
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| 208 |
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| 209 | <row>
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| 210 | <entry></entry>
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| 211 | <entry><userinput>IP: verbose route monitoring</userinput></entry>
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| 212 | <entry></entry>
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| 213 | <entry>=</entry>
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| 214 | <entry>CONFIG_IP_ROUTE_VERBOSE</entry>
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| 215 | </row>
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| 216 |
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| 217 | <row>
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| 218 | <entry></entry>
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| 219 | <entry><userinput>IP: TCP Explicit Congestion Notification support</userinput></entry>
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| 220 | <entry></entry>
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| 221 | <entry>=</entry>
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| 222 | <entry>CONFIG_INET_ECN</entry>
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| 223 | </row>
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| 224 |
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| 225 | <row>
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| 226 | <entry></entry>
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| 227 | <entry><userinput>IP: TCP syncookie support</userinput></entry>
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| 228 | <entry></entry>
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| 229 | <entry>=</entry>
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| 230 | <entry>CONFIG_SYN_COOKIES</entry>
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| 231 | </row>
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| 232 |
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| 233 | <row>
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| 234 | <entry></entry>
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| 235 | <entry align='center'>
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| 236 | <emphasis><userinput>IP: Netfilter Configuration:</userinput></emphasis></entry>
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| 237 | <entry align='left'><userinput>every option</userinput></entry>
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| 238 | <entry>=</entry>
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| 239 | <entry>CONFIG_IP_NF_*</entry>
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| 240 | </row>
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| 241 |
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| 242 | <row>
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| 243 | <entry></entry>
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| 244 | <entry align='right'><emphasis>WITHOUT:</emphasis></entry>
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| 245 | <entry align='left'><literallayout><userinput>ipchains (2.2-style) support
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| 246 | ipfw-adm (2.0-style) support</userinput></literallayout></entry>
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| 247 | <entry>w\</entry>
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| 248 | <entry>CONFIG_IP_NF_COMPAT_*</entry>
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| 249 | </row>
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| 250 |
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| 251 | <row>
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| 252 | <entry></entry>
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| 253 | <entry><userinput>Fast switching</userinput></entry>
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| 254 | <entry>Make sure to disable it because it would setup a bypass around
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| 255 | your firewall rules.</entry>
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| 256 | <entry>w\</entry>
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| 257 | <entry>CONFIG_NET_FASTROUTE</entry>
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| 258 | </row>
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| 259 |
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| 260 | </tbody>
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| 261 |
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| 262 | </tgroup>
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| 263 |
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| 264 | </table> -->
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| 265 |
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| 266 | </sect2>
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| 267 |
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| 268 |
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| 269 | <sect2 id="postlfs-security-fw-writing" xreflabel="writing the firewalling-setup-scripts">
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| 270 | <title>Now you can start to build your Firewall</title>
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| 271 |
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| 272 |
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| 273 | <sect3 id="postlfs-security-fw-persFw" xreflabel="Personal Firewall">
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| 274 | <title>Personal Firewall</title>
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| 275 |
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| 276 | <para>A Personal Firewall is supposed to let you access all the services
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| 277 | offered on the Internet, but keep your box secure and your data private.</para>
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| 278 |
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| 279 | <para>Below is a slightly modified version of Rusty Russell's recommendation
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| 280 | from the <ulink
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| 281 | url="http://www.netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html">Linux
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| 282 | 2.4 Packet Filtering HOWTO</ulink>:</para>
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| 283 |
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| 284 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall << "EOF"</command>
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| 285 | #!/bin/sh
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| 286 |
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| 287 | # Begin $rc_base/init.d/firewall
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| 288 |
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| 289 | # Insert connection-tracking modules (not needed if built into the kernel).
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| 290 | modprobe ip_tables
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| 291 | modprobe iptable_filter
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| 292 | modprobe ip_conntrack
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| 293 | modprobe ip_conntrack_ftp
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| 294 | modprobe ipt_state
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| 295 | modprobe ipt_LOG
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| 296 |
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| 297 | # allow local-only connections
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| 298 | iptables -A INPUT -i lo -j ACCEPT
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| 299 | # free output on any interface to any ip for any service (equal to -P ACCEPT)
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| 300 | iptables -A OUTPUT -j ACCEPT
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| 301 |
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| 302 | # permit answers on already established connections
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| 303 | # and permit new connections related to established ones (eg active-ftp)
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| 304 | iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
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| 305 |
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| 306 | # Log everything else: What's Windows' latest exploitable vulnerability?
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| 307 | iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
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| 308 |
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| 309 | # set a sane policy: everything not accepted > /dev/null
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| 310 | iptables -P INPUT DROP
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| 311 | iptables -P FORWARD DROP
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| 312 | iptables -P OUTPUT DROP
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| 313 |
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| 314 | # be verbose on dynamic ip-addresses (not needed in case of static IP)
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| 315 | echo 2 > /proc/sys/net/ipv4/ip_dynaddr
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| 316 |
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| 317 | # disable ExplicitCongestionNotification - too many routers are still ignorant
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| 318 | echo 0 > /proc/sys/net/ipv4/tcp_ecn
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| 319 |
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| 320 | # End $rc_base/init.d/firewall
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| 321 | <command>EOF</command></userinput></screen>
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| 322 |
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| 323 | <para>His script is quite simple, it drops all traffic coming in into your
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| 324 | computer that wasn't initiated from your box, but as long as you are simply
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| 325 | surfing the Internet you are unlikely to exceed its limits.</para>
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| 326 |
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| 327 | <para>If you frequently encounter certain delays at accessing ftp-servers,
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| 328 | please have a look at <xref linkend="postlfs-security-fw-busybox"/> -
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| 329 | <xref linkend="postlfs-security-fw-BB-4"/>.</para>
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| 330 |
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| 331 | <para>Even if you have daemons or services running on your box, these
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| 332 | should be inaccessible everywhere but from your box itself.
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| 333 | If you want to allow access to services on your machine, such as ssh or pinging,
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| 334 | take a look at <xref linkend="postlfs-security-fw-busybox"/>.</para>
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| 335 |
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| 336 | </sect3>
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| 337 |
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| 338 |
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| 339 | <sect3 id="postlfs-security-fw-masqRouter" xreflabel="Masquerading Router">
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| 340 | <title>Masquerading Router</title>
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| 341 |
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| 342 | <para>A true Firewall has two interfaces, one connected to an intranet,
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| 343 | in this example, <emphasis role="strong">eth0</emphasis>, and one
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| 344 | connected to the Internet, here, <emphasis role="strong">ppp0</emphasis>.
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| 345 | To provide the maximum security against the box itself being broken into,
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| 346 | make sure that there are no servers running on it, especially not
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| 347 | <application>X11</application> et
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| 348 | al. And, as a general principle, the box itself should not access any untrusted
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| 349 | service (Think of a name server giving answers that make your
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| 350 | bind crash, or, even worse, that implement a worm via a
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| 351 | buffer-overflow).</para>
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| 352 |
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| 353 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall << "EOF"</command>
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| 354 | #!/bin/sh
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| 355 |
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| 356 | # Begin $rc_base/init.d/firewall
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| 357 |
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| 358 | echo
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| 359 | echo "You're using the example-config for a setup of a firewall"
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| 360 | echo "from the firewalling-hint written for LinuxFromScratch."
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| 361 | echo "This example is far from being complete, it is only meant"
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| 362 | echo "to be a reference."
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| 363 | echo "Firewall security is a complex issue, that exceeds the scope"
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| 364 | echo "of the quoted configuration rules."
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| 365 | echo "You can find some quite comprehensive information"
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| 366 | echo "about firewalls in Chapter 4 of the BLFS book."
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| 367 | echo "http://www.linuxfromscratch.org/blfs"
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| 368 | echo
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| 369 |
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| 370 | # Insert iptables modules (not needed if built into the kernel).
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| 371 |
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| 372 | modprobe ip_tables
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| 373 | modprobe iptable_filter
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| 374 | modprobe ip_conntrack
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| 375 | modprobe ip_conntrack_ftp
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| 376 | modprobe ipt_state
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| 377 | modprobe iptable_nat
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| 378 | modprobe ip_nat_ftp
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| 379 | modprobe ipt_MASQUERADE
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| 380 | modprobe ipt_LOG
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| 381 | modprobe ipt_REJECT
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| 382 |
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| 383 | # allow local-only connections
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| 384 | iptables -A INPUT -i lo -j ACCEPT
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| 385 | iptables -A OUTPUT -o lo -j ACCEPT
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| 386 |
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| 387 | # allow forwarding
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| 388 | iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
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| 389 | iptables -A FORWARD -m state --state NEW -i ! ppp+ -j ACCEPT
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| 390 |
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| 391 | # do masquerading (not needed if intranet is not using private ip-addresses)
|
---|
| 392 | iptables -t nat -A POSTROUTING -o ppp+ -j MASQUERADE
|
---|
| 393 |
|
---|
| 394 | # Log everything for debugging (last of all rules, but before DROP/REJECT)
|
---|
| 395 | iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
|
---|
| 396 | iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD"
|
---|
| 397 | iptables -A OUTPUT -j LOG --log-prefix "FIREWALL:OUTPUT "
|
---|
| 398 |
|
---|
| 399 | # set a sane policy
|
---|
| 400 | iptables -P INPUT DROP
|
---|
| 401 | iptables -P FORWARD DROP
|
---|
| 402 | iptables -P OUTPUT DROP
|
---|
| 403 |
|
---|
| 404 | # be verbose on dynamic ip-addresses (not needed in case of static IP)
|
---|
| 405 | echo 2 > /proc/sys/net/ipv4/ip_dynaddr
|
---|
| 406 |
|
---|
| 407 | # disable ExplicitCongestionNotification
|
---|
| 408 | echo 0 > /proc/sys/net/ipv4/tcp_ecn
|
---|
| 409 |
|
---|
| 410 | # activate TCPsyncookies
|
---|
| 411 | echo 1 > /proc/sys/net/ipv4/tcp_syncookies
|
---|
| 412 |
|
---|
| 413 | # activate Route-Verification = IP-Spoofing_protection
|
---|
| 414 | for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
|
---|
| 415 | echo 1 > $f
|
---|
| 416 | done
|
---|
| 417 |
|
---|
| 418 | # activate IP-Forwarding
|
---|
| 419 | echo 1 > /proc/sys/net/ipv4/ip_forward
|
---|
| 420 | <command>EOF</command></userinput></screen>
|
---|
| 421 |
|
---|
| 422 | <para>With this script your intranet should be sufficiently secure against
|
---|
| 423 | external attacks. No one should be able to setup a new connection to any
|
---|
| 424 | internal service and, if it's masqueraded, it's even invisible. Furthermore,
|
---|
| 425 | your firewall should be nearly immune because there are no services running
|
---|
| 426 | that a cracker could attack.</para>
|
---|
| 427 |
|
---|
| 428 | <para>Note: if the interface you're connecting to the Internet
|
---|
| 429 | doesn't connect via ppp, you will need to change
|
---|
| 430 | <replaceable>ppp+</replaceable> to the name of the interface which you are
|
---|
| 431 | using. If you are using the same interface type to connect to both your
|
---|
| 432 | intranet and the Internet, you need to use the actual name of the
|
---|
| 433 | interface such as <emphasis role="strong">eth0</emphasis>,
|
---|
| 434 | on both interfaces.</para>
|
---|
| 435 |
|
---|
| 436 | <para>If you need stronger security (e.g., against DOS, connection
|
---|
| 437 | highjacking, spoofing, etc.), have a look at the list of
|
---|
| 438 | <xref linkend="postlfs-security-fw-library"/> at the end of this section.</para>
|
---|
| 439 |
|
---|
| 440 | </sect3>
|
---|
| 441 |
|
---|
| 442 | <sect3 id="postlfs-security-fw-busybox" xreflabel="BusyBox">
|
---|
| 443 | <title>BusyBox</title>
|
---|
| 444 |
|
---|
| 445 | <para>This scenario isn't too different from (<xref linkend="postlfs-security-fw-masqRouter"/>),
|
---|
| 446 | but in this case you want to offer some services to your intranet.
|
---|
| 447 | Examples of this can be when you want to admin your box from another host
|
---|
| 448 | on your intranet or use it as a proxy or a name server. Note: Outlining a true
|
---|
| 449 | concept of how to protect a server that offers services on the Internet
|
---|
| 450 | goes far beyond the scope of this document,
|
---|
| 451 | see <xref linkend="postlfs-security-fw-disclaimer"/>.</para>
|
---|
| 452 |
|
---|
| 453 | <para>Be cautious. Every service you offer and have enabled makes your
|
---|
| 454 | setup more complex and your box less secure. You induce the risks of
|
---|
| 455 | misconfigured services or running a service with an exploitable bug. A firewall
|
---|
| 456 | should generally not run any extra services. See the introduction to
|
---|
| 457 | <xref linkend="postlfs-security-fw-masqRouter"/> for some more details.</para>
|
---|
| 458 |
|
---|
| 459 | <para>If the services you'd like to offer do not need to access the Internet
|
---|
| 460 | themselves, like internal-only samba- or name-servers, it's quite
|
---|
| 461 | simple and should still be acceptable from a security standpoint.
|
---|
| 462 | Just add the following lines <emphasis>before</emphasis> the logging-rules
|
---|
| 463 | into the script.</para>
|
---|
| 464 |
|
---|
| 465 | <screen>iptables -A INPUT -i ! ppp+ -j ACCEPT
|
---|
| 466 | iptables -A OUTPUT -o ! ppp+ -j ACCEPT</screen>
|
---|
| 467 |
|
---|
| 468 | <para>If your daemons have to access the web themselves, like squid would need
|
---|
| 469 | to, you could open OUTPUT generally and restrict INPUT.</para>
|
---|
| 470 |
|
---|
| 471 | <screen>iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
|
---|
| 472 | iptables -A OUTPUT -j ACCEPT</screen>
|
---|
| 473 |
|
---|
| 474 | <para>However, it is generally not advisable to leave OUTPUT unrestricted. You lose
|
---|
| 475 | any control over trojans who'd like to "call home", and a bit of redundancy in case
|
---|
| 476 | you've (mis-)configured a service so that it does broadcast its existence to the
|
---|
| 477 | world.</para>
|
---|
| 478 |
|
---|
| 479 | <para>If you prefer to have this protection, you may restrict INPUT and OUTPUT
|
---|
| 480 | on all ports except those that it's absolutely necessary to have open.
|
---|
| 481 | Which ports you have to open depends on your needs: mostly you will find them
|
---|
| 482 | by looking for failed accesses in your log-files.</para>
|
---|
| 483 | <itemizedlist spacing="compact">
|
---|
| 484 | <!-- <orderedlist numeration="arabic" spacing="compact"> -->
|
---|
| 485 | <title>Have a look at the following examples:</title>
|
---|
| 486 |
|
---|
| 487 | <listitem><para>Squid is caching the web:</para>
|
---|
| 488 | <screen>iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
|
---|
| 489 | iptables -A INPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT</screen></listitem>
|
---|
| 490 |
|
---|
| 491 | <listitem><para>Your caching name server (e.g., dnscache) does its
|
---|
| 492 | lookups via udp:</para>
|
---|
| 493 | <screen>iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
|
---|
| 494 | iptables -A INPUT -p udp --sport 53 -m state --state ESTABLISHED -j ACCEPT</screen></listitem>
|
---|
| 495 |
|
---|
| 496 | <listitem><para>Alternatively, if you want to be able to ping your box to ensure
|
---|
| 497 | it's still alive:</para>
|
---|
| 498 | <screen>iptables -A INPUT -p icmp -m icmp --icmp-type echo-request -j ACCEPT
|
---|
| 499 | iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply -j ACCEPT</screen></listitem>
|
---|
| 500 |
|
---|
| 501 | <listitem><para><anchor id='postlfs-security-fw-BB-4' xreflabel="example no. 4"/>If you are
|
---|
| 502 | frequently accessing ftp-servers or enjoy chatting, you might notice certain
|
---|
| 503 | delays because some implementations of these daemons have the feature of
|
---|
| 504 | querying an identd on your box for logging usernames.
|
---|
| 505 | Although there's really no harm in this, having an identd running is not
|
---|
| 506 | recommended because some implementations are known to be vulnerable.</para>
|
---|
| 507 |
|
---|
| 508 | <para>To avoid these delays you could reject the requests
|
---|
| 509 | with a 'tcp-reset':</para>
|
---|
| 510 |
|
---|
| 511 | <screen>iptables -A INPUT -p tcp --dport 113 -j REJECT --reject-with tcp-reset
|
---|
| 512 | iptables -A OUTPUT -p tcp --sport 113 -m state --state RELATED -j ACCEPT</screen></listitem>
|
---|
| 513 |
|
---|
| 514 | <listitem><para>To log and drop invalid packets (harmless packets
|
---|
| 515 | that came in after netfilter's timeout or some types of network scans):</para>
|
---|
| 516 |
|
---|
| 517 | <screen>iptables -I INPUT 1 -p tcp -m state --state INVALID -j LOG --log-prefix \
|
---|
| 518 | "FIREWALL:INVALID"
|
---|
| 519 | iptables -I INPUT 2 -p tcp -m state --state INVALID -j DROP</screen></listitem>
|
---|
| 520 |
|
---|
| 521 | <listitem><para>Anything coming from the outside should not have a
|
---|
| 522 | private address, this is a common attack called IP-spoofing:</para>
|
---|
| 523 |
|
---|
| 524 | <screen>iptables -t nat -A PREROUTING -i ppp+ -s 10.0.0.0/8 -j DROP
|
---|
| 525 | iptables -t nat -A PREROUTING -i ppp+ -s 172.16.0.0/12 -j DROP
|
---|
| 526 | iptables -t nat -A PREROUTING -i ppp+ -s 192.168.0.0/16 -j DROP</screen></listitem>
|
---|
| 527 |
|
---|
| 528 | <listitem><para>To simplify debugging and be fair to anyone who'd like to
|
---|
| 529 | access a service you have disabled, purposely or by mistake, you should REJECT
|
---|
| 530 | those packets that are dropped.</para>
|
---|
| 531 |
|
---|
| 532 | <para>Obviously this must be done directly after logging as the very
|
---|
| 533 | last lines before the packets are dropped by policy:</para>
|
---|
| 534 |
|
---|
| 535 | <screen>iptables -A INPUT -j REJECT
|
---|
| 536 | iptables -A OUTPUT -p icmp --icmp-type 3 -j ACCEPT</screen></listitem>
|
---|
| 537 | </itemizedlist>
|
---|
| 538 | <!--</orderedlist>-->
|
---|
| 539 |
|
---|
| 540 | <para>These are only examples to show you some of the capabilities of the new
|
---|
| 541 | firewall code in Linux-Kernel 2.4. Have a look at the man page of
|
---|
| 542 | iptables.
|
---|
| 543 | There you will find more of them. The port-numbers you'll need for this
|
---|
| 544 | can be found in <filename>/etc/services</filename>, in case you didn't
|
---|
| 545 | find them by trial and error in your log file.</para>
|
---|
| 546 |
|
---|
| 547 | <para>If you add any of your offered or accessed services such as the above,
|
---|
| 548 | maybe even in FORWARD and for intranet-communication, and delete the
|
---|
| 549 | general clauses, you get an old fashioned packet filter.</para>
|
---|
| 550 |
|
---|
| 551 |
|
---|
| 552 | </sect3>
|
---|
| 553 |
|
---|
| 554 | </sect2>
|
---|
| 555 |
|
---|
| 556 |
|
---|
| 557 | <sect2 id="postlfs-security-fw-finale" xreflabel="Conclusion">
|
---|
| 558 | <title>Conclusion</title>
|
---|
| 559 |
|
---|
| 560 | <para>Finally, I'd like to remind you of one fact we must not forget:
|
---|
| 561 | The effort spent attacking a system corresponds to the value the cracker
|
---|
| 562 | expects to gain from it.
|
---|
| 563 | If you are responsible for such valuable assets that you expect great
|
---|
| 564 | effort to be made by potential crackers, you hopefully won't be in the
|
---|
| 565 | need of this hint!</para>
|
---|
| 566 |
|
---|
| 567 | <!-- <para><literallayout>Be cautious!
|
---|
| 568 |
|
---|
| 569 | Henning Rohde
|
---|
| 570 | <email>Henning.Rohde@uni-bayreuth.de</email></literallayout></para>
|
---|
| 571 |
|
---|
| 572 | <para>PS: And always do remember:
|
---|
| 573 | SecureIT is not a matter of a status-quo but one of never stopping
|
---|
| 574 | to take care!</para>
|
---|
| 575 |
|
---|
| 576 | <para>PPS: If any of these scripts fail, please tell me. I will try to trace
|
---|
| 577 | any faults.</para> -->
|
---|
| 578 |
|
---|
| 579 | </sect2>
|
---|
| 580 |
|
---|
| 581 |
|
---|
| 582 | <sect2 id="postlfs-security-fw-extra" xreflabel="Extra Information">
|
---|
| 583 | <title>Extra Information</title>
|
---|
| 584 |
|
---|
| 585 | <sect3 id="postlfs-security-fw-library" xreflabel="Links for further reading">
|
---|
| 586 | <title>Where to start with further reading on firewalls.</title>
|
---|
| 587 |
|
---|
| 588 | <para><blockquote><literallayout>
|
---|
| 589 | <ulink url="http://www.netfilter.org/">www.netfilter.org - Homepage of the netfilter/iptables project</ulink>
|
---|
| 590 | <ulink url="http://www.netfilter.org/documentation/FAQ/netfilter-faq.html">Netfilter related FAQ</ulink>
|
---|
| 591 | <ulink url="http://www.netfilter.org/documentation/index.html#HOWTO">Netfilter related HOWTO's</ulink>
|
---|
| 592 | <ulink url="http://en.tldp.org/LDP/nag2/x-087-2-firewall.html">en.tldp.org/LDP/nag2/x-087-2-firewall.html</ulink>
|
---|
| 593 | <ulink url="http://en.tldp.org/HOWTO/Security-HOWTO.html">en.tldp.org/HOWTO/Security-HOWTO.html</ulink>
|
---|
| 594 | <ulink url="http://en.tldp.org/HOWTO/Firewall-HOWTO.html">en.tldp.org/HOWTO/Firewall-HOWTO.html</ulink>
|
---|
| 595 | <ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire.html">www.ibm.com/developerworks/security/library/s-fire.html</ulink>
|
---|
| 596 | <ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire2.html">www.ibm.com/developerworks/security/library/s-fire2.html</ulink>
|
---|
| 597 | <ulink url="http://www.interhack.net/pubs/fw-faq/">www.interhack.net/pubs/fw-faq/</ulink>
|
---|
| 598 | <ulink url="http://www.linuxsecurity.com/docs/">www.linuxsecurity.com/docs/</ulink>
|
---|
| 599 | <ulink url="http://www.little-idiot.de/firewall">www.little-idiot.de/firewall (German & outdated, but very comprehensive)</ulink>
|
---|
| 600 | <ulink url="http://www.linuxgazette.com/issue65/stumpel.html">www.linuxgazette.com/issue65/stumpel.html</ulink>
|
---|
| 601 | <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>
|
---|
| 602 | <ulink url="http://staff.washington.edu/dittrich/misc/ddos">staff.washington.edu/dittrich/misc/ddos</ulink>
|
---|
| 603 | <ulink url="http://www.e-infomax.com/ipmasq">www.e-infomax.com/ipmasq</ulink>
|
---|
| 604 | <ulink url="http://www.circlemud.org/~jelson/writings/security/index.htm">www.circlemud.org/~jelson/writings/security/index.htm</ulink>
|
---|
| 605 | <ulink url="http://www.securityfocus.com">www.securityfocus.com</ulink>
|
---|
| 606 | <ulink url="http://www.cert.org/tech_tips/">www.cert.org - tech_tips</ulink>
|
---|
| 607 | <ulink url="http://security.ittoolbox.com/">security.ittoolbox.com</ulink>
|
---|
| 608 | <ulink url="http://www.linux-firewall-tools.com/linux/">www.linux-firewall-tools.com/linux/</ulink>
|
---|
| 609 | <ulink url="http://logi.cc/linux/athome-firewall.php3">logi.cc/linux/athome-firewall.php3</ulink>
|
---|
| 610 | <ulink url="http://www.insecure.org/reading.html">www.insecure.org/reading.html</ulink>
|
---|
| 611 | <ulink url="http://www.robertgraham.com/pubs/firewall-seen.html">www.robertgraham.com/pubs/firewall-seen.html</ulink>
|
---|
| 612 | </literallayout></blockquote></para>
|
---|
| 613 |
|
---|
| 614 | <!-- <para>If a link proves to be dead or if you think I missed one,
|
---|
| 615 | please mail!</para> -->
|
---|
| 616 |
|
---|
| 617 | </sect3>
|
---|
| 618 |
|
---|
| 619 | <sect3 id="postlfs-security-fw-status" xreflabel="/etc/rc.d/init.d/firewall.status">
|
---|
| 620 | <title>firewall.status</title>
|
---|
| 621 |
|
---|
| 622 | <para>If you'd like to have a look at the chains your firewall consists of and
|
---|
| 623 | the order in which the rules take effect:</para>
|
---|
| 624 |
|
---|
| 625 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall.status << "EOF"</command>
|
---|
| 626 | #!/bin/sh
|
---|
| 627 |
|
---|
| 628 | # Begin $rc_base/init.d/firewall.status
|
---|
| 629 |
|
---|
| 630 | echo "iptables.mangling:"
|
---|
| 631 | iptables -t mangle -v -L -n --line-numbers
|
---|
| 632 |
|
---|
| 633 | echo
|
---|
| 634 | echo "iptables.nat:"
|
---|
| 635 | iptables -t nat -v -L -n --line-numbers
|
---|
| 636 |
|
---|
| 637 | echo
|
---|
| 638 | echo "iptables.filter:"
|
---|
| 639 | iptables -v -L -n --line-numbers
|
---|
| 640 | <command>EOF</command></userinput></screen>
|
---|
| 641 | </sect3>
|
---|
| 642 |
|
---|
| 643 | <sect3 id="postlfs-security-fw-stop" xreflabel="/etc/rc.d/init.d/firewall.stop">
|
---|
| 644 | <title>firewall.stop</title>
|
---|
| 645 |
|
---|
| 646 | <para>If you need to turn the firewall off, this script will do it:</para>
|
---|
| 647 |
|
---|
| 648 | <screen><userinput><command>cat > /etc/rc.d/init.d/firewall.stop << "EOF"</command>
|
---|
| 649 | #!/bin/sh
|
---|
| 650 |
|
---|
| 651 | # Being $rc_base/init.d/firewall.stop
|
---|
| 652 |
|
---|
| 653 | # deactivate IP-Forwarding
|
---|
| 654 | echo 0 > /proc/sys/net/ipv4/ip_forward
|
---|
| 655 |
|
---|
| 656 | iptables -Z
|
---|
| 657 | iptables -F
|
---|
| 658 | iptables -t nat -F PREROUTING
|
---|
| 659 | iptables -t nat -F OUTPUT
|
---|
| 660 | iptables -t nat -F POSTROUTING
|
---|
| 661 | iptables -t mangle -F PREROUTING
|
---|
| 662 | iptables -t mangle -F OUTPUT
|
---|
| 663 | iptables -X
|
---|
| 664 | iptables -P INPUT ACCEPT
|
---|
| 665 | iptables -P FORWARD ACCEPT
|
---|
| 666 | iptables -P OUTPUT ACCEPT
|
---|
| 667 | <command>EOF</command></userinput></screen>
|
---|
| 668 |
|
---|
| 669 | </sect3>
|
---|
| 670 |
|
---|
| 671 | </sect2>
|
---|
[f45b1953] | 672 | </sect1>
|
---|
| 673 |
|
---|