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<h1 style="text-align: center;">Shorewall FAQs<br>
</h1>
<h2>Looking for Step by Step Configuration Instructions? Check out the <a
href="shorewall_quickstart_guide.htm">QuickStart Guides</a>. <br>
</h2>
<h1>PORT FORWARDING<br>
</h1>
<p align="left"><b>1. </b><a href="#faq1"> I want to <b>forward</b>
UDP <b> port</b> 7777 to my my personal PC with IP address
192.168.1.5. I've looked everywhere and can't find <b>how to do it</b>.</a></p>
<p align="left"><b>1a. </b><a href="#faq1a">Ok -- I followed those
instructions but it doesn't work.<br>
</a></p>
<p align="left"><b>1b. </b><a href="#faq1b">I'm still having problems
with port forwarding</a></p>
<p align="left"><b>1c. </b><a href="#faq1c">From the internet, I want
to <b>connect
to port 1022</b> on my firewall and have the <b>firewall forward the
connection
to port 22 on local system 192.168.1.3</b>. How do I do that?</a><br>
</p>
<p align="left"><span style="font-weight: bold;">30.<a
href="FAQ.htm#faq30"> </a></span><a href="FAQ.htm#faq30">I'm confused
about <span style="font-weight: bold;">when</span>
to use <span style="font-weight: bold;">DNAT</span> rules <span
style="font-weight: bold;">and when</span> to use <span
style="font-weight: bold;">ACCEPT</span> rules. </a> </p>
<h1><b>DNS and PORT FORWARDING/NAT<br>
</b></h1>
<p align="left"><b>2.</b> <a href="#faq2">I <b>port forward</b> www
requests to www.mydomain.com (IP 130.151.100.69)
to system 192.168.1.5 in my local network. <b>External clients can
browse</b> http://www.mydomain.com but <b>internal clients can't</b>.</a></p>
<p align="left"><b>2a. </b><a href="#faq2a">I have a zone "Z" with an
RFC1918 subnet and I use <b>one-to-one NAT</b> to
assign non-RFC1918 addresses to hosts in Z.
Hosts in Z cannot communicate with each other using their external
(non-RFC1918 addresses) so they <b>can't access
each other using their DNS names.</b></a></p>
<h1><b>NETMEETING/MSN<br>
</b></h1>
<p align="left"><b>3. </b><a href="#faq3">I want to use <b>Netmeeting</b>
or <b>MSN Instant Messenger </b>with Shorewall. What do I do?</a></p>
<h1><b>OPEN PORTS<br>
</b></h1>
<p align="left"><b>4. </b><a href="#faq4">I just used an online port
scanner to check my firewall and it shows <b>some ports as 'closed'
rather than 'blocked'.</b> Why?</a></p>
<p align="left"><b>4a. </b><a href="#faq4a">I just ran an <b>nmap UDP
scan</b> of my firewall and it showed 100s of ports as open!!!!<br>
</a></p>
<b>4b</b>. <a href="#faq4b">I have a port that I can't close no matter
how I change my rules. <br>
</a><b><br>
4c. </b><a href="#faq4c">How to I use Shorewall with <b>PortSentry</b>?</a><br>
<h1>CONNECTION PROBLEMS</h1>
<p align="left"><b>5. </b><a href="#faq5">I've installed Shorewall and
now I <b> can't ping</b> through the firewall</a><br>
<b><br>
15. </b><a href="#faq15"><b>My local systems can't see out to the net<br>
</b></a></p>
<b>29. <a href="#faq29">FTP Doesn't Work</a></b><br>
<h1>LOGGING<br>
</h1>
<p align="left"><b>6. </b><a href="#faq6">Where are the <b>log
messages</b> written and how do I <b>change the destination</b>?</a></p>
<p align="left"><b>6a. </b><a href="#faq6a">Are there any <b>log
parsers</b> that work with Shorewall?</a></p>
<p align="left"><b>6b. <a href="#faq6b">DROP messages</a></b><a
href="#faq6b"> on port 10619 are <b>flooding the logs</b> with their
connect requests. Can i exclude these error messages for this
port temporarily from logging in Shorewall?</a><br>
</p>
<p align="left"><b>6c. </b><a href="#faq6c">All day long I get a
steady flow of these <b>DROP messages from port 53</b> <b>to some
high numbered port</b>. They get dropped, but what the
heck are they?</a><br>
</p>
<p align="left"><b>6d.</b> <a href="#faq6d">Why is the <b>MAC address</b>
in Shorewall log messages <b>so long</b>? I thought MAC addresses were
only 6 bytes in length.</a><b><br>
</b></p>
<p align="left"><b>16. </b><a href="#faq16">Shorewall is writing <b>log
messages all over my console</b> making it unusable!<br>
</a></p>
<b>17</b>. <a href="#faq17">How do I find out <b>why this traffic is</b>
getting <b>logged?</b></a><br>
<b><br>
21. </b><a href="#faq21">I see these <b>strange log entries </b>occasionally;
what are they?</a><br>
<h1>ROUTING</h1>
<span style="font-weight: bold;">32. </span><a href="#faq32">My
firewall has <span style="font-weight: bold;">two connections to the
internet from two different ISPs</span>. How do I set this up in
Shorewall?</a><br>
<h1>STARTING AND STOPPING<br>
</h1>
<p align="left"><b>7. </b><a href="#faq7">When I stop Shorewall <b>using
'shorewall stop', I can't connect to anything</b>. Why doesn't that
command work?</a></p>
<p align="left"><b>8. </b><a href="#faq8">When I try to <b>start
Shorewall on RedHat</b> I get messages about insmod failing -- what's
wrong?<br>
</a></p>
<p align="left"><b>8a. </b><a href="#faq8a">When I try to <b>start
Shorewall on RedHat</b> I get a message referring me to <b>FAQ #8</b></a><br>
</p>
<p align="left"><b>9. </b><a href="FAQ.htm#faq9">Why can't Shorewall <b>detect
my interfaces </b>properly at startup?</a></p>
<b>22. </b><a href="#faq22">I have some <b>iptables commands </b>that
I want to <b>run when Shorewall starts.</b> Which file do I put them
in?</a><br>
<h1>ABOUT SHOREWALL<br>
</h1>
<p align="left"><b>10. </b><a href="#faq10">What <b>distributions</b>
does it work with?</a></p>
<p align="left"><b>11. </b><a href="#faq18">What <b>features</b> does
it
support?</a></p>
<p align="left"><b>12. </b><a href="#faq12">Is there a <b>GUI?</b></a></p>
<p align="left"><b>13. </b><a href="#faq13">Why do you call it <b>"Shorewall"?</b></a></p>
<b>23. </b><a href="#faq23">Why do you use such <b>ugly fonts</b> on
your <b>web site</b>?</a><br>
<b><br>
25. </b><a href="#faq25">How to I tell <b>which version of Shorewall</b>
I am <b>running</b>?</a><br>
<br>
<span style="font-weight: bold;">31. </span><a href="#faq31">Does
Shorewall provide protection against...</a><br>
<h1>RFC 1918<br>
</h1>
<p align="left"><b>14. </b><a href="#faq14">I'm connected via a cable
modem and it has an internel web server that
allows me to configure/monitor it but as expected
if I enable <b> rfc1918 blocking</b> for my eth0 interface, it also
blocks the <b>cable modems web server</b></a>.</p>
<p align="left"><b>14a. </b><a href="#faq14a">Even though it assigns
public IP addresses, my ISP's DHCP server has
an RFC 1918 address. If I enable RFC 1918 filtering on my external
interface, <b>my DHCP client cannot renew its lease</b>.</a></p>
<h1>ALIAS IP ADDRESSES/VIRTUAL INTERFACES<br>
</h1>
<b>18.</b> <a href="#faq18">Is there any way to use <b>aliased ip
addresses</b> with Shorewall, and maintain separate rulesets for
different IPs?</a><br>
<h1>MISCELLANEOUS<br>
</h1>
<b>19. </b><a href="#faq19">I have added <b>entries to
/etc/shorewall/tcrules</b> but they <b>don't </b>seem to <b>do
anything</b>. Why?</a><br>
<br>
<b>20. </b><a href="#faq20">I have just set up a server. <b>Do I have
to change Shorewall to allow access to my server from the internet?</b></a><br>
<br>
<b>24. </b><a href="#faq24">How
can I <b>allow conections</b> to let's say the ssh port
only<b> from specific IP Addresses</b> on the internet?</a><br>
<br>
<b>26. </b><a href="#faq26">When I try to use any of the <b>SYN
options in nmap</b> on or behind the firewall, I get "<b>operation not
permitted</b>". How can I use nmap with Shorewall?"</a><br>
<br>
<b><span style="font-weight: bold;">26a.&nbsp; </span></b><a
href="#faq26a">When I try
to use the <span style="font-weight: bold;">"-O" option of nmap</span>
from the firewall system, I get "<span style="font-weight: bold;">operation
not permitted". </span>How to I allow this option?</a><b><span
style="font-weight: bold;"><a href="#faq26a"> </a><br>
<br>
</span>27. </b><a href="#faq27">I am compiling a <b>new kernel</b>
for my
firewall<b>.</b> What should I look out for?</a><br>
<br>
<b>28. </b><a href="#faq28">How do I use Shorewall as a <b>Bridging
Firewall</b>?</a><br>
<a href="#faq30"></a><br>
<hr>
<h4 align="left"><a name="faq1"></a>1. I want to forward UDP port 7777
to my my personal PC with IP address 192.168.1.5. I've looked
everywhere and can't find how to do it.</h4>
<p align="left"><b>Answer: </b>The <a
href="Documentation.htm#PortForward"> first example</a> in the <a
href="Documentation.htm#Rules">rules file documentation</a> shows how
to do port forwarding under Shorewall. The
format of a port-forwarding rule to a local system
is as follows:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1" cellspacing="1">
<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>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:<i>&lt;local IP address&gt;</i>[:<i>&lt;local port</i>&gt;]</td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port #&gt;</i></td>
<td> <br>
</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">So to forward UDP port 7777 to internal system
192.168.1.5, the rule is:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1" cellspacing="1">
<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>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:192.168.1.5</td>
<td>udp</td>
<td>7777</td>
<td> <br>
</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<div align="left"> <font face="Courier"> </font>If you want to
forward requests directed to a particular address ( <i>&lt;external
IP&gt;</i> ) on your firewall to an internal system:</div>
<blockquote>
<table border="1" cellpadding="2" cellspacing="0"
style="border-collapse: collapse;">
<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>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:<i>&lt;local IP address&gt;</i>[:<i>&lt;local port</i>&gt;]</td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port #&gt;</i></td>
<td>-</td>
<td><i>&lt;external IP&gt;</i></td>
</tr>
</tbody>
</table>
</blockquote>
Finally, if you need to forward a range of ports, in the PORT column
specify the range as <i>low-port</i>:<i>high-port</i>.<br>
<h4 align="left"><a name="faq1a"></a>1a. Ok -- I followed those
instructions but it doesn't work</h4>
<p align="left"><b>Answer: </b>That is usually the result of one of
three things:</p>
<ul>
<li>You are trying to test from inside your firewall (no, that won't
work -- see <a href="#faq2">FAQ #2</a>).</li>
<li>You have a more basic problem with your local system (the one
that you are trying to forward to) such as an
incorrect default gateway (it should be set to the IP
address of your firewall's internal interface).</li>
<li>Your ISP is blocking that particular port inbound.<br>
</li>
</ul>
<h4 align="left"><a name="faq1b"></a>1b. I'm still having problems with
port forwarding</h4>
<b>Answer: </b>To further diagnose this problem:<br>
<ul>
<li>As root, type "iptables -t nat -Z". This clears the NetFilter
counters in the nat table.</li>
<li>Try to
connect to the redirected port from an external host.</li>
<li>As root type "shorewall show nat"</li>
<li>Locate
the appropriate DNAT rule. It will be in a chain called <i>&lt;source
zone&gt;</i>_dnat ('net_dnat' in the
above examples).</li>
<li>Is the
packet count in the first column non-zero? If so, the connection
request is reaching the firewall and is being redirected to the server.
In this case, the problem is
usually a missing or incorrect default gateway setting
on the local system (the system you are trying to forward to -- its
default gateway should be the
IP address of the firewall's interface to that system).</li>
<li>If the
packet count is zero:</li>
<ul>
<li>the connection request is not reaching your server (possibly it
is being blocked by your ISP); or</li>
<li>you are trying to connect to a secondary IP address on your
firewall and your rule is only redirecting the primary IP address (You
need to specify the secondary IP address in the
"ORIG. DEST." column in your DNAT rule); or</li>
<li>your
DNAT rule doesn't match the connection request in some other way. In
that case, you may have to use a packet sniffer such as tcpdump or
ethereal to further diagnose the problem.<br>
</li>
</ul>
</ul>
<h4 align="left"><a name="faq1c"></a><b>1c. </b>From the internet, I
want to connect to port 1022 on my firewall and have the firewall
forward the connection to port 22 on local system 192.168.1.3. How do I
do that?</h4>
In /etc/shorewall/rules:<br>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1">
<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>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net<br>
</td>
<td>loc:192.168.1.3:22</td>
<td>tcp</td>
<td>1022<br>
</td>
<td><br>
</td>
<td><br>
</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<h4 align="left"><a name="faq2"></a>2. I port forward www requests to
www.mydomain.com (IP 130.151.100.69) to system 192.168.1.5 in my local
network. External clients can browse http://www.mydomain.com but
internal clients can't.</h4>
<p align="left"><b>Answer: </b>I have two objections to this setup.</p>
<ul>
<li>Having an internet-accessible server in your local network is
like raising foxes in the corner of your hen house. If the server is
compromised, there's nothing between that server and your other
internal systems. For the cost of another NIC and a cross-over cable,
you can put your server in a DMZ such that it is isolated from your
local systems - assuming that the Server can be located near the
Firewall, of course :-)</li>
<li>The accessibility problem is best solved using <a
href="shorewall_setup_guide.htm#DNS">Bind Version 9 "views"</a> (or
using a separate DNS server for local clients) such that
www.mydomain.com resolves to 130.141.100.69 externally and 192.168.1.5
internally. That's what I do here at shorewall.net for my local systems
that use one-to-one NAT.</li>
</ul>
<p align="left">If you insist on an IP solution to the accessibility
problem rather than a DNS solution, then assuming that your external
interface is eth0 and your internal interface is eth1 and that eth1 has
IP address 192.168.1.254 with subnet 192.168.1.0/24.<br>
</p>
<p align="left">If you are running Shorewall 1.4.0 or earlier see <a
href="1.3/FAQ.htm#faq2">the 1.3 FAQ</a> for instructions suitable for
those releases.<br>
</p>
<p align="left">If you are running Shorewall 1.4.1 or Shorewall 1.4.1a,
please upgrade to Shorewall 1.4.2 or later.<br>
</p>
<p align="left">Otherwise:<br>
</p>
<ul>
</ul>
<ul>
</ul>
<ul>
<li>In /etc/shorewall/interfaces:</li>
</ul>
<blockquote>
<table cellpadding="2" border="1">
<tbody>
<tr>
<td valign="top">ZONE<br>
</td>
<td valign="top">INTERFACE<br>
</td>
<td valign="top">BROADCAST<br>
</td>
<td valign="top">OPTIONS<br>
</td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">eth1<br>
</td>
<td valign="top">detect<br>
</td>
<td valign="top"><b>routeback<br>
</b> </td>
</tr>
</tbody>
</table>
</blockquote>
<ul>
</ul>
<ul>
<li>In /etc/shorewall/rules: </li>
</ul>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;">
<tbody>
<tr>
<td><b>ACTION</b></td>
<td><b>SOURCE</b></td>
<td><b>DEST</b></td>
<td><b> PROTO</b></td>
<td><b>DEST<br>
PORT(S)</b></td>
<td><b>SOURCE<br>
PORT(S)</b></td>
<td><b>ORIGINAL<br>
DEST</b></td>
</tr>
<tr>
<td>DNAT<br>
</td>
<td>loc</td>
<td>web:192.168.1.5<br>
</td>
<td>tcp</td>
<td>www</td>
<td> -<br>
</td>
<td>130.151.100.69:192.168.1.254<br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<div align="left">
<p align="left">That rule only works of course if you have a static
external IP address. If you have a dynamic IP
address and are running Shorewall 1.3.4 or later
then include this in /etc/shorewall/init:</p>
</div>
<div align="left">
<pre> ETH0_IP=`find_interface_address eth0`</pre>
</div>
<div align="left">
<p align="left">and make your DNAT rule:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1">
<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>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>loc</td>
<td>web:192.168.1.5</td>
<td>tcp</td>
<td>www</td>
<td>-</td>
<td>$ETH0_IP:192.168.1.254</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Using this technique, you will want to configure your
DHCP/PPPoE client to automatically restart Shorewall each time that you
get a new IP address.</p>
</div>
<h4 align="left"><a name="faq2a"></a>2a. I have a zone "Z" with an
RFC1918 subnet and I use one-to-one NAT to assign non-RFC1918 addresses
to
hosts in Z. Hosts in Z cannot communicate with each other using their
external (non-RFC1918 addresses) so they can't access each other using
their DNS names.</h4>
<span style="font-weight: bold;">Note: </span>If the ALL INTERFACES
column in /etc/shorewall/nat is empty or contains "Yes", you will also
see log messages like the following when trying to access a host in Z
from another host in Z using the destination hosts's public address:<br>
<pre style="margin-left: 40px;"><font size="3"><tt>Oct&nbsp; 4 10:26:40 netgw kernel: Shorewall:FORWARD:REJECT:IN=eth1 OUT=eth1<br> SRC=192.168.118.200 DST=192.168.118.210 LEN=48 TOS=0x00 PREC=0x00 TTL=127<br> ID=1342 DF PROTO=TCP SPT=1494 DPT=1491 WINDOW=17472 RES=0x00 ACK SYN URGP=0</tt></font><br></pre>
<p align="left"><b>Answer: </b>This is another problem that is best
solved using Bind Version 9 "views". It allows
both external and internal clients to access
a NATed host using the host's DNS name.</p>
<p align="left">Another good way to approach this problem is to switch
from one-to-one NAT to Proxy ARP. That way, the
hosts in Z have non-RFC1918 addresses and can
be accessed externally and internally using the same address.</p>
<p align="left">If you don't like those solutions and prefer routing
all Z-&gt;Z
traffic through your firewall then:</p>
a) Set the Z-&gt;Z policy to ACCEPT.<br>
b)
Masquerade Z to itself.<br>
c) Set the routeback option on the interface to Z.<br>
d) Set the ALL INTERFACES column in the nat file to "Yes".<br>
<br>
<span style="font-weight: bold;">WARNING: In this configuration, all
Z-&gt;Z traffic will look to the server as if it came from the firewall
rather than from the original client! I DO NOT RECOMMEND THIS SETUP.</span><br>
<p align="left">Example:</p>
<p align="left">Zone: dmz<br>
Interface: eth2<br>
Subnet: 192.168.2.0/24</p>
<p align="left">In /etc/shorewall/interfaces:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber2">
<tbody>
<tr>
<td><u><b>ZONE</b></u></td>
<td><u><b>INTERFACE</b></u></td>
<td><u><b>BROADCAST</b></u></td>
<td><u><b>OPTIONS</b></u></td>
</tr>
<tr>
<td>dmz</td>
<td>eth2</td>
<td>192.168.2.255</td>
<td><span style="font-weight: bold;">routeback</span><br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">In /etc/shorewall/policy:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>SOURCE </b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>POLICY</b></u></td>
<td><u><b>LIMIT:BURST</b></u></td>
</tr>
<tr>
<td>dmz</td>
<td>dmz</td>
<td>ACCEPT</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">In /etc/shorewall/masq:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3" width="369">
<tbody>
<tr>
<td width="93"><u><b>INTERFACE </b></u></td>
<td width="31"><u><b>SUBNET</b></u></td>
<td width="120"><u><b>ADDRESS</b></u></td>
</tr>
<tr>
<td width="93">eth2</td>
<td width="31">192.168.2.0/24</td>
<td width="120"> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
In /etc/shorewall/nat, be sure that you have "Yes" in the ALL
INTERFACES column.<br>
<h4 align="left"><a name="faq3"></a>3. I want to use Netmeeting or MSN
Instant Messenger with Shorewall. What do I do?</h4>
<p align="left"><b>Answer: </b>There is an <a
href="http://www.kfki.hu/%7Ekadlec/sw/netfilter/newnat-suite/"> H.323
connection tracking/NAT module</a> that helps with Netmeeting. Look <a
href="http://linux-igd.sourceforge.net">here</a> for a solution for
MSN IM but be aware that there are significant security risks involved
with this solution. Also check the Netfilter mailing list archives at <a
href="http://www.netfilter.org">http://www.netfilter.org</a>. </p>
<h4 align="left"><a name="faq4"></a>4. I just used an online port
scanner to check my firewall and it shows some ports as 'closed' rather
than 'blocked'. Why?</h4>
<p align="left"><b>Answer: </b>The common.def included with version
1.3.x always rejects connection requests on TCP port 113 rather than
dropping them. This
is necessary to prevent outgoing connection problems
to services that use the 'Auth' mechanism for identifying requesting
users. Shorewall also rejects TCP ports
135, 137 and 139 as well as UDP ports 137-139. These are ports that are
used by Windows (Windows <u>can</u> be configured to use the DCE cell
locator on port 135). Rejecting these connection requests rather than
dropping them cuts down slightly on the
amount of Windows chatter on LAN segments connected to the
Firewall. </p>
<p align="left">If you are seeing port 80 being 'closed', that's
probably your ISP preventing you from running
a web server in violation of your Service Agreement.</p>
<h4 align="left"><a name="faq4a"></a>4a. I just ran an nmap UDP scan of
my firewall and it showed 100s of ports
as open!!!!</h4>
<p align="left"><b>Answer: </b>Take a deep breath and read the nmap
man page section about UDP scans. If nmap gets <b>nothing</b> back
from your firewall then
it reports the port as open. If you want to see which UDP ports are
really open, temporarily change your net-&gt;all policy to REJECT,
restart Shorewall and do the nmap UDP scan again.<br>
</p>
<h4><a name="faq4b"></a>4b. I have a port that I can't close no matter
how I change my rules.&nbsp;</h4>
I had a rule that allowed telnet from my local network to my firewall;
I removed that rule and restarted Shorewall but my telnet session still
works!!!<br>
<br>
<b>Answer: </b>&nbsp;Rules only govern the establishment of new
connections. Once a connection is established through the firewall
it will be usable until disconnected (tcp) or until it times out (other
protocols).&nbsp; If you stop telnet and try to establish a new session
your
firerwall will block that attempt.<br>
<h4 align="left"><a name="faq4c"></a><b>4c. </b>How to I use Shorewall
with PortSentry?</h4>
<a
href="http://www.shorewall.net/pub/shorewall/contrib/PortsentryHOWTO.txt">Here's
a writeup</a> on a nice integration of Shorewall and PortSentry.<br>
<h4 align="left"><a name="faq5"></a>5. I've installed Shorewall and now
I can't ping through the firewall</h4>
<p align="left"><b>Answer: </b>If you want your firewall to be totally
open for "ping", </p>
<p align="left">a) Create /etc/shorewall/common if it doesn't already
exist. <br>
b)
Be sure that the first command in the file is ".
/etc/shorewall/common.def"<br>
c)
Add the following to /etc/shorewall/common </p>
<blockquote>
<p align="left">run_iptables -A icmpdef -p ICMP --icmp-type
echo-request -j ACCEPT<br>
</p>
</blockquote>
For a complete description of Shorewall 'ping' management, see <a
href="ping.html">this page</a>.
<h4 align="left"><a name="faq6"></a>6. Where are the log messages
written and how do I change the destination?</h4>
<p align="left"><b>Answer: </b>NetFilter uses the kernel's equivalent
of syslog
(see "man syslog") to log messages. It always uses the LOG_KERN (kern)
facility
(see "man openlog") and you get to choose the log level (again, see
"man
syslog") in your <a href="Documentation.htm#Policy">policies</a> and <a
href="Documentation.htm#Rules">rules</a>. The destination for messaged
logged by syslog is controlled by /etc/syslog.conf (see "man
syslog.conf"). When you have changed /etc/syslog.conf, be sure to
restart syslogd (on a RedHat system, "service syslog restart"). </p>
<p align="left">By default, older versions of Shorewall ratelimited log
messages through <a href="Documentation.htm#Conf">settings</a> in
/etc/shorewall/shorewall.conf -- If you want to log all messages, set: </p>
<div align="left">
<pre align="left"> LOGLIMIT=""<br> LOGBURST=""<br></pre>
Beginning with Shorewall version 1.3.12, you can <a
href="shorewall_logging.html">set up Shorewall to log all of its
messages to a separate file</a>.<br>
</div>
<h4 align="left"><a name="faq6a"></a>6a. Are there any log parsers that
work with Shorewall?</h4>
<p align="left"><b>Answer: </b>Here are several links that may be
helpful: </p>
<blockquote>
<p align="left"><a
href="http://www.shorewall.net/pub/shorewall/parsefw/">http://www.shorewall.net/pub/shorewall/parsefw/</a><br>
<a href="http://www.fireparse.com">http://www.fireparse.com</a><br>
<a href="http://cert.uni-stuttgart.de/projects/fwlogwatch">http://cert.uni-stuttgart.de/projects/fwlogwatch</a><a
href="http://www.logwatch.org"><br>
http://www.logwatch.org</a><br>
<a href="http://gege.org/iptables">http://gege.org/iptables</a><br>
<a href="http://home.regit.org/ulogd-php.html"><font size="3">http://home.regit.org/ulogd-php.html</font></a><br>
</p>
</blockquote>
I personnaly use Logwatch. It emails me a report each day from my
various systems with each report summarizing the logged activity on the
corresponding system.
<h4 align="left"><b><a name="faq6b"></a>6b. DROP messages</b> on port
10619 are <b>flooding the logs</b> with their connect requests. Can i
exclude these error messages for this port temporarily from logging in
Shorewall?</h4>
Temporarily add the following rule:<br>
<pre> DROP net fw udp 10619</pre>
<h4 align="left"><a name="faq6c"></a>6c. All day long I get a steady
flow of these DROP messages from port 53 to some high numbered port.
They get dropped, but what the heck are they?</h4>
<pre>Jan 8 15:50:48 norcomix kernel: Shorewall:net2all:DROP:IN=eth0 OUT= MAC=00:40:c7:2e:09:c0:00:01:64:4a:70:00:08:00<br> SRC=208.138.130.16 DST=24.237.22.45 LEN=53 TOS=0x00 PREC=0x00<br> TTL=251 ID=8288 DF PROTO=UDP SPT=53 DPT=40275 LEN=33 </pre>
<b>Answer: </b>There are two possibilities:<br>
<ol>
<li>They are late-arriving replies to DNS queries.</li>
<li>They are corrupted reply packets.</li>
</ol>
You can distinguish the difference
by setting the <b>logunclean</b> option (<a
href="Documentation.htm#Interfaces">/etc/shorewall/interfaces</a>) on
your external interface (eth0 in the above example). If they get logged
twice, they are corrupted. I solve this problem by using an
/etc/shorewall/common file like this:<br>
<blockquote>
<pre>#<br># Include the standard common.def file<br>#<br>. /etc/shorewall/common.def<br>#<br># The following rule is non-standard and compensates for tardy<br># DNS replies<br>#<br>run_iptables -A common -p udp --sport 53 -mstate --state NEW -j DROP</pre>
</blockquote>
The above file is also include in all of my sample configurations
available in the <a href="shorewall_quickstart_guide.htm">Quick Start
Guides</a> and in the common.def file in Shorewall 1.4.0 and later.<br>
<h4 align="left"><a name="faq6d"></a><b>6d.</b> Why is the MAC address
in Shorewall log messages so long? I thought MAC addresses were only 6
bytes in length.</h4>
What is labeled as the MAC address in a Shorewall log message is
actually the Ethernet frame header. IT contains:<br>
<ul>
<li>the destination MAC address (6 bytes)</li>
<li>the source MAC address (6 bytes)</li>
<li>the ethernet frame type (2 bytes)</li>
</ul>
Example:<br>
<br>
MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00<br>
<ul>
<li>Destination MAC address = 00:04:4c:dc:e2:28</li>
<li>Source MAC address = 00:b0:8e:cf:3c:4c</li>
<li>Ethernet Frame Type = 08:00 (IP Version 4)</li>
</ul>
<h4 align="left"><a name="faq7"></a>7. When I stop Shorewall using
'shorewall stop', I can't connect to anything. Why doesn't that command
work?</h4>
<p align="left">The 'stop' command is intended to place your firewall
into a safe state whereby only those hosts listed in
/etc/shorewall/routestopped' are activated. If you want to totally open
up your firewall, you must use the 'shorewall clear' command. </p>
<h4 align="left"><a name="faq8"></a>8. When I try to start Shorewall on
RedHat, I get messages about insmod failing -- what's wrong?</h4>
<p align="left"><b>Answer: </b>The output you will see looks something
like this:</p>
<pre> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy<br> Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters<br> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod<br> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed<br> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed<br> iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?)<br> Perhaps iptables or your kernel needs to be upgraded.</pre>
<p align="left">This is usually cured by the following sequence of
commands: </p>
<div align="left">
<pre align="left"> <b><font color="#009900">service ipchains stop<br> chkconfig --delete ipchains<br> rmmod ipchains</font></b></pre>
</div>
<div align="left">
<p align="left">Also, be sure to check the <a href="errata.htm">errata</a>
for problems concerning the version of
iptables (v1.2.3) shipped with RH7.2.<br>
</p>
<h4><a name="faq8a"></a><b>8a. </b>When I try to start Shorewall on
RedHat I get a message referring me to FAQ #8</h4>
<b>Answer:</b> This is usually cured by the sequence of commands shown
above in FAQ #8 </div>
<h4 align="left"><a name="faq9"></a>9. Why can't Shorewall detect my
interfaces properly at startup?</h4>
<p align="left">I just installed Shorewall and when I issue the start
command, I see the following:</p>
<div align="left">
<pre> Processing /etc/shorewall/params ...<br> Processing /etc/shorewall/shorewall.conf ...<br> Starting Shorewall...<br> Loading Modules...<br> Initializing...<br> Determining Zones...<br> Zones: net loc<br> Validating interfaces file...<br> Validating hosts file...<br> Determining Hosts in Zones...<br><b> Net Zone: eth0:0.0.0.0/0<br> Local Zone: eth1:0.0.0.0/0<br></b> Deleting user chains...<br> Creating input Chains...<br> ...</pre>
</div>
<div align="left">
<p align="left">Why can't Shorewall detect my interfaces properly?</p>
</div>
<div align="left">
<p align="left"><b>Answer: </b>The above output is perfectly normal.
The Net zone is defined as all hosts that are connected through eth0
and the local zone is defined as all hosts connected through eth1</p>
</div>
<h4 align="left"><a name="faq10"></a>10. What Distributions does it
work with?</h4>
<p align="left">Shorewall works with any GNU/Linux distribution that
includes the <a href="shorewall_prerequisites.htm">proper prerequisites</a>.</p>
<h4 align="left">11. What Features does it have?</h4>
<p align="left"><b>Answer: </b>See the <a
href="shorewall_features.htm">Shorewall Feature List</a>.</p>
<h4 align="left"><a name="faq12"></a>12. Is there a GUI?</h4>
<p align="left"><b>Answer: </b>Yes. Shorewall support is included in
Webmin 1.060 and later versions. See <a href="http://www.webmin.com">http://www.webmin.com</a>
</p>
<h4 align="left"> <a name="faq13"></a>13. Why do you call it
"Shorewall"?</h4>
<p align="left"><b>Answer: </b>Shorewall is a concatenation of "<u>Shore</u>line"
(<a href="http://www.cityofshoreline.com">the city where I live</a>)
and "Fire<u>wall</u>". The full name of the product is actually
"Shoreline Firewall" but "Shorewall" is must more commonly used.</p>
<h4 align="left"> <a name="faq14"></a>14. I'm connected via a cable
modem and it has an internal web server that
allows me to configure/monitor it but as expected
if I enable rfc1918 blocking for my eth0 interface (the internet one),
it also blocks the cable modems web server.</h4>
<p align="left">Is there any way it can add a rule before the rfc1918
blocking that will let all traffic to and from the 192.168.100.1
address of the modem in/out but still block all other rfc1918 addresses?</p>
<p align="left"><b>Answer: </b>If you are running a version of
Shorewall earlier
than 1.3.1, create /etc/shorewall/start and in it, place the following:</p>
<div align="left">
<pre> run_iptables -I rfc1918 -s 192.168.100.1 -j ACCEPT</pre>
</div>
<div align="left">
<p align="left">If you are running version 1.3.1 or later, simply add
the following to<a href="Documentation.htm#rfc1918">
/etc/shorewall/rfc1918</a>:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>SUBNET </b></u></td>
<td><u><b>TARGET</b></u></td>
</tr>
<tr>
<td>192.168.100.1</td>
<td>RETURN</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Be sure that you add the entry ABOVE the entry for
192.168.0.0/16.<br>
</p>
<p align="left">Note: If you add a second IP address to your external
firewall interface to correspond to the modem address, you must also
make an entry in /etc/shorewall/rfc1918 for that address. For example,
if you configure the address 192.168.100.2 on your firewall, then you
would add two entries to /etc/shorewall/rfc1918: <br>
</p>
<blockquote>
<table cellpadding="2" border="1" style="border-collapse: collapse;">
<tbody>
<tr>
<td valign="top"><u><b>SUBNET</b></u><br>
</td>
<td valign="top"><u><b>TARGET</b></u><br>
</td>
</tr>
<tr>
<td valign="top">192.168.100.1<br>
</td>
<td valign="top">RETURN<br>
</td>
</tr>
<tr>
<td valign="top">192.168.100.2<br>
</td>
<td valign="top">RETURN<br>
</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<h4 align="left"><a name="faq14a"></a>14a. Even though it assigns
public IP addresses, my ISP's DHCP server has an RFC 1918 address. If I
enable RFC
1918 filtering on my external interface, my DHCP client cannot renew
its
lease.</h4>
</div>
<div align="left">
<p align="left">The solution is the same as FAQ 14 above. Simply
substitute the IP address of your ISPs DHCP server.</p>
</div>
<h4 align="left"><a name="faq15"></a>15. My local systems can't see out
to the net</h4>
<p align="left"><b>Answer: </b>Every time I read "systems can't see
out to the net", I wonder where the poster bought computers with eyes
and what those computers
will "see" when things are working properly. That aside, the most
common causes of this problem are:</p>
<ol>
<li>
<p align="left">The default gateway on each local system isn't set
to the IP address of the local firewall
interface.</p>
</li>
<li>
<p align="left">The entry for the local network in the
/etc/shorewall/masq file is wrong or missing.</p>
</li>
<li>
<p align="left">The DNS settings on the local systems are wrong or
the user is running a DNS server on the
firewall and hasn't enabled UDP and TCP port
53 from the firewall to the internet.</p>
</li>
</ol>
<h4 align="left"><a name="faq16"></a>16. Shorewall is writing log
messages all over my console making it unusable!</h4>
<p align="left"><b>Answer: </b>If you are running Shorewall version
1.4.4 or 1.4.4a then check the <a href="errata.htm">errata.</a>
Otherwise, see the 'dmesg' man page ("man dmesg"). You must add a
suitable 'dmesg' command to your startup scripts or place it in
/etc/shorewall/start. Under RedHat, the max log level that is sent to
the console is specified in /etc/sysconfig/init in the LOGLEVEL
variable.<br>
</p>
<h4><a name="faq17"></a>17. How do I find out why this traffic is
getting logged?</h4>
<b>Answer: </b>Logging occurs out of a number of chains (as indicated
in the log message) in Shorewall:<br>
<ol>
<li><b>man1918 </b>or <b>logdrop - </b>The destination address is
listed in /etc/shorewall/rfc1918 with a <b>logdrop </b>target -- see <a
href="Documentation.htm#rfc1918">/etc/shorewall/rfc1918.</a></li>
<li><b>rfc1918</b> or <b>logdrop </b>- The source address is listed
in /etc/shorewall/rfc1918 with a <b>logdrop </b>target -- see <a
href="Documentation.htm#rfc1918">/etc/shorewall/rfc1918.</a></li>
<li><b>all2&lt;zone&gt;</b>, <b>&lt;zone&gt;2all</b> or <b>all2all </b>-
You have a<a href="Documentation.htm#Policy"> policy</a> that specifies
a log level and this packet is
being logged under that policy. If you intend to ACCEPT
this traffic then you need a <a href="Documentation.htm#Rules">rule</a>
to that effect.<br>
</li>
<li><b>&lt;zone1&gt;2&lt;zone2&gt; </b>- Either you have a<a
href="Documentation.htm#Policy"> policy</a> for <b>&lt;zone1&gt; </b>to
<b>&lt;zone2&gt;</b> that specifies a log level and this packet is
being logged under that policy or this packet matches a <a
href="Documentation.htm#Rules">rule</a> that includes a log level.</li>
<li><b>&lt;interface&gt;_mac</b> - The packet is being logged under
the <b>maclist</b> <a href="Documentation.htm#Interfaces">interface
option</a>.<br>
</li>
<li><b>logpkt</b> - The packet is being logged under the <b>logunclean</b>
<a href="Documentation.htm#Interfaces">interface option</a>.</li>
<li><b>badpkt </b>- The packet is being logged under the <b>dropunclean</b>
<a href="Documentation.htm#Interfaces">interface option</a> as
specified in the <b>LOGUNCLEAN </b>setting in <a
href="Documentation.htm#Conf">/etc/shorewall/shorewall.conf</a>.</li>
<li><b>blacklst</b> - The packet is being logged because the source
IP is blacklisted in the<a href="Documentation.htm#Blacklist">
/etc/shorewall/blacklist </a>file.</li>
<li><b>newnotsyn </b>- The packet is being logged because it is a
TCP packet that is not part of any current connection yet it is not a
syn packet. Options affecting the logging of such packets include <b>NEWNOTSYN
</b>and <b>LOGNEWNOTSYN </b>in <a href="Documentation.htm#Conf">/etc/shorewall/shorewall.conf.</a></li>
<li><b>INPUT</b> or <b>FORWARD</b> - The packet has a source IP
address that isn't in any of your defined zones ("shorewall check" and
look at the printed zone definitions) or the chain is FORWARD and the
destination IP isn't in any of
your defined zones. Also see <a href="#faq2a">FAQ 2a</a> for another
cause of packets being logged in the FORWARD chain.<br>
</li>
<li><b>logflags </b>- The packet is being logged because it failed
the checks implemented by the <b>tcpflags </b><a
href="Documentation.htm#Interfaces">interface option</a>.</li>
</ol>
<p align="left">Here is an example:</p>
<font face="Century Gothic, Arial, Helvetica">
<p align="left"><font face="Courier">Jun 27 15:37:56 gateway kernel:
Shorewall:<span style="text-decoration: underline;">all2all:REJECT</span>:<span
style="text-decoration: underline;">IN=eth2</span> <span
style="text-decoration: underline;">OUT=eth1</span> <span
style="text-decoration: underline;">SRC=192.168.2.2</span>
<span style="text-decoration: underline;">DST=192.168.1.3</span> LEN=67
TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF <span
style="text-decoration: underline;">PROTO=UDP</span>
SPT=1803 <span style="text-decoration: underline;">DPT=53</span> LEN=47</font></p>
</font>
<p align="left">Let's look at the important parts of this message:</p>
<ul>
<li>all2all:REJECT - This packet was REJECTed out of the <span
style="font-weight: bold;">all2all</span> chain -- the packet
was rejected under the "all"-&gt;"all"
REJECT policy (number 3 above).</li>
<li>IN=eth2 - the packet entered the firewall via eth2. If you see
"IN=" with no interface name, the packet originated on the firewall
itself.<br>
</li>
<li>OUT=eth1 - if accepted, the packet would be sent on eth1. If you
see "OUT=" with no interface name, the packet would be processed by the
firewall itself.<br>
</li>
<li>SRC=192.168.2.2 - the packet was sent by 192.168.2.2</li>
<li>DST=192.168.1.3 - the packet is destined for 192.168.1.3</li>
<li>PROTO=UDP - UDP Protocol</li>
<li>DPT=53 - The destination port is 53 (DNS)<br>
</li>
</ul>
<p align="left">In this case, 192.168.2.2 was in the "dmz" zone and
192.168.1.3 is in the "loc" zone. I was missing the rule:</p>
&nbsp;&nbsp;&nbsp; ACCEPT&nbsp;&nbsp;&nbsp; dmz&nbsp;&nbsp;&nbsp;
loc&nbsp;&nbsp;&nbsp; udp&nbsp;&nbsp;&nbsp; 53
<h4><a name="faq18"></a>18. Is there any way to use <b>aliased ip
addresses</b> with Shorewall, and maintain separate rulesets for
different IPs?</h4>
<b>Answer: </b>Yes. See <a
href="Shorewall_and_Aliased_Interfaces.html">Shorewall and Aliased
Interfaces</a>.
<h4><b><a name="faq19"></a>19. </b>I have added entries to
/etc/shorewall/tcrules but they don't seem to do anything. Why?</h4>
You probably haven't set TC_ENABLED=Yes in
/etc/shorewall/shorewall.conf so the contents of the tcrules file are
simply being ignored.<br>
<h4><a name="faq20"></a><b>20. </b>I have just set up a server. <b>Do
I have to change Shorewall to allow access to my server from the
internet?</b><br>
</h4>
Yes. Consult the
<a href="shorewall_quickstart_guide.htm">QuickStart guide</a> that
you used during your initial setup for information about how to set up
rules for your server.<br>
<h4><a name="faq21"></a><b>21. </b>I see these <b>strange log entries
</b>occasionally; what are they?<br>
</h4>
<blockquote>
<pre>Nov 25 18:58:52 linux kernel: Shorewall:net2all:DROP:IN=eth1 OUT= MAC=00:60:1d:f0:a6:f9:00:60:1d:f6:35:50:08:00<br> SRC=206.124.146.179 DST=192.0.2.3 LEN=56 TOS=0x00 PREC=0x00 TTL=110 ID=18558 PROTO=ICMP TYPE=3 CODE=3 <br> [SRC=192.0.2.3 DST=172.16.1.10 LEN=128 TOS=0x00 PREC=0x00 TTL=47 ID=0 DF PROTO=UDP SPT=53 DPT=2857 LEN=108 ]<br></pre>
</blockquote>
192.0.2.3 is external on my firewall... 172.16.0.0/24 is my internal LAN<br>
<br>
<b>Answer: </b>While most people associate the Internet Control
Message Protocol (ICMP) with 'ping', ICMP is a key piece of the
internet. ICMP is used to report problems back to the sender of a
packet; this is what is happening here. Unfortunately, where NAT is
involved (including SNAT, DNAT and Masquerade), there are a lot of
broken implementations. That is what you are
seeing with these messages.<br>
<br>
Here is my interpretation of what is happening -- to confirm this
analysis, one would have to have packet sniffers placed a both ends of
the connection.<br>
<br>
Host 172.16.1.10 behind NAT gateway 206.124.146.179 sent a UDP DNS
query to 192.0.2.3 and your DNS server tried to send a response (the
response information is in the brackets -- note source port 53 which
marks this as a DNS reply). When the response was returned to to
206.124.146.179, it rewrote the destination IP TO 172.16.1.10 and
forwarded the packet to 172.16.1.10 who no longer had a connection on
UDP port 2857. This causes a port unreachable (type 3, code 3) to be
generated back to 192.0.2.3. As this packet is sent back through
206.124.146.179, that box correctly changes
the source address in the packet to 206.124.146.179 but doesn't reset
the DST IP in the original DNS response similarly. When
the ICMP reaches your firewall (192.0.2.3), your firewall has no record
of having sent a DNS reply to 172.16.1.10 so this ICMP doesn't appear
to be related to anything that was sent. The final result is that the
packet gets logged and dropped in the all2all chain. I have also seen
cases where the source IP in the ICMP itself isn't set
back to the external IP of the remote NAT gateway; that causes your
firewall to log and drop the packet out of the rfc1918 chain because
the source IP is reserved by RFC 1918.<br>
<h4><a name="faq22"></a><b>22. </b>I have some <b>iptables commands </b>that
I want to <b>run when Shorewall starts.</b> Which
file do I put them in?</h4>
You can place these commands in one of the <a
href="shorewall_extension_scripts.htm">Shorewall Extension Scripts</a>.
Be sure that you look at the contents of the chain(s) that you will be
modifying with your commands to be sure that the commands will do what
they are intended. Many iptables commands published in HOWTOs and other
instructional material use the -A command which adds the rules to the
end of the chain. Most chains that Shorewall constructs end with an
unconditional DROP, ACCEPT or REJECT rule and any rules that you add
after that will be ignored. Check "man iptables" and look at the -I
(--insert) command.<br>
<h4><a name="faq23"></a><b>23. </b>Why do you use such ugly fonts on
your web site?</h4>
The Shorewall web site is almost font neutral (it doesn't explicitly
specify fonts except on a few pages) so the fonts you see are largely
the default
fonts configured in your browser. If you don't like them then
reconfigure your browser.<br>
<h4><a name="faq24"></a>24. How can I <b>allow conections</b> to let's
say the ssh port only<b> from specific IP Addresses</b> on the internet?</h4>
In the SOURCE column of the rule, follow "net" by a colon and a list of
the host/subnet addresses as a comma-separated list.<br>
<pre> net:&lt;ip1&gt;,&lt;ip2&gt;,...<br></pre>
Example:<br>
<pre> ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22<br></pre>
<div align="left"> </div>
<h4><b><a name="faq25"></a>25. </b>How to I tell <b>which version of
Shorewall</b> I am <b>running</b>?<br>
</h4>
At the shell prompt, type:<br>
<br>
<font color="#009900"><b> /sbin/shorewall
version<br>
</b></font>
<h4><a name="faq26"></a><b>26. </b>When I try to use any of the SYN
options in nmap on or behind the firewall, I get "operation not
permitted".
How can I use nmap with Shorewall?"</h4>
Edit /etc/shorewall/shorewall.conf and change "NEWNOTSYN=No" to
"NEWNOTSYN=Yes" then restart Shorewall.<br>
<br>
<h4><a name="faq26a"></a><b><span style="font-weight: bold;">26a.&nbsp;
</span></b>When I try to use the <span style="font-weight: bold;">"-O"
option of nmap</span> from the firewall system, I get "<span
style="font-weight: bold;">operation not permitted". </span>How to I
allow this option?</h4>
Add this command to your /etc/shorewall/start file:<br>
<pre style="margin-left: 40px;"><tt>run_iptables -D OUTPUT -p ! icmp -m state --state INVALID -j DROP</tt><br></pre>
<h4><a name="faq27">27. I'm compiling a new kernel for my firewall.
What
should I look out for?</a></h4>
First take a look at the <a href="kernel.htm">Shorewall kernel
configuration page</a>. You probably also want to be sure that you have
selected the "<b>NAT of local connections (READ HELP)</b>" on the
Netfilter Configuration menu. Otherwise, DNAT rules with your firewall
as the source zone won't work with your new kernel.<br>
<h4><a name="faq28"></a>28. How do I use Shorewall as a Bridging
Firewall?<br>
</h4>
Basically, you don't. While there are kernel patches that allow you to
route bridge traffic through Netfilter, the environment is so different
from the Layer 3 firewalling environment that very little of Shorewall
works. In fact, so much of Shorewall doesn't work that my official
position
is that "Shorewall doesn't work with Layer 2 Bridging".<br>
<h4><a name="faq29"></a><b>29. FTP Doesn't Work</b><br>
</h4>
See the <a href="FTP.html">Shorewall and FTP page</a>.<br>
<h4><span style="font-weight: bold;"><a name="faq30"></a>30. </span>I'm
confused about when to use DNAT rules and when to use ACCEPT rules.</h4>
It would be a good idea to review the <a
href="shorewall_quickstart_guide.htm">QuickStart Guide </a>appropriate
for your setup; the guides cover this topic in a tutorial fashion. DNAT
rules should be used for connections that need to go the opposite
direction from SNAT/MASQUERADE. So if you masquerade or use SNAT from
your local network to the internet then you will need to use DNAT rules
to allow connections from the internet to your local network. In all
other cases, you use ACCEPT unless you need to hijack connections as
they go through your firewall and handle them on the firewall box
itself; in that case, you use a REDIRECT rule.<br>
<h4><a name="faq31"></a>31. Does Shorewall provide protection
against....</h4>
<ol>
<li>IP Spoofing: Sending packets over the WAN interface using an
internal LAP IP address as the source address? <span
style="font-weight: bold;">Answer: </span>Yes.</li>
<li>Tear Drop: Sending packets that contain overlapping fragments? <span
style="font-weight: bold;">Answer: </span>This is the responsibility
of the IP stack, not the Netfilter-based firewall since fragment
reassembly occurs before the stateful packet filter ever touches each
packet.</li>
<li>Smurf and Fraggle: Sending packets that use the WAN or LAN
broadcast address as the source address? <span
style="font-weight: bold;">Answer: </span>Shorewall can be configured
to do that using the <a href="blacklisting_support.htm">blacklisting</a>
facility.</li>
<li>Land Attack: Sending packets that use the same address as the
source and destination address? <span style="font-weight: bold;">Answer:
</span>Yes, if the <a href="Documentation.htm#Interfaces">routefilter
interface option</a> is selected.</li>
<li>DOS:<br>
&nbsp;&nbsp; - SYN Dos<br>
&nbsp;&nbsp; - ICMP Dos<br>
&nbsp;&nbsp; - Per-host Dos protection<br>
<span style="font-weight: bold;">Answer: </span>Shorewall has
facilities for limiting SYN and ICMP packets. Netfilter as included in
standard Linux kernels doesn't support per-remote-host limiting except
by explicit rule that specifies the host IP address; that form of
limiting is supported by Shorewall.</li>
</ol>
<h4><a name="faq32"></a><span style="font-weight: bold;">32. </span>My
firewall has two connections to the internet from two different ISPs.
How do I set this up in Shorewall?</h4>
Setting this up in Shorewall is easy; setting up the routing is a bit
harder.<br>
<br>
Assuming that eth0 and eth1 are the interfaces to the two ISPs then:<br>
<br>
/etc/shorewall/interfaces:<br>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber2">
<tbody>
<tr>
<td><u><b>ZONE</b></u></td>
<td><u><b>INTERFACE</b></u></td>
<td><u><b>BROADCAST</b></u></td>
<td><u><b>OPTIONS</b></u></td>
</tr>
<tr>
<td>net<br>
</td>
<td>eth0</td>
<td>detect<br>
</td>
<td>...<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">net<br>
</td>
<td style="vertical-align: top;">eth1<br>
</td>
<td style="vertical-align: top;">detect<br>
</td>
<td style="vertical-align: top;">...<br>
</td>
</tr>
</tbody>
</table>
</blockquote>
/etc/shorewall/policy:<br>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>SOURCE </b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>POLICY</b></u></td>
<td><u><b>LIMIT:BURST</b></u></td>
</tr>
<tr>
<td>net<br>
</td>
<td>net<br>
</td>
<td>DROP<br>
</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<hr style="width: 100%; height: 2px;">The following information
regarding setting up routing for this
configuration is reproduced from the <a href="http://www.lartc.org">LARTC
HOWTO</a> and has not been verified by the author. If you have
questions or problems with the instructions given below, please post to
the <a href="http://www.lartc.org/#mailinglist">LARTC mailing list</a>.<br>
<hr style="width: 100%; height: 2px;">A common configuration is the
following, in which there are two providers
that connect a local network (or even a single machine) to the big
Internet.
<pre class="SCREEN"> ________<br> +------------+ /<br> | | |<br> +-------------+ Provider 1 +-------<br> __ | | | /<br> ___/ \_ +------+-------+ +------------+ |<br> _/ \__ | if1 | /<br> / \ | | |<br>| Local network -----+ Linux router | | Internet<br> \_ __/ | | |<br> \__ __/ | if2 | \<br> \___/ +------+-------+ +------------+ |<br> | | | \<br> +-------------+ Provider 2 +-------<br> | | |<br> +------------+ \________</pre>
<p>There are usually two questions given this setup.</p>
<div class="SECT2">
<h2 class="SECT2">Split access</h2>
<p> The first is how to route answers to packets coming in over a
particular provider, say Provider 1, back out again over that same
provider. </p>
<p> Let us first set some symbolical names. Let <b class="COMMAND">$IF1</b>
be the name of the first interface (if1 in the picture above) and <b
class="COMMAND">$IF2</b> the name of the second interface. Then let <b
class="COMMAND">$IP1</b> be the IP address associated with <b
class="COMMAND">$IF1</b> and <b class="COMMAND">$IP2</b> the IP
address associated with <b class="COMMAND">$IF2</b>. Next, let <b
class="COMMAND">$P1</b> be the IP address of the gateway at Provider
1, and <b class="COMMAND">$P2</b> the IP address of the gateway at
provider 2. Finally, let <b class="COMMAND">$P1_NET</b> be the IP
network <b class="COMMAND">$P1</b> is in, and <b class="COMMAND">$P2_NET</b>
the IP network <b class="COMMAND">$P2</b> is in. </p>
<p> One creates two additional routing tables, say <b class="COMMAND">T1</b>
and <b class="COMMAND">T2</b>. These are added in
/etc/iproute2/rt_tables. Then you set up routing in these tables as
follows: </p>
<p> </p>
<pre class="SCREEN"> ip route add $P1_NET dev $IF1 src $IP1 table T1<br> ip route add default via $P1 table T1<br> ip route add $P2_NET dev $IF2 src $IP2 table T2<br> ip route add default via $P2 table T2<br> </pre>
Nothing spectacular, just build a route to the gateway and build a
default route via that gateway, as you would do in the case of a single
upstream provider, but put the routes in a separate table per provider.
Note that the network route suffices, as it tells you how to find any
host in that network, which includes the gateway, as specified above.
<p> Next you set up the main routing table. It is a good idea to route
things to the direct neighbour through the interface connected to that
neighbour. Note the `src' arguments, they make sure the right outgoing
IP address is chosen. </p>
<pre class="SCREEN"> ip route add $P1_NET dev $IF1 src $IP1<br> ip route add $P2_NET dev $IF2 src $IP2<br> </pre>
Then, your preference for default route:
<pre class="SCREEN"> ip route add default via $P1<br> </pre>
Next, you set up the routing rules. These actually choose what routing
table to route with. You want to make sure that you route out a given
interface if you already have the corresponding source address:
<pre class="SCREEN"> ip rule add from $IP1 table T1<br> ip rule add from $IP2 table T2<br> </pre>
This set of commands makes sure all answers to traffic coming in on a
particular interface get answered from that interface.
<p> </p>
<div class="WARNING">
<table class="WARNING" width="100%" border="0">
<tbody>
<tr>
<td width="25" align="center" valign="top"><img
src="images/BD21298_.gif" hspace="5" alt="Warning" title=""
style="width: 13px; height: 13px;"></td>
<td align="left" valign="top">
<p>Reader Rod Roark notes: 'If $P0_NET is the local network and
$IF0 is its interface,
the following additional entries are desirable: </p>
<pre class="SCREEN">ip route add $P0_NET dev $IF0 table T1<br>ip route add $P2_NET dev $IF2 table T1<br>ip route add 127.0.0.0/8 dev lo table T1<br>ip route add $P0_NET dev $IF0 table T2<br>ip route add $P1_NET dev $IF1 table T2<br>ip route add 127.0.0.0/8 dev lo table T2 </pre>
'</td>
</tr>
</tbody>
</table>
</div>
<p> Now, this is just the very basic setup. It will work for all
processes running on the router itself, and for the local network, if
it is masqueraded. If it is not, then you either have IP space from
both providers or you are going to want to masquerade to one of the two
providers. In both cases you will want to add rules selecting which
provider to route out from based on the IP address of the machine in
the local network. </p>
</div>
<div class="SECT2">
<h2 class="SECT2">Load balancing</h2>
<p> The second question is how to balance traffic going out over the
two providers. This is actually not hard if you already have set up
split access as above. </p>
<p> Instead of choosing one of the two providers as your default route,
you now set up the default route to be a multipath route. In the
default kernel this will balance routes over the two providers. It is
done as follows (once more building on the example in the section on
split-access): </p>
<pre class="SCREEN"> ip route add default scope global nexthop via $P1 dev $IF1 weight 1 \<br> nexthop via $P2 dev $IF2 weight 1<br> </pre>
This will balance the routes over both providers. The <b
class="COMMAND">weight</b> parameters can be tweaked to favor one
provider over the other.
<p> Note that balancing will not be perfect, as it is route based, and
routes are cached. This means that routes to often-used sites will
always be over the same provider. </p>
<p> Furthermore, if you really want to do this, you probably also want
to look at Julian Anastasov's patches at <a
href="http://www.ssi.bg/%7Eja/#routes" target="_top">http://www.ssi.bg/~ja/#routes
</a>, Julian's route patch page. They will make things nicer to work
with. </p>
</div>
<hr style="width: 100%; height: 2px;">End of information reproduced
from the LARTC HOWTO. If you have
questions or problems with the instructions given above, please post to
the <a href="http://www.lartc.org/#mailinglist">LARTC mailing list</a>.
<hr style="width: 100%; height: 2px;"><font size="2">Last updated
11/20/2003 - <a href="support.htm">Tom
Eastep</a></font>
<p><a href="copyright.htm"><font size="2">Copyright</font> <20> <font
size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a><br>
</p>
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