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<title>Shorewall FAQ</title>
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<td
width="100%">
<h1 align="center"><font color="#ffffff">Shorewall FAQs</font></h1>
</td>
</tr>
</tbody>
</table>
<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.<2E></a>
<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</b></a></p>
<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>
<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" cellspacing="1">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE
PORT</b></u></td>
<td><u><b>ORIG.
DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:<i>&lt;local
IP address&gt;</i>[:<i>&lt;local port</i>&gt;]</td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port
#&gt;</i></td>
<td> <br>
</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p align="left">So to forward UDP port 7777 to internal system 192.168.1.5,
the rule is:</p>
<blockquote>
<table border="1" cellpadding="2" style="border-collapse: collapse;"
id="AutoNumber1" cellspacing="1">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE
PORT</b></u></td>
<td><u><b>ORIG.
DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:192.168.1.5</td>
<td>udp</td>
<td>7777</td>
<td> <br>
</td>
<td> <br>
</td>
</tr>
</tbody>
</table>
</blockquote>
<div align="left"> <font face="Courier"> </font>If
you want to forward requests directed to a particular address
( <i>&lt;external IP&gt;</i> ) on your firewall to an internal
system:</div>
<blockquote>
<table border="1" cellpadding="2" cellspacing="0"
style="border-collapse: collapse;">
<tbody>
<tr>
<td><u><b>ACTION</b></u></td>
<td><u><b>SOURCE</b></u></td>
<td><u><b>DESTINATION</b></u></td>
<td><u><b>PROTOCOL</b></u></td>
<td><u><b>PORT</b></u></td>
<td><u><b>SOURCE
PORT</b></u></td>
<td><u><b>ORIG.
DEST.</b></u></td>
</tr>
<tr>
<td>DNAT</td>
<td>net</td>
<td>loc:<i>&lt;local
IP address&gt;</i>[:<i>&lt;local port</i>&gt;]</td>
<td><i>&lt;protocol&gt;</i></td>
<td><i>&lt;port
#&gt;</i></td>
<td>-</td>
<td><i>&lt;external
IP&gt;</i></td>
</tr>
</tbody>
</table>
</blockquote>
Finally, if you need to forward a range of ports, in
the PORT column specify the range as <i>low-port</i>:<i>high-port</i>.<br>
<h4 align="left"><a name="faq1a"></a>1a. Ok -- I followed those instructions
but it doesn't work</h4>
<p align="left"><b>Answer: </b>That is usually the result of one of three
things:</p>
<ul>
<li>You are trying
to test from inside your firewall (no, that won't
work -- see <a href="#faq2">FAQ #2</a>).</li>
<li>You have a
more basic problem with your local system 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>&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="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="file:///Z:/Shorewall-docs/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-&gt;Z traffic through your firewall then:</p>
<p align="left">a) Set the Z-&gt;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 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.<br>
</p>
<h4><a name="faq4b"></a>4b. I have a port that I can't close no matter how
I change my rules.<2E></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><EFBFBD>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).<2E> If you stop telnet
and try to establish a new session your firerwall will block that attempt.<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>
</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>"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. See <a
href="Shorewall_and_Aliased_Interfaces.html">Shorewall and Aliased Interfaces</a>.
<h4><b><a name="faq19"></a>19. </b>I have added entries to /etc/shorewall/tcrules
but they don't seem to do anything. Why?</h4>
You probably haven't set TC_ENABLED=Yes
in /etc/shorewall/shorewall.conf so the contents of the
tcrules file are simply being ignored.<br>
<h4><a name="faq20"></a><b>20. </b>I have just set up a server. <b>Do I have
to change Shorewall to allow access to my server from
the internet?</b><br>
</h4>
Yes. Consult the <a
href="shorewall_quickstart_guide.htm">QuickStart guide</a> that you
used during your initial setup for information about how to set up
rules for your server.<br>
<h4><a name="faq21"></a><b>21. </b>I see these <b>strange log entries </b>occasionally;
what are they?<br>
</h4>
<blockquote>
<pre>Nov 25 18:58:52 linux kernel: Shorewall:net2all:DROP:IN=eth1 OUT= MAC=00:60:1d:f0:a6:f9:00:60:1d:f6:35:50:08:00<br> SRC=206.124.146.179 DST=192.0.2.3 LEN=56 TOS=0x00 PREC=0x00 TTL=110 ID=18558 PROTO=ICMP TYPE=3 CODE=3 <br> [SRC=192.0.2.3 DST=172.16.1.10 LEN=128 TOS=0x00 PREC=0x00 TTL=47 ID=0 DF PROTO=UDP SPT=53 DPT=2857 LEN=108 ]<br></pre>
</blockquote>
192.0.2.3 is external on my firewall...
172.16.0.0/24 is my internal LAN<br>
<br>
<b>Answer: </b>While most people
associate the Internet Control Message Protocol (ICMP)
with 'ping', ICMP is a key piece of the internet. ICMP is
used to report problems back to the sender of a packet; this is
what is happening here. Unfortunately, where NAT is involved (including
SNAT, DNAT and Masquerade), there are a lot of broken implementations.
That is what you are seeing with these messages.<br>
<br>
Here is my interpretation of what
is happening -- to confirm this analysis, one would have to
have packet sniffers placed a both ends of the connection.<br>
<br>
Host 172.16.1.10 behind NAT gateway
206.124.146.179 sent a UDP DNS query to 192.0.2.3 and your
DNS server tried to send a response (the response information
is in the brackets -- note source port 53 which marks this as a
DNS reply). When the response was returned to to 206.124.146.179,
it rewrote the destination IP TO 172.16.1.10 and forwarded the packet
to 172.16.1.10 who no longer had a connection on UDP port 2857.
This causes a port unreachable (type 3, code 3) to be generated back
to 192.0.2.3. As this packet is sent back through 206.124.146.179,
that box correctly changes the source address in the packet to 206.124.146.179
but doesn't reset the DST IP in the original DNS response similarly.
When the ICMP reaches your firewall (192.0.2.3), your firewall has
no record of having sent a DNS reply to 172.16.1.10 so this ICMP doesn't
appear to be related to anything that was sent. The final result
is that the packet gets logged and dropped in the all2all chain. I have
also seen cases where the source IP in the ICMP itself isn't set back
to the external IP of the remote NAT gateway; that causes your firewall
to log and drop the packet out of the rfc1918 chain because the source
IP is reserved by RFC 1918.<br>
<h4><a name="faq22"></a><b>22. </b>I have some <b>iptables commands </b>that
I want to <b>run when Shorewall starts.</b> Which file do
I put them in?</h4>
You can place these commands in
one of the <a href="shorewall_extension_scripts.htm">Shorewall Extension
Scripts</a>. Be sure that you look at the contents of the chain(s) that
you will be modifying with your commands to be sure that the
commands will do what they are intended. Many iptables commands
published in HOWTOs and other instructional material use the -A
command which adds the rules to the end of the chain. Most chains
that Shorewall constructs end with an unconditional DROP, ACCEPT or
REJECT rule and any rules that you add after that will be ignored.
Check "man iptables" and look at the -I (--insert) command.<br>
<h4><a name="faq23"></a><b>23. </b>Why do you use such ugly fonts on your
web site?</h4>
The Shorewall web site is almost font neutral
(it doesn't explicitly specify fonts except on a few pages)
so the fonts you see are largely the default fonts configured in
your browser. If you don't like them then reconfigure your browser.<br>
<h4><a name="faq24"></a>24. How can I <b>allow conections</b> to let's say
the ssh port only<b> from specific IP Addresses</b> on the internet?</h4>
In the SOURCE column of the rule, follow "net"
by a colon and a list of the host/subnet addresses as a comma-separated
list.<br>
<pre> net:&lt;ip1&gt;,&lt;ip2&gt;,...<br></pre>
Example:<br>
<pre> ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22<br></pre>
<div align="left"> </div>
<h4><b><a name="faq25"></a>25. </b>How to I tell <b>which version of Shorewall</b>
I am <b>running</b>?<br>
</h4>
At the shell prompt, type:<br>
<br>
<font color="#009900"><b> /sbin/shorewall version</b></font><br>
<br>
<font size="2">Last updated 4/14/2003 - <a
href="support.htm">Tom Eastep</a></font>
<p><a href="copyright.htm"><font size="2">Copyright</font> <20> <font
size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a><br>
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