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git-svn-id: svn://svn.linuxfromscratch.org/BLFS/trunk/BOOK@3272 af4574ff-66df-0310-9fd7-8a98e5e911e0
679 lines
26 KiB
XML
679 lines
26 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
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"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../../general.ent">
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%general-entities;
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]>
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<sect1 id="fw-firewall" xreflabel="Firewalling">
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<sect1info>
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<othername>$LastChangedBy$</othername>
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<date>$Date$</date>
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</sect1info>
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<?dbhtml filename="firewall.html"?>
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<title>Setting up a network firewall</title>
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<para>Before you read this part of the chapter, note that we assume that you
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have already installed iptables as described in the previous section.</para>
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<sect2 id="fw-intro" xreflabel="Firewalling Introduction">
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<title>Introduction to Firewall Creation</title>
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<para>The general purpose of a firewall is to protect a network
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against malicious access by using a single machine as a firewall.
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This does imply that the firewall is to be considered a single point
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of failure, but it can make the administrator's life a lot easier.</para>
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<para>In a perfect world where you knew that every daemon or service
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on every machine was perfectly configured and was immune to, e.g.,
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buffer-overflows and any other imaginable problem regarding its
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security, and where you trusted every user accessing your services
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to aim no harm, you wouldn't need to have a firewall!
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In the real world however, daemons may be misconfigured,
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exploits against essential services are freely available, you
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may wish to choose which services are accessible by certain machines,
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you may wish to limit which machines or applications are allowed
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to have Internet access, or you may simply not trust some of your
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apps or users. In these situations you might benefit by using a
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firewall.</para>
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<para>Don't assume however, that having a firewall makes careful
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configuration redundant, or that it makes any negligent
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misconfiguration harmless. It also doesn't prevent anyone from exploiting a
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service you intentionally offer but haven't recently updated or patched
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after an exploit went public. Despite having a firewall, you need to
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keep applications and daemons on your system well-configured and
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up-to-date; a firewall is not a cure-all!</para>
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</sect2>
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<sect2>
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<title>Meaning of the word firewall.</title>
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<para>The word firewall can have several different meanings.</para>
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<sect3><title><xref linkend="fw-persFw"/></title>
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<para>This is a setup or program, for Windows commercially sold by
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companies such as Symantec, of which they claim or pretend that it
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secures a home or desktop-pc with Internet access. This topic is
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highly relevant for users who do not know the methods their computers
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might be accessed via the Internet or how to disable them,
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especially if they are always online and connected via
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broadband links.</para></sect3>
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<sect3><title><xref linkend="fw-masqRouter"/></title>
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<para>This is a box placed between the Internet and an intranet.
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To minimize the risk of compromising the firewall itself it
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should generally have only one role, that of protecting the intranet.
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Although not completely risk free, the tasks of doing the routing
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and eventually IP masquerading (rewriting IP-headers
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of the packets it routes from clients with private IP-addresses onto
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the Internet so that they seem to come from the firewall
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itself) are commonly considered harmless.</para></sect3>
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<sect3><title><xref linkend="fw-busybox"/></title>
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<para>This is often an old box you may have retired and nearly forgotten,
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performing masquerading or routing functions, but offering a bunch of
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services, e.g., web-cache, mail, etc. This may be very commonly used
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for home networks, but can definitely not be considered as secure
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anymore because the combining of server and router on one machine raises
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the complexity of the setup.</para></sect3>
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<sect3><title>Firewall with a demilitarized zone [not further described
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here]</title>
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<para>This box performs masquerading or routing, but grants public access to
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some branch of your network which, because of public IP's and a physically
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separated structure, is neither considered to be part of the inter- nor
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intranet. These servers are those which must be easily accessible
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from both the inter- and intranet. The firewall protects
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them all.</para></sect3>
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<sect3><title>Packetfilter / partly accessible net [partly described
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here, see <xref linkend="fw-busybox"/>]</title>
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<para>Doing routing or masquerading, but permitting only selected
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services to be accessible, sometimes only by selected internal users or boxes;
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mostly used in highly secure business contexts, sometimes by distrusting
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employers. This was the common configuration of a firewall at the time of
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the Linux 2.2 kernel. It's still possible to configure a firewall this way,
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but it makes the rules quite complex and lengthy.</para></sect3>
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</sect2>
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<sect2 id="postlfs-security-fw-disclaimer" xreflabel="Disclaimer">
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<title>Disclaimer</title>
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<!-- <para><emphasis>NEITHER THE AUTHOR NOR ANY OF THE LINUXFROMSCRATCH TEAM
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ARE RESPONSIBLE FOR ANY DAMAGES INCURRED DUE TO ACTIONS TAKEN BASED ON THIS
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DOCUMENT.</emphasis></para> -->
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<para>This document is meant as an introduction to how to setup a firewall. It
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is not a complete guide to securing systems. Firewalling is a complex issue
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that requires careful configuration. The scripts quoted here are simply
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intended to give examples as to how a firewall works, they are not intended to
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fit into any imaginable configuration and may not prevent any imaginable
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attack.</para>
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<para>The purpose of this text is simply to give you a hint on how to get
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started with a firewall.</para>
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<para>Customization of these scripts for your specific situation will
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be necessary for an optimal configuration, but you should make a serious
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study of the iptables documentation and creating firewalls in general before
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hacking away. Have a look at the list of
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<xref linkend="fw-library"/> at the end of this section for
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more details. Here you will find a list of URLs that contain quite
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comprehensive information about building your own firewall.</para>
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</sect2>
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<sect2 id="fw-kernel" xreflabel="getting a firewalling-enabled Kernel">
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<title>Getting a firewall enabled Kernel</title>
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<para>If you want your Linux-Box to have a firewall, you must first ensure
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that your kernel has been compiled with the relevant options turned on.
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<!-- <footnote><para>If you needed assistance how to configure, compile and
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install a new kernel, refer back to chapter VIII of the LinuxFromScratch book,
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<ulink url="http://www.linuxfromscratch.org/view/3.1/chapter08/kernel.html">
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Installing a kernel</ulink> and eventually
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<ulink url="http://www.linuxfromscratch.org/view/3.1/chapter08/lilo.html">
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Making the LFS system bootable</ulink>; note, that you'll need to reboot
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to actually run your new kernel.</para></footnote>-->
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</para>
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<para>How to configure your kernel, with enabling the options to be
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either compiled into the kernel or as modules, depends on your personal
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preferences and experience. Note, that for the quoted scripts it is assumed
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that the modules need to be loaded at first.</para>
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<screen>Network options menu
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Network packet filtering: Y
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Unix domain sockets: Y or M
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TCP/IP networking: Y
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IP: advanced router: Y
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IP: verbose route monitoring: Y
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IP: TCP Explicit Congestion Notification support: Y
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IP: TCP syncookie support: Y
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IP: Netfilter Configuration menu
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Every option except: Y or M
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ipchains (2.2-style) support N
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ipfwadm (2.0-style) support N
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Fast switching: N</screen>
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<!--
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<table frame='none'>
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<title>Essential config-options for a firewall enabled Kernel</title>
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<tgroup cols='5'>
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<colspec colnum='1' colwidth='8*' align='center'/>
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<colspec colnum='2' colwidth='19*' align='left'/>
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<colspec colnum='3' colwidth='11*' align='center'/>
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<colspec colnum='4' colwidth='1*' align='center'/>
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<colspec colnum='5' colwidth='14*' align='left'/>
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<tbody>
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<row>
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<entry><emphasis><userinput>Networking options:</userinput></emphasis></entry>
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<entry><userinput>Network packet filtering</userinput></entry>
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<entry></entry>
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<entry>=</entry>
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<entry>CONFIG_NETFILTER</entry>
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</row>
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<row>
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<entry></entry>
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<entry><userinput>Unix domain sockets</userinput></entry>
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<entry></entry>
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<entry>=</entry>
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<entry>CONFIG_UNIX</entry>
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</row>
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<row>
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<entry></entry>
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<entry><userinput>IP: TCP/IP networking</userinput></entry>
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<entry></entry>
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<entry>=</entry>
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<entry>CONFIG_INET</entry>
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</row>
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<row>
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<entry></entry>
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<entry><userinput>IP: advanced router</userinput></entry>
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<entry></entry>
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<entry>=</entry>
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<entry>CONFIG_IP_ADVANCED_ROUTER</entry>
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</row>
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<row>
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<entry></entry>
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<entry><userinput>IP: verbose route monitoring</userinput></entry>
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<entry></entry>
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<entry>=</entry>
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<entry>CONFIG_IP_ROUTE_VERBOSE</entry>
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</row>
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<row>
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<entry></entry>
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<entry><userinput>IP: TCP Explicit Congestion Notification support</userinput></entry>
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<entry></entry>
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<entry>=</entry>
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<entry>CONFIG_INET_ECN</entry>
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</row>
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<row>
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<entry></entry>
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<entry><userinput>IP: TCP syncookie support</userinput></entry>
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<entry></entry>
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<entry>=</entry>
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<entry>CONFIG_SYN_COOKIES</entry>
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</row>
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<row>
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<entry></entry>
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<entry align='center'>
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<emphasis><userinput>IP: Netfilter Configuration:</userinput></emphasis></entry>
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<entry align='left'><userinput>every option</userinput></entry>
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<entry>=</entry>
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<entry>CONFIG_IP_NF_*</entry>
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</row>
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<row>
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<entry></entry>
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<entry align='right'><emphasis>WITHOUT:</emphasis></entry>
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<entry align='left'><literallayout><userinput>ipchains (2.2-style) support
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ipfw-adm (2.0-style) support</userinput></literallayout></entry>
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<entry>w\</entry>
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<entry>CONFIG_IP_NF_COMPAT_*</entry>
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</row>
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<row>
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<entry></entry>
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<entry><userinput>Fast switching</userinput></entry>
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<entry>Make sure to disable it because it would setup a bypass around
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your firewall rules.</entry>
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<entry>w\</entry>
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<entry>CONFIG_NET_FASTROUTE</entry>
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</row>
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</tbody>
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</tgroup>
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</table> -->
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</sect2>
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<sect2 id="fw-writing" xreflabel="writing the firewalling-setup-scripts">
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<title>Now you can start to build your Firewall</title>
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<sect3 id="fw-persFw" xreflabel="Personal Firewall">
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<title>Personal Firewall</title>
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<para>A Personal Firewall is supposed to let you access all the services
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offered on the Internet, but keep your box secure and your data private.</para>
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<para>Below is a slightly modified version of Rusty Russell's recommendation
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from the <ulink
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url="http://www.netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html">
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Linux 2.4 Packet Filtering HOWTO</ulink>:</para>
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<screen><userinput><command>cat > /etc/rc.d/init.d/firewall << "EOF"</command>
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#!/bin/sh
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# Begin $rc_base/init.d/firewall
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# Insert connection-tracking modules
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# (not needed if built into the kernel)
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modprobe ip_tables
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modprobe iptable_filter
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modprobe ip_conntrack
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modprobe ip_conntrack_ftp
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modprobe ipt_state
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modprobe ipt_LOG
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# allow local-only connections
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iptables -A INPUT -i lo -j ACCEPT
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# free output on any interface to any ip for any service
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# (equal to -P ACCEPT)
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iptables -A OUTPUT -j ACCEPT
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# permit answers on already established connections
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# and permit new connections related to established ones
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# (eg active-ftp)
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iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
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# Log everything else: What's Windows' latest exploitable vulnerability?
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iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
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# set a sane policy: everything not accepted > /dev/null
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iptables -P INPUT DROP
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iptables -P FORWARD DROP
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iptables -P OUTPUT DROP
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# be verbose on dynamic ip-addresses (not needed in case of static IP)
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echo 2 > /proc/sys/net/ipv4/ip_dynaddr
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# disable ExplicitCongestionNotification
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# too many routers are still ignorant
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echo 0 > /proc/sys/net/ipv4/tcp_ecn
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# End $rc_base/init.d/firewall
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<command>EOF</command></userinput></screen>
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<para>His script is quite simple, it drops all traffic coming in into your
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computer that wasn't initiated from your box, but as long as you are simply
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surfing the Internet you are unlikely to exceed its limits.</para>
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<para>If you frequently encounter certain delays at accessing ftp-servers,
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please have a look at <xref linkend="fw-busybox"/> -
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<xref linkend="fw-BB-4"/>.</para>
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<para>Even if you have daemons or services running on your box, these
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should be inaccessible everywhere but from your box itself.
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If you want to allow access to services on your machine, such as ssh or
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pinging, take a look at <xref linkend="fw-busybox"/>.</para>
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</sect3>
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<sect3 id="fw-masqRouter" xreflabel="Masquerading Router">
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<title>Masquerading Router</title>
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<para>A true Firewall has two interfaces, one connected to an intranet,
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in this example, <emphasis role="strong">eth0</emphasis>, and one
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connected to the Internet, here, <emphasis role="strong">ppp0</emphasis>.
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To provide the maximum security against the box itself being broken into,
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make sure that there are no servers running on it, especially not
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<application>X11</application> et
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al. And, as a general principle, the box itself should not access any
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untrusted service (Think of a name server giving answers that make your
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bind crash, or, even worse, that implement a worm via a
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buffer-overflow).</para>
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<screen><userinput><command>cat > /etc/rc.d/init.d/firewall << "EOF"</command>
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#!/bin/sh
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# Begin $rc_base/init.d/firewall
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echo
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echo "You're using the example-config for a setup of a firewall"
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echo "from the firewalling-hint written for LinuxFromScratch."
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echo "This example is far from being complete, it is only meant"
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echo "to be a reference."
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echo "Firewall security is a complex issue, that exceeds the scope"
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echo "of the quoted configuration rules."
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echo "You can find some quite comprehensive information"
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echo "about firewalls in Chapter 4 of the BLFS book."
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echo "http://www.linuxfromscratch.org/blfs"
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echo
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# Insert iptables modules (not needed if built into the kernel).
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modprobe ip_tables
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modprobe iptable_filter
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modprobe ip_conntrack
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modprobe ip_conntrack_ftp
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modprobe ipt_state
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modprobe iptable_nat
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modprobe ip_nat_ftp
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modprobe ipt_MASQUERADE
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modprobe ipt_LOG
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modprobe ipt_REJECT
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# allow local-only connections
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iptables -A INPUT -i lo -j ACCEPT
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iptables -A OUTPUT -o lo -j ACCEPT
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# allow forwarding
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iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
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iptables -A FORWARD -m state --state NEW -i ! ppp+ -j ACCEPT
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# do masquerading
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# (not needed if intranet is not using private ip-addresses)
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iptables -t nat -A POSTROUTING -o ppp+ -j MASQUERADE
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# Log everything for debugging
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# (last of all rules, but before DROP/REJECT)
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iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "
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iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD"
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iptables -A OUTPUT -j LOG --log-prefix "FIREWALL:OUTPUT "
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# set a sane policy
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iptables -P INPUT DROP
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iptables -P FORWARD DROP
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iptables -P OUTPUT DROP
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# be verbose on dynamic ip-addresses
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# (not needed in case of static IP)
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echo 2 > /proc/sys/net/ipv4/ip_dynaddr
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# disable ExplicitCongestionNotification
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echo 0 > /proc/sys/net/ipv4/tcp_ecn
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# activate TCPsyncookies
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echo 1 > /proc/sys/net/ipv4/tcp_syncookies
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# activate Route-Verification = IP-Spoofing_protection
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for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
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echo 1 > $f
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done
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# activate IP-Forwarding
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echo 1 > /proc/sys/net/ipv4/ip_forward
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<command>EOF</command></userinput></screen>
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<para>With this script your intranet should be sufficiently secure against
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external attacks. No one should be able to setup a new connection to any
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internal service and, if it's masqueraded, it's even invisible. Furthermore,
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your firewall should be nearly immune because there are no services running
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that a cracker could attack.</para>
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<para>Note: if the interface you're connecting to the Internet
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doesn't connect via ppp, you will need to change
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<replaceable>ppp+</replaceable> to the name of the interface which you are
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using. If you are using the same interface type to connect to both your
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intranet and the Internet, you need to use the actual name of the
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interface such as <emphasis role="strong">eth0</emphasis>,
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on both interfaces.</para>
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<para>If you need stronger security (e.g., against DOS, connection
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highjacking, spoofing, etc.), have a look at the list of
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<xref linkend="fw-library"/> at the end of this section.</para>
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</sect3>
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<sect3 id="fw-busybox" xreflabel="BusyBox">
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<title>BusyBox</title>
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<para>This scenario isn't too different from (<xref linkend="fw-masqRouter"/>),
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but in this case you want to offer some services to your intranet.
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Examples of this can be when you want to admin your box from another host
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on your intranet or use it as a proxy or a name server. Note: Outlining a true
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concept of how to protect a server that offers services on the Internet
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goes far beyond the scope of this document,
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|
see <xref linkend="postlfs-security-fw-disclaimer"/>.</para>
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<para>Be cautious. Every service you offer and have enabled makes your
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setup more complex and your box less secure. You induce the risks of
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misconfigured services or running a service with an exploitable bug. A
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firewall should generally not run any extra services. See the introduction to
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<xref linkend="fw-masqRouter"/> for some more details.</para>
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|
|
<para>If the services you'd like to offer do not need to access the Internet
|
|
themselves, like internal-only samba- or name-servers, it's quite
|
|
simple and should still be acceptable from a security standpoint.
|
|
Just add the following lines <emphasis>before</emphasis> the logging-rules
|
|
into the script.</para>
|
|
|
|
<screen>iptables -A INPUT -i ! ppp+ -j ACCEPT
|
|
iptables -A OUTPUT -o ! ppp+ -j ACCEPT</screen>
|
|
|
|
<para>If your daemons have to access the web themselves, like squid would need
|
|
to, you could open OUTPUT generally and restrict INPUT.</para>
|
|
|
|
<screen>iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
|
|
iptables -A OUTPUT -j ACCEPT</screen>
|
|
|
|
<para>However, it is generally not advisable to leave OUTPUT unrestricted. You
|
|
lose any control over trojans who'd like to "call home", and a bit of
|
|
redundancy in case you've (mis-)configured a service so that it does broadcast
|
|
its existence to the world.</para>
|
|
|
|
<para>If you prefer to have this protection, you may restrict INPUT and OUTPUT
|
|
on all ports except those that it's absolutely necessary to have open.
|
|
Which ports you have to open depends on your needs: mostly you will find them
|
|
by looking for failed accesses in your log-files.</para>
|
|
<itemizedlist spacing="compact">
|
|
<!-- <orderedlist numeration="arabic" spacing="compact"> -->
|
|
<title>Have a look at the following examples:</title>
|
|
|
|
<listitem><para>Squid is caching the web:</para>
|
|
<screen>iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
|
|
iptables -A INPUT -p tcp --sport 80 -m state --state ESTABLISHED \
|
|
-j ACCEPT</screen>
|
|
</listitem>
|
|
|
|
<listitem><para>Your caching name server (e.g., dnscache) does its
|
|
lookups via udp:</para>
|
|
<screen>iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
|
|
iptables -A INPUT -p udp --sport 53 -m state --state ESTABLISHED \
|
|
-j ACCEPT</screen>
|
|
</listitem>
|
|
|
|
<listitem><para>Alternatively, if you want to be able to ping your box to
|
|
ensure it's still alive:</para>
|
|
|
|
<screen>iptables -A INPUT -p icmp -m icmp --icmp-type echo-request -j ACCEPT
|
|
iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply -j ACCEPT</screen>
|
|
</listitem>
|
|
|
|
<listitem><para><anchor id='fw-BB-4' xreflabel="example no. 4"/>If you are
|
|
frequently accessing ftp-servers or enjoy chatting, you might notice certain
|
|
delays because some implementations of these daemons have the feature of
|
|
querying an identd on your box for logging usernames.
|
|
Although there's really no harm in this, having an identd running is not
|
|
recommended because some implementations are known to be vulnerable.</para>
|
|
|
|
<para>To avoid these delays you could reject the requests
|
|
with a 'tcp-reset':</para>
|
|
|
|
<screen>iptables -A INPUT -p tcp --dport 113 -j REJECT --reject-with tcp-reset
|
|
iptables -A OUTPUT -p tcp --sport 113 -m state --state RELATED -j ACCEPT</screen>
|
|
</listitem>
|
|
|
|
<listitem><para>To log and drop invalid packets (harmless packets
|
|
that came in after netfilter's timeout or some types of network scans):</para>
|
|
|
|
<screen>iptables -I INPUT 1 -p tcp -m state --state INVALID -j LOG \
|
|
--log-prefix "FIREWALL:INVALID"
|
|
iptables -I INPUT 2 -p tcp -m state --state INVALID -j DROP</screen></listitem>
|
|
|
|
<listitem><para>Anything coming from the outside should not have a
|
|
private address, this is a common attack called IP-spoofing:</para>
|
|
|
|
<screen>iptables -t nat -A PREROUTING -i ppp+ -s 10.0.0.0/8 -j DROP
|
|
iptables -t nat -A PREROUTING -i ppp+ -s 172.16.0.0/12 -j DROP
|
|
iptables -t nat -A PREROUTING -i ppp+ -s 192.168.0.0/16 -j DROP</screen>
|
|
</listitem>
|
|
|
|
<listitem><para>To simplify debugging and be fair to anyone who'd like to
|
|
access a service you have disabled, purposely or by mistake, you should REJECT
|
|
those packets that are dropped.</para>
|
|
|
|
<para>Obviously this must be done directly after logging as the very
|
|
last lines before the packets are dropped by policy:</para>
|
|
|
|
<screen>iptables -A INPUT -j REJECT
|
|
iptables -A OUTPUT -p icmp --icmp-type 3 -j ACCEPT</screen></listitem>
|
|
</itemizedlist>
|
|
<!--</orderedlist>-->
|
|
|
|
<para>These are only examples to show you some of the capabilities of the new
|
|
firewall code in Linux-Kernel 2.4. Have a look at the man page of
|
|
iptables.
|
|
There you will find more of them. The port-numbers you'll need for this
|
|
can be found in <filename>/etc/services</filename>, in case you didn't
|
|
find them by trial and error in your log file.</para>
|
|
|
|
<para>If you add any of your offered or accessed services such as the above,
|
|
maybe even in FORWARD and for intranet-communication, and delete the
|
|
general clauses, you get an old fashioned packet filter.</para>
|
|
</sect3>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="fw-finale" xreflabel="Conclusion">
|
|
<title>Conclusion</title>
|
|
|
|
<para>Finally, I'd like to remind you of one fact we must not forget:
|
|
The effort spent attacking a system corresponds to the value the cracker
|
|
expects to gain from it.
|
|
If you are responsible for such valuable assets that you expect great
|
|
effort to be made by potential crackers, you hopefully won't be in the
|
|
need of this hint!</para>
|
|
|
|
<!-- <para><literallayout>Be cautious!
|
|
|
|
Henning Rohde
|
|
<email>Henning.Rohde@uni-bayreuth.de</email></literallayout></para>
|
|
|
|
<para>PS: And always do remember:
|
|
SecureIT is not a matter of a status-quo but one of never stopping
|
|
to take care!</para>
|
|
|
|
<para>PPS: If any of these scripts fail, please tell me. I will try to trace
|
|
any faults.</para> -->
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="postlfs-security-fw-extra" xreflabel="Extra Information">
|
|
<title>Extra Information</title>
|
|
|
|
<sect3 id="fw-library" xreflabel="Links for further reading">
|
|
<title>Where to start with further reading on firewalls.</title>
|
|
|
|
<para><blockquote><literallayout>
|
|
<ulink url="http://www.netfilter.org/">www.netfilter.org - Homepage of the netfilter/iptables project</ulink>
|
|
<ulink url="http://www.netfilter.org/documentation/FAQ/netfilter-faq.html">Netfilter related FAQ</ulink>
|
|
<ulink url="http://www.netfilter.org/documentation/index.html#HOWTO">Netfilter related HOWTO's</ulink>
|
|
<ulink url="http://en.tldp.org/LDP/nag2/x-087-2-firewall.html">en.tldp.org/LDP/nag2/x-087-2-firewall.html</ulink>
|
|
<ulink url="http://en.tldp.org/HOWTO/Security-HOWTO.html">en.tldp.org/HOWTO/Security-HOWTO.html</ulink>
|
|
<ulink url="http://en.tldp.org/HOWTO/Firewall-HOWTO.html">en.tldp.org/HOWTO/Firewall-HOWTO.html</ulink>
|
|
<ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire.html">www.ibm.com/developerworks/security/library/s-fire.html</ulink>
|
|
<ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire2.html">www.ibm.com/developerworks/security/library/s-fire2.html</ulink>
|
|
<ulink url="http://www.interhack.net/pubs/fw-faq/">www.interhack.net/pubs/fw-faq/</ulink>
|
|
<ulink url="http://www.linuxsecurity.com/docs/">www.linuxsecurity.com/docs/</ulink>
|
|
<ulink url="http://www.little-idiot.de/firewall">www.little-idiot.de/firewall (German & outdated, but very comprehensive)</ulink>
|
|
<ulink url="http://www.linuxgazette.com/issue65/stumpel.html">www.linuxgazette.com/issue65/stumpel.html</ulink>
|
|
<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>
|
|
<ulink url="http://staff.washington.edu/dittrich/misc/ddos">staff.washington.edu/dittrich/misc/ddos</ulink>
|
|
<ulink url="http://www.e-infomax.com/ipmasq">www.e-infomax.com/ipmasq</ulink>
|
|
<ulink url="http://www.circlemud.org/~jelson/writings/security/index.htm">www.circlemud.org/~jelson/writings/security/index.htm</ulink>
|
|
<ulink url="http://www.securityfocus.com">www.securityfocus.com</ulink>
|
|
<ulink url="http://www.cert.org/tech_tips/">www.cert.org - tech_tips</ulink>
|
|
<ulink url="http://security.ittoolbox.com/">security.ittoolbox.com</ulink>
|
|
<ulink url="http://www.linux-firewall-tools.com/linux/">www.linux-firewall-tools.com/linux/</ulink>
|
|
<ulink url="http://logi.cc/linux/athome-firewall.php3">logi.cc/linux/athome-firewall.php3</ulink>
|
|
<ulink url="http://www.insecure.org/reading.html">www.insecure.org/reading.html</ulink>
|
|
<ulink url="http://www.robertgraham.com/pubs/firewall-seen.html">www.robertgraham.com/pubs/firewall-seen.html</ulink>
|
|
</literallayout></blockquote></para>
|
|
</sect3>
|
|
|
|
<sect3 id="fw-status" xreflabel="/etc/rc.d/init.d/firewall.status">
|
|
<title>firewall.status</title>
|
|
|
|
<para>If you'd like to have a look at the chains your firewall consists of and
|
|
the order in which the rules take effect:</para>
|
|
|
|
<screen><userinput><command>cat > /etc/rc.d/init.d/firewall.status << "EOF"</command>
|
|
#!/bin/sh
|
|
|
|
# Begin $rc_base/init.d/firewall.status
|
|
|
|
echo "iptables.mangling:"
|
|
iptables -t mangle -v -L -n --line-numbers
|
|
|
|
echo
|
|
echo "iptables.nat:"
|
|
iptables -t nat -v -L -n --line-numbers
|
|
|
|
echo
|
|
echo "iptables.filter:"
|
|
iptables -v -L -n --line-numbers
|
|
<command>EOF</command></userinput></screen>
|
|
</sect3>
|
|
|
|
<sect3 id="postlfs-security-fw-stop" xreflabel="/etc/rc.d/init.d/firewall.stop">
|
|
<title>firewall.stop</title>
|
|
|
|
<para>If you need to turn the firewall off, this script will do it:</para>
|
|
|
|
<screen><userinput><command>cat > /etc/rc.d/init.d/firewall.stop << "EOF"</command>
|
|
#!/bin/sh
|
|
|
|
# Being $rc_base/init.d/firewall.stop
|
|
|
|
# deactivate IP-Forwarding
|
|
echo 0 > /proc/sys/net/ipv4/ip_forward
|
|
|
|
iptables -Z
|
|
iptables -F
|
|
iptables -t nat -F PREROUTING
|
|
iptables -t nat -F OUTPUT
|
|
iptables -t nat -F POSTROUTING
|
|
iptables -t mangle -F PREROUTING
|
|
iptables -t mangle -F OUTPUT
|
|
iptables -X
|
|
iptables -P INPUT ACCEPT
|
|
iptables -P FORWARD ACCEPT
|
|
iptables -P OUTPUT ACCEPT
|
|
<command>EOF</command></userinput></screen>
|
|
</sect3>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|