extern/shorewall_code
1
0
Fork 1
mirror of https://gitlab.com/shorewall/code.git synced 2024-12-26 08:08:59 +01:00
Code Issues Packages Projects Releases Wiki Activity
3f3e59f0ed
shorewall_code/Shorewall-docs/two-interface.htm
teastep c2ccd7fd3d Shorewall 1.4.8 
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@800 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
2003-12-02 23:51:46 +00:00

957 lines
36 KiB
HTML
Raw Blame History

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Language" content="en-us">
<meta name="GENERATOR" content="Microsoft FrontPage 5.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<meta http-equiv="Content-Type"
content="text/html; charset=windows-1252">
<title>Two-Interface Firewall</title>
<meta name="Microsoft Theme" content="none">
</head>
<body>
<h1 style="text-align: center;">Basic Two-Interface Firewall<br>
</h1>
<p align="left">Setting up a Linux system as a firewall for a small
network is a fairly straight-forward task if you understand the basics
and follow the documentation.</p>
<p>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 its most common configuration:</p>
<ul>
<li>Linux system used as a firewall/router for a small local network.</li>
<li style="font-weight: bold;">Single public IP address. If you have
more than one public IP address, this is not the guide you want -- see
the <a href="shorewall_setup_guide.htm">Shorewall Setup Guide</a>
instead.</li>
<li>Internet connection through cable modem, DSL, ISDN, Frame Relay,
dial-up ...</li>
</ul>
<p align="left">Here is a schematic of a typical installation.</p>
<p align="center"> <img border="0" src="images/basics.png" width="444"
height="635"> </p>
<p><b>If you are running Shorewall under Mandrake 9.0 or later, you can
easily configure the above setup using the Mandrake "Internet
Connection
Sharing" applet. From the Mandrake Control Center, select "Network
&amp; Internet" then "Connection Sharing".<br>
</b></p>
<p><b>Note however, that the Shorewall configuration produced by
Mandrake Internet Connection Sharing is strange and is apt to confuse
you if you use the rest of this documentation (it has two local zones;
"loc" and "masq" where "loc" is empty; this conflicts with this
documentation which assumes a single local zone "loc"). We therefore
recommend that once you have set up this sharing that you uninstall the
Mandrake Shorewall RPM and install the one from the <a
href="download.htm">download page</a> then follow the instructions in
this Guide.</b><br>
</p>
<p>Shorewall requires that you have the iproute/iproute2 package
installed (on RedHat, the package is called <i>iproute</i>)<i>. </i>You
can tell if this package is installed by the presence of an <b>ip</b>
program on your firewall system. As root, you can use the 'which'
command to check for this program:</p>
<pre> [root@gateway root]# which ip<br> /sbin/ip<br> [root@gateway root]#</pre>
<p>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. Points at which configuration changes are
recommended are flagged with <img border="0" src="images/BD21298_.gif"
width="13" height="13"> . Configuration notes that are unique to
LEAF/Bering are marked with&nbsp;<img src="images/leaflogo.gif"
alt="(LEAF Logo)" width="49" height="36"> </p>
<p><img border="0" src="images/j0213519.gif" width="60" height="60">
&nbsp;&nbsp;&nbsp; If you edit your configuration files on a Windows
system, you must save them as Unix files if your editor supports that
option or you must run them through dos2unix before trying
to use them. Similarly, if you copy a configuration file from your
Windows hard drive to a floppy disk, you must run dos2unix against the
copy before using it with Shorewall.</p>
<ul>
<li><a href="http://www.simtel.net/pub/pd/51438.html">Windows Version
of dos2unix</a></li>
<li><a href="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux
Version
of dos2unix</a></li>
</ul>
<h2 align="left">PPTP/ADSL</h2>
<img style="border: 0px solid ; width: 13px; height: 13px;"
src="images/BD21298_3.gif" title="" alt="">&nbsp;&nbsp;&nbsp; If you
have an ADSL Modem and you use PPTP to communicate with a server in
that modem, you must make the <a href="PPTP.htm#PPTP_ADSL">changes
recommended here</a> in addition to those detailed below. ADSL with
PPTP is most commonly found in Europe, notably in Austria.<br>
<h2 align="left">Shorewall Concepts</h2>
<p> <img border="0" src="images/BD21298_.gif" width="13" height="13"
alt=""> &nbsp;&nbsp;&nbsp; The configuration files for Shorewall are
contained in the directory /etc/shorewall -- for simple setups, you
will only need to deal with a few of these as described in this guide.
After you have <a href="Install.htm">installed Shorewall</a>, <b>download
the <a href="http://www1.shorewall.net/pub/shorewall/Samples/">two-interface
sample</a>, un-tar it (tar -zxvf two-interfaces.tgz) and and copy the
files
to /etc/shorewall (these files will replace files with the same
name).</b></p>
<p>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.</p>
<p>Shorewall views the network where it is running as being composed of
a set of <i>zones.</i> In the two-interface sample configuration, the
following zone names are used:</p>
<table border="0" style="border-collapse: collapse;" cellpadding="3"
cellspacing="0" id="AutoNumber2">
<tbody>
<tr>
<td><u><b>Name</b></u></td>
<td><u><b>Description</b></u></td>
</tr>
<tr>
<td><b>net</b></td>
<td><b>The Internet</b></td>
</tr>
<tr>
<td><b>loc</b></td>
<td><b>Your Local Network</b></td>
</tr>
</tbody>
</table>
<p>Zones are defined in the <a href="Documentation.htm#Zones">
/etc/shorewall/zones</a> file.</p>
<p>Shorewall also recognizes the firewall system as its own zone - by
default, the firewall itself is known as <b>fw.</b></p>
<p>Rules about what traffic to allow and what traffic to deny are
expressed in terms of zones.</p>
<ul>
<li>You express your default policy for connections from one zone to
another zone in the<a href="Documentation.htm#Policy">
/etc/shorewall/policy </a>file.</li>
<li>You define exceptions to those default policies in the <a
href="Documentation.htm#Rules">/etc/shorewall/rules </a>file.</li>
</ul>
<p>For each connection request entering the firewall, the request is
first checked against the /etc/shorewall/rules file. If no rule in
that file matches the connection request then the first policy
in /etc/shorewall/policy that matches the request is applied.
If that policy is REJECT or DROP&nbsp; the request is first checked
against
the rules in /etc/shorewall/common if that file exists; otherwise the
rules in /etc/shorewall/common.def are checked.</p>
<p>The /etc/shorewall/policy file included with the two-interface
sample
has the following policies:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>Source Zone</b></u></td>
<td><u><b>Destination Zone</b></u></td>
<td><u><b>Policy</b></u></td>
<td><u><b>Log Level</b></u></td>
<td><u><b>Limit:Burst</b></u></td>
</tr>
<tr>
<td>loc</td>
<td>net</td>
<td>ACCEPT</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>net</td>
<td>all</td>
<td>DROP</td>
<td>info</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>all</td>
<td>all</td>
<td>REJECT</td>
<td>info</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<blockquote>
<p>In the two-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.</p>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>Source Zone</b></u></td>
<td><u><b>Destination Zone</b></u></td>
<td><u><b>Policy</b></u></td>
<td><u><b>Log Level</b></u></td>
<td><u><b>Limit:Burst</b></u></td>
</tr>
<tr>
<td>fw</td>
<td>net</td>
<td>ACCEPT</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<p>The above policy will:</p>
<ol>
<li>allow all connection requests from your local network to the
internet</li>
<li>drop (ignore) all connection requests from the internet to your
firewall or local network</li>
<li>optionally accept all connection requests from the firewall to
the internet (if you uncomment the additional policy)</li>
<li>reject all other connection requests.</li>
</ol>
<p><img border="0" src="images/BD21298_.gif" width="13" height="13">
&nbsp;&nbsp;&nbsp; At this point, edit your /etc/shorewall/policy
and make any changes that you wish.</p>
<h2 align="left">Network Interfaces</h2>
<p align="center"> <img border="0" src="images/basics.png" width="444"
height="635"> </p>
<p align="left">The firewall has two network interfaces. Where Internet
connectivity
is through a cable or DSL "Modem", the <i>External Interface</i> will
be
the ethernet adapter that is connected to that "Modem" (e.g., <b>eth0</b>)&nbsp;
<u>unless</u> you connect via <i><u>P</u>oint-to-<u>P</u>oint
<u>P</u>rotocol over <u>E</u>thernet</i> (PPPoE) or <i><u>P</u>oint-to-<u>P</u>oint
<u>T</u>unneling <u>P</u>rotocol </i>(PPTP) in which case the
External Interface will be a ppp interface (e.g., <b>ppp0</b>). If you
connect via a regular modem, your External Interface will also be <b>ppp0</b>.
If you connect via ISDN, your external interface will be <b>ippp0.</b></p>
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; If your external interface is <b>ppp0</b>
or<b> ippp0</b>&nbsp; then you will want to set CLAMPMSS=yes in <a
href="Documentation.htm#Conf"> /etc/shorewall/shorewall.conf.</a></p>
<p align="left">Your <i>Internal Interface</i> will be an ethernet
adapter (eth1 or eth0) and will be connected to a hub or switch. Your
other computers will be connected to the same hub/switch (note:
If you have only a single internal system, you can connect the firewall
directly to the computer using a <i>cross-over </i> cable).</p>
<p align="left"><u><b> <img border="0" src="images/j0213519.gif"
width="60" height="60"> </b></u>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.&nbsp; When using these
recent versions, you can test using this kind of configuration if you
specify the <span style="font-weight: bold;">arp_filter</span> option
in /etc/shorewall/interfaces for all interfaces connected to the common
hub/switch. Using such a setup with a production firewall is strongly
recommended against.<br>
</p>
<p align="left"><img border="0" src="images/BD21298_.gif" align="left"
width="13" height="13"> &nbsp;&nbsp;&nbsp; The Shorewall two-interface
sample configuration assumes that the external interface is <b>eth0</b>
and the internal interface is <b>eth1</b>. If your configuration is
different, you will have to modify the sample <a
href="Documentation.htm#Interfaces">/etc/shorewall/interfaces</a> file
accordingly. While you are there, you may wish to review the list of
options that are specified for the interfaces. Some hints:</p>
<ul>
<li>
<p align="left">If your external interface is <b>ppp0</b> or <b>ippp0</b>,
you can replace the "detect" in the second column with "-". </p>
</li>
<li>
<p align="left">If your external interface is <b>ppp0</b> or <b>ippp0</b>
or if you have a static IP address, you can remove "dhcp" from the
option list. </p>
</li>
</ul>
<h2 align="left">IP Addresses</h2>
<p align="left">Before going further, we should say a few words about
Internet Protocol (IP) <i>addresses</i>. Normally, your ISP will
assign you a single <i> Public</i> IP address. This address may be
assigned via the<i> Dynamic Host Configuration Protocol</i> (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<i> static</i> IP address; that means that you configure
your firewall's external interface to use that address permanently.<i> </i>However
your external 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 in your internal network (the Internal Interface on your
firewall plus your other computers). RFC 1918 reserves several <i>Private
</i>IP address ranges for this purpose:</p>
<div align="left">
<pre> 10.0.0.0 - 10.255.255.255<br> 172.16.0.0 - 172.31.255.255<br> 192.168.0.0 - 192.168.255.255</pre>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; 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 'norfc1918' option from the
external interface's entry in /etc/shorewall/interfaces.</p>
</div>
<div align="left">
<p align="left">You will want to assign your addresses from the same <i>
sub-network </i>(<i>subnet)</i>.&nbsp; For our purposes, we can
consider a subnet to consists of a range of addresses x.y.z.0 -
x.y.z.255. Such a subnet will have a <i>Subnet Mask </i>of
255.255.255.0. The
address x.y.z.0 is reserved as the <i>Subnet Address</i> and x.y.z.255
is reserved as the <i>Subnet Broadcast</i> <i>Address</i>. In
Shorewall, a subnet is described using&nbsp;<a
href="shorewall_setup_guide.htm#Subnets"><i>Classless InterDomain
Routing </i>(CIDR) notation</a> with consists of the subnet address
followed by "/24". The "24" refers to the number of consecutive leading
"1" bits from the left of the subnet mask. </p>
</div>
<div align="left">
<p align="left">Example sub-network:</p>
</div>
<div align="left">
<blockquote>
<table border="1" style="border-collapse: collapse;" id="AutoNumber1"
cellpadding="2">
<tbody>
<tr>
<td><b>Range:</b></td>
<td>10.10.10.0 - 10.10.10.255</td>
</tr>
<tr>
<td><b>Subnet Address:</b></td>
<td>10.10.10.0</td>
</tr>
<tr>
<td><b>Broadcast Address:</b></td>
<td>10.10.10.255</td>
</tr>
<tr>
<td><b>CIDR&nbsp;Notation:</b></td>
<td>10.10.10.0/24</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">It is conventional to assign the internal interface
either the first usable address in the subnet (10.10.10.1 in the above
example) or the last usable address (10.10.10.254).</p>
</div>
<div align="left">
<p align="left">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<i>&nbsp; gateway</i>&nbsp;
(router).</p>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_1.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; Your local computers (computer 1 and
computer 2 in the above diagram) should be configured with their<i>
default gateway</i> to be the IP address of the firewall's internal
interface.<i>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </i> </p>
</div>
<p align="left">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 <i>"IP
Fundamentals: What Everyone Needs to Know about Addressing &amp;
Routing",</i> Thomas A. Maufer, Prentice-Hall, 1999, ISBN 0-13-975483-0.</p>
<p align="left">The remainder of this quide will assume that you have
configured your network as shown here:</p>
<p align="center"> <img border="0" src="images/basics1.png" width="444"
height="635"> </p>
<p align="left">The default gateway for computer's 1 &amp; 2 would be
10.10.10.254.<br>
</p>
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13" alt=""> &nbsp;&nbsp;&nbsp; <font color="#ff0000"><b>WARNING:
</b></font><b>Your ISP might assign your external interface an RFC 1918
address. If that address
is in the 10.10.10.0/24 subnet then you will need to select a DIFFERENT
RFC 1918 subnet for your local network.</b><br>
</p>
<h2 align="left">IP Masquerading (SNAT)</h2>
<p align="left">The addresses reserved by RFC 1918 are sometimes
referred to as <i>non-routable</i> 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 computer 1) sends
a
connection request to an internet host, the firewall must perform
<i>Network Address Translation </i>(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.&nbsp; 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 across the internet so the
remote host can't address its response to computer 1). When the
firewall
receives a return packet, it rewrites the destination address back to
10.10.10.1 and forwards the packet on to computer 1. </p>
<p align="left">On Linux systems, the above process is often referred
to
as<i> IP Masquerading</i> but you will also see the term <i>Source
Network
Address Translation </i>(SNAT) used. Shorewall follows the convention
used
with Netfilter:</p>
<ul>
<li>
<p align="left"><i>Masquerade</i> describes the case where you let
your firewall system automatically detect the external interface
address. </p>
</li>
<li>
<p align="left"><i>SNAT</i> refers to the case when you explicitly
specify the source address that you want outbound packets from your
local network to use. </p>
</li>
</ul>
<p align="left">In Shorewall, both Masquerading and SNAT are configured
with entries in the /etc/shorewall/masq file. You will normally use
Masquerading if your external IP is dynamic and SNAT if the IP
is static.</p>
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; If your external firewall interface is
<b>eth0</b>, you do not need to modify the file provided with the
sample. Otherwise, edit /etc/shorewall/masq and change the first column
to the name of your external interface and the second column to the
name of your internal interface.</p>
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; If your external IP is static, you can
enter it in the third column in the /etc/shorewall/masq 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.<br>
<br>
<img border="0" src="images/BD21298_.gif" width="13" height="13" alt="">
&nbsp;&nbsp;&nbsp; If you are using the Debian package, please check
your
shorewall.conf file to ensure that the following are set correctly;
if they are not, change them appropriately:<br>
</p>
<ul>
<li>NAT_ENABLED=Yes (Shorewall versions earlier than 1.4.6)</li>
<li>IP_FORWARDING=On<br>
</li>
</ul>
<h2 align="left">Port Forwarding (DNAT)</h2>
<p align="left">One of your goals may be to run one or more servers on
your local 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 the 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.</p>
<p align="left">The above process is called<i> Port Forwarding</i> or <i>
Destination Network Address Translation</i> (DNAT). You configure port
forwarding using DNAT rules in the /etc/shorewall/rules file.</p>
<p>The general form of a simple port forwarding rule in
/etc/shorewall/rules is:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:<i>&lt;server local ip address&gt; </i>[:<i>&lt;server
port&gt;</i>]</td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port&gt;</i></td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<p>Example 1 - you run a Web Server on computer 2 and you want to
forward
incoming TCP port 80 to that system:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:10.10.10.2</td>
<td>tcp</td>
<td>80</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<p>Example 2 - you run an FTP Server on computer 1 so you want to
forward
incoming TCP port 21 to that system:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:10.10.10.1</td>
<td>tcp</td>
<td>21<br>
</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<p>For FTP, you will also need to have FTP connection tracking and NAT
support
in your kernel. For vendor-supplied kernels, this means that the
ip_conntrack_ftp
and ip_nat_ftp modules must be loaded. Shorewall will automatically
load
these modules if they are available and located in the standard place
under
/lib/modules/&lt;<i>kernel version</i>&gt;/kernel/net/ipv4/netfilter.<br>
</p>
<p>A couple of important points to keep in mind:</p>
<ul>
<li>You must test the above rule from a client outside of your local
network (i.e., don't test from a browser running on computers 1 or 2 or
on the firewall). If you want to be able to access your web server
and/or FTP server from inside your firewall using the IP address of
your external interface, see <a href="FAQ.htm#faq2">Shorewall FAQ #2</a>.</li>
<li>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.</li>
</ul>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:10.10.10.2:80</td>
<td>tcp</td>
<td>5000</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<p> <img border="0" src="images/BD21298_.gif" width="13" height="13">
&nbsp;&nbsp;&nbsp; At this point, modify /etc/shorewall/rules to
add any DNAT rules that you require.</p>
<h2 align="left">Domain Name Server (DNS)</h2>
<p align="left">Normally, when you connect to your ISP, as part of
getting an IP address your firewall's <i>Domain Name Service </i>(DNS)
resolver will be automatically configured (e.g., the /etc/resolv.conf
file will be written). Alternatively, your ISP may have given you
the IP address of a pair of DNS <i> name servers</i> for you to
manually configure as your primary and secondary name servers.
Regardless
of how DNS gets configured on your firewall, it is <u>your</u>
responsibility to configure the resolver in your internal systems. You
can take
one of two approaches:</p>
<ul>
<li>
<p align="left">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 /etc/resolv.conf on your
firewall system -- the name servers are given in "nameserver" records
in that file. </p>
</li>
<li>
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; You can configure a<i> Caching Name
Server </i>on your firewall.<i> </i>Red Hat has an RPM for a caching
name server (the RPM also requires the 'bind' RPM) and for Bering
users, there is dnscache.lrp. If you take this approach, you configure
your internal systems to use the firewall itself as their primary (and
only) name server. You use the internal IP address of the firewall
(10.10.10.254 in the example above) for the name server address. 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
firewall; you do that by adding the following rules in
/etc/shorewall/rules. </p>
</li>
</ul>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>fw</td>
<td>tcp</td>
<td>53</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>fw</td>
<td>udp</td>
<td>53</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
<div align="left">
<h2 align="left">Other Connections</h2>
</div>
<div align="left">
<p align="left">The two-interface sample includes the following rules:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>net</td>
<td>tcp</td>
<td>53</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>fw</td>
<td>net</td>
<td>udp</td>
<td>53</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Those rules allow DNS access from your firewall and may
be removed if you uncommented the line in /etc/shorewall/policy
allowing all connections from the firewall to the internet.</p>
</div>
<div align="left">
<p align="left">The sample also includes:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>fw</td>
<td>tcp</td>
<td>22</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">That rule allows you to run an SSH server on your
firewall and connect to that server from your local systems.</p>
</div>
<div align="left">
<p align="left">If you wish to enable other connections between your
firewall and other systems, the general format is:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td><i>&lt;source zone&gt;</i></td>
<td><i>&lt;destination zone&gt;</i></td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port&gt;</i></td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Example - You want to run a Web Server on your firewall
system:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>fw</td>
<td>tcp</td>
<td>80</td>
<td>#Allow web access</td>
<td>from the internet</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>fw</td>
<td>tcp</td>
<td>80</td>
<td>#Allow web access</td>
<td>from the local network</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Those two rules would of course be in addition to the
rules listed above under "You can configure a Caching Name Server
on your firewall"</p>
</div>
<div align="left">
<p align="left">If you don't know what port and protocol a particular
application uses, look <a href="ports.htm">here</a>.</p>
</div>
<div align="left">
<p align="left"><b>Important: </b>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:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>net</td>
<td>fw</td>
<td>tcp</td>
<td>22</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left"><img src="images/leaflogo.gif" alt="(LEAF Logo)"
width="49" height="36"> &nbsp;&nbsp;&nbsp; Bering users will want to
add the following two rules to be
compatible with Jacques's Shorewall configuration.</p>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber4">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIGINAL ADDRESS</b></u></td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc<br>
</td>
<td>fw</td>
<td>udp<br>
</td>
<td>53<br>
</td>
<td>#Allow DNS Cache to</td>
<td>work<br>
</td>
</tr>
<tr>
<td>ACCEPT</td>
<td>loc</td>
<td>fw</td>
<td>tcp</td>
<td>80</td>
<td>#Allow weblet to work</td>
<td><br>
</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<p align="left"><br>
<img border="0" src="images/BD21298_.gif" width="13" height="13">
&nbsp;&nbsp;&nbsp; Now edit your /etc/shorewall/rules file to add or
delete other connections as required.</p>
</div>
<div align="left">
<h2 align="left">Starting and Stopping Your Firewall</h2>
</div>
<div align="left">
<p align="left"> <img border="0" src="images/BD21298_2.gif" width="13"
height="13" alt="Arrow"> &nbsp;&nbsp;&nbsp; The <a href="Install.htm">installation
procedure </a> configures your system to start Shorewall at system
boot&nbsp; 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 /etc/shorewall/startup_disabled.<br>
</p>
<p align="left"><font color="#ff0000"><b>IMPORTANT</b>: </font><font
color="#ff0000">Users of the .deb package must edit
/etc/default/shorewall and set 'startup=1'.</font><br>
</p>
</div>
<div align="left">
<p align="left">The firewall is started using the "shorewall start"
command and stopped using "shorewall stop". When the firewall is
stopped, routing is enabled on those hosts that have an entry in <a
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped</a>.
A running firewall may be restarted using the "shorewall restart"
command. If you want to totally remove any trace of Shorewall from your
Netfilter configuration, use "shorewall clear".</p>
</div>
<div align="left">
<p align="left"><img border="0" src="images/BD21298_.gif" width="13"
height="13"> &nbsp;&nbsp;&nbsp; The two-interface sample assumes that
you want
to enable routing to/from <b>eth1 </b>(the local network) when
Shorewall is stopped. If your local network isn't connected to <b>eth1</b>
or if you wish to enable access to/from other hosts, change
/etc/shorewall/routestopped accordingly.</p>
</div>
<div align="left">
<p align="left"><b>WARNING: </b>If you are connected to your firewall
from the internet, do not issue a "shorewall stop" command unless you
have added an entry for the IP address that you are connected from to <a
href="Documentation.htm#Routestopped">/etc/shorewall/routestopped</a>.
Also, I don't recommend using "shorewall restart"; it is better
to create an <i><a href="configuration_file_basics.htm#Configs">alternate
configuration</a></i> and test it using the <a
href="starting_and_stopping_shorewall.htm">"shorewall try" command</a>.<br>
</p>
<h2>Additional Recommended Reading</h2>
I highly recommend that you review the <a
href="configuration_file_basics.htm">Common Configuration File
Features page</a> -- it contains helpful tips about Shorewall features
than make administering your firewall easier.
</div>
<p align="left"><font size="2">Last updated 11/15/2003 - <a
href="support.htm">Tom Eastep</a></font></p>
<p align="left"><a href="copyright.htm"><font size="2">Copyright 2002,
2003 Thomas M. Eastep</font></a><br>
</p>
</body>
</html>
Reference in New Issue View Git Blame Copy Permalink