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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<article>
<!--$Id$-->
<articleinfo>
<title>Shorewall FAQs</title>
<authorgroup>
<corpauthor>Shorewall Community</corpauthor>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate>2004-01-25</pubdate>
<copyright>
<year>2001-2004</year>
<holder>Thomas M. Eastep</holder>
</copyright>
<legalnotice>
<para>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
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation License</ulink></quote>.</para>
</legalnotice>
</articleinfo>
<section>
<title>Installing Shorewall</title>
<section>
<title>Where do I find Step by Step Installation and Configuration
Instructions?</title>
<para><emphasis role="bold">Answer:</emphasis> Check out the <ulink
url="shorewall_quickstart_guide.htm">QuickStart Guides</ulink>.</para>
</section>
</section>
<section>
<title>Port Forwarding</title>
<section id="faq1">
<title>(FAQ 1) I want to forward UDP port 7777 to my my personal PC with
IP address 192.168.1.5. I&#39;ve looked everywhere and can&#39;t find
how to do it.</title>
<para><emphasis role="bold">Answer:</emphasis> The first example in the
<ulink url="Documentation.htm#Rules">rules file documentation</ulink>
shows how to do port forwarding under Shorewall. The format of a
port-forwarding rule to a local system is as follows:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT
DNAT net loc:&#60;l<emphasis>ocal IP address</emphasis>&#62;[:&#60;<emphasis>local port</emphasis>&#62;] &#60;<emphasis>protocol</emphasis>&#62; &#60;<emphasis>port #</emphasis>&#62;</programlisting>
<para>So to forward UDP port 7777 to internal system 192.168.1.5, the
rule is:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT
DNAT net loc:192.168.1.5 udp 7777</programlisting>
<para>If you want to forward requests directed to a particular address (
<emphasis>&#60;external IP&#62;</emphasis> ) on your firewall to an
internal system:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL
# PORT DEST.
DNAT net loc:&#60;l<emphasis>ocal IP address</emphasis>&#62;[:&#60;<emphasis>local port</emphasis>&#62;] &#60;<emphasis>protocol</emphasis>&#62; &#60;<emphasis>port #</emphasis>&#62; - &#60;<emphasis>external IP</emphasis>&#62;</programlisting>
<para>Finally, if you need to forward a range of ports, in the PORT
column specify the range as <emphasis>&#60;low-port&#62;:&#60;high-port&#62;</emphasis>.</para>
<section id="faq1a">
<title>(FAQ 1a) Ok -- I followed those instructions but it doesn&#39;t
work</title>
<para><emphasis role="bold">Answer:</emphasis> That is usually the
result of one of four things:</para>
<itemizedlist>
<listitem>
<para>You are trying to test from inside your firewall (no, that
won&#39;t work -- see <xref linkend="faq2" />).</para>
</listitem>
<listitem>
<para>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&#39;s internal interface).</para>
</listitem>
<listitem>
<para>Your ISP is blocking that particular port inbound.</para>
</listitem>
<listitem>
<para>You are running Mandrake Linux and have configured Internet
Connection Sharing. In that case, the name of your local zone is
&#39;masq&#39; rather than &#39;loc&#39; (change all instances of
&#39;loc&#39; to &#39;masq&#39; in your rules). You may want to
consider re-installing Shorewall in a configuration which matches
the Shorewall documentation. See the <ulink
url="two-interface.htm">two-interface QuickStart Guide</ulink> for
details.</para>
</listitem>
</itemizedlist>
</section>
<section id="faq1b">
<title>(FAQ 1b) I&#39;m still having problems with port forwarding</title>
<para><emphasis role="bold">Answer:</emphasis> To further diagnose
this problem:</para>
<itemizedlist>
<listitem>
<para>As root, type <quote><command>iptables -t nat -Z</command></quote>.
This clears the NetFilter counters in the nat table.</para>
</listitem>
<listitem>
<para>Try to connect to the redirected port from an external host.</para>
</listitem>
<listitem>
<para>As root type <quote><command>shorewall show nat</command></quote></para>
</listitem>
<listitem>
<para>Locate the appropriate DNAT rule. It will be in a chain
called <emphasis>&#60;source zone&#62;</emphasis>_dnat (<quote>net_dnat</quote>
in the above examples).</para>
</listitem>
<listitem>
<para>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&#39;s interface to that
system).</para>
</listitem>
<listitem>
<para>If the packet count is zero:</para>
<itemizedlist>
<listitem>
<para>the connection request is not reaching your server
(possibly it is being blocked by your ISP); or</para>
</listitem>
<listitem>
<para>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
<quote>ORIG. DEST.</quote> column in your DNAT rule); or</para>
</listitem>
<listitem>
<para>your DNAT rule doesn&#39;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.</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</section>
<section id="faq1c">
<title>(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?</title>
<para>In /<filename>etc/shorewall/rules</filename>:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT
DNAT net loc:192.168.3:22 tcp 1022</programlisting>
</section>
</section>
<section id="faq30">
<title>(FAQ 30) I&#39;m confused about when to use DNAT rules and when
to use ACCEPT rules.</title>
<para>It would be a good idea to review the <ulink
url="shorewall_quickstart_guide.htm">QuickStart Guide</ulink>
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.</para>
</section>
</section>
<section>
<title>DNS and Port Forwarding/NAT</title>
<section id="faq2">
<title>(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&#39;t.</title>
<para><emphasis role="bold">Answer:</emphasis> I have two objections to
this setup.</para>
<itemizedlist>
<listitem>
<para>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&#39;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 :-)</para>
</listitem>
<listitem>
<para>The accessibility problem is best solved using <ulink
url="shorewall_setup_guide.htm#DNS">Bind Version 9 <quote>views</quote></ulink>
(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&#39;s what I do here at shorewall.net
for my local systems that use one-to-one NAT.</para>
</listitem>
</itemizedlist>
<para>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.</para>
<para>If you are running Shorewall 1.4.0 or earlier see the <ulink
url="1.3/FAQ.htm#faq2">1.3 FAQ</ulink> for instructions suitable for
those releases.</para>
<para>If you are running Shorewall 1.4.1 or Shorewall 1.4.1a, please
upgrade to Shorewall 1.4.2 or later.</para>
<para>Otherwise:<warning><para>In this configuration, all loc-&#62;loc
traffic will look to the server as if it came from the firewall rather
than from the original client!</para></warning></para>
<itemizedlist>
<listitem>
<para>In <filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
loc eth1 detect <emphasis role="bold">routeback</emphasis></programlisting>
</listitem>
<listitem>
<para>In <filename>/etc/shorewall/rules</filename>:</para>
<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</programlisting>
<para>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 <filename>/etc/shorewall/init</filename>:</para>
<programlisting><command>ETH0_IP=`find_interface_address eth0`</command></programlisting>
<para>and make your DNAT rule:</para>
<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</programlisting>
<para>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.</para>
</listitem>
</itemizedlist>
<section id="faq2a">
<title>(FAQ 2a) I have a zone <quote>Z</quote> 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&#39;t access each other using
their DNS names.</title>
<note>
<para>If the ALL INTERFACES column in /etc/shorewall/nat is empty or
contains <quote>Yes</quote>, 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&#39;s public address:</para>
<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</programlisting>
</note>
<para><emphasis role="bold">Answer:</emphasis> This is another problem
that is best solved using Bind Version 9 <quote>views</quote>. It
allows both external and internal clients to access a NATed host using
the host&#39;s DNS name.</para>
<para>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.</para>
<para>If you don&#39;t like those solutions and prefer routing all
Z-&#62;Z traffic through your firewall then:</para>
<orderedlist>
<listitem>
<para>Set the Z-&#62;Z policy to ACCEPT.</para>
</listitem>
<listitem>
<para>Masquerade Z to itself.</para>
</listitem>
<listitem>
<para>Set the routeback option on the interface to Z.</para>
</listitem>
<listitem>
<para>Set the ALL INTERFACES column in the nat file to
<quote>Yes</quote>.</para>
<warning>
<para>In this configuration, all Z-&#62;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.</para>
</warning>
</listitem>
</orderedlist>
<example>
<title>Example:</title>
<literallayout>Zone: dmz Interface: eth2 Subnet: 192.168.2.0/24</literallayout>
<para>In <filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
loc eth2 192.168.2.255 <emphasis role="bold">routeback</emphasis></programlisting>
<para>In <filename>/etc/shorewall/policy</filename>:</para>
<programlisting>#SOURCE DESTINATION POLICY LIMIT:BURST
dmz dmz ACCEPT</programlisting>
<para>In <filename>/etc/shorewall/masq</filename>:</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth2 192.168.2.0/24</programlisting>
<para>In <filename>/etc/shorewall/na</filename>t, be sure that you
have <quote>Yes</quote> in the ALL INTERFACES column.</para>
</example>
</section>
</section>
</section>
<section>
<title>Netmeeting/MSN</title>
<section id="faq3">
<title>(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with
Shorewall. What do I do?</title>
<para><emphasis role="bold">Answer:</emphasis> There is an <ulink
url="http://www.kfki.hu/%7Ekadlec/sw/netfilter/newnat-suite/">H.323
connection tracking/NAT module</ulink> that helps with Netmeeting. Look
<ulink url="http://linux-igd.sourceforge.net">here</ulink> 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 <ulink url="http://www.netfilter.org">http://www.netfilter.org</ulink>.</para>
</section>
</section>
<section>
<title>Open Ports</title>
<section id="faq4">
<title>(FAQ 4) I just used an online port scanner to check my firewall
and it shows some ports as <quote>closed</quote> rather than
<quote>blocked</quote>. Why?</title>
<para><emphasis role="bold">Answer:</emphasis> 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 <quote>Auth</quote>
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 <emphasis>can</emphasis> 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.</para>
<para>If you are seeing port 80 being <quote>closed</quote>, that&#39;s
probably your ISP preventing you from running a web server in violation
of your Service Agreement.</para>
<tip>
<para>You can change the default behavior of Shorewall through use of
an /etc/shorewall/common file. See the <ulink
url="shorewall_extension_scripts.htm">Extension Script Section</ulink>.</para>
</tip>
<tip>
<para>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.</para>
</tip>
<section id="faq4a">
<title>(FAQ 4a) I just ran an nmap UDP scan of my firewall and it
showed 100s of ports as open!!!!</title>
<para><emphasis role="bold">Answer:</emphasis> Take a deep breath and
read the nmap man page section about UDP scans. If nmap gets <emphasis
role="bold">nothing</emphasis> 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-&#62;all policy to REJECT, restart
Shorewall and do the nmap UDP scan again.</para>
</section>
<section id="faq4b">
<title>(FAQ 4b) I have a port that I can&#39;t close no matter how I
change my rules.</title>
<para>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!!!</para>
<para><emphasis role="bold">Answer:</emphasis> 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.</para>
</section>
<section id="faq4c">
<title>(FAQ 4c) How to I use Shorewall with PortSentry?</title>
<para><ulink
url="http://www.shorewall.net/pub/shorewall/contrib/PortsentryHOWTO.txt">Here&#39;s
a writeup</ulink> on a nice integration of Shorewall and PortSentry.</para>
</section>
</section>
</section>
<section>
<title>Connection Problems</title>
<section id="faq5">
<title>(FAQ 5) I&#39;ve installed Shorewall and now I can&#39;t ping
through the firewall</title>
<para><emphasis role="bold">Answer:</emphasis> If you want your firewall
to be totally open for <quote>ping</quote>,</para>
<orderedlist>
<listitem>
<para>Create <filename>/etc/shorewall/common</filename> if it
doesn&#39;t already exist.</para>
</listitem>
<listitem>
<para>Be sure that the first command in the file is <quote>.
<filename>/etc/shorewall/common.de</filename>f</quote></para>
</listitem>
<listitem>
<para>Add the following to <filename>/etc/shorewall/common</filename></para>
<programlisting><command>run_iptables -A icmpdef -p ICMP --icmp-type echo-request -j ACCEPT</command></programlisting>
</listitem>
</orderedlist>
<para>For a complete description of Shorewall <quote>ping</quote>
management, see <ulink url="ping.html">this page</ulink>.</para>
</section>
<section id="faq15">
<title>(FAQ 15) My local systems can&#39;t see out to the net</title>
<para><emphasis role="bold">Answer:</emphasis> Every time I read
<quote>systems can&#39;t see out to the net</quote>, I wonder where the
poster bought computers with eyes and what those computers will
<quote>see</quote> when things are working properly. That aside, the
most common causes of this problem are:</para>
<orderedlist>
<listitem>
<para>The default gateway on each local system isn&#39;t set to the
IP address of the local firewall interface.</para>
</listitem>
<listitem>
<para>The entry for the local network in the /etc/shorewall/masq
file is wrong or missing.</para>
</listitem>
<listitem>
<para>The DNS settings on the local systems are wrong or the user is
running a DNS server on the firewall and hasn&#39;t enabled UDP and
TCP port 53 from the firewall to the internet.</para>
</listitem>
</orderedlist>
</section>
<section id="faq29">
<title>(FAQ 29) FTP Doesn&#39;t Work</title>
<para>See the <ulink url="FTP.html">Shorewall and FTP page</ulink>.</para>
</section>
</section>
<section>
<title>Logging</title>
<section id="faq6">
<title>(FAQ 6) Where are the log messages written and how do I change
the destination?</title>
<para><emphasis role="bold">Answer:</emphasis> NetFilter uses the
kernel&#39;s equivalent of syslog (see <quote>man syslog</quote>) to log
messages. It always uses the LOG_KERN (kern) facility (see
<quote>man openlog</quote>) and you get to choose the log level (again,
see <quote>man syslog</quote>) in your <ulink
url="Documentation.htm#Policy">policies</ulink> and <ulink
url="Documentation.htm#Rules">rules</ulink>. The destination for
messaged logged by syslog is controlled by <filename>/etc/syslog.conf</filename>
(see <quote>man syslog.conf</quote>). When you have changed
/etc/syslog.conf, be sure to restart syslogd (on a RedHat system,
<quote>service syslog restart</quote>).</para>
<para>By default, older versions of Shorewall ratelimited log messages
through <ulink url="Documentation.htm#Conf">settings</ulink> in
<filename>/etc/shorewall/shorewall.conf</filename> -- If you want to log
all messages, set:</para>
<programlisting>LOGLIMIT=&#34;&#34;
LOGBURST=&#34;&#34;</programlisting>
<para>Beginning with Shorewall version 1.3.12, you can <ulink
url="shorewall_logging.html">set up Shorewall to log all of its messages
to a separate file</ulink>.</para>
<section id="faq6a">
<title>(FAQ 6a) Are there any log parsers that work with Shorewall?</title>
<para><emphasis role="bold">Answer:</emphasis> Here are several links
that may be helpful:</para>
<literallayout><ulink
url="http://www.shorewall.net/pub/shorewall/parsefw/">http://www.shorewall.net/pub/shorewall/parsefw/</ulink>
<ulink url="http://www.fireparse.com">http://www.fireparse.com</ulink>
<ulink url="http://cert.uni-stuttgart.de/projects/fwlogwatch">http://cert.uni-stuttgart.de/projects/fwlogwatch</ulink>
<ulink url="http://www.logwatch.org">http://www.logwatch.org</ulink>
<ulink url="http://gege.org/iptables">http://gege.org/iptables</ulink>
<ulink url="http://home.regit.org/ulogd-php.html">http://home.regit.org/ulogd-php.html</ulink></literallayout>
<para>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.</para>
</section>
<section id="faq6b">
<title>(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?</title>
<para>Temporarily add the following rule:</para>
<programlisting>DROP net fw udp 10619</programlisting>
</section>
<section id="faq6c">
<title>(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?</title>
<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</programlisting>
<para><emphasis role="bold">Answer:</emphasis> There are two
possibilities:</para>
<orderedlist>
<listitem>
<para>They are late-arriving replies to DNS queries.</para>
</listitem>
<listitem>
<para>They are corrupted reply packets.</para>
</listitem>
</orderedlist>
<para>You can distinguish the difference by setting the <emphasis
role="bold">logunclean</emphasis> option (<filename><ulink
url="Documentation.htm#Interfaces">/etc/shorewall/interfaces</ulink></filename>)
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:</para>
<programlisting>#
# Include the standard common.def file
#
<command>. /etc/shorewall/common.def</command>
#
# The following rule is non-standard and compensates for tardy
# DNS replies
#
<command>run_iptables -A common -p udp --sport 53 -mstate --state NEW -j DROP</command></programlisting>
<para>The above file is also include in all of my sample
configurations available in the <ulink
url="shorewall_quickstart_guide.htm">Quick Start Guides</ulink> and in
the common.def file in Shorewall 1.4.0 and later.</para>
</section>
<section id="faq6d">
<title>(FAQ 6d) Why is the MAC address in Shorewall log messages so
long? I thought MAC addresses were only 6 bytes in length.</title>
<para>What is labeled as the MAC address in a Shorewall log message is
actually the Ethernet frame header. It contains:</para>
<itemizedlist>
<listitem>
<para>the destination MAC address (6 bytes)</para>
</listitem>
<listitem>
<para>the source MAC address (6 bytes)</para>
</listitem>
<listitem>
<para>the ethernet frame type (2 bytes)</para>
</listitem>
</itemizedlist>
<para><example><title>Example</title><para><programlisting>MAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00</programlisting><itemizedlist><listitem><para>Destination
MAC address = 00:04:4c:dc:e2:28</para></listitem><listitem><para>Source
MAC address = 00:b0:8e:cf:3c:4c</para></listitem><listitem><para>Ethernet
Frame Type = 08:00 (IP Version 4)</para></listitem></itemizedlist></para></example></para>
</section>
</section>
<section id="faq16">
<title>(FAQ 16) Shorewall is writing log messages all over my console
making it unusable!</title>
<para><emphasis role="bold">Answer:</emphasis> If you are running
Shorewall version 1.4.4 or 1.4.4a then check the <ulink url="errata.htm">errata</ulink>.
Otherwise:</para>
<itemizedlist>
<listitem>
<para>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
<quote>-c <emphasis>&#60;n&#62;</emphasis></quote> where
<emphasis>&#60;n&#62;</emphasis> is a log level of 5 or less; or</para>
</listitem>
<listitem>
<para>See the <quote>dmesg</quote> man page (<quote>man dmesg</quote>).
You must add a suitable <quote>dmesg</quote> command to your startup
scripts or place it in /etc/shorewall/start.</para>
</listitem>
</itemizedlist>
<tip>
<para>Under RedHat and Mandrake, the max <ulink
url="shorewall_logging.html">log level</ulink> that is sent to the
console is specified in /etc/sysconfig/init in the LOGLEVEL variable.
Set <quote>LOGLEVEL=5</quote> to suppress info (log level 6) messages
on the console.</para>
</tip>
<tip>
<para>Under Debian, you can set KLOGD=<quote>-c 5</quote> in
<filename>/etc/init.d/klogd</filename> to suppress info (log level 6)
messages on the console.</para>
</tip>
<tip>
<para>Under SuSE, add <quote>-c 5</quote> to KLOGD_PARAMS in
/etc/sysconfig/syslog to suppress info (log level 6) messages on the
console.</para>
</tip>
</section>
<section id="faq17">
<title>(FAQ 17) What does this log message mean?</title>
<para><emphasis role="bold">Answer:</emphasis> Logging occurs out of a
number of chains (as indicated in the log message) in Shorewall:</para>
<variablelist>
<varlistentry>
<term>man1918 or logdrop</term>
<listitem>
<para>The destination address is listed in <filename>/etc/shorewall/rfc1918</filename>
with a <emphasis role="bold">logdrop</emphasis> target -- see
<filename><ulink url="Documentation.htm#rfc1918">/etc/shorewall/rfc1918</ulink></filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>rfc1918 or logdrop</term>
<listitem>
<para>The source address is listed in <filename>/etc/shorewall/rfc1918</filename>
with a <emphasis role="bold">logdrop</emphasis> target -- see
<filename><ulink url="Documentation.htm#rfc1918">/etc/shorewall/rfc1918</ulink></filename>.</para>
</listitem>
</varlistentry>
<varlistentry id="all2all">
<term>all2&#60;zone&#62;, &#60;zone&#62;2all or all2all</term>
<listitem>
<para>You have a <ulink url="Documentation.htm#Policy">policy</ulink>
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
<ulink url="Documentation.htm#Rules">rule</ulink> to that effect.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>&#60;zone1&#62;2&#60;zone2&#62;</term>
<listitem>
<para>Either you have a <ulink url="Documentation.htm#Policy">policy</ulink>
for <emphasis role="bold">&#60;zone1&#62;</emphasis> to <emphasis
role="bold">&#60;zone2&#62;</emphasis> that specifies a log level
and this packet is being logged under that policy or this packet
matches a <ulink url="Documentation.htm#Rules">rule</ulink> that
includes a log level.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>&#60;interface&#62;_mac</term>
<listitem>
<para>The packet is being logged under the <emphasis role="bold">maclist</emphasis>
<ulink url="Documentation.htm#Interfaces">interface option</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>logpkt</term>
<listitem>
<para>The packet is being logged under the <emphasis role="bold">logunclean</emphasis>
<ulink url="Documentation.htm#Interfaces">interface option</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>badpkt</term>
<listitem>
<para>The packet is being logged under the <emphasis role="bold">dropunclean</emphasis>
<ulink url="Documentation.htm#Interfaces">interface option</ulink>
as specified in the <emphasis role="bold">LOGUNCLEAN</emphasis>
setting in <ulink url="Documentation.htm#Conf"><filename>/etc/shorewall/shorewall.conf</filename></ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>blacklst</term>
<listitem>
<para>The packet is being logged because the source IP is
blacklisted in the <filename><ulink
url="Documentation.htm#Blacklist">/etc/shorewall/blacklist</ulink></filename>
file.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>newnotsyn</term>
<listitem>
<para>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 <emphasis
role="bold">NEWNOTSYN</emphasis> and <emphasis role="bold">LOGNEWNOTSYN</emphasis>
in <ulink url="Documentation.htm#Conf"><filename>/etc/shorewall/shorewall.conf</filename></ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>INPUT or FORWARD</term>
<listitem>
<para>The packet has a source IP address that isn&#39;t in any of
your defined zones (<quote>shorewall check</quote> and look at the
printed zone definitions) or the chain is FORWARD and the
destination IP isn&#39;t in any of your defined zones. Also see
<xref linkend="faq2a" /> for another cause of packets being logged
in the FORWARD chain.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>logflags</term>
<listitem>
<para>The packet is being logged because it failed the checks
implemented by the <emphasis role="bold">tcpflags</emphasis>
<ulink url="Documentation.htm#Interfaces">interface option</ulink>.</para>
</listitem>
</varlistentry>
</variablelist>
<example>
<title>Here is an example:</title>
<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</programlisting>
<para>Let&#39;s look at the important parts of this message:</para>
<variablelist>
<varlistentry>
<term>all2all:REJECT</term>
<listitem>
<para>This packet was REJECTed out of the <emphasis role="bold">all2all</emphasis>
chain -- the packet was rejected under the <quote>all</quote>-&#62;<quote>all</quote>
REJECT policy (<xref linkend="all2all" /> above).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>IN=eth2</term>
<listitem>
<para>the packet entered the firewall via eth2. If you see
<quote>IN=</quote> with no interface name, the packet originated
on the firewall itself.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OUT=eth1</term>
<listitem>
<para>if accepted, the packet would be sent on eth1. If you see
<quote>OUT=</quote> with no interface name, the packet would be
processed by the firewall itself.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SRC=192.168.2.2</term>
<listitem>
<para>the packet was sent by 192.168.2.2</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DST=192.168.1.3</term>
<listitem>
<para>the packet is destined for 192.168.1.3</para>
</listitem>
</varlistentry>
<varlistentry>
<term>PROTO=UDP</term>
<listitem>
<para>UDP Protocol</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DPT=53</term>
<listitem>
<para>The destination port is 53 (DNS)</para>
</listitem>
</varlistentry>
</variablelist>
<para>For additional information about the log message, see <ulink
url="http://logi.cc/linux/netfilter-log-format.php3">http://logi.cc/linux/netfilter-log-format.php3</ulink>.</para>
<para>In this case, 192.168.2.2 was in the <quote>dmz</quote> zone and
192.168.1.3 is in the <quote>loc</quote> zone. I was missing the rule:</para>
<programlisting>ACCEPT dmz loc udp 53</programlisting>
</example>
</section>
<section id="faq21">
<title>(FAQ 21) I see these strange log entries occasionally; what are
they?</title>
<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 ]</programlisting>
<para>192.0.2.3 is external on my firewall... 172.16.0.0/24 is my
internal LAN</para>
<para><emphasis role="bold">Answer:</emphasis> While most people
associate the Internet Control Message Protocol (ICMP) with
<quote>ping</quote>, 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.</para>
<para>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.</para>
<para>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&#39;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&#39;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&#39;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.</para>
</section>
</section>
<section>
<title>Routing</title>
<section id="faq32">
<title>(FAQ 32) My firewall has two connections to the internet from two
different ISPs. How do I set this up in Shorewall?</title>
<para>Setting this up in Shorewall is easy; setting up the routing is a
bit harder.</para>
<para>Assuming that <filename class="devicefile">eth0</filename> and
<filename class="devicefile">eth1</filename> are the interfaces to the
two ISPs then:</para>
<para><filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect
net eth1 detect</programlisting>
<para><filename>/etc/shorewall/policy</filename>:</para>
<programlisting>#SOURCE DESTINATION POLICY LIMIT:BURST
net net DROP</programlisting>
<para>If you have masqueraded hosts, be sure to update
<filename>/etc/shorewall/masq</filename> to masquerade to both ISPs. For
example, if you masquerade all hosts connected to <filename
class="devicefile">eth2</filename> then:</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 eth2
eth1 eth2</programlisting>
<para><citetitle>There was an article in SysAdmin covering this topic.
It may be found at <ulink
url="http://www.samag.com/documents/s=1824/sam0201h/">http://www.samag.com/documents/s=1824/sam0201h/</ulink></citetitle></para>
<para><citetitle>The following information regarding setting up routing
for this configuration is reproduced from the <ulink
url="http://www.lartc.org">LARTC HOWTO</ulink> and has not been verified
by the author. If you have questions or problems with the instructions
given below, please post to the <ulink
url="http://www.lartc.org/#mailinglist">LARTC mailing list</ulink>.</citetitle></para>
<sidebar>
<para>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.</para>
<programlisting> ________
+------------+ /
| | |
+-------------+ Provider 1 +-------
__ | | | /
___/ \_ +------+-------+ +------------+ |
_/ \__ | if1 | /
/ \ | | |
| Local network -----+ Linux router | | Internet
\_ __/ | | |
\__ __/ | if2 | \
\___/ +------+-------+ +------------+ |
| | | \
+-------------+ Provider 2 +-------
| | |
+------------+ \________
</programlisting>
<para>There are usually two questions given this setup.</para>
<para><emphasis role="bold">Split access</emphasis></para>
<para>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.</para>
<para>Let us first set some symbolical names. Let <emphasis
role="bold">$IF1</emphasis> be the name of the first interface (if1 in
the picture above) and <emphasis role="bold">$IF2</emphasis> the name
of the second interface. Then let <emphasis role="bold">$IP1</emphasis>
be the IP address associated with <emphasis role="bold">$IF1</emphasis>
and <emphasis role="bold">$IP2</emphasis> the IP address associated
with <emphasis role="bold">$IF2</emphasis>. Next, let <emphasis
role="bold">$P1</emphasis> be the IP address of the gateway at
Provider 1, and <emphasis role="bold">$P2</emphasis> the IP address of
the gateway at provider 2. Finally, let <emphasis role="bold">$P1_NET</emphasis>
be the IP network <emphasis role="bold">$P1</emphasis> is in, and
<emphasis role="bold">$P2_NET</emphasis> the IP network <emphasis
role="bold">$P2</emphasis> is in.</para>
<para>One creates two additional routing tables, say <emphasis
role="bold">T1</emphasis> and <emphasis role="bold">T2</emphasis>.
These are added in /etc/iproute2/rt_tables. Then you set up routing in
these tables as follows:</para>
<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</programlisting>
<para>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.</para>
<para>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&#39; arguments, they make sure the
right outgoing IP address is chosen.</para>
<programlisting>ip route add $P1_NET dev $IF1 src $IP1
ip route add $P2_NET dev $IF2 src $IP2</programlisting>
<para>Then, your preference for default route:</para>
<programlisting>ip route add default via $P1</programlisting>
<para>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:</para>
<programlisting>ip rule add from $IP1 table T1
ip rule add from $IP2 table T2</programlisting>
<para>This set of commands makes sure all answers to traffic coming in
on a particular interface get answered from that interface.</para>
<note>
<para>&#39;If $P0_NET is the local network and $IF0 is its
interface, the following additional entries are desirable:</para>
<programlisting format="linespecific">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</programlisting>
</note>
<para>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.</para>
<para><emphasis role="bold">Load balancing</emphasis></para>
<para>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.</para>
<para>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):</para>
<programlisting>ip route add default scope global nexthop via $P1 dev $IF1 weight 1 \
nexthop via $P2 dev $IF2 weight 1</programlisting>
<para>This will balance the routes over both providers. The <emphasis
role="bold">weight</emphasis> parameters can be tweaked to favor one
provider over the other.</para>
<note>
<para>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.</para>
</note>
<para>Furthermore, if you really want to do this, you probably also
want to look at Julian Anastasov&#39;s patches at <ulink
url="http://www.ssi.bg/%7Eja/#routes">http://www.ssi.bg/~ja/#routes</ulink>
, Julian&#39;s route patch page. They will make things nicer to work
with.</para>
</sidebar>
</section>
</section>
<section>
<title>Starting and Stopping</title>
<section id="faq7">
<title>(FAQ 7) When I stop Shorewall using <quote>shorewall stop</quote>,
I can&#39;t connect to anything. Why doesn&#39;t that command work?</title>
<para>The <quote><command>stop</command></quote> command is intended to
place your firewall into a safe state whereby only those hosts listed in
<filename>/etc/shorewall/routestopped</filename>&#39; are activated. If
you want to totally open up your firewall, you must use the
<quote><command>shorewall clear</command></quote> command.</para>
</section>
<section id="faq8">
<title>(FAQ 8) When I try to start Shorewall on RedHat, I get messages
about insmod failing -- what&#39;s wrong?</title>
<para><emphasis role="bold">Answer:</emphasis> The output you will see
looks something like this:</para>
<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&#39;t initialize iptables table `nat&#39;: iptables who? (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.</programlisting>
<para>This problem is usually corrected through the following sequence
of commands</para>
<programlisting>service ipchains stop
chkconfig --delete ipchains
rmmod ipchains</programlisting>
<para>Also, be sure to check the <ulink url="errata.htm">errata</ulink>
for problems concerning the version of iptables (v1.2.3) shipped with
RH7.2.</para>
<section id="faq8a">
<title>(FAQ 8a) When I try to start Shorewall on RedHat I get a
message referring me to FAQ #8</title>
<para><emphasis role="bold">Answer:</emphasis> This is usually cured
by the sequence of commands shown above in <xref linkend="faq8" />.</para>
</section>
</section>
<section id="faq9">
<title>(FAQ 9) Why can&#39;t Shorewall detect my interfaces properly at
startup?</title>
<para>I just installed Shorewall and when I issue the start command, I
see the following:</para>
<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...
<emphasis role="bold">Net Zone: eth0:0.0.0.0/0
</emphasis><emphasis role="bold">Local Zone: eth1:0.0.0.0/0</emphasis>
Deleting user chains...
Creating input Chains...
...</programlisting>
<para>Why can&#39;t Shorewall detect my interfaces properly?</para>
<para><emphasis role="bold">Answer:</emphasis> 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 <filename class="devicefile">eth1</filename>. If you
are running Shorewall 1.4.10 or later, you can consider setting the
<ulink url="Documentation.htm#Interfaces"><emphasis role="bold">detectnets</emphasis>
interface option</ulink> on your local interface (<filename
class="devicefile">eth1</filename> in the above example). That will
cause Shorewall to restrict the local zone to only those networks routed
through that interface.</para>
</section>
<section id="faq22">
<title>(FAQ 22) I have some iptables commands that I want to run when
Shorewall starts. Which file do I put them in?</title>
<para>You can place these commands in one of the <ulink
url="shorewall_extension_scripts.htm">Shorewall Extension Scripts</ulink>.
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 <quote>man iptables</quote> and look
at the -I (--insert) command.</para>
</section>
</section>
<section>
<title>About Shorewall</title>
<section id="faq10">
<title>(FAQ 10) What Distributions does it work with?</title>
<para>Shorewall works with any GNU/Linux distribution that includes the
<ulink url="shorewall_prerequisites.htm">proper prerequisites</ulink>.</para>
</section>
<section id="faq11">
<title>(FAQ 11) What Features does it have?</title>
<para><emphasis role="bold">Answer:</emphasis> See the <ulink
url="shorewall_features.htm">Shorewall Feature List</ulink>.</para>
</section>
<section id="faq12">
<title>(FAQ 12) Is there a GUI?</title>
<para><emphasis role="bold">Answer:</emphasis> Yes. Shorewall support is
included in Webmin 1.060 and later versions. See <ulink
url="http://www.webmin.com">http://www.webmin.com</ulink></para>
</section>
<section id="faq13">
<title>(FAQ 13) Why do you call it <quote>Shorewall</quote>?</title>
<para><emphasis role="bold">Answer:</emphasis> Shorewall is a
concatenation of <quote><emphasis>Shore</emphasis>line</quote> (<ulink
url="http://www.cityofshoreline.com">the city where I live</ulink>) and
<quote>Fire<emphasis>wall</emphasis></quote>. The full name of the
product is actually <quote>Shoreline Firewall</quote> but
<quote>Shorewall</quote> is must more commonly used.</para>
</section>
<section id="faq23">
<title>(FAQ 23) Why do you use such ugly fonts on your web site?</title>
<para>The Shorewall web site is almost font neutral (it doesn&#39;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&#39;t
like them then reconfigure your browser.</para>
</section>
<section id="faq25">
<title>(FAQ 25) How to I tell which version of Shorewall I am running?</title>
<para>At the shell prompt, type:</para>
<programlisting><command>/sbin/shorewall version</command></programlisting>
</section>
<section id="faq31">
<title>(FAQ 31) Does Shorewall provide protection against....</title>
<variablelist>
<varlistentry>
<term>IP Spoofing: Sending packets over the WAN interface using an
internal LAP IP address as the source address?</term>
<listitem>
<para>Answer: Yes.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Tear Drop: Sending packets that contain overlapping fragments?</term>
<listitem>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Smurf and Fraggle: Sending packets that use the WAN or LAN
broadcast address as the source address?</term>
<listitem>
<para>Answer: Shorewall can be configured to do that using the
<ulink url="blacklisting_support.htm">blacklisting</ulink>
facility.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Land Attack: Sending packets that use the same address as the
source and destination address?</term>
<listitem>
<para>Answer: Yes, if the <ulink
url="Documentation.htm#Interfaces">routefilter interface option</ulink>
is selected.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DOS: - SYN Dos - ICMP Dos - Per-host Dos protection</term>
<listitem>
<para>Answer: Shorewall has facilities for limiting SYN and ICMP
packets. Netfilter as included in standard Linux kernels
doesn&#39;t support per-remote-host limiting except by explicit
rule that specifies the host IP address; that form of limiting is
supported by Shorewall.</para>
</listitem>
</varlistentry>
</variablelist>
</section>
</section>
<section>
<title>RFC 1918</title>
<section id="faq14">
<title>(FAQ 14) I&#39;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.</title>
<para>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?</para>
<para><emphasis role="bold">Answer:</emphasis> If you are running a
version of Shorewall earlier than 1.3.1, create /etc/shorewall/start and
in it, place the following:</para>
<programlisting><command>run_iptables -I rfc1918 -s 192.168.100.1 -j ACCEPT</command></programlisting>
<para>If you are running version 1.3.1 or later, simply add the
following to <ulink url="Documentation.htm#rfc1918">/etc/shorewall/rfc1918</ulink>:</para>
<para>Be sure that you add the entry ABOVE the entry for 192.168.0.0/16.</para>
<programlisting>#SUBNET TARGET
192.168.100.1 RETURN</programlisting>
<note>
<para>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:</para>
<programlisting>#SUBNET TARGET
192.168.100.1 RETURN
192.168.100.2 RETURN</programlisting>
</note>
<section id="faq14a">
<title>(FAQ 14a) Even though it assigns public IP addresses, my
ISP&#39;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.</title>
<para>The solution is the same as <xref linkend="faq14" /> above.
Simply substitute the IP address of your ISPs DHCP server.</para>
</section>
</section>
</section>
<section>
<title>Alias IP Addresses/Virtual Interfaces</title>
<section id="faq18">
<title>(FAQ 18) Is there any way to use aliased ip addresses with
Shorewall, and maintain separate rulesets for different IPs?</title>
<para><emphasis role="bold">Answer:</emphasis> Yes. See <ulink
url="Shorewall_and_Aliased_Interfaces.html">Shorewall and Aliased
Interfaces</ulink>.</para>
</section>
</section>
<section>
<title>Miscellaneous</title>
<section id="faq19">
<title>(FAQ 19) I have added entries to /etc/shorewall/tcrules but they
don&#39;t seem to do anything. Why?</title>
<para>You probably haven&#39;t set TC_ENABLED=Yes in
/etc/shorewall/shorewall.conf so the contents of the tcrules file are
simply being ignored.</para>
</section>
<section id="faq20">
<title>(FAQ 20) I have just set up a server. Do I have to change
Shorewall to allow access to my server from the internet?</title>
<para>Yes. Consult the <ulink url="shorewall_quickstart_guide.htm">QuickStart
guide</ulink> that you used during your initial setup for information
about how to set up rules for your server.</para>
</section>
<section id="faq24">
<title>(FAQ 24) How can I allow conections to let&#39;s say the ssh port
only from specific IP Addresses on the internet?</title>
<para>In the SOURCE column of the rule, follow <quote>net</quote> by a
colon and a list of the host/subnet addresses as a comma-separated list.</para>
<programlisting>net:&#60;ip1&#62;,&#60;ip2&#62;,...</programlisting>
<example>
<title>Example:</title>
<programlisting>ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22</programlisting>
</example>
</section>
<section id="faq26">
<title>(FAQ 26) When I try to use any of the SYN options in nmap on or
behind the firewall, I get <quote>operation not permitted</quote>. How
can I use nmap with Shorewall?&#34;</title>
<para>Edit /etc/shorewall/shorewall.conf and change <quote>NEWNOTSYN=No</quote>
to <quote>NEWNOTSYN=Yes</quote> then restart Shorewall.</para>
<section id="faq26a">
<title>(FAQ 26a) When I try to use the <quote>-O</quote> option of
nmap from the firewall system, I get <quote>operation not permitted</quote>.
How do I allow this option?</title>
<para>Add this command to your /etc/shorewall/start file:</para>
<programlisting><command>run_iptables -D OUTPUT -p ! icmp -m state --state INVALID -j DROP</command></programlisting>
</section>
</section>
<section id="faq27">
<title>(FAQ 27) I&#39;m compiling a new kernel for my firewall. What
should I look out for?</title>
<para>First take a look at the <ulink url="kernel.htm">Shorewall kernel
configuration page</ulink>. You probably also want to be sure that you
have selected the <quote><emphasis role="bold">NAT of local connections
(READ HELP)</emphasis></quote> on the Netfilter Configuration menu.
Otherwise, DNAT rules with your firewall as the source zone won&#39;t
work with your new kernel.</para>
<section id="faq27a">
<title>(FAQ 27a) I just built and installed a new kernel and now
Shorewall won&#39;t start. I know that my kernel options are correct.</title>
<para>The last few lines of <ulink url="troubleshoot.htm">a startup
trace</ulink> are these:</para>
<programlisting>+ run_iptables2 -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE
+ &#39;[&#39; &#39;x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j
MASQUERADE&#39; = &#39;x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.
0/0 -j MASQUERADE&#39; &#39;]&#39;
+ 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
+ &#39;[&#39; -z &#39;&#39; &#39;]&#39;
+ stop_firewall
+ set +x</programlisting>
<para><emphasis role="bold">Answer:</emphasis> Your new kernel
contains headers that are incompatible with the ones used to compile
your <command>iptables</command> utility. You need to rebuild
<command>iptables</command> using your new kernel source.</para>
</section>
</section>
<section id="faq28">
<title>(FAQ 28) How do I use Shorewall as a Bridging Firewall?</title>
<para>Basically, you don&#39;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&#39;t work that
my official position is that <quote>Shorewall doesn&#39;t work with
Layer 2 Bridging</quote>.</para>
</section>
</section>
<appendix>
<title>Revision History</title>
<para><revhistory><revision><revnumber>1.15</revnumber><date>2004-01-25</date><authorinitials>TE</authorinitials><revremark>Updated
FAQ 32 to mention masquerading. Remove tables.</revremark></revision><revision><revnumber>1.14</revnumber><date>2004-01-24</date><authorinitials>TE</authorinitials><revremark>Added
FAQ 27a regarding kernel/iptables incompatibility.</revremark></revision><revision><revnumber>1.13</revnumber><date>2004-01-24</date><authorinitials>TE</authorinitials><revremark>Add
a note about the <emphasis role="bold">detectnets</emphasis> interface
option in FAQ 9.</revremark></revision><revision><revnumber>1.12</revnumber><date>2004-01-20</date><authorinitials>TE</authorinitials><revremark>Improve
FAQ 16 answer.</revremark></revision><revision><revnumber>1.11</revnumber><date>2004-01-14</date><authorinitials>TE</authorinitials><revremark>Corrected
broken link</revremark></revision><revision><revnumber>1.10</revnumber><date>2004-01-09</date><authorinitials>TE</authorinitials><revremark>Added
a couple of more legacy FAQ numbers.</revremark></revision><revision><revnumber>1.9</revnumber><date>2004-01-08</date><authorinitials>TE</authorinitials><revremark>Corrected
typo in FAQ 26a. Added warning to FAQ 2 regarding source address of
redirected requests.</revremark></revision><revision><revnumber>1.8</revnumber><date>2003-12-31</date><authorinitials>TE</authorinitials><revremark>Additions
to FAQ 4.</revremark></revision><revision><revnumber>1.7</revnumber><date>2003-12-30</date><authorinitials>TE</authorinitials><revremark>Remove
dead link from FAQ 1.</revremark></revision><revision><revnumber>1.6</revnumber><date>2003.12-18</date><authorinitials>TE</authorinitials><revremark>Add
external link reference to FAQ 17.</revremark></revision><revision><revnumber>1.5</revnumber><date>2003-12-16</date><authorinitials>TE</authorinitials><revremark>Added
a link to a Sys Admin article about multiple internet interfaces. Added
Legal Notice. Moved &#34;abstract&#34; to the body of the document. Moved
Revision History to this Appendix.</revremark></revision><revision><revnumber>1.4</revnumber><date>2003-12-13</date><authorinitials>TE</authorinitials><revremark>Corrected
formatting problems</revremark></revision><revision><revnumber>1.3</revnumber><date>2003-12-10</date><authorinitials>TE</authorinitials><revremark>Changed
the title of FAQ 17</revremark></revision><revision><revnumber>1.2</revnumber><date>2003-12-09</date><authorinitials>TE</authorinitials><revremark>Added
Copyright and legacy FAQ numbers</revremark></revision><revision><revnumber>1.1</revnumber><date>2003-12-04</date><authorinitials>MN</authorinitials><revremark>Converted
to Simplified DocBook XML</revremark></revision><revision><revnumber>1.0</revnumber><date>2002-08-13</date><authorinitials>TE</authorinitials><revremark>Initial
revision</revremark></revision></revhistory></para>
</appendix>
</article>
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