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<html>
<head>
<meta http-equiv="Content-Language" content="en-us">
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<meta name="GENERATOR" content="Microsoft FrontPage 5.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<title>Shorewall FAQ</title>
<meta name="Microsoft Theme" content="none">
</head>
<body>
<table border="0" cellpadding="0" cellspacing="0" style="border-collapse: collapse" width="100%" id="AutoNumber4" bgcolor="#400169" height="90">
<tr>
<td width="100%">
<h1 align="center"><font color="#FFFFFF">Shorewall FAQs</font></h1>
</td>
</tr>
</table>
<p align="left"><b>1. </b><a href="#faq1">&nbsp;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.</a></p>
<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="#faq3">I have a zone &quot;Z&quot; with an RFC1918
subnet and I use <b>static 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>
<p align="left"><b>3. </b><a href="#faq3">I want to use <b>Netmeeting </b>with
Shorewall. What do I do?</a></p>
<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!!!!</a></p>
<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></p>
<p align="left"><b>6. </b><a href="#faq6">Where are the <b>log messages</b>
written and&nbsp; 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>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 7.x</b>, I
get messages about insmod failing -- what's wrong?</a></p>
<p align="left"><b>9. </b><a href="#faq9"><b>Why </b>does Shorewall <b>only accept IP addresses</b> as
opposed to FQDNs?</a></p>
<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">Why isn't there a <b>GUI</b></a></p>
<p align="left"><b>13. </b><a href="#faq13">Why do you call it <b>&quot;Shorewall&quot;?</b></a></p>
<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>
<p align="left"><b>15. </b><a href="#faq15"><b>My local systems can't see out to
the net</b></a></p>
<p align="left"><b>16. </b><a href="#faq16">Shorewall is writing <b>log messages
all over my console</b> making it unusable!</a></p>
<p align="left"><b>17. </b><a href="#faq17">Why can't Shorewall <b>detect my
interfaces </b>properly?</a></p>
<blockquote>
<p align="left">&nbsp;</p>
</blockquote>
<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. Assuming
that you have a dynamic external IP address, 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">
<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>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</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">
<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>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</table>
</blockquote>
<div align="left">
<pre align="left"><font face="Courier"> DNAT net loc:192.168.1.5 udp 7777</font></pre>
</div>
<p align="left">If you want to forward requests directed to a particular
address ( <i>&lt;external IP&gt;</i> ) on your firewall to an internal system:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse" id="AutoNumber1">
<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>
</table>
</blockquote>
<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 two 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 such as an
incorrect default gateway configured (it should be set to the IP address of your
firewall's internal interface).</li>
</ul>
<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 &quot;views&quot;</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 static 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, do the following:</p>
<p align="left">a) In /etc/shorewall/interfaces, specify &quot;multi&quot; as an option
for eth1.</p>
<div align="left">
<p align="left">b) In /etc/shorewall/rules, add:</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse" id="AutoNumber1">
<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:192.168.1.0/24</td>
<td>loc:192.168.1.5</td>
<td>tcp</td>
<td>www</td>
<td>-</td>
<td>130.151.100.69:192.168.1.254</td>
</tr>
</table>
</blockquote>
</div>
<div align="left">
<pre align="left"> <font face="Courier">DNAT&nbsp;&nbsp;&nbsp; loc:192.168.1.0/24&nbsp;&nbsp;&nbsp; loc:192.168.1.5&nbsp;&nbsp;&nbsp; tcp&nbsp;&nbsp;&nbsp; www&nbsp;&nbsp;&nbsp; -&nbsp;&nbsp;&nbsp; 130.151.100.69:192.168.1.254</font></pre>
</div>
<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/params:</div>
<div align="left">
<pre> ETH0_IP=`find_interface_address eth0`</pre>
</div>
<div align="left">
<p align="left">and make your DNAT rule:</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse" id="AutoNumber1">
<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:192.168.1.0/24</td>
<td>loc:192.168.1.5</td>
<td>tcp</td>
<td>www</td>
<td>-</td>
<td>$ETH0_IP:192.168.1.254</td>
</tr>
</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.</div>
<h4 align="left"><a name="faq2a"></a>2a. I have a zone &quot;Z&quot; with an RFC1918 subnet and I
use static 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>
<p align="left"><b>Answer: </b>This is another problem that is best solved using Bind Version 9
&quot;views&quot;. 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
static 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.&nbsp;</p>
<p align="left">If you don't like those solutions and prefer routing all Z-&gt;Z
traffic through your firewall then:</p>
<p align="left">a) Specify &quot;multi&quot; on the entry for Z's interface in
/etc/shorewall/interfaces.<br>
b) Set the Z-&gt;Z policy to ACCEPT.<br>
c) Masquerade Z to itself.<br>
<br>
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">
<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>multi</td>
</tr>
</table>
</blockquote>
<p align="left">In /etc/shorewall/policy:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse" id="AutoNumber3">
<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>&nbsp;</td>
</tr>
</table>
</blockquote>
<div align="left">
<pre align="left"> dmz&nbsp;&nbsp;&nbsp; dmz&nbsp;&nbsp;&nbsp; ACCEPT</pre>
</div>
<p align="left">In /etc/shorewall/masq:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse" id="AutoNumber3" width="369">
<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">&nbsp;</td>
</tr>
</table>
</blockquote>
<h4 align="left"><a name="faq3"></a>3. I want to use Netmeeting with Shorewall. What do I do?</h4>
<p align="left"><b>Answer: </b>There is an <a href="http://www.kfki.hu/~kadlec/sw/netfilter/newnat-suite/"> H.323 connection tracking/NAT module</a> that may help.
Also check the Netfilter mailing list archives at <a href="http://netfilter.samba.org">http://netfilter.samba.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.</p>
<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
&quot;ping&quot;: </p>
<p align="left">a) Do NOT specify 'noping' on any interface in
/etc/shorewall/interfaces.<br>
b) Copy /etc/shorewall/icmp.def to /etc/shorewall/icmpdef<br>
c) Add the following to /etc/shorewall/icmpdef: </p>
<blockquote>
<p align="left">run_iptables -A icmpdef -p ICMP --icmp-type echo-request -j
ACCEPT </p>
</blockquote>
<h4 align="left"><a name="faq6"></a>6. Where are the log messages written
and&nbsp; how do I change the destination?</h4>
<p align="left"><b>Answer: </b>NetFilter uses the kernel's equivalent of syslog (see &quot;man
syslog&quot;) to log messages. It always uses the LOG_KERN (kern) facility (see
&quot;man openlog&quot;) and you get to choose the log level (again, see
&quot;man syslog&quot;) 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 &quot;man
syslog.conf&quot;). When you have changed /etc/syslog.conf, be sure to restart
syslogd (on a RedHat system, &quot;service syslog restart&quot;). </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=&quot;&quot;
LOGBURST=&quot;&quot;</pre>
</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></p>
</blockquote>
<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 interfaces/hosts having the 'routestopped' option
in /etc/shorewall/interfaces and /etc/shorewall/hosts 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
7.x, 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
Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed
iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?)
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"> service ipchains stop
chkconfig --delete ipchains
rmmod ipchains</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.</div>
<h4 align="left"> <a name="faq9"></a>9. Why does Shorewall only accept IP
addresses as opposed to FQDNs?</h4><p align="left"> <b>Answer: </b>FQDNs in iptables rules
aren't nearly as useful as they first appear. When a DNS name appears in a rule,
the iptables utility resolves the name to one or more IP addresses and inserts
those addresses into the rule. So change in the DNS-&gt;IP address relationship
that occur after the firewall has started have absolutely no effect on the
firewall's ruleset.</p>
<p align="left"> I'm also trying to protect
people from themselves. If your firewall rules include FQDN's then:</p>
<ul>
<li>If your /etc/resolv.conf is wrong then your firewall won't
start.</li>
<li>If your /etc/nsswitch.conf is wrong then your firewall won't
start.</li>
<li>If your Name Server(s) is(are) down then your firewall won't
start.</li>
<li>Factors totally outside your control (your ISP's router is
down for example), can prevent your firewall from starting.</li>
</ul>
<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>.<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>.<h4 align="left"><a name="faq12"></a>12. Why isn't there a GUI?</h4>
<p align="left"><b>Answer: </b>Every time I've started to work on one, I find myself doing
other things. I guess I just don't care enough if Shorewall has a GUI to
invest the effort to create one myself. There are several Shorewall GUI
projects underway however and I will publish links to them when the authors
feel that they are ready. <h4 align="left">
<a name="faq13"></a>13. Why do you call it &quot;Shorewall&quot;?</h4>
<p align="left"><b>Answer: </b>Shorewall is a concatenation of &quot;<u>Shore</u>line&quot; (<a href="http://www.cityofshoreline.com">the
city where I live</a>) and &quot;Fire<u>wall</u>&quot;.<h4 align="left">
<a name="faq14"></a>14.&nbsp; 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:<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>:</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse" id="AutoNumber3">
<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>
</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.</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.</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 &quot;systems can't see out to the net&quot;, I wonder
where the poster bought computers with eyes and what those computers will &quot;see&quot;
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>&quot;man dmesg&quot; -- 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.</p>
<h4 align="left"><a name="faq17"></a>17. Why can't Shorewall detect my
interfaces properly?</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/shorewall.conf ...
Processing /etc/shorewall/params ...
Starting Shorewall...
Loading Modules...
Initializing...
Determining Zones...
Zones: net loc
Validating interfaces file...
Validating hosts file...
Determining Hosts in Zones...
<b> Net Zone: eth0:0.0.0.0/0
Local Zone: eth1:0.0.0.0/0
</b> Deleting user chains...
Creating input Chains...
...</pre>
</div>
<div align="left">
<p align="left">Why can't Shorewall detect my interfaces properly?</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.</div>
<p align="left"><font size="2">Last updated
8/24/2002 - <a href="support.htm">Tom
Eastep</a></font></p>
<p><font face="Trebuchet MS"><a href="copyright.htm"><font size="2">Copyright</font>
<EFBFBD> <font size="2">2001, 2002 Thomas M. Eastep.</font></a></font></p>
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