mirror of
https://gitlab.com/shorewall/code.git
synced 2024-11-24 16:43:21 +01:00
f4c9a0cc23
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@1067 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
760 lines
36 KiB
XML
760 lines
36 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<article id="three-interface">
|
|
<!--$Id$-->
|
|
|
|
<articleinfo>
|
|
<title>Three-Interface Firewall</title>
|
|
|
|
<authorgroup>
|
|
<author>
|
|
<firstname>Tom</firstname>
|
|
|
|
<surname>Eastep</surname>
|
|
</author>
|
|
</authorgroup>
|
|
|
|
<pubdate>2003-11-15</pubdate>
|
|
|
|
<copyright>
|
|
<year>2002</year>
|
|
|
|
<year>2003</year>
|
|
|
|
<holder>Thomas M. Eastep</holder>
|
|
</copyright>
|
|
|
|
<legalnotice>
|
|
<para>Permission is granted to copy, distribute and/or modify this
|
|
document under the terms of the GNU Free Documentation License, Version
|
|
1.2 or any later version published by the Free Software Foundation; with
|
|
no Invariant Sections, with no Front-Cover, and with no Back-Cover
|
|
Texts. A copy of the license is included in the section entitled
|
|
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation License</ulink></quote>.</para>
|
|
</legalnotice>
|
|
</articleinfo>
|
|
|
|
<section>
|
|
<title>Introduction</title>
|
|
|
|
<para>Setting up a Linux system as a firewall for a small network with DMZ
|
|
is a fairly straight-forward task if you understand the basics and follow
|
|
the documentation.</para>
|
|
|
|
<para>This guide doesn't attempt to acquaint you with all of the
|
|
features of Shorewall. It rather focuses on what is required to configure
|
|
Shorewall in one of its more popular configurations:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>Linux system used as a firewall/router for a small local
|
|
network.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Single public IP address.</para>
|
|
|
|
<note>
|
|
<para>If you have more than one public IP address, this is not the
|
|
guide you want -- see the <ulink url="shorewall_setup_guide.htm">Shorewall
|
|
Setup Guide</ulink> instead.</para>
|
|
</note>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>DMZ connected to a separate ethernet interface.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Connection through DSL, Cable Modem, ISDN, Frame Relay, dial-up,
|
|
...</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>Here is a schematic of a typical installation.</para>
|
|
|
|
<figure>
|
|
<title>schematic of a typical installation</title>
|
|
|
|
<mediaobject>
|
|
<imageobject>
|
|
<imagedata align="center" fileref="images/dmz1.png" format="PNG" />
|
|
</imageobject>
|
|
</mediaobject>
|
|
</figure>
|
|
|
|
<section>
|
|
<title>Requirements</title>
|
|
|
|
<para>Shorewall requires that you have the <command>iproute</command>/<command>iproute2</command>
|
|
package installed (on <trademark>RedHat</trademark>, the package is
|
|
called <command>iproute</command>). You can tell if this package is
|
|
installed by the presence of an <command>ip</command> program on your
|
|
firewall system. As <systemitem class="username">root</systemitem>, you
|
|
can use the <command>which</command> command to check for this program:</para>
|
|
|
|
<programlisting>[root@gateway root]# <command>which ip</command>
|
|
/sbin/ip
|
|
[root@gateway root]#</programlisting>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Before you start</title>
|
|
|
|
<para>I recommend that you first read through the guide to familiarize
|
|
yourself with what's involved then go back through it again making
|
|
your configuration changes.</para>
|
|
|
|
<caution>
|
|
<para>If you edit your configuration files on a
|
|
<trademark>Windows</trademark> system, you must save them as
|
|
<trademark>Unix</trademark> files if your editor supports that option
|
|
or you must run them through <command>dos2unix</command> before trying
|
|
to use them. Similarly, if you copy a configuration file from your
|
|
<trademark>Windows</trademark> hard drive to a floppy disk, you must
|
|
run <command>dos2unix</command> against the copy before using it with
|
|
Shorewall.</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para><ulink url="http://www.simtel.net/pub/pd/51438.html">Windows
|
|
Version of dos2unix</ulink></para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para><ulink url="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
|
|
Version of dos2unix</ulink></para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</caution>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Conventions</title>
|
|
|
|
<para>Points at which configuration changes are recommended are flagged
|
|
with <inlinegraphic fileref="images/BD21298_.gif" format="GIF" />.</para>
|
|
|
|
<para>Configuration notes that are unique to LEAF/Bering are marked with
|
|
<inlinegraphic fileref="images/leaflogo.gif" format="GIF" />.</para>
|
|
</section>
|
|
</section>
|
|
|
|
<section>
|
|
<title>PPTP/ADSL</title>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>If you have an ADSL Modem and you use PPTP to communicate with a
|
|
server in that modem, you must make the <ulink url="PPTP.htm#PPTP_ADSL">changes
|
|
recommended here</ulink> in addition to those detailed below. ADSL with
|
|
PPTP is most commonly found in Europe, notably in Austria.</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Shorewall Concepts</title>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>The configuration files for Shorewall are contained in the directory
|
|
<filename>/etc/shorewall</filename> -- for simple setups, you will only
|
|
need to deal with a few of these as described in this guide. After you
|
|
have installed Shorewall, download the three-interface sample, un-tar it (<command>tar
|
|
<option>-zxvf</option> <filename>three-interfaces.tgz</filename></command>)
|
|
and and copy the files to <filename>/etc/shorewall</filename> (the files
|
|
will replace files with the same names that were placed in
|
|
<filename>/etc/shorewall</filename> when Shorewall was installed).</para>
|
|
|
|
<para>As each file is introduced, I suggest that you look through the
|
|
actual file on your system -- each file contains detailed configuration
|
|
instructions and default entries.</para>
|
|
|
|
<para>Shorewall views the network where it is running as being composed of
|
|
a set of zones. In the three-interface sample configuration, the following
|
|
zone names are used:</para>
|
|
|
|
<informaltable>
|
|
<tgroup align="left" cols="2">
|
|
<thead valign="middle">
|
|
<row>
|
|
<entry align="center">Name</entry>
|
|
|
|
<entry align="center">Description</entry>
|
|
</row>
|
|
</thead>
|
|
|
|
<tbody>
|
|
<row>
|
|
<entry>net</entry>
|
|
|
|
<entry>The Internet</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry>loc</entry>
|
|
|
|
<entry>Your Local Network</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry>dmz</entry>
|
|
|
|
<entry>Demilitarized Zone</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</informaltable>
|
|
|
|
<para>Zone names are defined in <filename>/etc/shorewall/zones</filename>.</para>
|
|
|
|
<para>Shorewall also recognizes the firewall system as its own zone - by
|
|
default, the firewall itself is known as <varname>fw</varname>.</para>
|
|
|
|
<para>Rules about what traffic to allow and what traffic to deny are
|
|
expressed in terms of zones.</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>You express your default policy for connections from one zone to
|
|
another zone in the <filename>/etc/shorewall/policy</filename> file.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>You define exceptions to those default policies in the
|
|
<filename>/etc/shorewall/rules</filename> file.</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>For each connection request entering the firewall, the request is
|
|
first checked against the <filename>/etc/shorewall/rules</filename> file.
|
|
If no rule in that file matches the connection request then the first
|
|
policy in <filename>/etc/shorewall/policy</filename> that matches the
|
|
request is applied. If that policy is REJECT or DROP the request is first
|
|
checked against the rules in <filename>/etc/shorewall/common</filename> if
|
|
that file exists; otherwise the file <filename>/etc/shorewall/common.def</filename>
|
|
is checked</para>
|
|
|
|
<para>The <filename>/etc/shorewall/policy</filename> file included with
|
|
the three-interface sample has the following policies:</para>
|
|
|
|
<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
|
|
loc net ACCEPT
|
|
net all DROP info
|
|
all all REJECT info</programlisting>
|
|
|
|
<important>
|
|
<para>In the three-interface sample, the line below is included but
|
|
commented out. If you want your firewall system to have full access to
|
|
servers on the internet, uncomment that line.</para>
|
|
|
|
<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
|
|
fw net ACCEPT</programlisting>
|
|
</important>
|
|
|
|
<para>The above policy will:</para>
|
|
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>allow all connection requests from your local network to the
|
|
internet</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>drop (ignore) all connection requests from the internet to your
|
|
firewall or local network</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>optionally accept all connection requests from the firewall to
|
|
the internet (if you uncomment the additional policy)</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>reject all other connection requests.</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>At this point, edit your <filename>/etc/shorewall/policy</filename>
|
|
file and make any changes that you wish.</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Network Interfaces</title>
|
|
|
|
<figure>
|
|
<title>DMZ</title>
|
|
|
|
<mediaobject>
|
|
<imageobject>
|
|
<imagedata align="center" fileref="images/dmz1.png" format="PNG" />
|
|
</imageobject>
|
|
</mediaobject>
|
|
</figure>
|
|
|
|
<para>The firewall has three network interfaces. Where Internet
|
|
connectivity is through a cable or DSL <quote>Modem</quote>, the External
|
|
Interface will be the ethernet adapter that is connected to that
|
|
<quote>Modem</quote> (e.g., <filename class="devicefile">eth0</filename>)
|
|
unless you connect via <emphasis>Point-to-Point Protocol</emphasis> over
|
|
Ethernet (PPPoE) or <emphasis>Point-to-Point Tunneling Protocol</emphasis>
|
|
(PPTP) in which case the External Interface will be a <literal>ppp</literal>
|
|
interface (e.g., <filename class="devicefile">ppp0</filename>). If you
|
|
connect via a regular modem, your External Interface will also be
|
|
<filename class="devicefile">ppp0</filename>. If you connect using ISDN,
|
|
you external interface will be <filename class="devicefile">ippp0</filename>.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>If your external interface is <filename class="devicefile">ppp0</filename>
|
|
or <filename class="devicefile">ippp0</filename> then you will want to set
|
|
<varname>CLAMPMSS=yes</varname> in <filename>/etc/shorewall/shorewall.conf</filename>.</para>
|
|
|
|
<para>Your Local Interface will be an ethernet adapter (<filename
|
|
class="devicefile">eth0</filename>, <filename class="devicefile">eth1</filename>
|
|
or <filename class="devicefile">eth2</filename>) and will be connected to
|
|
a hub or switch. Your local computers will be connected to the same switch
|
|
(note: If you have only a single local system, you can connect the
|
|
firewall directly to the computer using a cross-over cable).</para>
|
|
|
|
<para>Your DMZ Interface will also be an ethernet adapter (<filename
|
|
class="devicefile">eth0</filename>, <filename class="devicefile">eth1</filename>
|
|
or <filename class="devicefile">eth2</filename>) and will be connected to
|
|
a hub or switch. Your DMZ computers will be connected to the same switch
|
|
(note: If you have only a single DMZ system, you can connect the firewall
|
|
directly to the computer using a cross-over cable).</para>
|
|
|
|
<caution>
|
|
<para>Do not connect the internal and external interface to the same hub
|
|
or switch except for testing AND you are running Shorewall version 1.4.7
|
|
or later. When using these recent versions, you can test using this kind
|
|
of configuration if you specify the arp_filter option in
|
|
<filename>/etc/shorewall/interfaces</filename> for all interfaces
|
|
connected to the common hub/switch. Using such a setup with a production
|
|
firewall is strongly recommended against.</para>
|
|
</caution>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>The Shorewall three-interface sample configuration assumes that the
|
|
external interface is <filename class="devicefile">eth0</filename>, the
|
|
local interface is <filename class="devicefile">eth1</filename> and the
|
|
DMZ interface is <filename class="devicefile">eth2</filename>. If your
|
|
configuration is different, you will have to modify the sample
|
|
<filename>/etc/shorewall/interfaces</filename> file accordingly. While you
|
|
are there, you may wish to review the list of options that are specified
|
|
for the interfaces. Some hints:</para>
|
|
|
|
<tip>
|
|
<para>If your external interface is <filename class="devicefile">ppp0</filename>
|
|
or <filename class="devicefile">ippp0</filename>, you can replace the
|
|
<quote>detect</quote> in the second column with <quote>-</quote>
|
|
(without the quotes).</para>
|
|
</tip>
|
|
|
|
<tip>
|
|
<para>If your external interface is <filename class="devicefile">ppp0</filename>
|
|
or <filename class="devicefile">ippp0</filename> or if you have a static
|
|
IP address, you can remove <quote>dhcp</quote> from the option list.</para>
|
|
</tip>
|
|
</section>
|
|
|
|
<section>
|
|
<title>IP Addresses</title>
|
|
|
|
<para>Before going further, we should say a few words about Internet
|
|
Protocol (IP) addresses. Normally, your ISP will assign you a single
|
|
Public IP address. This address may be assigned via the Dynamic Host
|
|
Configuration Protocol (DHCP) or as part of establishing your connection
|
|
when you dial in (standard modem) or establish your PPP connection. In
|
|
rare cases, your ISP may assign you a static IP address; that means that
|
|
you configure your firewall's external interface to use that address
|
|
permanently. Regardless of how the address is assigned, it will be shared
|
|
by all of your systems when you access the Internet. You will have to
|
|
assign your own addresses for your internal network (the local and DMZ
|
|
Interfaces on your firewall plus your other computers). RFC 1918 reserves
|
|
several Private IP address ranges for this purpose:</para>
|
|
|
|
<programlisting>10.0.0.0 - 10.255.255.255
|
|
172.16.0.0 - 172.31.255.255
|
|
192.168.0.0 - 192.168.255.255</programlisting>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>Before starting Shorewall, you should look at the IP address of your
|
|
external interface and if it is one of the above ranges, you should remove
|
|
the <varname>norfc1918</varname> option from the external interface's
|
|
entry in <filename>/etc/shorewall/interfaces</filename>.</para>
|
|
|
|
<para>You will want to assign your local addresses from one sub-network or
|
|
subnet and your DMZ addresses from another subnet. For our purposes, we
|
|
can consider a subnet to consists of a range of addresses <systemitem
|
|
class="ipaddress">x.y.z.0</systemitem> - <systemitem class="ipaddress">x.y.z.255</systemitem>.
|
|
Such a subnet will have a Subnet Mask of <systemitem class="netmask">255.255.255.0</systemitem>.
|
|
The address <systemitem class="ipaddress">x.y.z.0</systemitem> is reserved
|
|
as the Subnet Address and <systemitem class="netmask">x.y.z.255</systemitem>
|
|
is reserved as the Subnet Broadcast Address. In Shorewall, a subnet is
|
|
described using Classless InterDomain Routing (CIDR) notation with
|
|
consists of the subnet address followed by <varname>/24</varname>. The
|
|
<varname>24</varname> refers to the number of consecutive <quote>1</quote>
|
|
bits from the left of the subnet mask.</para>
|
|
|
|
<table>
|
|
<title>Example sub-network</title>
|
|
|
|
<tgroup cols="2">
|
|
<colspec align="left" />
|
|
|
|
<tbody>
|
|
<row>
|
|
<entry>Range:</entry>
|
|
|
|
<entry><systemitem class="ipaddress">10.10.10.0</systemitem> -
|
|
<systemitem class="ipaddress">10.10.10.255</systemitem></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry>Subnet Address:</entry>
|
|
|
|
<entry><systemitem class="ipaddress">10.10.10.0</systemitem></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry>Broadcast Address:</entry>
|
|
|
|
<entry><systemitem class="ipaddress">10.10.10.255</systemitem></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry>CIDR Notation:</entry>
|
|
|
|
<entry><systemitem class="ipaddress">10.10.10.0/24</systemitem></entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<para>It is conventional to assign the internal interface either the first
|
|
usable address in the subnet (<systemitem class="ipaddress">10.10.10.1</systemitem>
|
|
in the above example) or the last usable address (<systemitem
|
|
class="ipaddress">10.10.10.254</systemitem>).</para>
|
|
|
|
<para>One of the purposes of subnetting is to allow all computers in the
|
|
subnet to understand which other computers can be communicated with
|
|
directly. To communicate with systems outside of the subnetwork, systems
|
|
send packets through a gateway (router).</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>Your local computers (Local Computers 1 & 2) should be
|
|
configured with their default gateway set to the IP address of the
|
|
firewall's internal interface and your DMZ computers (DMZ Computers 1
|
|
& 2) should be configured with their default gateway set to the IP
|
|
address of the firewall's DMZ interface.</para>
|
|
|
|
<para>The foregoing short discussion barely scratches the surface
|
|
regarding subnetting and routing. If you are interested in learning more
|
|
about IP addressing and routing, I highly recommend <quote>IP
|
|
Fundamentals: What Everyone Needs to Know about Addressing & Routing</quote>,
|
|
Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</para>
|
|
|
|
<para>The remainder of this quide will assume that you have configured
|
|
your network as shown here:</para>
|
|
|
|
<figure>
|
|
<title>DMZ</title>
|
|
|
|
<mediaobject>
|
|
<imageobject>
|
|
<imagedata fileref="images/dmz2.png" />
|
|
</imageobject>
|
|
|
|
<caption>
|
|
<para>The default gateway for the DMZ computers would be <systemitem
|
|
class="ipaddress">10.10.11.254</systemitem> and the default gateway
|
|
for the Local computers would be <systemitem class="ipaddress">10.10.10.254</systemitem>.</para>
|
|
|
|
<warning>
|
|
<para>Your ISP might assign your external interface an RFC 1918
|
|
address. If that address is in the <systemitem class="ipaddress">10.10.10.0/24</systemitem>
|
|
subnet then you will need to select a DIFFERENT RFC 1918 subnet
|
|
for your local network and if it is in the <systemitem
|
|
class="ipaddress">10.10.11.0/24</systemitem> subnet then you will
|
|
need to select a different RFC 1918 subnet for your DMZ.</para>
|
|
</warning>
|
|
</caption>
|
|
</mediaobject>
|
|
</figure>
|
|
</section>
|
|
|
|
<section>
|
|
<title>IP Masquerading (SNAT)</title>
|
|
|
|
<para>The addresses reserved by RFC 1918 are sometimes referred to as
|
|
non-routable because the Internet backbone routers don't forward
|
|
packets which have an RFC-1918 destination address. When one of your local
|
|
systems (let's assume local computer 1) sends a connection request to
|
|
an internet host, the firewall must perform Network Address Translation
|
|
(NAT). The firewall rewrites the source address in the packet to be the
|
|
address of the firewall's external interface; in other words, the
|
|
firewall makes it look as if the firewall itself is initiating the
|
|
connection. This is necessary so that the destination host will be able to
|
|
route return packets back to the firewall (remember that packets whose
|
|
destination address is reserved by RFC 1918 can't be routed accross
|
|
the internet). When the firewall receives a return packet, it rewrites the
|
|
destination address back to 10.10.10.1 and forwards the packet on to local
|
|
computer 1.</para>
|
|
|
|
<para>On Linux systems, the above process is often referred to as IP
|
|
Masquerading and you will also see the term Source Network Address
|
|
Translation (SNAT) used. Shorewall follows the convention used with
|
|
Netfilter: <itemizedlist><listitem><para><emphasis>Masquerade</emphasis>
|
|
describes the case where you let your firewall system automatically detect
|
|
the external interface address.</para></listitem><listitem><para><emphasis>SNAT</emphasis>
|
|
refers to the case when you explicitly specify the source address that you
|
|
want outbound packets from your local network to use.</para></listitem></itemizedlist>
|
|
In Shorewall, both Masquerading and SNAT are configured with entries in
|
|
the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
|
|
file.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>If your external firewall interface is <filename class="devicefile">eth0</filename>,
|
|
your local interface <filename class="devicefile">eth1</filename> and your
|
|
DMZ interface is <filename class="devicefile">eth2</filename> then you do
|
|
not need to modify the file provided with the sample. Otherwise, edit
|
|
<filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
|
|
and change it to match your configuration.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>If your external IP is static, you can enter it in the third column
|
|
in the <filename class="directory">/etc/shorewall/</filename><filename>masq</filename>
|
|
entry if you like although your firewall will work fine if you leave that
|
|
column empty. Entering your static IP in column 3 makes processing
|
|
outgoing packets a little more efficient.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>If you are using the Debian package, please check your
|
|
<filename>shorewall.conf</filename> file to ensure that the following are
|
|
set correctly; if they are not, change them appropriately:
|
|
<itemizedlist><listitem><para><varname>NAT_ENABLED=Yes</varname>
|
|
(Shorewall versions earlier than 1.4.6)</para></listitem><listitem><para><varname>IP_FORWARDING=On</varname></para></listitem></itemizedlist></para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Port Forwarding (DNAT)</title>
|
|
|
|
<para>One of your goals will be to run one or more servers on your DMZ
|
|
computers. Because these computers have RFC-1918 addresses, it is not
|
|
possible for clients on the internet to connect directly to them. It is
|
|
rather necessary for those clients to address their connection requests to
|
|
your firewall who rewrites the destination address to the address of your
|
|
server and forwards the packet to that server. When your server responds,
|
|
the firewall automatically performs SNAT to rewrite the source address in
|
|
the response.</para>
|
|
|
|
<para>The above process is called <emphasis>Port Forwarding</emphasis> or
|
|
<emphasis>Destination Network Address Translation</emphasis> (DNAT). You
|
|
configure port forwarding using DNAT rules in the <filename
|
|
class="directory">/etc/shorewall/</filename><filename>rules</filename>
|
|
file.</para>
|
|
|
|
<para>The general form of a simple port forwarding rule in <filename
|
|
class="directory">/etc/shorewall/</filename><filename>rules</filename> is:
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
DNAT net dmz:<emphasis><server local ip address></emphasis>[:<emphasis><server port></emphasis>] <emphasis><protocol></emphasis> <emphasis><port></emphasis></programlisting>
|
|
If you don't specify the <emphasis><varname><server port></varname></emphasis>,
|
|
it is assumed to be the same as <emphasis><varname><port></varname></emphasis>.</para>
|
|
|
|
<example>
|
|
<title>You run a Web Server on DMZ Computer 2 and you want to forward
|
|
incoming TCP port 80 to that system</title>
|
|
|
|
<para><programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
DNAT net dmz:10.10.11.2 tcp 80
|
|
ACCEPT loc dmz:10.10.11.2 tcp 80</programlisting><itemizedlist><listitem><para>Entry
|
|
1 forwards port 80 from the Internet.</para></listitem><listitem><para>Entry
|
|
2 allows connections from the local network.</para></listitem></itemizedlist>
|
|
Several important points to keep in mind:<itemizedlist><listitem><para>When
|
|
you are connecting to your server from your local systems, you must use
|
|
the server's internal IP address (<systemitem class="ipaddress">10.10.11.2</systemitem>).</para></listitem><listitem><para>Many
|
|
ISPs block incoming connection requests to port 80. If you have problems
|
|
connecting to your web server, try the following rule and try connecting
|
|
to port 5000 (e.g., connect to <literal>http://w.x.y.z:5000 where
|
|
w.x.y.z</literal> is your external IP).<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE
|
|
# PORT(S)
|
|
DNAT net dmz:10.10.11.2:80 tcp 80 5000</programlisting></para></listitem><listitem><para>If
|
|
you want to be able to access your server from the local network using
|
|
your external address, then if you have a static external IP you can
|
|
replace the loc->dmz rule above with:<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
|
|
# PORT(S) DEST
|
|
DNAT loc dmz:10.10.11.2 tcp 80 - <emphasis><external ip></emphasis></programlisting>If
|
|
you have a dynamic ip then you must ensure that your external interface
|
|
is up before starting Shorewall and you must take steps as follows
|
|
(assume that your external interface is <filename class="devicefile">eth0</filename>):<orderedlist><listitem><para>Include
|
|
the following in /etc/shorewall/params:</para><para><command>ETH0_IP=$(find_interface_address
|
|
eth0)</command></para></listitem><listitem><para>Make your
|
|
<literal>loc->dmz</literal> rule: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
|
|
# PORT(S) DEST
|
|
DNAT loc dmz:10.10.11.2 tcp 80 - $ETH0_IP</programlisting></para></listitem></orderedlist></para></listitem><listitem><para>If
|
|
you want to access your server from the DMZ using your external IP
|
|
address, see <ulink url="FAQ.htm#faq2a">FAQ 2a</ulink>.</para></listitem></itemizedlist></para>
|
|
</example>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>At this point, add the DNAT and ACCEPT rules for your servers.</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Domain Name Server (DNS)</title>
|
|
|
|
<para>Normally, when you connect to your ISP, as part of getting an IP
|
|
address your firewall's <emphasis>Domain Name Service</emphasis> (DNS)
|
|
resolver will be automatically configured (e.g., the <filename>/etc/resolv.conf</filename>
|
|
file will be written). Alternatively, your ISP may have given you the IP
|
|
address of a pair of DNS name servers for you to manually configure as
|
|
your primary and secondary name servers. It is your responsibility to
|
|
configure the resolver in your internal systems. You can take one of two
|
|
approaches: <itemizedlist><listitem><para>You can configure your internal
|
|
systems to use your ISP's name servers. If you ISP gave you the
|
|
addresses of their servers or if those addresses are available on their
|
|
web site, you can configure your internal systems to use those addresses.
|
|
If that information isn't available, look in <filename>/etc/resolv.conf</filename>
|
|
on your firewall system -- the name servers are given in <quote>nameserver</quote>
|
|
records in that file.</para></listitem><listitem><para><inlinegraphic
|
|
fileref="images/BD21298_.gif" format="GIF" /></para><para>You can
|
|
configure a <emphasis>Caching Name Server</emphasis> on your firewall or
|
|
in your DMZ. <trademark>Red Hat</trademark> has an RPM for a caching name
|
|
server (which also requires the '<command>bind</command>' RPM) and
|
|
for Bering users, there is <filename>dnscache.lrp</filename>. If you take
|
|
this approach, you configure your internal systems to use the caching name
|
|
server as their primary (and only) name server. You use the internal IP
|
|
address of the firewall (<systemitem class="ipaddress">10.10.10.254</systemitem>
|
|
in the example above) for the name server address if you choose to run the
|
|
name server on your firewall. To allow your local systems to talk to your
|
|
caching name server, you must open port 53 (both UDP and TCP) from the
|
|
local network to the server; you do that by adding the rules in
|
|
<filename>/etc/shorewall/rules</filename>.</para></listitem></itemizedlist>
|
|
If you run the name server on the firewall:
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT loc fw tcp 53
|
|
ACCEPT loc fw udp 53
|
|
ACCEPT dmz fw tcp 53
|
|
ACCEPT dmz fw udp 53 </programlisting>
|
|
Run name server on DMZ computer 1: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT loc dmz:10.10.11.1 tcp 53
|
|
ACCEPT loc dmz:10.10.11.1 udp 53
|
|
ACCEPT dmz dmz:10.10.11.1 tcp 53
|
|
ACCEPT dmz dmz:10.10.11.1 udp 53 </programlisting></para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Other Connections</title>
|
|
|
|
<para>The three-interface sample includes the following rules:
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT fw net udp 53
|
|
ACCEPT fw net tcp 53 </programlisting>Those
|
|
rules allow DNS access from your firewall and may be removed if you
|
|
commented out the line in <filename>/etc/shorewall/policy</filename>
|
|
allowing all connections from the firewall to the internet.</para>
|
|
|
|
<para>The sample also includes: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT loc fw tcp 22
|
|
ACCEPT loc fw tcp 22 </programlisting>That
|
|
rule allows you to run an SSH server on your firewall and in each of your
|
|
DMZ systems and to connect to those servers from your local systems.</para>
|
|
|
|
<para>If you wish to enable other connections between your systems, the
|
|
general format is: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT <emphasis><source zone> <destination zone> <protocol> <port> </emphasis></programlisting></para>
|
|
|
|
<example>
|
|
<title>You want to run a publicly-available DNS server on your firewall
|
|
system</title>
|
|
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT net fw tcp 53
|
|
ACCEPT net fw udp 53 </programlisting>
|
|
|
|
<para>Those two rules would of course be in addition to the rules listed
|
|
above under "If you run the name server on your firewall".</para>
|
|
</example>
|
|
|
|
<para>If you don't know what port and protocol a particular
|
|
application uses, <ulink url="ports.htm">look here</ulink>.</para>
|
|
|
|
<important>
|
|
<para>I don't recommend enabling telnet to/from the internet because
|
|
it uses clear text (even for login!). If you want shell access to your
|
|
firewall from the internet, use SSH: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT net fw tcp 22</programlisting></para>
|
|
</important>
|
|
|
|
<para><inlinegraphic fileref="images/leaflogo.gif" format="GIF" /> Bering
|
|
users will want to add the following two rules to be compatible with
|
|
Jacques's Shorewall configuration: <programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
|
|
ACCEPT loc fw udp 53
|
|
ACCEPT net fw tcp 80 </programlisting><itemizedlist><listitem><para>Entry
|
|
1 allows the DNS Cache to be used.</para></listitem><listitem><para>Entry
|
|
2 allows the <quote>weblet</quote> to work.</para></listitem></itemizedlist><inlinegraphic
|
|
fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>Now modify <filename>/etc/shorewall/rules</filename> to add or
|
|
remove other connections as required.</para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Starting and Stopping Your Firewall</title>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>The <ulink url="Install.htm">installation procedure</ulink>
|
|
configures your system to start Shorewall at system boot but beginning
|
|
with Shorewall version 1.3.9 startup is disabled so that your system
|
|
won't try to start Shorewall before configuration is complete. Once
|
|
you have completed configuration of your firewall, you can enable
|
|
Shorewall startup by removing the file <filename>/etc/shorewall/startup_disabled</filename>.
|
|
<important><para>Users of the <filename>.deb</filename> package must edit
|
|
<filename>/etc/default/shorewall</filename> and set <varname>startup=1</varname>.</para></important>
|
|
The firewall is started using the <command>shorewall start</command>
|
|
command and stopped using <command>shorewall stop</command>. When the
|
|
firewall is stopped, routing is enabled on those hosts that have an entry
|
|
in <ulink url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
|
|
A running firewall may be restarted using the <command>shorewall restart</command>
|
|
command. If you want to totally remove any trace of Shorewall from your
|
|
Netfilter configuration, use <command>shorewall clear</command>.</para>
|
|
|
|
<para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para>
|
|
|
|
<para>The three-interface sample assumes that you want to enable routing
|
|
to/from <filename class="devicefile">eth1</filename> (your local network)
|
|
and <filename class="devicefile">eth2</filename> (DMZ) when Shorewall is
|
|
stopped. If these two interfaces don't connect to your local network
|
|
and DMZ or if you want to enable a different set of hosts, modify
|
|
<filename>/etc/shorewall/routestopped</filename> accordingly.
|
|
<warning><para>If you are connected to your firewall from the internet, do
|
|
not issue a <command>shorewall stop</command> command unless you have
|
|
added an entry for the IP address that you are connected from to <ulink
|
|
url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>.
|
|
Also, I don't recommend using <command>shorewall restart</command>; it
|
|
is better to create an <ulink url="configuration_file_basics.htm#Configs">alternate
|
|
configuration</ulink> and test it using the <ulink
|
|
url="starting_and_stopping_shorewall.htm"><command>shorewall try</command>
|
|
command</ulink>.</para></warning></para>
|
|
</section>
|
|
|
|
<section>
|
|
<title>Additional Recommended Reading</title>
|
|
|
|
<para>I highly recommend that you review the <ulink
|
|
url="configuration_file_basics.htm">Common Configuration File Features</ulink>
|
|
page -- it contains helpful tips about Shorewall features than make
|
|
administering your firewall easier.</para>
|
|
</section>
|
|
</article> |