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shorewall_code/Shorewall-docs/FAQ.htm
teastep 10b51d1991 Shorewall 1.3.14 Release 
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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>
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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</b>
or <b>MSN Instant 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>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>.<2E>
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>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">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>
<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>
<br>
<b>23. </b><a href="#faq23">Why do you use such <b>ugly fonts</b>
on your <b>web site</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>
<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>
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 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>&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
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>
<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 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 may help 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.</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="shorewall_logging.html">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>
<a href="http://gege.org/iptables">http://gege.org/iptables</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<4F><50><EFBFBD> net<65><74><EFBFBD> fw<66><77><EFBFBD> udp<64><70><EFBFBD> 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.<2E> They
get dropped, but what the heck are they?</h4>
<pre>Jan<EFBFBD> 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>.<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. What is labeled as the MAC address in a Shorewall log message
is actually the Ethernet frame header. In contains:<br>
</h4>
<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"> 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. 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>"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
-- 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 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><EFBFBD><EFBFBD><EFBFBD> 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>
<h4></h4>
<div align="left"> </div>
<font size="2">Last updated 2/6/2003 - <a
href="support.htm">Tom Eastep</a></font>
<p><a href="copyright.htm"><font size="2">Copyright</font>
<EFBFBD> <font size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a><br>
</p>
</body>
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