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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 and Multiple Internet Connections</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2005</year>
<year>2006</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>
<warning>
<para>Make sure you are<emphasis role="bold"> running a current,
vendor-supported distribution</emphasis>, before attempting to perform
this setup, older distributions do not meet the minimum requirements,and
you will need to recompile iptables, kernel and other software on your
system. If you don't follow this advice,<emphasis role="bold">we will
<emphasis role="bold">not</emphasis> be able to help
you</emphasis>.</para>
</warning>
<warning>
<para>Reading just Shorewall documentation is probably not going to give
you enough background to use this material. Shorewall may make iptables
easy but the Shorewall team doesn't have the resources to be able to
spoon-feed Linux policy routing to you (please remember that the user's
manual for a tractor doesn't teach you to grow corn either). You will
likely need to refer to the following additional information:</para>
<itemizedlist>
<listitem>
<para>The LARTC HOWTO: <ulink
url="http://www.lartc.org">http://www.lartc.org</ulink></para>
</listitem>
<listitem>
<para>Output of <command>man ip</command></para>
</listitem>
<listitem>
<para>Output of <command>ip route help</command> and <command>ip rule
help</command></para>
</listitem>
</itemizedlist>
</warning>
<section>
<title>Multiple Internet Connection Support</title>
<para>Beginning with Shorewall 2.3.2, limited support is included for
multiple internet connections. Limitations of this support are as
follows:</para>
<itemizedlist>
<listitem>
<para>It utilizes static routing configuration. As such, there is no
provision for reacting to the failure of either of the uplinks.</para>
</listitem>
<listitem>
<para>The routing changes are made and the route cache is purged when
Shorewall is started <emphasis role="bold">and when Shorewall is
restarted</emphasis> (unless you specify the "-n" option to
<command>shorewall restart</command>). Ideally, restarting the packet
filter should have no effect on routing.</para>
</listitem>
<listitem>
<para>Prior to Shorewall 3.4.0, the routes and route rules added by
this support were not completely removed during <command>shorewall
stop</command>, <command>shorewall clear</command> or
<command>shorewall restart</command>.</para>
</listitem>
</itemizedlist>
<section>
<title>Overview</title>
<para>Let's assume that a firewall is connected via two separate
ethernet interfaces to two different ISPs as in the following
diagram.</para>
<graphic align="center" fileref="images/TwoISPs.png" valign="middle" />
<itemizedlist>
<listitem>
<para>eth0 connects to ISP1. The IP address of eth0 is
206.124.146.176 and the ISP's gateway router has IP address
206.124.146.254.</para>
</listitem>
<listitem>
<para>eth1 connects to ISP 2. The IP address of eth1 is
130.252.99.27 and the ISP's gateway router has IP address
130.252.99.254.</para>
</listitem>
<listitem>
<para>eth2 connects to the local LAN. Its IP configuration is not
relevant to this discussion.</para>
</listitem>
</itemizedlist>
<para>Each of these <firstterm>providers</firstterm> is described in an
entry in the file <filename>/etc/shorewall/providers</filename>.</para>
<para>Entries in <filename>/etc/shorewall/providers</filename> can
specify that outgoing connections are to be load-balanced between the
two ISPs. Entries in <filename>/etc/shorewall/tcrules</filename> can be
used to direct particular outgoing connections to one ISP or the other.
Use of <filename>/etc/shorewall/tcrules</filename> is not required for
<filename>/etc/shorewall/providers</filename> to work, but you must
select a unique MARK value for each provider so Shorewall can set up the
correct marking rules for you.</para>
<para>When you use the <emphasis role="bold">track</emphasis> option in
<filename>/etc/shorewall/providers</filename>, connections from the
internet are automatically routed back out of the correct interface and
through the correct ISP gateway. This works whether the connection is
handled by the firewall itself or if it is routed or port-forwarded to a
system behind the firewall.</para>
<para>Shorewall will set up the routing and will update the
<filename>/etc/iproute2/rt_tables</filename> to include the table names
and numbers of the tables that it adds.</para>
<caution>
<para>This feature uses <ulink url="traffic_shaping.htm">packet
marking</ulink> to control the routing. As a consequence, there are
some restrictions concerning entries in
<filename>/etc/shorewall/tcrules</filename>:</para>
<itemizedlist>
<listitem>
<para>Packet marking for traffic control purposes may not be done
in the PREROUTING table for connections involving providers with
'track' specified (see below).</para>
</listitem>
<listitem>
<para>You may not use the SAVE or RESTORE options.</para>
</listitem>
<listitem>
<para>You may not use connection marking.</para>
</listitem>
</itemizedlist>
</caution>
<para>The <filename>/etc/shorewall/providers</filename> file can also be
used in other routing scenarios. See the <ulink
url="Shorewall_Squid_Usage.html">Squid documentation</ulink> for an
example.</para>
</section>
<section>
<title>/etc/shorewall/providers File</title>
<para>Entries in this file have the following columns. As in all
Shorewall configuration files, enter "-" in a column if you don't want
to enter any value.</para>
<variablelist>
<varlistentry>
<term>NAME</term>
<listitem>
<para>The provider name. Must begin with a letter and consist of
letters and digits. The provider name becomes the name of the
generated routing table for this provider.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>NUMBER</term>
<listitem>
<para>A number between 1 and 252. This becomes the routing table
number for the generated table for this provider.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>MARK</term>
<listitem>
<para>A mark value used in your /etc/shorewall/tcrules file to
direct packets to this provider. Shorewall will also mark
connections that have seen input from this provider with this
value and will restore the packet mark in the PREROUTING CHAIN.
Mark values must be in the range 1-255.</para>
<para>Beginning with Shorewall version 3.2.0 Beta 6, you may use
may set HIGH_ROUTE_MARKS=Yes in
<filename>/etc/shorewall/shorewall.conf</filename>. This allows
you to:</para>
<itemizedlist>
<listitem>
<para>Use connection marks for traffic shaping, provided that
you assign those marks in the FORWARD chain.</para>
</listitem>
<listitem>
<para>Use mark values &gt; 255 for provider marks in this
column. These mark values must be a multiple of 256 in the
range 256-65280 (hex equivalent 0x100 - 0xFF00 with the
low-order 8 bits being zero).</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>DUPLICATE</term>
<listitem>
<para>Gives the name or number of a routing table to duplicate.
May be 'main' or the name or number of a previously declared
provider. For most applications, you want to specify 'main'
here.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>INTERFACE</term>
<listitem>
<para>The name of the interface to the provider.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>GATEWAY</term>
<listitem>
<para>The IP address of the provider's Gateway router.</para>
<para>You can enter <emphasis role="bold">detect</emphasis> here
and Shorewall will attempt to automatically determine the gateway
IP address.</para>
<para><emphasis role="bold">Hint:</emphasis> <emphasis
role="bold">"detect"</emphasis> is appropriate for use in cases
where the interface named in the INTERFACE column is dynamically
configured via DHCP etc.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OPTIONS</term>
<listitem>
<para>A comma-separated list from the following:</para>
<variablelist>
<varlistentry>
<term>track</term>
<listitem>
<para>If specified, connections FROM this interface are to
be tracked so that responses may be routed back out this
same interface.</para>
<para>You want to specify 'track' if internet hosts will be
connecting to local servers through this provider. Any time
that you specify 'track', you will also want to specify
'balance' (see below).</para>
<para>Use of this feature requires that your kernel and
iptables support CONNMARK target and connmark match support.
It does not require the ROUTE target extension.</para>
<warning>
<para>iptables 1.3.1 is broken with respect to CONNMARK
and iptables-save/iptables-restore. This means that if you
configure multiple ISPs, <command>shorewall
restore</command> may fail. If it does, you may patch your
iptables using the patch at <ulink
url="http://shorewall.net/pub/shorewall/contrib/iptables/CONNMARK.diff">http://shorewall.net/pub/shorewall/contrib/iptables/CONNMARK.diff</ulink>.</para>
</warning>
<important>
<para>If you are using
<filename>/etc/shorewall/providers</filename> because you
have multiple internet connections, we recommend that you
specify 'track' even if you don't need it. It helps
maintain long-term connections in which there are
significant periods with no traffic.</para>
</important>
</listitem>
</varlistentry>
<varlistentry>
<term>balance</term>
<listitem>
<para>The providers that have 'balance' specified will get
outbound traffic load-balanced among them. 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>
<para>By default, each provider is given the same weight (1)
. You can change the weight of a given provider by following
<emphasis>balance</emphasis> with "=" and the desired weight
(e.g., balance=2). The weights reflect the relative
bandwidth of the providers connections and should be small
numbers since the kernel actually creates additional default
routes for each weight increment.</para>
<important>
<para>If you are using
<filename>/etc/shorewall/providers</filename> because you
have multiple internet connections, we recommend that you
specify 'balance' even if you don't need it. You can still
use entries in <filename>/etc/shorewall/tcrules</filename>
to force traffic to one provider or another.<note>
<para>If you don't heed this advice then be prepared
to read <ulink url="FAQ.htm#faq57">FAQ 57</ulink> and
<ulink url="FAQ.htm#faq58">FAQ 58</ulink>.</para>
</note></para>
</important>
<important>
<para>If you specify 'balance' and still find that all
traffic is going out through only one provider, you may
need to install a kernel built with
CONFIG_IP_ROUTE_MULTIPATH_CACHED=n. Several users have
reported that this change has corrected similar
problems.</para>
<para>The SuSE 10.0 kernel is subject to this problem, and
<ulink
url="https://bugzilla.novell.com/show_bug.cgi?id=190908">
a kernel oops may result in this circumstance.</ulink>
SUSE 10.1 and SLES 10 have
CONFIG_IP_ROUTE_MULTIPATH_CACHED=n set by default. The
source of the problem seems to be <ulink
url="http://news.gmane.org/find-root.php?message_id=%3c00da01c5b35a%24b12b9860%241b00a8c0%40cruncher%3e">an
incompatibility between the LARTC patches and
CONFIG_IP_ROUTE_MULTIPATH_CACHED.</ulink></para>
</important>
</listitem>
</varlistentry>
<varlistentry>
<term>loose</term>
<listitem>
<para>Do not include routing rules that force traffic whose
source IP is an address of the INTERFACE to be routed to
this provider. Useful for defining providers that are to be
used only when the appropriate packet mark is applied.
Should not be specified together with <emphasis
role="bold">balance</emphasis>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>optional (added in Shorewall 3.2.2)</term>
<listitem>
<para>Shorewall will determine of this interface is up and
has a configured IPv4 address. If it is not, a warning is
issued and this provider is not configured.</para>
<note>
<para>'optional' is designed to detect interface states
that will cause <command>shorewall start</command> or
<command>shorewall restart</command> to fail; just because
an interface is in a state that Shorewall can [re]start
without error doesn't mean that traffic can actually be
sent through the interface.</para>
</note>
</listitem>
</varlistentry>
</variablelist>
<para>For those of you who are termnally confused between<emphasis
role="bold"> track</emphasis> and <emphasis
role="bold">balance</emphasis>:</para>
<itemizedlist>
<listitem>
<para><emphasis role="bold">track</emphasis> governs incoming
connections.</para>
</listitem>
<listitem>
<para><emphasis role="bold">balance</emphasis> governs
outgoing connections.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term>COPY</term>
<listitem>
<para>When you specify an existing table in the DUPLICATE column,
Shorewall copies all routes through the interface specified in the
INTERFACE column plus the interfaces listed in this column. At a
minumum, you should list all interfaces on your firewall in this
column except those internet interfaces specified in the INTERFACE
column of entries in this file.</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section>
<title>What an entry in the Providers File Does</title>
<para>Adding another entry in the providers file simply creates an
alternate routing table for you. In addition:</para>
<orderedlist>
<listitem>
<para>Unless <emphasis role="bold">loose</emphasis> is specified, an
ip rule is generated for each IP address on the INTERFACE that
routes traffic from that address through the associated routing
table.</para>
</listitem>
<listitem>
<para>If you specify <emphasis role="bold">track</emphasis>, then
connections which have had at least one packet arrive on the
interface listed in the INTERFACE column have their connection mark
set to the value in the MARK column. In the PREROUTING chain,
packets with a connection mark have their packet mark set to the
value of the associated connection mark; packets marked in this way
bypass any prerouting rules that you create in
<filename>/etc/shorewall/tcrules</filename>. This ensures that
packets associated with connections from outside are always routed
out of the correct interface.</para>
</listitem>
<listitem>
<para>If you specify <emphasis role="bold">balance</emphasis>, then
Shorewall will replace the 'default' route with weight 100 in the
'main' routing table with a load-balancing route among those
gateways where <emphasis role="bold">balance</emphasis> was
specified. So if you configure default routes, be sure that their
weight is less than 100 or the route added by Shorewall will not be
used.</para>
</listitem>
</orderedlist>
<para>That's <emphasis role="bold">all</emphasis> that these entries do.
You still have to follow the principle stated in the <ulink
url="Shorewall_and_Routing.html">Shorewall Routing
documentation</ulink>:</para>
<orderedlist>
<listitem>
<para>Routing determines where packets are to be sent.</para>
</listitem>
<listitem>
<para>Once routing determines where the packet is to go, the
firewall (Shorewall) determines if the packet is allowed to go
there.</para>
</listitem>
</orderedlist>
<para>The bottom line is that if you want traffic to go out through a
particular provider then you <emphasis>must </emphasis>mark that traffic
with the provider's MARK value in
<filename>/etc/shorewall/tcrules</filename> and you must do that marking
in the PREROUTING chain; or, you must provide the appropriate rules in
<filename>/etc/shorewall/route_rules</filename>.</para>
<warning id="Undo">
<para>If you are running a Shorewall version prior to 3.4.0, entries
in <filename>/etc/shorewall/providers</filename> permanently alter
your firewall/gateway's routing; that is, the effect of these changes
is not reversed by <command>shorewall stop</command> or
<command>shorewall clear</command>. To restore routing to its original
state, you may have to restart your network. This can usually be done
by <command>/etc/init.d/network restart</command> or
<command>/etc/init.d/networking restart</command>. Check your
distribution's networking documentation.</para>
<para>Here are some additional things to consider:</para>
<itemizedlist>
<listitem>
<para>You can mitigate the effect of the Shorewall-generated
changes to your routing table by specifying a
<emphasis>metric</emphasis> for each default route that you
configure. Shorewall will generate a load-balancing default route
(assuming that <emphasis role="bold">balance</emphasis> has been
specified for some of the providers) that does not include a
metric and that will therefore not replace any existing route that
has a non-zero metric.</para>
</listitem>
<listitem>
<para>The <command>-n</command> option to <command>shorewall
restart</command> and <command>shorewall restore</command> can be
used to prevent the command from changing your routing.</para>
</listitem>
<listitem>
<para>The <filename>/etc/shorewall/stopped</filename> file can
also be used to restore routing when you stop Shorewall. With your
firewall in its normal (single-table) routing configuration, you
can capture the contents as follows:</para>
<programlisting>ip route ls &gt; routes</programlisting>
<para>Here's what the <filename>routes</filename> file looked like
after I did that on my firewall:</para>
<programlisting>192.168.1.1 dev eth3 scope link
206.124.146.177 dev eth1 scope link
192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1
192.168.2.0/24 via 192.168.2.2 dev tun0
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.254
206.124.146.0/24 dev eth3 proto kernel scope link src 206.124.146.176
169.254.0.0/16 dev eth0 scope link
127.0.0.0/8 dev lo scope link
default via 206.124.146.254 dev eth3</programlisting>
<para>Now edit the file as shown below:</para>
<programlisting><command>ip route flush table main
ip route add</command> 192.168.1.1 dev eth3 scope link
<command>ip route add </command>206.124.146.177 dev eth1 scope link
<command>ip route add </command>192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1
<command>ip route add </command>192.168.2.0/24 via 192.168.2.2 dev tun0
<command>ip route add </command>192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.254
<command>ip route add </command>206.124.146.0/24 dev eth3 proto kernel scope link src 206.124.146.176
<command>ip route add </command>169.254.0.0/16 dev eth0 scope link
<command>ip route add </command>127.0.0.0/8 dev lo scope link
<command>ip route add </command>default via 206.124.146.254 dev eth3
<command>ip route flush cache</command></programlisting>
<para>Now paste the contents of that file into
<filename>/etc/shorewall/stopped</filename>.</para>
<para>You might also want to consider adding the following to the
file:</para>
<programlisting><command>ip rule ls</command> | while read priority rule; do
case ${priority%:} in
0|3276[67])
;;
*)
ip rule del $rule
;;
esac
done</programlisting>
<para>That code will delete all but the default routing
rules.</para>
</listitem>
</itemizedlist>
</warning>
</section>
<section>
<title>What an entry in the Providers File Does NOT Do</title>
<para>Given that Shorewall is simply a tool to configure Netfilter and
does not run continuously in your system, entries in the providers file
<emphasis role="bold">do not provide any automatic failover in the event
of failure of one of your Internet connections</emphasis>.</para>
</section>
<section id="Martians">
<title>Martians</title>
<para>One problem that often arises with Multi-ISP configuration is
'Martians'. If your internet interfaces are configured with the
<emphasis role="bold">routefilter</emphasis> option in
<filename>/etc/shorewall/interfaces</filename> (remember that if you set
that option, you should also select <emphasis
role="bold">logmartians</emphasis>), then things may not work correctly
and you will see messages like this:</para>
<programlisting>Feb 9 17:23:45 gw.ilinx kernel: martian source 206.124.146.176 from 64.86.88.116, on dev eth1
Feb 9 17:23:45 gw.ilinx kernel: ll header: 00:a0:24:2a:1f:72:00:13:5f:07:97:05:08:00</programlisting>
<para>The above message is somewhat awkwardly phrased. The source IP in
this incoming packet was 64.86.88.116 and the destination IP address is
206.124.146.176. Another gotcha is that the incoming packet has already
had the destination IP address changed for DNAT or because the original
outgoing connection was altered by an entry in
<filename>/etc/shorewall/masq</filename> (SNAT or Masquerade). So the
destination IP address (206.124.146.176) may not have been the
destination IP address in the packet as it was initially
received.</para>
<para>There a couple of common causes for these problems:</para>
<orderedlist>
<listitem>
<para>You have connected both of your external interfaces to the
same hub/switch. Connecting multiple firewall interfaces to a common
hub or switch is always a bad idea that will result in
hard-to-diagnose problems.</para>
</listitem>
<listitem>
<para>You are specifying both the <emphasis
role="bold">loose</emphasis> and <emphasis
role="bold">balance</emphasis> options on your provider(s). This
causes individual connections to ping-pong back and forth between
the interfaces which is guaranteed to cause problems.</para>
</listitem>
<listitem>
<para>You are redirecting traffic from the local system out of one
interface or the other using packet marking in your
<filename>/etc/shorewall/tcrules</filename> file. A better approach
is to configure the application to use the appropriate local IP
address (the IP address of the interface that you want the
application to use). See <link linkend="Local">below</link>.</para>
</listitem>
</orderedlist>
<para>If all else fails, remove the <emphasis
role="bold">routefilter</emphasis> option from your external interfaces.
If you do this, you may wish to add rules to log and drop packets from
the Internet that have source addresses in your local networks. For
example, if the local LAN in the above diagram is 192.168.1.0/24, then
you would add this rule:</para>
<programlisting>#ACTION SOURCE DEST
DROP:info net:192.168.1.0/24 all</programlisting>
<para>Be sure the above rule is added before any other rules with
<emphasis>net</emphasis> in the SOURCE column.</para>
</section>
<section>
<title>Example</title>
<para>The configuration in the figure at the top of this section would
be specified in <filename>/etc/shorewall/providers</filename> as
follows.</para>
<programlisting>#NAME NUMBER MARK DUPLICATE INTERFACE GATEWAY OPTIONS COPY
ISP1 1 1 main eth0 206.124.146.254 track,balance eth2
ISP2 2 2 main eth1 130.252.99.254 track,balance eth2</programlisting>
<para>Other configuration files go something like this:</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>Regardless of whether you have masqueraded hosts or not, the
following entries are required in
<filename>/etc/shorewall/masq</filename> if you plan to redirect
connections from the firewall using entries in
<filename>/etc/shorewall/tcrules</filename> or if you specify <emphasis
role="bold">balance</emphasis> on your providers.</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 130.252.99.27 206.124.146.176
eth1 206.124.146.176 130.252.99.27</programlisting>
<para>Those entries ensure that traffic originating on the firewall and
redirected via packet marks always has the source IP address
corresponding to the interface that it is routed out of.</para>
<note>
<para>If you have a Dynamic IP address on either of the interfaces,
you can use shell variables to construct the above rules. For example,
if <filename class="devicefile">eth0</filename> had a dynamic IP
address, then:</para>
<para><filename>/etc/shorewall/params</filename>:</para>
<programlisting>ETH0_IP=$(find_first_interface_address eth0)</programlisting>
<para>/etc/shorewall/masq:</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 130.252.99.27 $ETH0_IP
eth1 $ETH0_IP 130.252.99.27</programlisting>
</note>
<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 206.124.146.176
eth1 eth2 130.252.99.27</programlisting>
<warning>
<para>Entries in <filename>/etc/shorewall/masq</filename> have no
effect on which ISP a particular connection will be sent through. That
is rather the purpose of entries in
<filename>/etc/shorewall/tcrules</filename> or
<filename>/etc/shorewall/route_rules</filename>.</para>
</warning>
<para>Now suppose that you want to route all outgoing SMTP traffic from
your local network through ISP 2. You would make this entry in <ulink
url="traffic_shaping.htm">/etc/shorewall/tcrules</ulink> (and if you are
running a version of Shorewall earlier than 3.0.0, you would set
TC_ENABLED=Yes in <ulink
url="???">/etc/shorewall/shorewall.conf</ulink>).</para>
<programlisting>#MARK SOURCE DEST PROTO PORT(S) CLIENT USER TEST
# PORT(S)
2:P &lt;local network&gt; 0.0.0.0/0 tcp 25</programlisting>
</section>
<section id="Local">
<title>Applications running on the Firewall</title>
<para>Experience has shown that in some cases, problems occur with
applications running on the firewall itself. This is especially true
when you have specified <emphasis role="bold">routefilter</emphasis> on
your external interfaces in /etc/shorewall/interfaces (see <link
linkend="Martians">above</link>). When this happens, it is suggested
that you have the application use specific local IP addresses rather
than 0.</para>
<para>Examples:</para>
<itemizedlist>
<listitem>
<para>Squid: In <filename>squid.conf</filename>, set <emphasis
role="bold">tcp_outgoing_address</emphasis> to the IP address of the
interface that you want Squid to use.</para>
</listitem>
<listitem>
<para>In OpenVPN, set <emphasis role="bold">local
</emphasis>(<emphasis role="bold">--local</emphasis> on the command
line) to the IP address that you want the server to receive
connections on.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title>/etc/shorewall/route_rules</title>
<para>The <filename>/etc/shorewall/route_rules</filename> file was added
in Shorewall version 3.2.0. The <filename>route_rules</filename> file
allows assigning certain traffic to a particular provider just as
entries in the <filename>tcrules</filename> file. The difference between
the two files is that entries in <filename>route_rules</filename> are
independent of Netfilter.</para>
<warning>
<para>Prior to Shorewall version 3.4.0, entries in
<filename>/etc/shorewall/route_rules</filename> permanently alter your
firewall/gateway's routing; that is, the effect of these changes is
not reversed by <command>shorewall stop</command> or
<command>shorewall clear</command>. To restore routing to its original
state, you may have to restart your network. This can usually be done
by <command>/etc/init.d/network restart</command> or
<command>/etc/init.d/networking restart</command>. Check your
distribution's networking documentation.</para>
<para>Please also see the <link linkend="Undo">warning in the section
above entitled <emphasis>What an Entry in the Providers File
Does</emphasis></link>.</para>
</warning>
<section>
<title>Routing Rules</title>
<para>Routing rules are maintained by the Linux kernel and can be
displayed using the <command>ip rule ls</command> command. When
routing a packet, the rules are processed in turn until the packet is
successfully routed.</para>
<programlisting>gateway:~ # <command>ip rule ls</command>
0: from all lookup local &lt;=== Local (to the firewall) IP addresses
10001: from all fwmark 0x1 lookup Blarg &lt;=== This and the next rule are generated by the
10002: from all fwmark 0x2 lookup Comcast 'MARK' values in /etc/shorewall/providers.
20000: from 206.124.146.176 lookup Blarg &lt;=== This and the next rule are generated unless
20256: from 24.12.22.33 lookup Comcast 'loose' is specified; based in the output of 'ip addr ls'
32766: from all lookup main &lt;=== This is the routing table shown by 'iproute -n'
32767: from all lookup default &lt;=== This table is usually empty
gateway:~ #</programlisting>
<para>In the above example, there are two providers: Blarg and Comcast
with MARK 1 going to Blarg and mark 2 going to Comcast.</para>
</section>
<section>
<title>Columns in the route_rules file</title>
<para>Columns in the file are:</para>
<variablelist>
<varlistentry>
<term>SOURCE (Optional)</term>
<listitem>
<para>An ip address (network or host) that matches the source IP
address in a packet. May also be specified as an interface name
optionally followed by ":" and an address. If the device 'lo' is
specified, the packet must originate from the firewall
itself.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DEST (Optional)</term>
<listitem>
<para>An ip address (network or host) that matches the
destination IP address in a packet.</para>
<para>If you choose to omit either SOURCE or DEST, place "-" in
that column. Note that you may not omit both SOURCE and
DEST.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>PROVIDER</term>
<listitem>
<para>The provider to route the traffic through. May be
expressed either as the provider name or the provider
number.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>PRIORITY</term>
<listitem>
<para>The rule's priority which determines the order in which
the rules are processed.</para>
<para>1000-1999 Before Shorewall-generated 'MARK' rules</para>
<para>11000- 11999 After 'MARK' rules but before
Shorewall-generated rules for ISP interfaces.</para>
<para>26000-26999 After ISP interface rules but before 'default'
rule.</para>
<para>Rules with equal priority are applied in the order in
which they appear in the file.</para>
</listitem>
</varlistentry>
</variablelist>
<para>Example 1: You want all traffic entering the firewall on eth1 to
be routed through Comcast.</para>
<programlisting>#SOURCE DEST PROVIDER PRIORITY
eth1 - Comcast 1000</programlisting>
<para>With this entry, the output of <command>ip rule ls</command>
would be as follows.</para>
<para><programlisting>gateway:~ # <command>ip rule ls</command>
0: from all lookup local
1000: from all iif eth1 lookup Comcast
10001: from all fwmark 0x1 lookup Blarg
10002: from all fwmark 0x2 lookup Comcast
20000: from 206.124.146.176 lookup Blarg
20256: from 24.12.22.33 lookup Comcast
32766: from all lookup main
32767: from all lookup default
gateway:~ #</programlisting>Note that because we used a priority of 1000, the
test for <filename class="devicefile">eth1</filename> is inserted
before the fwmark tests.</para>
<para>Example 2: You use OpenVPN (routed setup w/tunX) in combination
with multiple providers. In this case you have to set up a rule to
ensure that the OpenVPN traffic is routed back through the tunX
interface(s) rather than through any of the providers. 10.8.0.0/24 is
the subnet choosen in your OpenVPN configuration (server 10.8.0.0
255.255.255.0).</para>
<programlisting>#SOURCE DEST PROVIDER PRIORITY
- 10.8.0.0/24 main 1000</programlisting>
</section>
</section>
</section>
</article>
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