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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Shorewall FAQs</title><meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /></head><body><div class="article" lang="en" xml:lang="en"><div class="titlepage"><div><div><h1 class="title"><a id="id2590562"></a>Shorewall FAQs</h1></div><div><div class="authorgroup"><h3 class="corpauthor">Shorewall Community</h3><div class="author"><h3 class="author"><span class="firstname">Tom</span> <span class="surname">Eastep</span></h3></div></div></div><div><p class="copyright">Copyright © 2001-2004 Thomas M. Eastep</p></div><div><div class="legalnotice"><p>Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
“<span class="quote"><a href="GnuCopyright.htm" target="_self">GNU Free Documentation License</a></span>”.</p></div></div><div><p class="pubdate">2004-02-03</p></div></div><div></div><hr /></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="#id2807299">Installing Shorewall</a></span></dt><dd><dl><dt><span class="section"><a href="#id2807598">Where do I find Step by Step Installation and Configuration
Instructions?</a></span></dt></dl></dd><dt><span class="section"><a href="#id2807623">Port Forwarding</a></span></dt><dd><dl><dt><span class="section"><a href="#faq1">(FAQ 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.</a></span></dt><dd><dl><dt><span class="section"><a href="#faq1a">(FAQ 1a) Ok -- I followed those instructions but it doesn't
work</a></span></dt><dt><span class="section"><a href="#faq1b">(FAQ 1b) I'm still having problems with port forwarding</a></span></dt><dt><span class="section"><a href="#faq1c">(FAQ 1c) 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?</a></span></dt></dl></dd><dt><span class="section"><a href="#faq30">(FAQ 30) I'm confused about when to use DNAT rules and when
to use ACCEPT rules.</a></span></dt></dl></dd><dt><span class="section"><a href="#id2859491">DNS and Port Forwarding/NAT</a></span></dt><dd><dl><dt><span class="section"><a href="#faq2">(FAQ 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.</a></span></dt><dd><dl><dt><span class="section"><a href="#faq2a">(FAQ 2a) I have a zone Z with an RFC1918 subnet
and I use one-to-one 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.</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="#id2859698">Netmeeting/MSN</a></span></dt><dd><dl><dt><span class="section"><a href="#faq3">(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with
Shorewall. What do I do?</a></span></dt></dl></dd><dt><span class="section"><a href="#id2859801">Open Ports</a></span></dt><dd><dl><dt><span class="section"><a href="#faq4">(FAQ 4) I just used an online port scanner to check my firewall
and it shows some ports as closed rather than
blocked. Why?</a></span></dt><dd><dl><dt><span class="section"><a href="#faq4a">(FAQ 4a) I just ran an nmap UDP scan of my firewall and it
showed 100s of ports as open!!!!</a></span></dt><dt><span class="section"><a href="#faq4b">(FAQ 4b) I have a port that I can't close no matter how I
change my rules.</a></span></dt><dt><span class="section"><a href="#faq4c">(FAQ 4c) How to I use Shorewall with PortSentry?</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="#id2806601">Connection Problems</a></span></dt><dd><dl><dt><span class="section"><a href="#faq5">(FAQ 5) I've installed Shorewall and now I can't ping
through the firewall</a></span></dt><dt><span class="section"><a href="#faq15">(FAQ 15) My local systems can't see out to the net</a></span></dt><dt><span class="section"><a href="#faq29">(FAQ 29) FTP Doesn't Work</a></span></dt><dt><span class="section"><a href="#id2806829">(FAQ 33) From clients behind the firewall, connections to some
sites fail. Connections to the same sites from the firewall itself work
fine. What's wrong.</a></span></dt></dl></dd><dt><span class="section"><a href="#id2806863">Logging</a></span></dt><dd><dl><dt><span class="section"><a href="#faq6">(FAQ 6) Where are the log messages written and how do I change
the destination?</a></span></dt><dd><dl><dt><span class="section"><a href="#faq6a">(FAQ 6a) Are there any log parsers that work with Shorewall?</a></span></dt><dt><span class="section"><a href="#faq6b">(FAQ 2b) DROP messages on port 10619 are flooding the logs with
their connect requests. Can i exclude these error messages for this
port temporarily from logging in Shorewall?</a></span></dt><dt><span class="section"><a href="#faq6c">(FAQ 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?</a></span></dt><dt><span class="section"><a href="#faq6d">(FAQ 6d) Why is the MAC address in Shorewall log messages so
long? I thought MAC addresses were only 6 bytes in length.</a></span></dt></dl></dd><dt><span class="section"><a href="#faq16">(FAQ 16) Shorewall is writing log messages all over my console
making it unusable!</a></span></dt><dt><span class="section"><a href="#faq17">(FAQ 17) What does this log message mean?</a></span></dt><dt><span class="section"><a href="#faq21">(FAQ 21) I see these strange log entries occasionally; what are
they?</a></span></dt></dl></dd><dt><span class="section"><a href="#id2875168">Routing</a></span></dt><dd><dl><dt><span class="section"><a href="#faq32">(FAQ 32) My firewall has two connections to the internet from two
different ISPs. How do I set this up in Shorewall?</a></span></dt></dl></dd><dt><span class="section"><a href="#id2875693">Starting and Stopping</a></span></dt><dd><dl><dt><span class="section"><a href="#faq7">(FAQ 7) When I stop Shorewall using shorewall stop,
I can't connect to anything. Why doesn't that command work?</a></span></dt><dt><span class="section"><a href="#faq8">(FAQ 8) When I try to start Shorewall on RedHat, I get messages
about insmod failing -- what's wrong?</a></span></dt><dd><dl><dt><span class="section"><a href="#faq8a">(FAQ 8a) When I try to start Shorewall on RedHat I get a
message referring me to FAQ #8</a></span></dt></dl></dd><dt><span class="section"><a href="#faq9">(FAQ 9) Why can't Shorewall detect my interfaces properly at
startup?</a></span></dt><dt><span class="section"><a href="#faq22">(FAQ 22) I have some iptables commands that I want to run when
Shorewall starts. Which file do I put them in?</a></span></dt></dl></dd><dt><span class="section"><a href="#id2876004">About Shorewall</a></span></dt><dd><dl><dt><span class="section"><a href="#faq10">(FAQ 10) What Distributions does it work with?</a></span></dt><dt><span class="section"><a href="#faq11">(FAQ 11) What Features does it have?</a></span></dt><dt><span class="section"><a href="#faq12">(FAQ 12) Is there a GUI?</a></span></dt><dt><span class="section"><a href="#faq13">(FAQ 13) Why do you call it Shorewall?</a></span></dt><dt><span class="section"><a href="#faq23">(FAQ 23) Why do you use such ugly fonts on your web site?</a></span></dt><dt><span class="section"><a href="#faq25">(FAQ 25) How to I tell which version of Shorewall I am running?</a></span></dt><dt><span class="section"><a href="#faq31">(FAQ 31) Does Shorewall provide protection against....</a></span></dt><dt><span class="section"><a href="#id2865341">Given that the Debian Stable Release includes Shorewall 1.2.12,
how can you not support that version?</a></span></dt></dl></dd><dt><span class="section"><a href="#id2865362">RFC 1918</a></span></dt><dd><dl><dt><span class="section"><a href="#faq14">(FAQ 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.</a></span></dt><dd><dl><dt><span class="section"><a href="#faq14a">(FAQ 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.</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="#id2865509">Alias IP Addresses/Virtual Interfaces</a></span></dt><dd><dl><dt><span class="section"><a href="#faq18">(FAQ 18) Is there any way to use aliased ip addresses with
Shorewall, and maintain separate rulesets for different IPs?</a></span></dt></dl></dd><dt><span class="section"><a href="#id2865549">Miscellaneous</a></span></dt><dd><dl><dt><span class="section"><a href="#faq19">(FAQ 19) I have added entries to /etc/shorewall/tcrules but they
don't seem to do anything. Why?</a></span></dt><dt><span class="section"><a href="#faq20">(FAQ 20) I have just set up a server. Do I have to change
Shorewall to allow access to my server from the internet?</a></span></dt><dt><span class="section"><a href="#faq24">(FAQ 24) How can I allow conections to let's say the ssh port
only from specific IP Addresses on the internet?</a></span></dt><dt><span class="section"><a href="#faq26">(FAQ 26) 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?&quot;</a></span></dt><dd><dl><dt><span class="section"><a href="#faq26a">(FAQ 26a) When I try to use the -O option of
nmap from the firewall system, I get operation not permitted.
How do I allow this option?</a></span></dt></dl></dd><dt><span class="section"><a href="#faq27">(FAQ 27) I'm compiling a new kernel for my firewall. What
should I look out for?</a></span></dt><dd><dl><dt><span class="section"><a href="#faq27a">(FAQ 27a) I just built and installed a new kernel and now
Shorewall won't start. I know that my kernel options are correct.</a></span></dt></dl></dd><dt><span class="section"><a href="#faq28">(FAQ 28) How do I use Shorewall as a Bridging Firewall?</a></span></dt></dl></dd><dt><span class="appendix"><a href="#id2865882">A. Revision History</a></span></dt></dl></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2807299"></a>Installing Shorewall</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2807598"></a>Where do I find Step by Step Installation and Configuration
Instructions?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Check out the <a href="shorewall_quickstart_guide.htm" target="_self">QuickStart Guides</a>.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2807623"></a>Port Forwarding</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq1"></a>(FAQ 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.</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> The first example in the
<a href="Documentation.htm#Rules" target="_self">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><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT
DNAT net loc:&lt;l<span class="emphasis"><em>ocal IP address</em></span>&gt;[:&lt;<span class="emphasis"><em>local port</em></span>&gt;] &lt;<span class="emphasis"><em>protocol</em></span>&gt; &lt;<span class="emphasis"><em>port #</em></span>&gt;</pre></td></tr></table><p>So to forward UDP port 7777 to internal system 192.168.1.5, the
rule is:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT
DNAT net loc:192.168.1.5 udp 7777</pre></td></tr></table><p>If you want to forward requests directed to a particular address (
<span class="emphasis"><em>&lt;external IP&gt;</em></span> ) on your firewall to an
internal system:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL
# PORT DEST.
DNAT net loc:&lt;l<span class="emphasis"><em>ocal IP address</em></span>&gt;[:&lt;<span class="emphasis"><em>local port</em></span>&gt;] &lt;<span class="emphasis"><em>protocol</em></span>&gt; &lt;<span class="emphasis"><em>port #</em></span>&gt; - &lt;<span class="emphasis"><em>external IP</em></span>&gt;</pre></td></tr></table><p>Finally, if you need to forward a range of ports, in the PORT
column specify the range as <span class="emphasis"><em>&lt;low-port&gt;:&lt;high-port&gt;</em></span>.</p><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq1a"></a>(FAQ 1a) Ok -- I followed those instructions but it doesn't
work</h4></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> That is usually the
result of one of four things:</p><div class="itemizedlist"><ul type="disc"><li><p>You are trying to test from inside your firewall (no, that
won't work -- see <a href="#faq2" title="(FAQ 2) I port forward www requests to www.mydomain.com (IP&#10; 130.151.100.69) to system 192.168.1.5 in my local network. External&#10; clients can browse http://www.mydomain.com but internal clients&#10; can't.">the section called “(FAQ 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.”</a>).</p></li><li><p>You have a more basic problem with your local system (the
one that you are trying to forward to) such as an incorrect
default gateway (it should be set to the IP address of your
firewall's internal interface).</p></li><li><p>Your ISP is blocking that particular port inbound.</p></li><li><p>You are running Mandrake Linux and have configured Internet
Connection Sharing. In that case, the name of your local zone is
'masq' rather than 'loc' (change all instances of
'loc' to 'masq' in your rules). You may want to
consider re-installing Shorewall in a configuration which matches
the Shorewall documentation. See the <a href="two-interface.htm" target="_self">two-interface QuickStart Guide</a> for
details.</p></li></ul></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq1b"></a>(FAQ 1b) I'm still having problems with port forwarding</h4></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> To further diagnose
this problem:</p><div class="itemizedlist"><ul type="disc"><li><p>As root, type “<span class="quote"><span><b class="command">iptables -t nat -Z</b></span></span>”.
This clears the NetFilter counters in the nat table.</p></li><li><p>Try to connect to the redirected port from an external host.</p></li><li><p>As root type “<span class="quote"><span><b class="command">shorewall show nat</b></span></span>”</p></li><li><p>Locate the appropriate DNAT rule. It will be in a chain
called <span class="emphasis"><em>&lt;source zone&gt;</em></span>_dnat (“<span class="quote">net_dnat</span>”
in the above examples).</p></li><li><p>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 local system
(the system you are trying to forward to -- its default gateway
should be the IP address of the firewall's interface to that
system).</p></li><li><p>If the packet count is zero:</p><div class="itemizedlist"><ul type="circle"><li><p>the connection request is not reaching your server
(possibly it is being blocked by your ISP); or</p></li><li><p>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
“<span class="quote">ORIG. DEST.</span>” column in your DNAT rule); or</p></li><li><p>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.</p></li></ul></div></li></ul></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq1c"></a>(FAQ 1c) 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></div><div></div></div><p>In /<tt class="filename">etc/shorewall/rules</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT
DNAT net loc:192.168.3:22 tcp 1022</pre></td></tr></table></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq30"></a>(FAQ 30) I'm confused about when to use DNAT rules and when
to use ACCEPT rules.</h3></div></div><div></div></div><p>It would be a good idea to review the <a href="shorewall_quickstart_guide.htm" target="_self">QuickStart Guide</a>
appropriate for your setup; the guides cover this topic in a tutorial
fashion. DNAT rules should be used for connections that need to go the
opposite direction from SNAT/MASQUERADE. So if you masquerade or use
SNAT from your local network to the internet then you will need to use
DNAT rules to allow connections from the internet to your local network.
In all other cases, you use ACCEPT unless you need to hijack connections
as they go through your firewall and handle them on the firewall box
itself; in that case, you use a REDIRECT rule.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2859491"></a>DNS and Port Forwarding/NAT</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq2"></a>(FAQ 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.</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> I have two objections to
this setup.</p><div class="itemizedlist"><ul type="disc"><li><p>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 :-)</p></li><li><p>The accessibility problem is best solved using <a href="shorewall_setup_guide.htm#DNS" target="_self">Bind Version 9 “<span class="quote">views</span>”</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 one-to-one NAT.</p></li></ul></div><p>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.</p><p>If you are running Shorewall 1.4.0 or earlier see the <a href="1.3/FAQ.htm#faq2" target="_self">1.3 FAQ</a> for instructions suitable for
those releases.</p><p>If you are running Shorewall 1.4.1 or Shorewall 1.4.1a, please
upgrade to Shorewall 1.4.2 or later.</p><p>Otherwise:</p><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Warning</h3><p>In this configuration, all loc-&gt;loc
traffic will look to the server as if it came from the firewall rather
than from the original client!</p></div><div class="itemizedlist"><ul type="disc"><li><p>In <tt class="filename">/etc/shorewall/interfaces</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ZONE INTERFACE BROADCAST OPTIONS
loc eth1 detect <span class="bold"><b>routeback</b></span></pre></td></tr></table></li><li><p>In <tt class="filename">/etc/shorewall/rules</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL
# PORT DEST.
DNAT loc loc:192.168.1.5 tcp www - 130.151.100.69:192.168.1.254</pre></td></tr></table><p>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 <tt class="filename">/etc/shorewall/init</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting"><span><b class="command">ETH0_IP=`find_interface_address eth0`</b></span></pre></td></tr></table><p>and make your DNAT rule:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL
# PORT DEST.
DNAT loc loc:192.168.1.5 tcp www - $ETH0_IP:192.168.1.254</pre></td></tr></table><p>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></li></ul></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq2a"></a>(FAQ 2a) I have a zone “<span class="quote">Z</span>” with an RFC1918 subnet
and I use one-to-one 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></div></div><div></div></div><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>If the ALL INTERFACES column in /etc/shorewall/nat is empty or
contains “<span class="quote">Yes</span>”, you will also see log messages like the
following when trying to access a host in Z from another host in Z
using the destination hosts's public address:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">Oct 4 10:26:40 netgw kernel:
Shorewall:FORWARD:REJECT:IN=eth1 OUT=eth1 SRC=192.168.118.200
DST=192.168.118.210 LEN=48 TOS=0x00 PREC=0x00 TTL=127 ID=1342 DF
PROTO=TCP SPT=1494 DPT=1491 WINDOW=17472 RES=0x00 ACK SYN URGP=0</pre></td></tr></table></div><p><span class="bold"><b>Answer:</b></span> This is another problem
that is best solved using Bind Version 9 “<span class="quote">views</span>”. It
allows both external and internal clients to access a NATed host using
the host's DNS name.</p><p>Another good way to approach this problem is to switch from
one-to-one 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>If you don't like those solutions and prefer routing all
Z-&gt;Z traffic through your firewall then:</p><div class="orderedlist"><ol type="1"><li><p>Set the Z-&gt;Z policy to ACCEPT.</p></li><li><p>Masquerade Z to itself.</p></li><li><p>Set the routeback option on the interface to Z.</p></li><li><p>Set the ALL INTERFACES column in the nat file to
“<span class="quote">Yes</span>”.</p><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Warning</h3><p>In this configuration, all Z-&gt;Z traffic will look to
the server as if it came from the firewall rather than from the
original client! I DO NOT RECOMMEND THIS SETUP.</p></div></li></ol></div><div class="example"><a id="id2810035"></a><p class="title"><b>Example 1. Example:</b></p><div class="literallayout"><p>Zone: dmz Interface: eth2 Subnet: 192.168.2.0/24</p></div><p>In <tt class="filename">/etc/shorewall/interfaces</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ZONE INTERFACE BROADCAST OPTIONS
loc eth2 192.168.2.255 <span class="bold"><b>routeback</b></span></pre></td></tr></table><p>In <tt class="filename">/etc/shorewall/policy</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#SOURCE DESTINATION POLICY LIMIT:BURST
dmz dmz ACCEPT</pre></td></tr></table><p>In <tt class="filename">/etc/shorewall/masq</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#INTERFACE SUBNET ADDRESS
eth2 192.168.2.0/24</pre></td></tr></table><p>In <tt class="filename">/etc/shorewall/na</tt>t, be sure that you
have “<span class="quote">Yes</span>” in the ALL INTERFACES column.</p></div></div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2859698"></a>Netmeeting/MSN</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq3"></a>(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with
Shorewall. What do I do?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> There is an <a href="http://www.kfki.hu/%7Ekadlec/sw/netfilter/newnat-suite/" target="_self">H.323
connection tracking/NAT module</a> that helps with Netmeeting. Note
however that one of the Netfilter developers recently posted the
following:</p><div class="blockquote"><blockquote class="blockquote"><p>&gt; I know PoM -ng is going to address this issue, but till it
is ready, and &gt; all the extras are ported to it, is there any way
to use the h.323 &gt; contrack module kernel patch with a 2.6 kernel?
&gt; Running 2.6.1 - no 2.4 kernel stuff on the system, so downgrade
is not &gt; an option... The module is not ported yet to 2.6, sorry.
&gt; Do I have any options besides a gatekeeper app (does not work in
my &gt; network) or a proxy (would prefer to avoid them)? I suggest
everyone to setup a proxy (gatekeeper) instead: the module is really
dumb and does not deserve to exist at all. It was an excellent tool to
debug/develop the newnat interface.</p></blockquote></div><p>Look <a href="http://linux-igd.sourceforge.net" target="_self">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" target="_self">http://www.netfilter.org</a>.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2859801"></a>Open Ports</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq4"></a>(FAQ 4) I just used an online port scanner to check my firewall
and it shows some ports as “<span class="quote">closed</span>” rather than
“<span class="quote">blocked</span>”. Why?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> (Shorewall versions prior
to 2.0.0 only). 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 “<span class="quote">Auth</span>” mechanism for identifying requesting
users. Shorewall also rejects TCP ports 135, 137, 139 and 445 as well as
UDP ports 137-139. These are ports that are used by Windows (Windows
<span class="emphasis"><em>can</em></span> 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>If you are seeing port 80 being “<span class="quote">closed</span>”, that's
probably your ISP preventing you from running a web server in violation
of your Service Agreement.</p><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>You can change the default behavior of Shorewall through use of
an /etc/shorewall/common file. See the <a href="shorewall_extension_scripts.htm" target="_self">Extension Script Section</a>.</p></div><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>Beginning with Shorewall 1.4.9, Shorewall no longer rejects the
Windows SMB ports (135-139 and 445) by default and silently drops them
instead.</p></div><p><span class="bold"><b>Answer:</b></span> (Shorewall versions 2.0.0
and later). The default Shorewall setup invokes the <span class="bold"><b>Drop</b></span> action prior to enforcing a DROP policy and
the default policy to all zone from the internet is DROP. The Drop
action is defined in <tt class="filename">/etc/shorewall/action.Drop</tt>
which in turn invokes the <span class="bold"><b>RejectAuth</b></span>
action (defined in <tt class="filename">/etc/shorewall/action.RejectAuth</tt>).
This is necessary to prevent outgoing connection problems to services
that use the “<span class="quote">Auth</span>” mechanism for identifying requesting
users. That is the only service which the default setup rejects.</p><p>If you are seeing closed TCP ports other than 113 (auth) then
either you have added rules to REJECT those ports or a router outside of
your firewall is responding to connection requests on those ports.</p><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq4a"></a>(FAQ 4a) I just ran an nmap UDP scan of my firewall and it
showed 100s of ports as open!!!!</h4></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Take a deep breath and
read the nmap man page section about UDP scans. If nmap gets <span class="bold"><b>nothing</b></span> 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></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq4b"></a>(FAQ 4b) I have a port that I can't close no matter how I
change my rules.</h4></div></div><div></div></div><p>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!!!</p><p><span class="bold"><b>Answer:</b></span> 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.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq4c"></a>(FAQ 4c) How to I use Shorewall with PortSentry?</h4></div></div><div></div></div><p><a href="http://www.shorewall.net/pub/shorewall/contrib/PortsentryHOWTO.txt" target="_self">Here's
a writeup</a> on a nice integration of Shorewall and PortSentry.</p></div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2806601"></a>Connection Problems</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq5"></a>(FAQ 5) I've installed Shorewall and now I can't ping
through the firewall</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> If you want your firewall
to be totally open for “<span class="quote">ping</span>”,</p><div class="orderedlist"><ol type="1"><li><p>Create <tt class="filename">/etc/shorewall/common</tt> if it
doesn't already exist.</p></li><li><p>Be sure that the first command in the file is “<span class="quote">.
<tt class="filename">/etc/shorewall/common.de</tt>f</span>”</p></li><li><p>Add the following to <tt class="filename">/etc/shorewall/common</tt></p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting"><span><b class="command">run_iptables -A icmpdef -p ICMP --icmp-type echo-request -j ACCEPT</b></span></pre></td></tr></table></li></ol></div><p>For a complete description of Shorewall “<span class="quote">ping</span>”
management, see <a href="ping.html" target="_self">this page</a>.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq15"></a>(FAQ 15) My local systems can't see out to the net</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Every time I read
“<span class="quote">systems can't see out to the net</span>”, I wonder where the
poster bought computers with eyes and what those computers will
“<span class="quote">see</span>” when things are working properly. That aside, the
most common causes of this problem are:</p><div class="orderedlist"><ol type="1"><li><p>The default gateway on each local system isn't set to the
IP address of the local firewall interface.</p></li><li><p>The entry for the local network in the /etc/shorewall/masq
file is wrong or missing.</p></li><li><p>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></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq29"></a>(FAQ 29) FTP Doesn't Work</h3></div></div><div></div></div><p>See the <a href="FTP.html" target="_self">Shorewall and FTP page</a>.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2806829"></a>(FAQ 33) From clients behind the firewall, connections to some
sites fail. Connections to the same sites from the firewall itself work
fine. What's wrong.</h3></div></div><div></div></div><p><span class="bold"><b>Answer</b></span>: Most likely, you need to
set CLAMPMSS=Yes in <a href="Documentation.htm#Conf" target="_self">/etc/shorewall/shorewall.conf</a>.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2806863"></a>Logging</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq6"></a>(FAQ 6) Where are the log messages written and how do I change
the destination?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> NetFilter uses the
kernel's equivalent of syslog (see “<span class="quote">man syslog</span>”) to log
messages. It always uses the LOG_KERN (kern) facility (see
“<span class="quote">man openlog</span>”) and you get to choose the log level (again,
see “<span class="quote">man syslog</span>”) in your <a href="Documentation.htm#Policy" target="_self">policies</a> and <a href="Documentation.htm#Rules" target="_self">rules</a>. The destination for
messaged logged by syslog is controlled by <tt class="filename">/etc/syslog.conf</tt>
(see “<span class="quote">man syslog.conf</span>”). When you have changed
/etc/syslog.conf, be sure to restart syslogd (on a RedHat system,
“<span class="quote">service syslog restart</span>”).</p><p>By default, older versions of Shorewall ratelimited log messages
through <a href="Documentation.htm#Conf" target="_self">settings</a> in
<tt class="filename">/etc/shorewall/shorewall.conf</tt> -- If you want to log
all messages, set:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">LOGLIMIT=&quot;&quot;
LOGBURST=&quot;&quot;</pre></td></tr></table><p>Beginning with Shorewall version 1.3.12, you can <a href="shorewall_logging.html" target="_self">set up Shorewall to log all of its messages
to a separate file</a>.</p><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq6a"></a>(FAQ 6a) Are there any log parsers that work with Shorewall?</h4></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Here are several links
that may be helpful:</p><div class="literallayout"><p><a href="http://www.shorewall.net/pub/shorewall/parsefw/" target="_self">http://www.shorewall.net/pub/shorewall/parsefw/</a><br />
<a href="http://www.fireparse.com" target="_self">http://www.fireparse.com</a><br />
<a href="http://cert.uni-stuttgart.de/projects/fwlogwatch" target="_self">http://cert.uni-stuttgart.de/projects/fwlogwatch</a><br />
<a href="http://www.logwatch.org" target="_self">http://www.logwatch.org</a><br />
<a href="http://gege.org/iptables" target="_self">http://gege.org/iptables</a><br />
<a href="http://home.regit.org/ulogd-php.html" target="_self">http://home.regit.org/ulogd-php.html</a></p></div><p>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.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq6b"></a>(FAQ 2b) DROP messages on port 10619 are flooding the logs with
their connect requests. Can i exclude these error messages for this
port temporarily from logging in Shorewall?</h4></div></div><div></div></div><p>Temporarily add the following rule:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">DROP net fw udp 10619</pre></td></tr></table></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq6c"></a>(FAQ 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></div></div><div></div></div><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">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 SRC=208.138.130.16
DST=24.237.22.45 LEN=53 TOS=0x00 PREC=0x00 TTL=251 ID=8288 DF
PROTO=UDP SPT=53 DPT=40275 LEN=33</pre></td></tr></table><p><span class="bold"><b>Answer:</b></span> There are two
possibilities:</p><div class="orderedlist"><ol type="1"><li><p>They are late-arriving replies to DNS queries.</p></li><li><p>They are corrupted reply packets.</p></li></ol></div><p>You can distinguish the difference by setting the <span class="bold"><b>logunclean</b></span> option (<tt class="filename"><a href="Documentation.htm#Interfaces" target="_self">/etc/shorewall/interfaces</a></tt>)
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:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#
# Include the standard common.def file
#
<span><b class="command">. /etc/shorewall/common.def</b></span>
#
# The following rule is non-standard and compensates for tardy
# DNS replies
#
<span><b class="command">run_iptables -A common -p udp --sport 53 -mstate --state NEW -j DROP</b></span></pre></td></tr></table><p>The above file is also include in all of my sample
configurations available in the <a href="shorewall_quickstart_guide.htm" target="_self">Quick Start Guides</a> and in
the common.def file in Shorewall 1.4.0 and later.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq6d"></a>(FAQ 6d) Why is the MAC address in Shorewall log messages so
long? I thought MAC addresses were only 6 bytes in length.</h4></div></div><div></div></div><p>What is labeled as the MAC address in a Shorewall log message is
actually the Ethernet frame header. It contains:</p><div class="itemizedlist"><ul type="disc"><li><p>the destination MAC address (6 bytes)</p></li><li><p>the source MAC address (6 bytes)</p></li><li><p>the ethernet frame type (2 bytes)</p></li></ul></div><div class="example"><a id="id2806108"></a><p class="title"><b>Example 2. Example</b></p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00</pre></td></tr></table><div class="itemizedlist"><ul type="disc"><li><p>Destination
MAC address = 00:04:4c:dc:e2:28</p></li><li><p>Source
MAC address = 00:b0:8e:cf:3c:4c</p></li><li><p>Ethernet
Frame Type = 08:00 (IP Version 4)</p></li></ul></div></div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq16"></a>(FAQ 16) Shorewall is writing log messages all over my console
making it unusable!</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> If you are running
Shorewall version 1.4.4 or 1.4.4a then check the <a href="errata.htm" target="_self">errata</a>.
Otherwise:</p><div class="itemizedlist"><ul type="disc"><li><p>Find where klogd is being started (it will be from one of the
files in /etc/init.d -- sysklogd, klogd, ...). Modify that file or
the appropriate configuration file so that klogd is started with
“<span class="quote">-c <span class="emphasis"><em>&lt;n&gt;</em></span></span>” where
<span class="emphasis"><em>&lt;n&gt;</em></span> is a log level of 5 or less; or</p></li><li><p>See the “<span class="quote">dmesg</span>” man page (“<span class="quote">man dmesg</span>”).
You must add a suitable “<span class="quote">dmesg</span>” command to your startup
scripts or place it in /etc/shorewall/start.</p></li></ul></div><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>Under RedHat and Mandrake, the max <a href="shorewall_logging.html" target="_self">log level</a> that is sent to the
console is specified in /etc/sysconfig/init in the LOGLEVEL variable.
Set “<span class="quote">LOGLEVEL=5</span>” to suppress info (log level 6) messages
on the console.</p></div><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>Under Debian, you can set KLOGD=“<span class="quote">-c 5</span>” in
<tt class="filename">/etc/init.d/klogd</tt> to suppress info (log level 6)
messages on the console.</p></div><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Tip</h3><p>Under SuSE, add “<span class="quote">-c 5</span>” to KLOGD_PARAMS in
/etc/sysconfig/syslog to suppress info (log level 6) messages on the
console.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq17"></a>(FAQ 17) What does this log message mean?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Logging occurs out of a
number of chains (as indicated in the log message) in Shorewall:</p><div class="variablelist"><dl><dt><span class="term">man1918 or logdrop</span></dt><dd><p>The destination address is listed in <tt class="filename">/etc/shorewall/rfc1918</tt>
with a <span class="bold"><b>logdrop</b></span> target -- see
<tt class="filename"><a href="Documentation.htm#rfc1918" target="_self">/etc/shorewall/rfc1918</a></tt>.</p></dd><dt><span class="term">rfc1918 or logdrop</span></dt><dd><p>The source address is listed in <tt class="filename">/etc/shorewall/rfc1918</tt>
with a <span class="bold"><b>logdrop</b></span> target -- see
<tt class="filename"><a href="Documentation.htm#rfc1918" target="_self">/etc/shorewall/rfc1918</a></tt>.</p></dd><dt><a id="all2all"></a><span class="term">all2&lt;zone&gt;, &lt;zone&gt;2all or all2all</span></dt><dd><p>You have a <a href="Documentation.htm#Policy" target="_self">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" target="_self">rule</a> to that effect.</p></dd><dt><span class="term">&lt;zone1&gt;2&lt;zone2&gt;</span></dt><dd><p>Either you have a <a href="Documentation.htm#Policy" target="_self">policy</a>
for <span class="bold"><b>&lt;zone1&gt;</b></span> to <span class="bold"><b>&lt;zone2&gt;</b></span> that specifies a log level
and this packet is being logged under that policy or this packet
matches a <a href="Documentation.htm#Rules" target="_self">rule</a> that
includes a log level.</p></dd><dt><span class="term">&lt;interface&gt;_mac</span></dt><dd><p>The packet is being logged under the <span class="bold"><b>maclist</b></span>
<a href="Documentation.htm#Interfaces" target="_self">interface option</a>.</p></dd><dt><span class="term">logpkt</span></dt><dd><p>The packet is being logged under the <span class="bold"><b>logunclean</b></span>
<a href="Documentation.htm#Interfaces" target="_self">interface option</a>.</p></dd><dt><span class="term">badpkt</span></dt><dd><p>The packet is being logged under the <span class="bold"><b>dropunclean</b></span>
<a href="Documentation.htm#Interfaces" target="_self">interface option</a>
as specified in the <span class="bold"><b>LOGUNCLEAN</b></span>
setting in <a href="Documentation.htm#Conf" target="_self"><tt class="filename">/etc/shorewall/shorewall.conf</tt></a>.</p></dd><dt><span class="term">blacklst</span></dt><dd><p>The packet is being logged because the source IP is
blacklisted in the <tt class="filename"><a href="Documentation.htm#Blacklist" target="_self">/etc/shorewall/blacklist</a></tt>
file.</p></dd><dt><span class="term">newnotsyn</span></dt><dd><p>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 <span class="bold"><b>NEWNOTSYN</b></span> and <span class="bold"><b>LOGNEWNOTSYN</b></span>
in <a href="Documentation.htm#Conf" target="_self"><tt class="filename">/etc/shorewall/shorewall.conf</tt></a>.</p></dd><dt><span class="term">INPUT or FORWARD</span></dt><dd><p>The packet has a source IP address that isn't in any of
your defined zones (“<span class="quote">shorewall check</span>” and look at the
printed zone definitions) or the chain is FORWARD and the
destination IP isn't in any of your defined zones. Also see
<a href="#faq2a" title="(FAQ 2a) I have a zone Z with an RFC1918 subnet&#10; and I use one-to-one NAT to assign non-RFC1918 addresses to hosts in&#10; Z. Hosts in Z cannot communicate with each other using their external&#10; (non-RFC1918 addresses) so they can't access each other using&#10; their DNS names.">the section called “(FAQ 2a) I have a zone Z with an RFC1918 subnet
and I use one-to-one 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.”</a> for another cause of packets being logged
in the FORWARD chain.</p></dd><dt><span class="term">logflags</span></dt><dd><p>The packet is being logged because it failed the checks
implemented by the <span class="bold"><b>tcpflags</b></span>
<a href="Documentation.htm#Interfaces" target="_self">interface option</a>.</p></dd></dl></div><div class="example"><a id="id2874848"></a><p class="title"><b>Example 3. Here is an example:</b></p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">Jun 27 15:37:56 gateway kernel:
Shorewall:all2all:REJECT:IN=eth2 OUT=eth1 SRC=192.168.2.2
DST=192.168.1.3 LEN=67 TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF PROTO=UDP
SPT=1803 DPT=53 LEN=47</pre></td></tr></table><p>Let's look at the important parts of this message:</p><div class="variablelist"><dl><dt><span class="term">all2all:REJECT</span></dt><dd><p>This packet was REJECTed out of the <span class="bold"><b>all2all</b></span>
chain -- the packet was rejected under the “<span class="quote">all</span>”-&gt;“<span class="quote">all</span>”
REJECT policy (<a href="#all2all">all2&lt;zone&gt;, &lt;zone&gt;2all or all2all</a> above).</p></dd><dt><span class="term">IN=eth2</span></dt><dd><p>the packet entered the firewall via eth2. If you see
“<span class="quote">IN=</span>” with no interface name, the packet originated
on the firewall itself.</p></dd><dt><span class="term">OUT=eth1</span></dt><dd><p>if accepted, the packet would be sent on eth1. If you see
“<span class="quote">OUT=</span>” with no interface name, the packet would be
processed by the firewall itself.</p></dd><dt><span class="term">SRC=192.168.2.2</span></dt><dd><p>the packet was sent by 192.168.2.2</p></dd><dt><span class="term">DST=192.168.1.3</span></dt><dd><p>the packet is destined for 192.168.1.3</p></dd><dt><span class="term">PROTO=UDP</span></dt><dd><p>UDP Protocol</p></dd><dt><span class="term">DPT=53</span></dt><dd><p>The destination port is 53 (DNS)</p></dd></dl></div><p>For additional information about the log message, see <a href="http://logi.cc/linux/netfilter-log-format.php3" target="_self">http://logi.cc/linux/netfilter-log-format.php3</a>.</p><p>In this case, 192.168.2.2 was in the “<span class="quote">dmz</span>” zone and
192.168.1.3 is in the “<span class="quote">loc</span>” zone. I was missing the rule:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ACCEPT dmz loc udp 53</pre></td></tr></table></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq21"></a>(FAQ 21) I see these strange log entries occasionally; what are
they?</h3></div></div><div></div></div><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">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 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 [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 ]</pre></td></tr></table><p>192.0.2.3 is external on my firewall... 172.16.0.0/24 is my
internal LAN</p><p><span class="bold"><b>Answer:</b></span> While most people
associate the Internet Control Message Protocol (ICMP) with
“<span class="quote">ping</span>”, 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.</p><p>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.</p><p>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.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2875168"></a>Routing</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq32"></a>(FAQ 32) My firewall has two connections to the internet from two
different ISPs. How do I set this up in Shorewall?</h3></div></div><div></div></div><p>Setting this up in Shorewall is easy; setting up the routing is a
bit harder.</p><p>Assuming that <tt class="filename">eth0</tt> and
<tt class="filename">eth1</tt> are the interfaces to the
two ISPs then:</p><p><tt class="filename">/etc/shorewall/interfaces</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect
net eth1 detect</pre></td></tr></table><p><tt class="filename">/etc/shorewall/policy</tt>:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#SOURCE DESTINATION POLICY LIMIT:BURST
net net DROP</pre></td></tr></table><p>If you have masqueraded hosts, be sure to update
<tt class="filename">/etc/shorewall/masq</tt> to masquerade to both ISPs. For
example, if you masquerade all hosts connected to <tt class="filename">eth2</tt> then:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#INTERFACE SUBNET ADDRESS
eth0 eth2
eth1 eth2</pre></td></tr></table><p><i class="citetitle">There was an article in SysAdmin covering this topic.
It may be found at <a href="http://www.samag.com/documents/s=1824/sam0201h/" target="_self">http://www.samag.com/documents/s=1824/sam0201h/</a></i></p><p><i class="citetitle">The following information regarding setting up routing
for this configuration is reproduced from the <a href="http://www.lartc.org" target="_self">LARTC HOWTO</a> and has not been verified
by the author. If you have questions or problems with the instructions
given below, please post to the <a href="http://www.lartc.org/#mailinglist" target="_self">LARTC mailing list</a>.</i></p><div class="sidebar"><p>A common configuration is the following, in which there are two
providers that connect a local network (or even a single machine) to
the big Internet.</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting"> ________
+------------+ /
| | |
+-------------+ Provider 1 +-------
__ | | | /
___/ \_ +------+-------+ +------------+ |
_/ \__ | if1 | /
/ \ | | |
| Local network -----+ Linux router | | Internet
\_ __/ | | |
\__ __/ | if2 | \
\___/ +------+-------+ +------------+ |
| | | \
+-------------+ Provider 2 +-------
| | |
+------------+ \________
</pre></td></tr></table><p>There are usually two questions given this setup.</p><p><span class="bold"><b>Split access</b></span></p><p>The first is how to route answers to packets coming in over a
particular provider, say Provider 1, back out again over that same
provider.</p><p>Let us first set some symbolical names. Let <span class="bold"><b>$IF1</b></span> be the name of the first interface (if1 in
the picture above) and <span class="bold"><b>$IF2</b></span> the name
of the second interface. Then let <span class="bold"><b>$IP1</b></span>
be the IP address associated with <span class="bold"><b>$IF1</b></span>
and <span class="bold"><b>$IP2</b></span> the IP address associated
with <span class="bold"><b>$IF2</b></span>. Next, let <span class="bold"><b>$P1</b></span> be the IP address of the gateway at
Provider 1, and <span class="bold"><b>$P2</b></span> the IP address of
the gateway at provider 2. Finally, let <span class="bold"><b>$P1_NET</b></span>
be the IP network <span class="bold"><b>$P1</b></span> is in, and
<span class="bold"><b>$P2_NET</b></span> the IP network <span class="bold"><b>$P2</b></span> is in.</p><p>One creates two additional routing tables, say <span class="bold"><b>T1</b></span> and <span class="bold"><b>T2</b></span>.
These are added in /etc/iproute2/rt_tables. Then you set up routing in
these tables as follows:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ip route add $P1_NET dev $IF1 src $IP1 table T1
ip route add default via $P1 table T1
ip route add $P2_NET dev $IF2 src $IP2 table T2
ip route add default via $P2 table T2</pre></td></tr></table><p>Nothing spectacular, just build a route to the gateway and build
a default route via that gateway, as you would do in the case of a
single upstream provider, but put the routes in a separate table per
provider. Note that the network route suffices, as it tells you how to
find any host in that network, which includes the gateway, as
specified above.</p><p>Next you set up the main routing table. It is a good idea to
route things to the direct neighbour through the interface connected
to that neighbour. Note the `src' arguments, they make sure the
right outgoing IP address is chosen.</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ip route add $P1_NET dev $IF1 src $IP1
ip route add $P2_NET dev $IF2 src $IP2</pre></td></tr></table><p>Then, your preference for default route:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ip route add default via $P1</pre></td></tr></table><p>Next, you set up the routing rules. These actually choose what
routing table to route with. You want to make sure that you route out
a given interface if you already have the corresponding source
address:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ip rule add from $IP1 table T1
ip rule add from $IP2 table T2</pre></td></tr></table><p>This set of commands makes sure all answers to traffic coming in
on a particular interface get answered from that interface.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>'If $P0_NET is the local network and $IF0 is its
interface, the following additional entries are desirable:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ip route add $P0_NET dev $IF0 table T1
ip route add $P2_NET dev $IF2 table T1
ip route add 127.0.0.0/8 dev lo table T1
ip route add $P0_NET dev $IF0 table T2
ip route add $P1_NET dev $IF1 table T2
ip route add 127.0.0.0/8 dev lo table T2</pre></td></tr></table></div><p>Now, this is just the very basic setup. It will work for all
processes running on the router itself, and for the local network, if
it is masqueraded. If it is not, then you either have IP space from
both providers or you are going to want to masquerade to one of the
two providers. In both cases you will want to add rules selecting
which provider to route out from based on the IP address of the
machine in the local network.</p><p><span class="bold"><b>Load balancing</b></span></p><p>The second question is how to balance traffic going out over the
two providers. This is actually not hard if you already have set up
split access as above.</p><p>Instead of choosing one of the two providers as your default
route, you now set up the default route to be a multipath route. In
the default kernel this will balance routes over the two providers. It
is done as follows (once more building on the example in the section
on split-access):</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ip route add default scope global nexthop via $P1 dev $IF1 weight 1 \
nexthop via $P2 dev $IF2 weight 1</pre></td></tr></table><p>This will balance the routes over both providers. The <span class="bold"><b>weight</b></span> parameters can be tweaked to favor one
provider over the other.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>balancing will not be perfect, as it is route based, and
routes are cached. This means that routes to often-used sites will
always be over the same provider.</p></div><p>Furthermore, if you really want to do this, you probably also
want to look at Julian Anastasov's patches at <a href="http://www.ssi.bg/%7Eja/#routes" target="_self">http://www.ssi.bg/~ja/#routes</a>
, Julian's route patch page. They will make things nicer to work
with.</p></div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2875693"></a>Starting and Stopping</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq7"></a>(FAQ 7) When I stop Shorewall using “<span class="quote">shorewall stop</span>”,
I can't connect to anything. Why doesn't that command work?</h3></div></div><div></div></div><p>The “<span class="quote"><span><b class="command">stop</b></span></span>” command is intended to
place your firewall into a safe state whereby only those hosts listed in
<tt class="filename">/etc/shorewall/routestopped</tt>' are activated. If
you want to totally open up your firewall, you must use the
“<span class="quote"><span><b class="command">shorewall clear</b></span></span>” command.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq8"></a>(FAQ 8) When I try to start Shorewall on RedHat, I get messages
about insmod failing -- what's wrong?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> The output you will see
looks something like this:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy
Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed
/lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed
iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.</pre></td></tr></table><p>This problem is usually corrected through the following sequence
of commands</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">service ipchains stop
chkconfig --delete ipchains
rmmod ipchains</pre></td></tr></table><p>Also, be sure to check the <a href="errata.htm" target="_self">errata</a>
for problems concerning the version of iptables (v1.2.3) shipped with
RH7.2.</p><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq8a"></a>(FAQ 8a) When I try to start Shorewall on RedHat I get a
message referring me to FAQ #8</h4></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> This is usually cured
by the sequence of commands shown above in <a href="#faq8" title="(FAQ 8) When I try to start Shorewall on RedHat, I get messages&#10; about insmod failing -- what's wrong?">the section called “(FAQ 8) When I try to start Shorewall on RedHat, I get messages
about insmod failing -- what's wrong?”</a>.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq9"></a>(FAQ 9) Why can't Shorewall detect my interfaces properly at
startup?</h3></div></div><div></div></div><p>I just installed Shorewall and when I issue the start command, I
see the following:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf ...
Starting Shorewall...
Loading Modules...
Initializing...
Determining Zones...
Zones: net loc
Validating interfaces file...
Validating hosts file...
Determining Hosts in Zones...
<span class="bold"><b>Net Zone: eth0:0.0.0.0/0
</b></span><span class="bold"><b>Local Zone: eth1:0.0.0.0/0</b></span>
Deleting user chains...
Creating input Chains...
...</pre></td></tr></table><p>Why can't Shorewall detect my interfaces properly?</p><p><span class="bold"><b>Answer:</b></span> 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 <tt class="filename">eth1</tt>. If you
are running Shorewall 1.4.10 or later, you can consider setting the
<a href="Documentation.htm#Interfaces" target="_self"><span class="bold"><b>detectnets</b></span>
interface option</a> on your local interface (<tt class="filename">eth1</tt> in the above example). That will
cause Shorewall to restrict the local zone to only those networks routed
through that interface.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq22"></a>(FAQ 22) I have some iptables commands that I want to run when
Shorewall starts. Which file do I put them in?</h3></div></div><div></div></div><p>You can place these commands in one of the <a href="shorewall_extension_scripts.htm" target="_self">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 “<span class="quote">man iptables</span>” and look
at the -I (--insert) command.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2876004"></a>About Shorewall</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq10"></a>(FAQ 10) What Distributions does it work with?</h3></div></div><div></div></div><p>Shorewall works with any GNU/Linux distribution that includes the
<a href="shorewall_prerequisites.htm" target="_self">proper prerequisites</a>.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq11"></a>(FAQ 11) What Features does it have?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> See the <a href="shorewall_features.htm" target="_self">Shorewall Feature List</a>.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq12"></a>(FAQ 12) Is there a GUI?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Yes. Shorewall support is
included in Webmin 1.060 and later versions. See <a href="http://www.webmin.com" target="_self">http://www.webmin.com</a></p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq13"></a>(FAQ 13) Why do you call it “<span class="quote">Shorewall</span>”?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Shorewall is a
concatenation of “<span class="quote"><span class="emphasis"><em>Shore</em></span>line</span>” (<a href="http://www.cityofshoreline.com" target="_self">the city where I live</a>) and
“<span class="quote">Fire<span class="emphasis"><em>wall</em></span></span>”. The full name of the
product is actually “<span class="quote">Shoreline Firewall</span>” but
“<span class="quote">Shorewall</span>” is must more commonly used.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq23"></a>(FAQ 23) Why do you use such ugly fonts on your web site?</h3></div></div><div></div></div><p>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.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq25"></a>(FAQ 25) How to I tell which version of Shorewall I am running?</h3></div></div><div></div></div><p>At the shell prompt, type:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting"><span><b class="command">/sbin/shorewall version</b></span></pre></td></tr></table></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq31"></a>(FAQ 31) Does Shorewall provide protection against....</h3></div></div><div></div></div><div class="variablelist"><dl><dt><span class="term">IP Spoofing: Sending packets over the WAN interface using an
internal LAP IP address as the source address?</span></dt><dd><p>Answer: Yes.</p></dd><dt><span class="term">Tear Drop: Sending packets that contain overlapping fragments?</span></dt><dd><p>Answer: This is the responsibility of the IP stack, not the
Netfilter-based firewall since fragment reassembly occurs before
the stateful packet filter ever touches each packet.</p></dd><dt><span class="term">Smurf and Fraggle: Sending packets that use the WAN or LAN
broadcast address as the source address?</span></dt><dd><p>Answer: Shorewall can be configured to do that using the
<a href="blacklisting_support.htm" target="_self">blacklisting</a>
facility.</p></dd><dt><span class="term">Land Attack: Sending packets that use the same address as the
source and destination address?</span></dt><dd><p>Answer: Yes, if the <a href="Documentation.htm#Interfaces" target="_self">routefilter interface option</a>
is selected.</p></dd><dt><span class="term">DOS: - SYN Dos - ICMP Dos - Per-host Dos protection</span></dt><dd><p>Answer: Shorewall has facilities for limiting SYN and ICMP
packets. Netfilter as included in standard Linux kernels
doesn't support per-remote-host limiting except by explicit
rule that specifies the host IP address; that form of limiting is
supported by Shorewall.</p></dd></dl></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id2865341"></a>Given that the Debian Stable Release includes Shorewall 1.2.12,
how can you not support that version?</h3></div></div><div></div></div><p>The first release of Shorewall was in March of 2001. Shorewall
1.2.12 was released in May of 2002. It is now the year 2004 and soon
Shorewall 2.0 will be available. Shorewall 1.2.12 is poorly documented
and is missing many of the features that Shorewall users find essential
today.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2865362"></a>RFC 1918</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq14"></a>(FAQ 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.</h3></div></div><div></div></div><p>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><span class="bold"><b>Answer:</b></span> If you are running a
version of Shorewall earlier than 1.3.1, create /etc/shorewall/start and
in it, place the following:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting"><span><b class="command">run_iptables -I rfc1918 -s 192.168.100.1 -j ACCEPT</b></span></pre></td></tr></table><p>If you are running version 1.3.1 or later, simply add the
following to <a href="Documentation.htm#rfc1918" target="_self">/etc/shorewall/rfc1918</a>:</p><p>Be sure that you add the entry ABOVE the entry for 192.168.0.0/16.</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#SUBNET TARGET
192.168.100.1 RETURN</pre></td></tr></table><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>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:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">#SUBNET TARGET
192.168.100.1 RETURN
192.168.100.2 RETURN</pre></td></tr></table></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq14a"></a>(FAQ 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><div></div></div><p>The solution is the same as <a href="#faq14" title="(FAQ 14) I'm connected via a cable modem and it has an&#10; internal web server that allows me to configure/monitor it but as&#10; expected if I enable rfc1918 blocking for my eth0 interface (the&#10; internet one), it also blocks the cable modems web server.">the section called “(FAQ 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.”</a> above.
Simply substitute the IP address of your ISPs DHCP server.</p></div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2865509"></a>Alias IP Addresses/Virtual Interfaces</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq18"></a>(FAQ 18) Is there any way to use aliased ip addresses with
Shorewall, and maintain separate rulesets for different IPs?</h3></div></div><div></div></div><p><span class="bold"><b>Answer:</b></span> Yes. See <a href="Shorewall_and_Aliased_Interfaces.html" target="_self">Shorewall and Aliased
Interfaces</a>.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="id2865549"></a>Miscellaneous</h2></div></div><div></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq19"></a>(FAQ 19) I have added entries to /etc/shorewall/tcrules but they
don't seem to do anything. Why?</h3></div></div><div></div></div><p>You probably haven't set TC_ENABLED=Yes in
/etc/shorewall/shorewall.conf so the contents of the tcrules file are
simply being ignored.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq20"></a>(FAQ 20) I have just set up a server. Do I have to change
Shorewall to allow access to my server from the internet?</h3></div></div><div></div></div><p>Yes. Consult the <a href="shorewall_quickstart_guide.htm" target="_self">QuickStart
guide</a> that you used during your initial setup for information
about how to set up rules for your server.</p></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq24"></a>(FAQ 24) How can I allow conections to let's say the ssh port
only from specific IP Addresses on the internet?</h3></div></div><div></div></div><p>In the SOURCE column of the rule, follow “<span class="quote">net</span>” by a
colon and a list of the host/subnet addresses as a comma-separated list.</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">net:&lt;ip1&gt;,&lt;ip2&gt;,...</pre></td></tr></table><div class="example"><a id="id2865645"></a><p class="title"><b>Example 4. Example:</b></p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22</pre></td></tr></table></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq26"></a>(FAQ 26) When I try to use any of the SYN options in nmap on or
behind the firewall, I get “<span class="quote">operation not permitted</span>”. How
can I use nmap with Shorewall?&quot;</h3></div></div><div></div></div><p>Edit /etc/shorewall/shorewall.conf and change “<span class="quote">NEWNOTSYN=No</span>”
to “<span class="quote">NEWNOTSYN=Yes</span>” then restart Shorewall.</p><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq26a"></a>(FAQ 26a) When I try to use the “<span class="quote">-O</span>” option of
nmap from the firewall system, I get “<span class="quote">operation not permitted</span>”.
How do I allow this option?</h4></div></div><div></div></div><p>Add this command to your /etc/shorewall/start file:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting"><span><b class="command">run_iptables -D OUTPUT -p ! icmp -m state --state INVALID -j DROP</b></span></pre></td></tr></table></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq27"></a>(FAQ 27) I'm compiling a new kernel for my firewall. What
should I look out for?</h3></div></div><div></div></div><p>First take a look at the <a href="kernel.htm" target="_self">Shorewall kernel
configuration page</a>. You probably also want to be sure that you
have selected the “<span class="quote"><span class="bold"><b>NAT of local connections
(READ HELP)</b></span></span>” on the Netfilter Configuration menu.
Otherwise, DNAT rules with your firewall as the source zone won't
work with your new kernel.</p><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="faq27a"></a>(FAQ 27a) I just built and installed a new kernel and now
Shorewall won't start. I know that my kernel options are correct.</h4></div></div><div></div></div><p>The last few lines of <a href="troubleshoot.htm" target="_self">a startup
trace</a> are these:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">+ run_iptables2 -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
+ '[' 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE' = 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.
0/0 -j MASQUERADE' ']'
+ run_iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
+ iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
iptables: Invalid argument
+ '[' -z '' ']'
+ stop_firewall
+ set +x</pre></td></tr></table><p><span class="bold"><b>Answer:</b></span> Your new kernel
contains headers that are incompatible with the ones used to compile
your <span><b class="command">iptables</b></span> utility. You need to rebuild
<span><b class="command">iptables</b></span> using your new kernel source.</p></div></div><div class="section" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="faq28"></a>(FAQ 28) How do I use Shorewall as a Bridging Firewall?</h3></div></div><div></div></div><p>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 “<span class="quote">Shorewall doesn't work with
Layer 2 Bridging</span>”.</p></div></div><div class="appendix" lang="en" xml:lang="en"><h2 class="title" style="clear: both"><a id="id2865882"></a>A. Revision History</h2><div class="revhistory"><table border="0" width="100%" summary="Revision history"><tr><th align="left" valign="top" colspan="3"><b>Revision History</b></th></tr><tr><td align="left">Revision 1.17</td><td align="left">2004-02-03</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Added
FAQ 33.</td></tr><tr><td align="left">Revision 1.16</td><td align="left">2004-02-03</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Updated
for Shorewall 2.0.</td></tr><tr><td align="left">Revision 1.15</td><td align="left">2004-01-25</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Updated
FAQ 32 to mention masquerading. Remove tables.</td></tr><tr><td align="left">Revision 1.14</td><td align="left">2004-01-24</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Added
FAQ 27a regarding kernel/iptables incompatibility.</td></tr><tr><td align="left">Revision 1.13</td><td align="left">2004-01-24</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Add
a note about the <span class="bold"><b>detectnets</b></span> interface
option in FAQ 9.</td></tr><tr><td align="left">Revision 1.12</td><td align="left">2004-01-20</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Improve
FAQ 16 answer.</td></tr><tr><td align="left">Revision 1.11</td><td align="left">2004-01-14</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Corrected
broken link</td></tr><tr><td align="left">Revision 1.10</td><td align="left">2004-01-09</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Added
a couple of more legacy FAQ numbers.</td></tr><tr><td align="left">Revision 1.9</td><td align="left">2004-01-08</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Corrected
typo in FAQ 26a. Added warning to FAQ 2 regarding source address of
redirected requests.</td></tr><tr><td align="left">Revision 1.8</td><td align="left">2003-12-31</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Additions
to FAQ 4.</td></tr><tr><td align="left">Revision 1.7</td><td align="left">2003-12-30</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Remove
dead link from FAQ 1.</td></tr><tr><td align="left">Revision 1.6</td><td align="left">2003.12-18</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Add
external link reference to FAQ 17.</td></tr><tr><td align="left">Revision 1.5</td><td align="left">2003-12-16</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Added
a link to a Sys Admin article about multiple internet interfaces. Added
Legal Notice. Moved &quot;abstract&quot; to the body of the document. Moved
Revision History to this Appendix.</td></tr><tr><td align="left">Revision 1.4</td><td align="left">2003-12-13</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Corrected
formatting problems</td></tr><tr><td align="left">Revision 1.3</td><td align="left">2003-12-10</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Changed
the title of FAQ 17</td></tr><tr><td align="left">Revision 1.2</td><td align="left">2003-12-09</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Added
Copyright and legacy FAQ numbers</td></tr><tr><td align="left">Revision 1.1</td><td align="left">2003-12-04</td><td align="left">MN</td></tr><tr><td align="left" colspan="3">Converted
to Simplified DocBook XML</td></tr><tr><td align="left">Revision 1.0</td><td align="left">2002-08-13</td><td align="left">TE</td></tr><tr><td align="left" colspan="3">Initial
revision</td></tr></table></div></div></div></body></html>
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