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shorewall_code/Shorewall-docs/FAQ.htm
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<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">
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<body>
<table border="0" cellpadding="0" cellspacing="0"
style="border-collapse: collapse;" width="100%" id="AutoNumber4"
bgcolor="#400169" height="90">
<tbody>
<tr>
<td width="100%">
<h1 align="center"><font color="#ffffff">Shorewall FAQs</font></h1>
</td>
</tr>
</tbody>
</table>
<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>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 "Z" 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/MSN
Messenger </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 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</b> I get messages about insmod failing -- what's
wrong?</a></p>
<p align="left"><b>9. </b><a href="FAQ.htm#faq9">Why can't Shorewall <b>detect
my interfaces </b>properly?</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>"Shorewall"?</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!<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>
<br>
<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>
<br>
<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?<br>
<br>
</b></a><b>21. </b><a href="#faq21">I see these <b>strange log entries
</b>occasionally; what are they?<br>
</a><br>
<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>
<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">
<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">
<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" 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</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>
<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="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>zone</i>_dnat where <i>zone</i> is the zone that includes
the<68> ('net' 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 server (the server's default gateway
should be the IP address of the firewall's interface to the server).</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="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
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 "multi" as an option
for eth1 (No longer required as of Shorewall version 1.3.9).</p>
<div align="left">
<p align="left">b) In /etc/shorewall/rules, add:</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: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>
</tbody>
</table>
</blockquote>
</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:</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: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>
</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 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 "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
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. </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 "multi" on the entry for Z's interface in /etc/shorewall/interfaces
(If you are running a Shorewall version earlier than 1.3.9).<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">
<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>multi</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>
<div align="left">
<pre align="left"> dmz dmz 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">
<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>
<h4 align="left"><a name="faq3"></a>3. I want to use Netmeeting/MSN 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 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 "ping": </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<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><br>Beginning with Shorewall version 1.3.12, you can <a
href="configuration_file_basics.htm#Levels">set up Shorewall to log all of its messages to a separate file</a>.<br></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><a
href="http://www.logwatch.org"><br>
http://www.logwatch.org</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.<2E>
<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"> service ipchains stop<br> chkconfig --delete ipchains<br> 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.</p>
</div>
<h4 align="left"> </h4>
<h4 align="left"><a name="faq9"></a>9. 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 ...<br> Processing /etc/shorewall/params ...<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. 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. </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>"man dmesg" -- 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>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> - The source address is listed in
/etc/shorewall/rfc1918 with a <b>logdrop </b>target<EFBFBD>-- 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.</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>.<br>
</li>
</ol>
<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. You simply use the IP address in your
rules (or if you use NAT, use the local IP address in your rules).
<b>Note:</b> The ":n" notation (e.g., eth0:0) is deprecated and will
disappear eventually. Neither iproute (ip and tc) nor iptables supports
that notation so neither does Shorewall. <br>
<br>
<b>Example 1:</b><br>
<br>
/etc/shorewall/rules
<pre wrap=""><span class="moz-txt-citetags"></span> # Accept AUTH but only on address 192.0.2.125<br><span
class="moz-txt-citetags"></span><br><span class="moz-txt-citetags"></span> ACCEPT net fw:192.0.2.125 tcp auth<br><span
class="moz-txt-citetags"></span></pre>
<span class="moz-txt-citetags"></span><b>Example 2 (NAT):</b><br>
<br>
<span class="moz-txt-citetags"></span>/etc/shorewall/nat<br>
<pre wrap=""><span class="moz-txt-citetags"></span><span
class="moz-txt-citetags"></span> 192.0.2.126 eth0 10.1.1.126</pre>
/etc/shorewall/rules
<pre wrap=""><span class="moz-txt-citetags"></span> # Accept HTTP on 192.0.2.126 (a.k.a. 10.1.1.126)<br><span
class="moz-txt-citetags"></span><br> <span class="moz-txt-citetags"></span>ACCEPT net loc:10.1.1.126 tcp www<span
class="moz-txt-citetags"></span><br></pre>
<b>Example 3 (DNAT):<br>
</b>
<pre> # Forward SMTP on external address 192.0.2.127 to local system 10.1.1.127<br><br> DNAT net loc:10.1.1.127 tcp smtp - 192.0.2.127<br></pre>
<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 <20>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>
<br>
<div align="left"> </div>
<font size="2">Last updated 12/13/2002 - <a
href="support.htm">Tom Eastep</a></font>
<p><font face="Trebuchet MS"><a href="copyright.htm"><font size="2">Copyright</font>
<20> <font size="2">2001, 2002 Thomas M. Eastep.</font></a></font><br>
</p>
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