[b4b71892] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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[6732c094] | 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.5/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 | <sect2 id="fw-intro" xreflabel="Firewalling Introduction">
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| 19 | <title>Introduction to Firewall Creation</title>
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| 20 |
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[14c0be2f] | 21 | <para>
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| 22 | The purpose of a firewall is to protect a computer or a network against
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| 23 | malicious access. In a perfect world every daemon or service, on every
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| 24 | machine, is perfectly configured and immune to security flaws, and all
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| 25 | users are trusted implicitly to use the equipment as intended. However,
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| 26 | this is rarely, if ever, the case. Daemons may be misconfigured, or
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| 27 | updates may not have been applied for known exploits against essential
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| 28 | services. Additionally, you may wish to choose which services are
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| 29 | accessible by certain machines or users, or you may wish to limit which
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| 30 | machines or applications are allowed external access. Alternatively, you
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| 31 | simply may not trust some of your applications or users. For these
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| 32 | reasons, a carefully designed firewall should be an essential part of
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| 33 | system security.
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| 34 | </para>
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| 35 |
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| 36 | <para>
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| 37 | While a firewall can greatly limit the scope of the above issues, do not
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| 38 | assume that having a firewall makes careful configuration redundant, or
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| 39 | that any negligent misconfiguration is harmless. A firewall does not
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| 40 | prevent the exploitation of any service you offer outside of it. Despite
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| 41 | having a firewall, you need to keep applications and daemons properly
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| 42 | configured and up to date.
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| 43 | </para>
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[8920dfa] | 44 |
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| 45 | </sect2>
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| 46 |
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| 47 | <sect2>
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| 48 | <title>Meaning of the Word "Firewall"</title>
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| 49 |
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[14c0be2f] | 50 | <para>
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| 51 | The word firewall can have several different meanings.
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| 52 | </para>
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[8920dfa] | 53 |
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| 54 | <sect3>
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[14c0be2f] | 55 | <title>Personal Firewall</title>
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[8920dfa] | 56 |
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[14c0be2f] | 57 | <para>
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| 58 | This is a hardware device or software program, intended to secure a
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| 59 | home or desktop computer connected to the Internet. This type of
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| 60 | firewall is highly relevant for users who do not know how their
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| 61 | computers might be accessed via the Internet or how to disable
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| 62 | that access, especially if they are always online and connected
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| 63 | via broadband links.
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| 64 | </para>
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| 65 |
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| 66 | <para>
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| 67 | An example configuration for a personal firewall is provided at
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| 68 | <xref linkend="fw-persFw-ipt"/>.
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| 69 | </para>
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[8920dfa] | 70 |
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| 71 | </sect3>
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| 72 |
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| 73 | <sect3>
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[14c0be2f] | 74 | <title>Masquerading Router</title>
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[8920dfa] | 75 |
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[14c0be2f] | 76 | <para>
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| 77 | This is a system placed between the Internet and an intranet.
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| 78 | To minimize the risk of compromising the firewall itself, it should
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| 79 | generally have only one role—that of protecting the intranet.
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| 80 | Although not completely risk-free, the tasks of doing the routing and
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| 81 | IP masquerading (rewriting IP headers of the packets it routes from
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| 82 | clients with private IP addresses onto the Internet so that they seem
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| 83 | to come from the firewall itself) are commonly considered relatively
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| 84 | secure.
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| 85 | </para>
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| 86 |
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| 87 | <para>
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| 88 | Example configurations for a masquerading firewall are provided at
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| 89 | <xref linkend="fw-masqRouter-ipt"/> and
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| 90 | <xref linkend="fw-masqRouter-nft"/>.
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| 91 | </para>
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[8920dfa] | 92 |
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| 93 | </sect3>
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| 94 |
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[d612b9f] | 95 | <sect3>
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[14c0be2f] | 96 | <title>BusyBox</title>
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| 97 |
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| 98 | <para>
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| 99 | This is often an old computer you may have retired and nearly
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| 100 | forgotten, performing masquerading or routing functions, but offering
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| 101 | non-firewall services such as a web-cache or mail. This may be used
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| 102 | for home networks, but is not to be considered as secure as a firewall
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| 103 | only machine because the combination of server and router/firewall on
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| 104 | one machine raises the complexity of the setup.
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| 105 | </para>
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[8920dfa] | 106 |
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[14c0be2f] | 107 | <para>
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| 108 | An example configuration for a BusyBox is provided at
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| 109 | <xref linkend="fw-busybox-ipt"/>.
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| 110 | </para>
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[8920dfa] | 111 |
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| 112 | </sect3>
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| 113 |
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| 114 | <sect3>
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[14c0be2f] | 115 | <title>Firewall with a Demilitarized Zone</title>
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| 116 |
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| 117 | <para>
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| 118 | This type of firewall performs masquerading or routing, but grants
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| 119 | public access to some branch of your network that is physically
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| 120 | separated from your regular intranet and is essentially a separate
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| 121 | network with direct Internet access. The servers on this network are
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| 122 | those which must be easily accessible from both the Internet and
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| 123 | intranet. The firewall protects both networks. This type of firewall
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| 124 | has a minimum of three network interfaces.
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| 125 | </para>
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[8920dfa] | 126 |
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| 127 | </sect3>
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| 128 |
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| 129 | <sect3>
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| 130 | <title>Packetfilter</title>
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| 131 |
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[14c0be2f] | 132 | <para>
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| 133 | This type of firewall does routing or masquerading but does
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| 134 | not maintain a state table of ongoing communication streams. It is
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| 135 | fast but quite limited in its ability to block undesired packets
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| 136 | without blocking desired packets.
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| 137 | </para>
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[8920dfa] | 138 |
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| 139 | </sect3>
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| 140 |
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| 141 | </sect2>
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| 142 |
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[14c0be2f] | 143 | <sect2>
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| 144 | <title>Conclusion</title>
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[8920dfa] | 145 |
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| 146 | <caution>
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[14c0be2f] | 147 | <para>
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| 148 | The example configurations provided for <xref linkend="iptables"/> and
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| 149 | <xref linkend="nftables"/> are not intended to be a complete guide to
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| 150 | securing systems. Firewalling is a complex issue that requires careful
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| 151 | configuration. The configurations provided by BLFS are intended only to
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| 152 | give examples of how a firewall works. They are not intended to fit any
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| 153 | particular configuration and may not provide complete protection from
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| 154 | an attack.
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| 155 | </para>
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[8920dfa] | 156 | </caution>
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| 157 |
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[14c0be2f] | 158 | <para>
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| 159 | BLFS provides two utilities to manage the kernel Netfilter interface,
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| 160 | <xref linkend="iptables"/> and <xref linkend="nftables"/>.
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| 161 | </para>
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| 162 |
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| 163 | <para>
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| 164 | <xref linkend="iptables"/> has been around since early 2.4 kernels, and
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| 165 | has been the standard since. If you plan not to use a configuration
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| 166 | utility, this is likely the set of tools that will be most familiar to
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| 167 | existing admins.
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| 168 | </para>
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| 169 |
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| 170 | <para>
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| 171 | <xref linkend="nftables"/> is the successor to <xref linkend="iptables"/>
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| 172 | and provies all of the same functionality with a single userspace tool,
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| 173 | <command>nft</command>, that uses similar syntax to BSD's
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| 174 | <application>pf</application> utility, and may be easier for new users or
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| 175 | admins already familiar with that platform.
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| 176 | </para>
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| 177 |
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| 178 | <para>
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| 179 | While both can be used in tandem, that is an advanced configuration and
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| 180 | you should decide on one or the other. Both pages include very simple
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| 181 | example configurations, and customization of the provided configurations
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| 182 | for your specific environment will be necessary if you elect to use
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| 183 | either without a configuration tool.
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| 184 | </para>
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| 185 |
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| 186 | <para>
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| 187 | Additionally, a firewall management tool, <xref linkend="firewalld"/>, is
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| 188 | provided to greatly ease firewall configuration for both simple and
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| 189 | complex environments, and can be used with either tool. You should not
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| 190 | use the example configurations if you intend to use
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| 191 | <application>firewalld</application> to manage your firewall rules.
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| 192 | </para>
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| 193 |
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| 194 | <para>
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| 195 | If you elect to configure manually, have a look at the
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| 196 | list of further reading below for more details. Here you will find a
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| 197 | list of URLs that contain comprehensive information about building
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| 198 | firewalls and further securing your system.
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| 199 | </para>
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[b4b71892] | 200 |
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[8920dfa] | 201 | </sect2>
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[b4b71892] | 202 |
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[14c0be2f] | 203 | <sect2 id="fw-extra-info">
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[8920dfa] | 204 | <title>Extra Information</title>
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[b4b71892] | 205 |
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[14c0be2f] | 206 | <sect3>
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| 207 | <title>Further Reading on Firewalls</title>
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[8920dfa] | 208 |
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| 209 | <blockquote>
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| 210 | <literallayout>
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[14c0be2f] | 211 | <ulink url="http://www.netfilter.org/">www.netfilter.org - Homepage of the netfilter/iptables/nftables projects</ulink>
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[b4b71892] | 212 | <ulink url="http://www.netfilter.org/documentation/FAQ/netfilter-faq.html">Netfilter related FAQ</ulink>
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| 213 | <ulink url="http://www.netfilter.org/documentation/index.html#HOWTO">Netfilter related HOWTO's</ulink>
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[14c0be2f] | 214 | <ulink url="https://wiki.nftables.org/wiki-nftables/index.php/Main_Page">nftables HOWTO</ulink>
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[b4b71892] | 215 | <ulink url="http://en.tldp.org/LDP/nag2/x-087-2-firewall.html">en.tldp.org/LDP/nag2/x-087-2-firewall.html</ulink>
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| 216 | <ulink url="http://en.tldp.org/HOWTO/Security-HOWTO.html">en.tldp.org/HOWTO/Security-HOWTO.html</ulink>
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| 217 | <ulink url="http://en.tldp.org/HOWTO/Firewall-HOWTO.html">en.tldp.org/HOWTO/Firewall-HOWTO.html</ulink>
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| 218 | <ulink url="http://www.linuxsecurity.com/docs/">www.linuxsecurity.com/docs/</ulink>
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| 219 | <ulink url="http://www.little-idiot.de/firewall">www.little-idiot.de/firewall (German & outdated, but very comprehensive)</ulink>
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| 220 | <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>
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| 221 | <ulink url="http://staff.washington.edu/dittrich/misc/ddos">staff.washington.edu/dittrich/misc/ddos</ulink>
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| 222 | <ulink url="http://www.e-infomax.com/ipmasq">www.e-infomax.com/ipmasq</ulink>
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| 223 | <ulink url="http://www.circlemud.org/~jelson/writings/security/index.htm">www.circlemud.org/~jelson/writings/security/index.htm</ulink>
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| 224 | <ulink url="http://www.securityfocus.com">www.securityfocus.com</ulink>
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| 225 | <ulink url="http://www.cert.org/tech_tips/">www.cert.org - tech_tips</ulink>
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| 226 | <ulink url="http://security.ittoolbox.com/">security.ittoolbox.com</ulink>
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| 227 | <ulink url="http://www.insecure.org/reading.html">www.insecure.org/reading.html</ulink>
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[8920dfa] | 228 | </literallayout>
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| 229 | </blockquote>
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| 230 |
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| 231 | </sect3>
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| 232 |
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| 233 | </sect2>
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[b4b71892] | 234 |
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[f45b1953] | 235 | </sect1>
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