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360 lines
15 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
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<article id="Shorewall_and_Aliased_Interfaces">
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<!--$Id$-->
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<articleinfo>
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<title>Shorewall and Aliased Interfaces</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
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<copyright>
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<year>2001-2009</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<caution>
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<para><emphasis role="bold">This article applies to Shorewall 4.3 and
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later. If you are running a version of Shorewall earlier than Shorewall
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4.3.5 then please see the documentation for that
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release.</emphasis></para>
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</caution>
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<section id="Background">
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<title>Background</title>
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<para>The traditional net-tools contain a program called
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<emphasis>ifconfig</emphasis> which is used to configure network devices.
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ifconfig introduced the concept of <emphasis>aliased</emphasis> or
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<emphasis>virtual</emphasis> interfaces. These virtual interfaces have
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names of the form <emphasis>interface:integer</emphasis> (e.g., <filename
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class="devicefile">eth0:0</filename>) and ifconfig treats them more or
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less like real interfaces.</para>
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<example id="ifconfig">
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<title>ifconfig</title>
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<programlisting>[root@gateway root]# <command>ifconfig eth0:0</command>
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eth0:0 Link encap:Ethernet HWaddr 02:00:08:3:FA:55
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inet addr:206.124.146.178 Bcast:206.124.146.255 Mask:255.255.255.0
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UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
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Interrupt:11 Base address:0x2000
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[root@gateway root]# </programlisting>
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</example>
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<para>The ifconfig utility is being gradually phased out in favor of the
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<firstterm>ip</firstterm> utility which is part of the
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<emphasis>iproute</emphasis> package. The ip utility does not use the
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concept of aliases or virtual interfaces but rather treats additional
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addresses on an interface as objects in their own right. The ip utility
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does provide for interaction with ifconfig in that it allows addresses to
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be <emphasis>labeled</emphasis> where these labels take the form of
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ipconfig virtual interfaces.</para>
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<example id="ip">
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<title>ip</title>
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<programlisting>[root@gateway root]# <command>ip addr show dev eth0</command>
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2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc htb qlen 100
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link/ether 02:00:08:e3:fa:55 brd ff:ff:ff:ff:ff:ff
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inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0
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inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0:0
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[root@gateway root]# </programlisting>
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<para><note>
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<para>One <emphasis role="bold">cannot</emphasis> type
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<quote><command>ip addr show dev eth0:0</command></quote> because
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<quote><filename class="devicefile">eth0:0</filename></quote> is a
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label for a particular address rather than a device name.</para>
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<programlisting>[root@gateway root]# <command>ip addr show dev eth0:0</command>
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Device "eth0:0" does not exist.
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[root@gateway root]#</programlisting>
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</note></para>
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</example>
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<para>The iptables program doesn't support virtual interfaces in either
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its <quote>-i</quote> or <quote>-o</quote> command options; as a
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consequence, Shorewall does not allow them to be used in the
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/etc/shorewall/interfaces file or anywhere else except as described in the
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discussion below.</para>
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</section>
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<section id="Adding">
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<title>Adding Addresses to Interfaces</title>
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<para>Most distributions have a facility for adding additional addresses
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to interfaces. If you have already used your distribution's capability to
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add your required addresses, you can skip this section.</para>
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<para>Shorewall provides facilities for automatically adding addresses to
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interfaces as described in the following section. It is also easy to add
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them yourself using the <emphasis role="bold">ip</emphasis> utility. The
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above alias was added using:</para>
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<programlisting><command>ip addr add 206.124.146.178/24 brd 206.124.146.255 dev eth0 label eth0:0</command></programlisting>
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<para>You probably want to arrange to add these addresses when the device
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is started rather than placing commands like the above in one of the
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Shorewall extension scripts. For example, on RedHat systems, you can place
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the commands in /sbin/ifup-local:</para>
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<programlisting>#!/bin/sh
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case $1 in
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eth0)
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/sbin/ip addr add 206.124.146.178 dev eth0 label eth0:0
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;;
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esac</programlisting>
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<para>RedHat systems also allow adding such aliases from the network
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administration GUI (which only works well if you have a graphical
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environment on your firewall).</para>
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<para>On Debian and LEAF/Bering systems, it is as simple as adding the
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command to the interface definition as follows:</para>
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<programlisting># Internet interface
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auto eth0
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iface eth0 inet static
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address 206.124.146.176
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netmask 255.255.255.0
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gateway 206.124.146.254
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<command>up ip addr add 206.124.146.178/24 brd 206.124.146.255 dev eth0 label eth0:0</command></programlisting>
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</section>
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<section id="How">
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<title>So how do I handle more than one address on an interface?</title>
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<para>The answer depends on what you are trying to do with the interfaces.
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In the sub-sections that follow, we'll take a look at common
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scenarios.</para>
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<note>
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<para>The examples in the following sub-sections assume that the local
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network is 192.168.1.0/24.</para>
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</note>
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<section id="Rules">
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<title>Separate Rules</title>
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<para>If you need to make a rule for traffic to/from the firewall itself
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that only applies to a particular IP address, simply qualify the $FW
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zone with the IP address.</para>
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<example id="SSH">
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<title>allow SSH from net to eth0:0 above</title>
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<para><optional><filename>/etc/shorewall/rules</filename></optional><programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
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ACCEPT net $FW:206.124.146.178 tcp 22</programlisting></para>
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</example>
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</section>
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<section id="DNAT">
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<title>DNAT</title>
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<para>Suppose that I had set up eth0:0 as above and I wanted to port
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forward from that virtual interface to a web server running in my local
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zone at 192.168.1.3. That is accomplished by a single rule in the
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<filename>/etc/shorewall/rules</filename> file:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
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# PORT(S) DEST
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DNAT net loc:192.168.1.3 tcp 80 - 206.124.146.178 </programlisting>
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<para>If I wished to forward tcp port 10000 on that virtual interface to
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port 22 on local host 192.168.1.3, the rule would be:</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
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# PORT(S) DEST
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DNAT net loc:192.168.1.3:22 tcp 22 - 206.124.146.178 </programlisting>
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</section>
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<section id="SNAT">
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<title>SNAT</title>
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<para>If you wanted to use eth0:0 as the IP address for outbound
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connections from your local zone (eth1), then in
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<filename>/etc/shorewall/masq</filename>:</para>
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<programlisting>#INTERFACE SUBNET ADDRESS
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eth0 192.168.1.0/24 206.124.146.178</programlisting>
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<para>Similarly, you want SMTP traffic from local system 192.168.1.22 to
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have source IP 206.124.146.178:<programlisting>#INTERFACE SUBNET ADDRESS PROTO DEST PORT(S)
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eth0 192.168.1.22 206.124.146.178 tcp 25</programlisting></para>
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<para>Shorewall can create the alias (additional address) for you if you
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set ADD_SNAT_ALIASES=Yes in
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<filename>/etc/shorewall/shorewall.con</filename>f.</para>
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<warning>
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<para>Addresses added by ADD_SNAT_ALIASES=Yes are deleted and re-added
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during <command>shorewall restart</command>. As a consequence,
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connections using those addresses may be severed.</para>
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</warning>
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<para>Shorewall can create the <quote>label</quote> (virtual interface)
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so that you can see the created address using ifconfig. In addition to
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setting ADD_SNAT_ALIASES=Yes, you specify the virtual interface name in
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the INTERFACE column as follows.</para>
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<para><filename>/etc/shorewall/masq</filename><programlisting>#INTERFACE SUBNET ADDRESS
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eth0:0 192.168.1.0/24 206.124.146.178</programlisting>Shorewall
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can also set up SNAT to round-robin over a range of IP addresses. To do
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that, you specify a range of IP addresses in the ADDRESS column. If you
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specify a label in the INTERFACE column, Shorewall will use that label
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for the first address of the range and will increment the label by one
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for each subsequent label.</para>
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<para><filename>/etc/shorewall/masq</filename><programlisting>#INTERFACE SUBNET ADDRESS
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eth0:0 192.168.1.0/24 206.124.146.178-206.124.146.180</programlisting></para>
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<para>The above would create three IP addresses:</para>
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<programlisting>eth0:0 = 206.124.146.178
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eth0:1 = 206.124.146.179
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eth0:2 = 206.124.146.180</programlisting>
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</section>
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<section id="NAT">
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<title>One-to-one NAT</title>
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<para>If you wanted to use one-to-one NAT to link <filename
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class="devicefile">eth0:0</filename> with local address 192.168.1.3, you
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would have the following in
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<filename>/etc/shorewall/nat</filename>:</para>
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<programlisting>#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL
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206.124.146.178 eth0 192.168.1.3 no no</programlisting>
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<para>Shorewall can create the alias (additional address) for you if you
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set ADD_IP_ALIASES=Yes in /etc/shorewall/shorewall.conf.</para>
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<warning>
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<para>Addresses added by ADD_IP_ALIASES=Yes are deleted and re-added
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during <command>shorewall restart</command>. As a consequence,
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connections using those addresses may be severed.</para>
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</warning>
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<para>Shorewall can create the <quote>label</quote> (virtual interface)
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so that you can see the created address using ifconfig. In addition to
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setting ADD_IP_ALIASES=Yes, you specify the virtual interface name in
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the INTERFACE column as follows.</para>
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<para><filename>/etc/shorewall/nat</filename><programlisting>#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL
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206.124.146.178 eth0:0 192.168.1.3 no no</programlisting></para>
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<para>In either case, to create rules in
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<filename>/etc/shorewall/rules</filename> that pertain only to this NAT
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pair, you simply qualify the local zone with the internal IP
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address.</para>
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<example id="SSH1">
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<title>You want to allow SSH from the net to 206.124.146.178 a.k.a.
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192.168.1.3.</title>
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<para><programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
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ACCEPT net loc:192.168.1.3 tcp 22</programlisting></para>
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</example>
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</section>
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<section id="Subnets">
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<title>MULTIPLE SUBNETS</title>
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<para>Sometimes multiple IP addresses are used because there are
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multiple subnetworks configured on a LAN segment. This technique does
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not provide for any security between the subnetworks if the users of the
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systems have administrative privileges because in that case, the users
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can simply manipulate their system's routing table to bypass your
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firewall/router. Nevertheless, there are cases where you simply want to
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consider the LAN segment itself as a zone and allow your firewall/router
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to route between the two subnetworks.</para>
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<example id="subnets">
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<title>Local interface eth1 interfaces to 192.168.1.0/24 and
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192.168.20.0/24. The primary IP address of eth1 is 192.168.1.254 and
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eth1:0 is 192.168.20.254. You simply want your firewall to route
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between these two subnetworks.</title>
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<para>In <filename>/etc/shorewall/zones</filename>:</para>
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<programlisting>#ZONE TYPE OPTIONS
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loc ipv4</programlisting>
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<para>In <filename>/etc/shorewall/interfaces</filename>:</para>
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<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
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loc eth1 - <emphasis role="bold">routeback</emphasis> </programlisting>
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<para>In <filename>/etc/shorewall/rules</filename>, simply specify
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ACCEPT rules for the traffic that you want to permit.</para>
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</example>
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<example id="subnets1">
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<title>Local interface eth1 interfaces to 192.168.1.0/24 and
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192.168.20.0/24. The primary IP address of eth1 is 192.168.1.254 and
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eth1:0 is 192.168.20.254. You want to make these subnetworks into
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separate zones and control the access between them (the users of the
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systems do not have administrative privileges).</title>
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<para>In <filename>/etc/shorewall/zones</filename>:</para>
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<programlisting>#ZONE TYPE OPTIONS
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loc ipv4
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loc2 ipv4</programlisting>
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<para>In <filename>/etc/shorewall/interfaces</filename>:</para>
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<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
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- eth1 - </programlisting>
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<para>In <filename>/etc/shorewall/hosts</filename>:</para>
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<programlisting>#ZONE HOSTS OPTIONS
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loc eth1:192.168.1.0/24
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loc2 eth1:192.168.20.0/24</programlisting>
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<para>In <filename>/etc/shorewall/rules</filename>, simply specify
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ACCEPT rules for the traffic that you want to permit.</para>
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<para>For more information on handling multiple networks through a
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single interface, see <ulink
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url="Multiple_Zones.html"><emphasis>Routing on One
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Interface</emphasis></ulink>.</para>
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</example>
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</section>
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<section>
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<title>Defining a Zone-per-Address</title>
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<para><ulink url="Vserver.html">Shorewall's support for Linux
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Vservers</ulink> can (mis-)used to create a separate zone per alias.
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Note that this results in a <emphasis>partitioning of the firewall
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zone</emphasis>. In this usage, you probably want to define an ACCEPT
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policy between your vserver zones and the firewall zone.</para>
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</section>
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</section>
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</article>
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