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<h1 align="center"><font color="#ffffff">Shorewall FAQs</font></h1>
</td>
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<h1>Looking for Step by Step Configuration Instructions? Check out the <a
href="shorewall_quickstart_guide.htm">QuickStart Guides</a>. <br>
</h1>
<h1>PORT FORWARDING<br>
</h1>
<p align="left"><b>1. </b><a href="#faq1"> I want to <b>forward</b> UDP <b>
port</b> 7777 to my my personal PC with
IP address 192.168.1.5. I've looked everywhere
and can't find <b>how to do it</b>.</a></p>
<p align="left"><b>1a. </b><a href="#faq1a">Ok -- I followed those instructions
but it doesn't work.<br>
</a></p>
<p align="left"><b>1b. </b><a href="#faq1b">I'm still having problems with
port forwarding</a></p>
<p align="left"><b>1c. </b><a href="#faq1c">From the internet, I want to <b>connect
to port 1022</b> on my firewall and have the <b>firewall forward the connection
to port 22 on local system 192.168.1.3</b>. How do I do that?</a><br>
</p>
<h1><b>DNS and PORT FORWARDING/NAT<br>
</b></h1>
<p align="left"><b>2.</b> <a href="#faq2">I <b>port forward</b> www requests
to www.mydomain.com (IP 130.151.100.69)
to system 192.168.1.5 in my local network. <b>External
clients can browse</b> http://www.mydomain.com but
<b>internal clients can't</b>.</a></p>
<p align="left"><b>2a. </b><a href="#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>
<h1><b>NETMEETING/MSN<br>
</b></h1>
<p align="left"><b>3. </b><a href="#faq3">I want to use <b>Netmeeting</b>
or <b>MSN Instant Messenger </b>with Shorewall.
What do I do?</a></p>
<h1><b>OPEN PORTS<br>
</b></h1>
<p align="left"><b>4. </b><a href="#faq4">I just used an online port scanner
to check my firewall and it shows <b>some
ports as 'closed' rather than 'blocked'.</b>
Why?</a></p>
<p align="left"><b>4a. </b><a href="#faq4a">I just ran an <b>nmap UDP scan</b>
of my firewall and it showed 100s of ports
as open!!!!<br>
</a></p>
<b>4b</b>. <a href="#faq4b">I have a port that I can't close
no matter how I change my rules. <br>
</a><b><br>
4c. </b><a href="#faq4c">How to I use Shorewall with <b>PortSentry</b>?</a><br>
<h1>CONNECTION PROBLEMS</h1>
<p align="left"><b>5. </b><a href="#faq5">I've installed Shorewall and now
I <b> can't ping</b> through the firewall</a><br>
<b><br>
15. </b><a href="#faq15"><b>My local systems can't see
out to the net<br>
</b></a></p>
<b>29. <a href="#faq29">FTP Doesn't Work</a></b><br>
<h1>LOGGING<br>
</h1>
<p align="left"><b>6. </b><a href="#faq6">Where are the <b>log messages</b>
written and how do I <b>change the destination</b>?</a></p>
<p align="left"><b>6a. </b><a href="#faq6a">Are there any <b>log parsers</b>
that work with Shorewall?</a></p>
<p align="left"><b>6b. <a href="#faq6b">DROP messages</a></b><a
href="#faq6b"> on port 10619 are <b>flooding the logs</b> with their connect
requests. Can i exclude these error messages for this
port temporarily from logging in Shorewall?</a><br>
</p>
<p align="left"><b>6c. </b><a href="#faq6c">All day long I get a steady flow
of these <b>DROP messages from port 53</b> <b>to some
high numbered port</b>. They get dropped, but what the
heck are they?</a><br>
</p>
<p align="left"><b>6d.</b> <a href="#faq6d">Why is the <b>MAC address</b>
in Shorewall log messages <b>so long</b>? I thought MAC addresses
were only 6 bytes in length.</a><b><br>
</b></p>
<p align="left"><b>16. </b><a href="#faq16">Shorewall is writing <b>log messages
all over my console</b> making it unusable!<br>
</a></p>
<b>17</b>. <a href="#faq17">How do I find out <b>why
this traffic is</b> getting <b>logged?</b></a><br>
<b><br>
21. </b><a href="#faq21">I see these <b>strange
log entries </b>occasionally; what are they?</a><br>
<h1>STARTING AND STOPPING<br>
</h1>
<p align="left"><b>7. </b><a href="#faq7">When I stop Shorewall <b>using
'shorewall stop', I can't connect to anything</b>. Why doesn't that command
work?</a></p>
<p align="left"><b>8. </b><a href="#faq8">When I try to <b>start Shorewall
on RedHat</b> I get messages about insmod
failing -- what's wrong?<br>
</a></p>
<p align="left"><b>8a. </b><a href="#faq8a">When I try to <b>start Shorewall
on RedHat</b> I get a message referring me to <b>FAQ #8</b></a><br>
</p>
<p align="left"><b>9. </b><a href="FAQ.htm#faq9">Why can't Shorewall <b>detect
my interfaces </b>properly at startup?</a></p>
<b>22. </b><a
href="#faq22">I have some <b>iptables commands </b>that I
want to <b>run when Shorewall starts.</b> Which file do I put
them in?</a><br>
<h1>ABOUT SHOREWALL<br>
</h1>
<p align="left"><b>10. </b><a href="#faq10">What <b>distributions</b> does
it work with?</a></p>
<p align="left"><b>11. </b><a href="#faq18">What <b>features</b> does it
support?</a></p>
<p align="left"><b>12. </b><a href="#faq12">Is there a <b>GUI?</b></a></p>
<p align="left"><b>13. </b><a href="#faq13">Why do you call it <b>"Shorewall"?</b></a></p>
<b>23. </b><a
href="#faq23">Why do you use such <b>ugly fonts</b> on
your <b>web site</b>?</a><br>
<b><br>
25. </b><a href="#faq25">How to I tell <b>which version
of Shorewall</b> I am <b>running</b>?</a><br>
<h1>RFC 1918<br>
</h1>
<p align="left"><b>14. </b><a href="#faq14">I'm connected via a cable modem
and it has an internel web server that
allows me to configure/monitor it but as expected
if I enable <b> rfc1918 blocking</b> for my eth0 interface,
it also blocks the <b>cable modems web server</b></a>.</p>
<p align="left"><b>14a. </b><a href="#faq14a">Even though it assigns public
IP addresses, my ISP's DHCP server has
an RFC 1918 address. If I enable RFC 1918 filtering
on my external interface, <b>my DHCP client cannot renew
its lease</b>.</a></p>
<h1>ALIAS IP ADDRESSES/VIRTUAL INTERFACES<br>
</h1>
<b>18.</b> <a
href="#faq18">Is there any way to use <b>aliased ip addresses</b>
with Shorewall, and maintain separate rulesets for
different IPs?</a><br>
<h1>MISCELLANEOUS<br>
</h1>
<b>19. </b><a href="#faq19">I have added <b>entries
to /etc/shorewall/tcrules</b> but they <b>don't </b>seem
to <b>do anything</b>. Why?</a><br>
<br>
<b>20. </b><a
href="#faq20">I have just set up a server. <b>Do I have
to change Shorewall to allow access to my server from the
internet?</b></a><br>
<br>
<b>24. </b><a href="#faq24">How
can I <b>allow conections</b> to let's say the ssh port
only<b> from specific IP Addresses</b> on the internet?</a><br>
<br>
<b>26. </b><a href="#faq26">When I try to use
any of the <b>SYN options in nmap</b> on or behind the firewall, I get
"<b>operation not permitted</b>". How can I use nmap with Shorewall?"</a><br>
<br>
<b>27. </b><a href="#faq27">I am compiling a <b>new kernel</b> for
my firewall<b>.</b> What should I look out for?</a><br>
<br>
<b>28. </b><a href="#faq28">How do I use Shorewall as a <b>Bridging
Firewall</b>?</a><br>
<hr>
<h4 align="left"><a name="faq1"></a>1. I want to forward UDP port 7777 to
my my personal PC with IP address 192.168.1.5.
I've looked everywhere and can't find how to
do it.</h4>
<p align="left"><b>Answer: </b>The <a
href="Documentation.htm#PortForward"> first example</a> in the <a
href="Documentation.htm#Rules">rules file documentation</a> shows how to
do port forwarding under Shorewall. The
format of a port-forwarding rule to a local system
is as follows:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1" cellspacing="1">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:<i><local IP address></i>[:<i><local
port</i>>]</td>
<td><i><protocol></i></td>
<td><i><port #></i></td>
<td> <br>
</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">So to forward UDP port 7777 to internal system 192.168.1.5,
the rule is:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1" cellspacing="1">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:192.168.1.5</td>
<td>udp</td>
<td>7777</td>
<td> <br>
</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<div align="left"> <font face="Courier"> </font>If
you want to forward requests directed to a particular
address ( <i><external IP></i> ) on your firewall
to an internal system:</div>
<blockquote>
<table border="1" cellpadding="2" cellspacing="0"
style="border-collapse: collapse;">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:<i><local IP address></i>[:<i><local
port</i>>]</td>
<td><i><protocol></i></td>
<td><i><port #></i></td>
<td>-</td>
<td><i><external IP></i></td>
</tr>
</tbody>
</table>
</blockquote>
Finally, if you need to forward a range
of ports, in the PORT column specify the range as <i>low-port</i>:<i>high-port</i>.<br>
<h4 align="left"><a name="faq1a"></a>1a. Ok -- I followed those instructions
but it doesn't work</h4>
<p align="left"><b>Answer: </b>That is usually the result of one of three
things:</p>
<ul>
<li>You
are trying to test from inside your firewall (no,
that won't work -- see <a href="#faq2">FAQ #2</a>).</li>
<li>You
have a more basic problem with your local system
such as an incorrect default gateway configured (it
should be set to the IP address of your firewall's internal
interface).</li>
<li>Your ISP is blocking that particular port inbound.<br>
</li>
</ul>
<h4 align="left"><a name="faq1b"></a>1b. I'm still having problems with port
forwarding</h4>
<b>Answer: </b>To
further diagnose this problem:<br>
<ul>
<li>As root,
type "iptables -t nat -Z". This clears the NetFilter
counters in the nat table.</li>
<li>Try to
connect to the redirected port from an external host.</li>
<li>As root
type "shorewall show nat"</li>
<li>Locate
the appropriate DNAT rule. It will be in a chain called
<i><source zone></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="faq1c"></a><b>1c. </b>From the internet, I want
to connect to port 1022 on my firewall and have the firewall forward
the connection to port 22 on local system 192.168.1.3. How do I do
that?</h4>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net<br>
</td>
<td>loc:192.168.1.3:22</td>
<td>tcp</td>
<td>1022<br>
</td>
<td><br>
</td>
<td><br>
</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<h4 align="left"><a name="faq2"></a>2. I port forward www requests to www.mydomain.com
(IP 130.151.100.69) to system 192.168.1.5
in my local network. External clients can browse
http://www.mydomain.com but internal clients can't.</h4>
<p align="left"><b>Answer: </b>I have two objections to this setup.</p>
<ul>
<li>Having
an internet-accessible server in your local network
is like raising foxes in the corner of your hen house.
If the server is compromised, there's nothing between
that server and your other internal systems. For the
cost of another NIC and a cross-over cable, you can put
your server in a DMZ such that it is isolated from your local
systems - assuming that the Server can be located near the
Firewall, of course :-)</li>
<li>The
accessibility problem is best solved using <a
href="shorewall_setup_guide.htm#DNS">Bind Version 9 "views"</a>
(or using a separate DNS server for local clients) such that www.mydomain.com
resolves to 130.141.100.69 externally and 192.168.1.5
internally. That's what I do here at shorewall.net
for my local systems that use 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.<br>
</p>
<p align="left">If you are running Shorewall 1.4.0 or earlier see <a
href="1.3/FAQ.htm#faq2">the 1.3 FAQ</a> for instructions suitable for
those releases.<br>
</p>
<p align="left">If you are running Shorewall 1.4.1 or Shorewall 1.4.1a, please
upgrade to Shorewall 1.4.2 or later.<br>
</p>
<p align="left">Otherwise:<br>
</p>
<ul>
</ul>
<ul>
</ul>
<ul>
<li>In /etc/shorewall/interfaces:</li>
</ul>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top">ZONE<br>
</td>
<td valign="top">INTERFACE<br>
</td>
<td valign="top">BROADCAST<br>
</td>
<td valign="top">OPTIONS<br>
</td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">eth1<br>
</td>
<td valign="top">detect<br>
</td>
<td valign="top"><b>routeback<br>
</b> </td>
</tr>
</tbody>
</table>
</blockquote>
<ul>
</ul>
<ul>
<li>In /etc/shorewall/rules: </li>
</ul>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;">
<tbody>
<tr>
<td><b>ACTION</b></td>
<td><b>SOURCE</b></td>
<td><b>DEST</b></td>
<td><b> PROTO</b></td>
<td><b>DEST<br>
PORT(S)</b></td>
<td><b>SOURCE<br>
PORT(S)</b></td>
<td><b>ORIGINAL<br>
DEST</b></td>
</tr>
<tr>
<td>DNAT<br>
</td>
<td>loc</td>
<td>web:192.168.1.5<br>
</td>
<td>tcp</td>
<td>www</td>
<td> -<br>
</td>
<td>130.151.100.69:192.168.1.254<br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<div align="left">
<p align="left">That rule only works of course if you have a static external
IP address. If you have a dynamic IP
address and are running Shorewall 1.3.4 or later
then include this in /etc/shorewall/init:</p>
</div>
<div align="left">
<pre> ETH0_IP=`find_interface_address eth0`</pre>
</div>
<div align="left">
<p align="left">and make your DNAT rule:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE PORT</b></u></td>
<td><u><b>ORIG. DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>loc</td>
<td>web:192.168.1.5</td>
<td>tcp</td>
<td>www</td>
<td>-</td>
<td>$ETH0_IP:192.168.1.254</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Using this technique, you will want to configure your DHCP/PPPoE
client to automatically restart Shorewall
each time that you get a new IP address.</p>
</div>
<h4 align="left"><a name="faq2a"></a>2a. I have a zone "Z" with an RFC1918
subnet and I use 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->Z
traffic through your firewall then:</p>
<p align="left">a) Set the Z->Z policy to ACCEPT.<br>
b)
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><br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">In /etc/shorewall/policy:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>SOURCE </b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>POLICY</b></u></td>
<td><u><b>LIMIT:BURST</b></u></td>
</tr>
<tr>
<td>dmz</td>
<td>dmz</td>
<td>ACCEPT</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">In /etc/shorewall/masq:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3" width="369">
<tbody>
<tr>
<td width="93"><u><b>INTERFACE </b></u></td>
<td width="31"><u><b>SUBNET</b></u></td>
<td width="120"><u><b>ADDRESS</b></u></td>
</tr>
<tr>
<td width="93">eth2</td>
<td width="31">192.168.2.0/24</td>
<td width="120"> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<h4 align="left"><a name="faq3"></a>3. I want to use Netmeeting or MSN Instant
Messenger with Shorewall. What do I do?</h4>
<p align="left"><b>Answer: </b>There is an <a
href="http://www.kfki.hu/%7Ekadlec/sw/netfilter/newnat-suite/"> H.323 connection
tracking/NAT module</a> that helps with
Netmeeting. Look <a
href="http://linux-igd.sourceforge.net">here</a> for a solution
for MSN IM but be aware that there are significant security risks
involved with this solution. Also check the Netfilter mailing
list archives at <a href="http://www.netfilter.org">http://www.netfilter.org</a>.
</p>
<h4 align="left"><a name="faq4"></a>4. I just used an online port scanner
to check my firewall and it shows some
ports as 'closed' rather than 'blocked'. Why?</h4>
<p align="left"><b>Answer: </b>The common.def included with version 1.3.x
always rejects connection requests on
TCP port 113 rather than dropping them. This
is necessary to prevent outgoing connection problems
to services that use the 'Auth' mechanism for identifying
requesting users. Shorewall also rejects TCP ports
135, 137 and 139 as well as UDP ports 137-139. These are
ports that are used by Windows (Windows <u>can</u> be configured
to use the DCE cell locator on port 135). Rejecting these connection
requests rather than dropping them cuts down slightly on the
amount of Windows chatter on LAN segments connected to the
Firewall. </p>
<p align="left">If you are seeing port 80 being 'closed', that's probably
your ISP preventing you from running
a web server in violation of your Service Agreement.</p>
<h4 align="left"><a name="faq4a"></a>4a. I just ran an nmap UDP scan of my
firewall and it showed 100s of ports
as open!!!!</h4>
<p align="left"><b>Answer: </b>Take a deep breath and read the nmap man page
section about UDP scans. If nmap gets
<b>nothing</b> back from your firewall then
it reports the port as open. If you want to see which
UDP ports are really open, temporarily change your net->all
policy to REJECT, restart Shorewall and do the nmap
UDP scan again.<br>
</p>
<h4><a name="faq4b"></a>4b. I have a port that I can't close no matter how
I change my rules.�</h4>
I had a rule that allowed telnet from my local network to my
firewall; I removed that rule and restarted Shorewall but my telnet
session still works!!!<br>
<br>
<b>Answer: </b>�Rules only govern the establishment of new
connections. Once a connection is established through the firewall
it will be usable until disconnected (tcp) or until it times out (other
protocols).� If you stop telnet and try to establish a new session your
firerwall will block that attempt.<br>
<h4 align="left"><a name="faq4c"></a><b>4c. </b>How to I use Shorewall with
PortSentry?</h4>
<a
href="http://www.shorewall.net/pub/shorewall/contrib/PortsentryHOWTO.txt">Here's
a writeup</a> on a nice integration of Shorewall and PortSentry.<br>
<h4 align="left"><a name="faq5"></a>5. I've installed Shorewall and now I
can't ping through the firewall</h4>
<p align="left"><b>Answer: </b>If you want your firewall to be totally open
for "ping", </p>
<p align="left">a) Create /etc/shorewall/common if it doesn't already exist.
<br>
b)
Be sure that the first command in the file is ". /etc/shorewall/common.def"<br>
c)
Add the following to /etc/shorewall/common </p>
<blockquote>
<p align="left">run_iptables -A icmpdef -p ICMP --icmp-type echo-request
-j ACCEPT<br>
</p>
</blockquote>
For a complete description
of Shorewall 'ping' management, see <a href="ping.html">this
page</a>.
<h4 align="left"><a name="faq6"></a>6. Where are the log messages written
and how do I change the destination?</h4>
<p align="left"><b>Answer: </b>NetFilter uses the kernel's equivalent of syslog
(see "man syslog") to log messages. It always uses the LOG_KERN (kern) facility
(see "man openlog") and you get to choose the log level (again, see "man
syslog") in your <a href="Documentation.htm#Policy">policies</a> and <a
href="Documentation.htm#Rules">rules</a>. The destination for messaged
logged by syslog is controlled by /etc/syslog.conf (see "man syslog.conf").
When you have changed /etc/syslog.conf, be
sure to restart syslogd (on a RedHat system, "service
syslog restart"). </p>
<p align="left">By default, older versions of Shorewall ratelimited log messages
through <a
href="Documentation.htm#Conf">settings</a> in /etc/shorewall/shorewall.conf
-- If you want to log all messages, set: </p>
<div align="left">
<pre align="left"> LOGLIMIT=""<br> LOGBURST=""<br></pre>
Beginning with Shorewall version 1.3.12, you can <a
href="shorewall_logging.html">set up Shorewall to log all of its messages
to a separate file</a>.<br>
</div>
<h4 align="left"><a name="faq6a"></a>6a. Are there any log parsers that work
with Shorewall?</h4>
<p align="left"><b>Answer: </b>Here are several links that may be helpful:
</p>
<blockquote>
<p align="left"><a
href="http://www.shorewall.net/pub/shorewall/parsefw/"> http://www.shorewall.net/pub/shorewall/parsefw/</a><br>
<a
href="http://www.fireparse.com">http://www.fireparse.com</a><br>
<a
href="http://cert.uni-stuttgart.de/projects/fwlogwatch">http://cert.uni-stuttgart.de/projects/fwlogwatch</a><a
href="http://www.logwatch.org"><br>
http://www.logwatch.org</a><br>
<a href="http://gege.org/iptables">http://gege.org/iptables</a><br>
<a href="http://home.regit.org/ulogd-php.html"><font size="3">http://home.regit.org/ulogd-php.html</font></a><br>
</p>
</blockquote>
I personnaly use Logwatch.
It emails me a report each day from my various systems
with each report summarizing the logged activity on the corresponding
system.
<h4 align="left"><b><a name="faq6b"></a>6b. DROP messages</b> on port 10619
are <b>flooding the logs</b> with their connect requests.
Can i exclude these error messages for this port temporarily
from logging in Shorewall?</h4>
Temporarily add the following rule:<br>
<pre> DROP net fw udp 10619</pre>
<h4 align="left"><a name="faq6c"></a>6c. All day long I get a steady flow
of these DROP messages from port 53 to some high numbered
port. They get dropped, but what the heck are they?</h4>
<pre>Jan 8 15:50:48 norcomix kernel: Shorewall:net2all:DROP:IN=eth0 OUT= MAC=00:40:c7:2e:09:c0:00:01:64:4a:70:00:08:00<br> SRC=208.138.130.16 DST=24.237.22.45 LEN=53 TOS=0x00 PREC=0x00<br> TTL=251 ID=8288 DF PROTO=UDP SPT=53 DPT=40275 LEN=33 </pre>
<b>Answer: </b>There are two possibilities:<br>
<ol>
<li>They are late-arriving replies
to DNS queries.</li>
<li>They are corrupted reply packets.</li>
</ol>
You can distinguish the difference
by setting the <b>logunclean</b> option (<a
href="Documentation.htm#Interfaces">/etc/shorewall/interfaces</a>)
on your external interface (eth0 in the above example). If they get
logged twice, they are corrupted. I solve this problem by using
an /etc/shorewall/common file like this:<br>
<blockquote>
<pre>#<br># Include the standard common.def file<br>#<br>. /etc/shorewall/common.def<br>#<br># The following rule is non-standard and compensates for tardy<br># DNS replies<br>#<br>run_iptables -A common -p udp --sport 53 -mstate --state NEW -j DROP</pre>
</blockquote>
The above file is also include in all
of my sample configurations available in the <a
href="shorewall_quickstart_guide.htm">Quick Start Guides</a> and in
the common.def file in Shorewall 1.4.0 and later.<br>
<h4 align="left"><a name="faq6d"></a><b>6d.</b> Why is the MAC address in
Shorewall log messages so long? I thought MAC addresses were
only 6 bytes in length.</h4>
What is labeled as the MAC address in a Shorewall
log message is actually the Ethernet frame header. IT contains:<br>
<ul>
<li>the destination MAC address (6 bytes)</li>
<li>the source MAC address (6 bytes)</li>
<li>the ethernet frame type (2 bytes)</li>
</ul>
Example:<br>
<br>
MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00<br>
<ul>
<li>Destination MAC address = 00:04:4c:dc:e2:28</li>
<li>Source MAC address = 00:b0:8e:cf:3c:4c</li>
<li>Ethernet Frame Type = 08:00 (IP Version
4)</li>
</ul>
<h4 align="left"><a name="faq7"></a>7. When I stop Shorewall using 'shorewall
stop', I can't connect to anything. Why
doesn't that command work?</h4>
<p align="left">The 'stop' command is intended to place your firewall into
a safe state whereby only those hosts listed
in /etc/shorewall/routestopped' are activated.
If you want to totally open up your firewall, you must
use the 'shorewall clear' command. </p>
<h4 align="left"><a name="faq8"></a>8. When I try to start Shorewall on RedHat,
I get messages about insmod failing -- what's wrong?</h4>
<p align="left"><b>Answer: </b>The output you will see looks something like
this:</p>
<pre> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy<br> Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters<br> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod<br> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed<br> /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed<br> iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?)<br> Perhaps iptables or your kernel needs to be upgraded.</pre>
<p align="left">This is usually cured by the following sequence of commands:
</p>
<div align="left">
<pre align="left"> <b><font color="#009900">service ipchains stop<br> chkconfig --delete ipchains<br> rmmod ipchains</font></b></pre>
</div>
<div align="left">
<p align="left">Also, be sure to check the <a href="errata.htm">errata</a>
for problems concerning the version of
iptables (v1.2.3) shipped with RH7.2.<br>
</p>
<h4><a name="faq8a"></a><b>8a. </b>When I try to start Shorewall on RedHat
I get a message referring me to FAQ #8</h4>
<b>Answer:</b> This is usually cured by the sequence
of commands shown above in FAQ #8
</div>
<h4 align="left"><a name="faq9"></a>9. Why can't Shorewall detect my interfaces
properly at startup?</h4>
<p align="left">I just installed Shorewall and when I issue the start command,
I see the following:</p>
<div align="left">
<pre> Processing /etc/shorewall/params ...<br> Processing /etc/shorewall/shorewall.conf ...<br> Starting Shorewall...<br> Loading Modules...<br> Initializing...<br> Determining Zones...<br> Zones: net loc<br> Validating interfaces file...<br> Validating hosts file...<br> Determining Hosts in Zones...<br><b> Net Zone: eth0:0.0.0.0/0<br> Local Zone: eth1:0.0.0.0/0<br></b> Deleting user chains...<br> Creating input Chains...<br> ...</pre>
</div>
<div align="left">
<p align="left">Why can't Shorewall detect my interfaces properly?</p>
</div>
<div align="left">
<p align="left"><b>Answer: </b>The above output is perfectly normal. The Net
zone is defined as all hosts that are connected through eth0 and the local
zone is defined as all hosts connected through eth1</p>
</div>
<h4 align="left"><a name="faq10"></a>10. What Distributions does it work
with?</h4>
<p align="left">Shorewall works with any GNU/Linux distribution that includes
the <a
href="shorewall_prerequisites.htm">proper prerequisites</a>.</p>
<h4 align="left">11. What Features does it have?</h4>
<p align="left"><b>Answer: </b>See the <a href="shorewall_features.htm">Shorewall
Feature List</a>.</p>
<h4 align="left"><a name="faq12"></a>12. Is there a GUI?</h4>
<p align="left"><b>Answer: </b>Yes. Shorewall support is included in Webmin
1.060 and later versions. See <a
href="http://www.webmin.com">http://www.webmin.com</a> </p>
<h4 align="left"> <a name="faq13"></a>13. Why do you call it "Shorewall"?</h4>
<p align="left"><b>Answer: </b>Shorewall is a concatenation of "<u>Shore</u>line"
(<a
href="http://www.cityofshoreline.com">the city where I live</a>)
and "Fire<u>wall</u>". The full name of the product
is actually "Shoreline Firewall" but "Shorewall" is must more
commonly used.</p>
<h4 align="left"> <a name="faq14"></a>14. I'm connected via a cable modem
and it has an internal web server that
allows me to configure/monitor it but as expected
if I enable rfc1918 blocking for my eth0 interface
(the internet one), it also blocks the cable modems web server.</h4>
<p align="left">Is there any way it can add a rule before the rfc1918 blocking
that will let all traffic to and from the
192.168.100.1 address of the modem in/out but
still block all other rfc1918 addresses?</p>
<p align="left"><b>Answer: </b>If you are running a version of Shorewall earlier
than 1.3.1, create /etc/shorewall/start and in it, place the following:</p>
<div align="left">
<pre> run_iptables -I rfc1918 -s 192.168.100.1 -j ACCEPT</pre>
</div>
<div align="left">
<p align="left">If you are running version 1.3.1 or later, simply add the
following to<a
href="Documentation.htm#rfc1918"> /etc/shorewall/rfc1918</a>:</p>
</div>
<div align="left">
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber3">
<tbody>
<tr>
<td><u><b>SUBNET </b></u></td>
<td><u><b>TARGET</b></u></td>
</tr>
<tr>
<td>192.168.100.1</td>
<td>RETURN</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<p align="left">Be sure that you add the entry ABOVE the entry for 192.168.0.0/16.<br>
</p>
<p align="left">Note: If you add a second IP address to your external firewall
interface to correspond to the modem address,
you must also make an entry in /etc/shorewall/rfc1918
for that address. For example, if you configure the
address 192.168.100.2 on your firewall, then you would add
two entries to /etc/shorewall/rfc1918: <br>
</p>
<blockquote>
<table cellpadding="2" border="1" style="border-collapse: collapse;">
<tbody>
<tr>
<td valign="top"><u><b>SUBNET</b></u><br>
</td>
<td valign="top"><u><b>TARGET</b></u><br>
</td>
</tr>
<tr>
<td valign="top">192.168.100.1<br>
</td>
<td valign="top">RETURN<br>
</td>
</tr>
<tr>
<td valign="top">192.168.100.2<br>
</td>
<td valign="top">RETURN<br>
</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div align="left">
<h4 align="left"><a name="faq14a"></a>14a. Even though it assigns public IP
addresses, my ISP's DHCP server has an RFC 1918 address. If I enable RFC
1918 filtering on my external interface, my DHCP client cannot renew its
lease.</h4>
</div>
<div align="left">
<p align="left">The solution is the same as FAQ 14 above. Simply substitute
the IP address of your ISPs DHCP server.</p>
</div>
<h4 align="left"><a name="faq15"></a>15. My local systems can't see out to
the net</h4>
<p align="left"><b>Answer: </b>Every time I read "systems can't see out to
the net", I wonder where the poster bought
computers with eyes and what those computers
will "see" when things are working properly. That
aside, the most common causes of this problem are:</p>
<ol>
<li>
<p align="left">The default gateway on each local system isn't set to
the IP address of the local firewall
interface.</p>
</li>
<li>
<p align="left">The entry for the local network in the /etc/shorewall/masq
file is wrong or missing.</p>
</li>
<li>
<p align="left">The DNS settings on the local systems are wrong or the
user is running a DNS server on the
firewall and hasn't enabled UDP and TCP port
53 from the firewall to the internet.</p>
</li>
</ol>
<h4 align="left"><a name="faq16"></a>16. Shorewall is writing log messages
all over my console making it unusable!</h4>
<p align="left"><b>Answer: </b>If you are running Shorewall version 1.4.4
or 1.4.4a then check the <a href="errata.htm">errata.</a> Otherwise,
see the 'dmesg' man page ("man dmesg"). You must add a suitable 'dmesg'
command to your startup scripts or place
it in /etc/shorewall/start. Under RedHat, the
max log level that is sent to the console is specified
in /etc/sysconfig/init in the LOGLEVEL variable.<br>
</p>
<h4><a name="faq17"></a>17. How do I find out why this traffic is getting
logged?</h4>
<b>Answer:
</b>Logging occurs out of a number of chains (as indicated
in the log message) in Shorewall:<br>
<ol>
<li><b>man1918
</b>or <b>logdrop - </b>The destination address
is listed in /etc/shorewall/rfc1918 with a <b>logdrop
</b>target -- see <a href="Documentation.htm#rfc1918">/etc/shorewall/rfc1918.</a></li>
<li><b>rfc1918</b>
or <b>logdrop </b>- The source address is listed in /etc/shorewall/rfc1918
with a <b>logdrop </b>target -- see <a
href="Documentation.htm#rfc1918">/etc/shorewall/rfc1918.</a></li>
<li><b>all2<zone></b>,
<b><zone>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><zone1>2<zone2>
</b>- Either you have a<a
href="Documentation.htm#Policy"> policy</a> for <b><zone1>
</b>to <b><zone2></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><interface>_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.
See <a href="Shorewall_and_Aliased_Interfaces.html">Shorewall and Aliased
Interfaces</a>.
<h4><b><a name="faq19"></a>19. </b>I have added entries to /etc/shorewall/tcrules
but they don't seem to do anything. Why?</h4>
You probably haven't
set TC_ENABLED=Yes in /etc/shorewall/shorewall.conf
so the contents of the tcrules file are simply being ignored.<br>
<h4><a name="faq20"></a><b>20. </b>I have just set up a server. <b>Do I have
to change Shorewall to allow access to my server
from the internet?</b><br>
</h4>
Yes. Consult the
<a href="shorewall_quickstart_guide.htm">QuickStart guide</a> that
you used during your initial setup for information about how to set
up rules for your server.<br>
<h4><a name="faq21"></a><b>21. </b>I see these <b>strange log entries </b>occasionally;
what are they?<br>
</h4>
<blockquote>
<pre>Nov 25 18:58:52 linux kernel: Shorewall:net2all:DROP:IN=eth1 OUT= MAC=00:60:1d:f0:a6:f9:00:60:1d:f6:35:50:08:00<br> SRC=206.124.146.179 DST=192.0.2.3 LEN=56 TOS=0x00 PREC=0x00 TTL=110 ID=18558 PROTO=ICMP TYPE=3 CODE=3 <br> [SRC=192.0.2.3 DST=172.16.1.10 LEN=128 TOS=0x00 PREC=0x00 TTL=47 ID=0 DF PROTO=UDP SPT=53 DPT=2857 LEN=108 ]<br></pre>
</blockquote>
192.0.2.3 is external
on my firewall... 172.16.0.0/24 is my internal LAN<br>
<br>
<b>Answer: </b>While
most people associate the Internet Control Message
Protocol (ICMP) with 'ping', ICMP is a key piece of the
internet. ICMP is used to report problems back to the sender
of a packet; this is what is happening here. Unfortunately,
where NAT is involved (including SNAT, DNAT and Masquerade),
there are a lot of broken implementations. That is what you are
seeing with these messages.<br>
<br>
Here is my interpretation
of what is happening -- to confirm this analysis, one
would have to have packet sniffers placed a both ends of the
connection.<br>
<br>
Host 172.16.1.10 behind
NAT gateway 206.124.146.179 sent a UDP DNS query
to 192.0.2.3 and your DNS server tried to send a response
(the response information is in the brackets -- note source
port 53 which marks this as a DNS reply). When the response was
returned to to 206.124.146.179, it rewrote the destination IP
TO 172.16.1.10 and forwarded the packet to 172.16.1.10 who no longer
had a connection on UDP port 2857. This causes a port unreachable
(type 3, code 3) to be generated back to 192.0.2.3. As this packet
is sent back through 206.124.146.179, that box correctly changes
the source address in the packet to 206.124.146.179 but doesn't
reset the DST IP in the original DNS response similarly. When
the ICMP reaches your firewall (192.0.2.3), your firewall has no
record of having sent a DNS reply to 172.16.1.10 so this ICMP doesn't
appear to be related to anything that was sent. The final result
is that the packet gets logged and dropped in the all2all chain. I
have also seen cases where the source IP in the ICMP itself isn't set
back to the external IP of the remote NAT gateway; that causes your
firewall to log and drop the packet out of the rfc1918 chain because
the source IP is reserved by RFC 1918.<br>
<h4><a name="faq22"></a><b>22. </b>I have some <b>iptables commands </b>that
I want to <b>run when Shorewall starts.</b> Which
file do I put them in?</h4>
You can place these
commands in one of the <a
href="shorewall_extension_scripts.htm">Shorewall Extension Scripts</a>.
Be sure that you look at the contents of the chain(s) that you will
be modifying with your commands to be sure that the commands
will do what they are intended. Many iptables commands published
in HOWTOs and other instructional material use the -A command
which adds the rules to the end of the chain. Most chains that Shorewall
constructs end with an unconditional DROP, ACCEPT or REJECT rule
and any rules that you add after that will be ignored. Check "man iptables"
and look at the -I (--insert) command.<br>
<h4><a name="faq23"></a><b>23. </b>Why do you use such ugly fonts on your
web site?</h4>
The Shorewall web site is almost
font neutral (it doesn't explicitly specify fonts except
on a few pages) so the fonts you see are largely the default
fonts configured in your browser. If you don't like them then
reconfigure your browser.<br>
<h4><a name="faq24"></a>24. How can I <b>allow conections</b> to let's say
the ssh port only<b> from specific IP Addresses</b> on
the internet?</h4>
In the SOURCE column of the rule,
follow "net" by a colon and a list of the host/subnet addresses
as a comma-separated list.<br>
<pre> net:<ip1>,<ip2>,...<br></pre>
Example:<br>
<pre> ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22<br></pre>
<div align="left"> </div>
<h4><b><a name="faq25"></a>25. </b>How to I tell <b>which version of Shorewall</b>
I am <b>running</b>?<br>
</h4>
At the shell prompt, type:<br>
<br>
<font color="#009900"><b> /sbin/shorewall
version</b></font><br>
<h4><a name="faq26"></a><b>26. </b>When I try to use any of the SYN options
in nmap on or behind the firewall, I get "operation not permitted".
How can I use nmap with Shorewall?"</h4>
Edit /etc/shorewall/shorewall.conf and change "NEWNOTSYN=No" to
"NEWNOTSYN=Yes" then restart Shorewall.<br>
<h4><a name="faq27">27. I'm compiling a new kernel for my firewall. What
should I look out for?</a></h4>
First take a look at the <a href="kernel.htm">Shorewall kernel configuration
page</a>. You probably also want to be sure that you have selected the
"<b>NAT of local connections (READ HELP)</b>" on the Netfilter Configuration
menu. Otherwise, DNAT rules with your firewall as the source zone won't
work with your new kernel.<br>
<h4><a name="faq28"></a>28. How do I use Shorewall as a Bridging Firewall?<br>
</h4>
Basically, you don't. While there are kernel patches that allow you
to route bridge traffic through Netfilter, the environment is so different
from the Layer 3 firewalling environment that very little of Shorewall
works. In fact, so much of Shorewall doesn't work that my official position
is that "Shorewall doesn't work with Layer 2 Bridging".<br>
<h4><a name="faq29"></a><b>29. FTP Doesn't Work</b><br>
</h4>
See the <a href="FTP.html">Shorewall and FTP page</a>.<br>
<br>
<font size="2">Last updated 7/30/2003 - <a
href="support.htm">Tom Eastep</a></font>
<p><a href="copyright.htm"><font size="2">Copyright</font> � <font
size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a><br>
</p>
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