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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article id="Shorewall_and_Aliased_Interfaces">
<!--$Id$-->
<articleinfo>
<title>Shorewall and Aliased Interfaces</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2001-2009</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>
<caution>
<para><emphasis role="bold">This article applies to Shorewall 4.3 and
later. If you are running a version of Shorewall earlier than Shorewall
4.3.5 then please see the documentation for that
release.</emphasis></para>
</caution>
<section id="Background">
<title>Background</title>
<para>The traditional net-tools contain a program called
<emphasis>ifconfig</emphasis> which is used to configure network devices.
ifconfig introduced the concept of <emphasis>aliased</emphasis> or
<emphasis>virtual</emphasis> interfaces. These virtual interfaces have
names of the form <emphasis>interface:integer</emphasis> (e.g., <filename
class="devicefile">eth0:0</filename>) and ifconfig treats them more or
less like real interfaces.</para>
<example id="ifconfig">
<title>ifconfig</title>
<programlisting>[root@gateway root]# <command>ifconfig eth0:0</command>
eth0:0 Link encap:Ethernet HWaddr 02:00:08:3:FA:55
inet addr:206.124.146.178 Bcast:206.124.146.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:11 Base address:0x2000
[root@gateway root]# </programlisting>
</example>
<para>The ifconfig utility is being gradually phased out in favor of the
<firstterm>ip</firstterm> utility which is part of the
<emphasis>iproute</emphasis> package. The ip utility does not use the
concept of aliases or virtual interfaces but rather treats additional
addresses on an interface as objects in their own right. The ip utility
does provide for interaction with ifconfig in that it allows addresses to
be <emphasis>labeled</emphasis> where these labels take the form of
ipconfig virtual interfaces.</para>
<example id="ip">
<title>ip</title>
<programlisting>[root@gateway root]# <command>ip addr show dev eth0</command>
2: eth0: &lt;BROADCAST,MULTICAST,UP&gt; mtu 1500 qdisc htb qlen 100
link/ether 02:00:08:e3:fa:55 brd ff:ff:ff:ff:ff:ff
inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0
inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0:0
[root@gateway root]# </programlisting>
<para><note>
<para>One <emphasis role="bold">cannot</emphasis> type
<quote><command>ip addr show dev eth0:0</command></quote> because
<quote><filename class="devicefile">eth0:0</filename></quote> is a
label for a particular address rather than a device name.</para>
<programlisting>[root@gateway root]# <command>ip addr show dev eth0:0</command>
Device "eth0:0" does not exist.
[root@gateway root]#</programlisting>
</note></para>
</example>
<para>The iptables program doesn't support virtual interfaces in either
its <quote>-i</quote> or <quote>-o</quote> command options; as a
consequence, Shorewall does not allow them to be used in the
/etc/shorewall/interfaces file or anywhere else except as described in the
discussion below.</para>
</section>
<section id="Adding">
<title>Adding Addresses to Interfaces</title>
<para>Most distributions have a facility for adding additional addresses
to interfaces. If you have already used your distribution's capability to
add your required addresses, you can skip this section.</para>
<para>Shorewall provides facilities for automatically adding addresses to
interfaces as described in the following section. It is also easy to add
them yourself using the <emphasis role="bold">ip</emphasis> utility. The
above alias was added using:</para>
<programlisting><command>ip addr add 206.124.146.178/24 brd 206.124.146.255 dev eth0 label eth0:0</command></programlisting>
<para>You probably want to arrange to add these addresses when the device
is started rather than placing commands like the above in one of the
Shorewall extension scripts. For example, on RedHat systems, you can place
the commands in /sbin/ifup-local:</para>
<programlisting>#!/bin/sh
case $1 in
eth0)
/sbin/ip addr add 206.124.146.178 dev eth0 label eth0:0
;;
esac</programlisting>
<para>RedHat systems also allow adding such aliases from the network
administration GUI (which only works well if you have a graphical
environment on your firewall).</para>
<para>On Debian and LEAF/Bering systems, it is as simple as adding the
command to the interface definition as follows:</para>
<programlisting># Internet interface
auto eth0
iface eth0 inet static
address 206.124.146.176
netmask 255.255.255.0
gateway 206.124.146.254
<command>up ip addr add 206.124.146.178/24 brd 206.124.146.255 dev eth0 label eth0:0</command></programlisting>
</section>
<section id="How">
<title>So how do I handle more than one address on an interface?</title>
<para>The answer depends on what you are trying to do with the interfaces.
In the sub-sections that follow, we'll take a look at common
scenarios.</para>
<note>
<para>The examples in the following sub-sections assume that the local
network is 192.168.1.0/24.</para>
</note>
<section id="Rules">
<title>Separate Rules</title>
<para>If you need to make a rule for traffic to/from the firewall itself
that only applies to a particular IP address, simply qualify the $FW
zone with the IP address.</para>
<example id="SSH">
<title>allow SSH from net to eth0:0 above</title>
<para><optional><filename>/etc/shorewall/rules</filename></optional><programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT net $FW:206.124.146.178 tcp 22</programlisting></para>
</example>
</section>
<section id="DNAT">
<title>DNAT</title>
<para>Suppose that I had set up eth0:0 as above and I wanted to port
forward from that virtual interface to a web server running in my local
zone at 192.168.1.3. That is accomplished by a single rule in the
<filename>/etc/shorewall/rules</filename> file:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S) SOURCE ORIGINAL
# PORT(S) DEST
DNAT net loc:192.168.1.3 tcp 80 - 206.124.146.178 </programlisting>
</section>
<section id="SNAT">
<title>SNAT</title>
<para>If you wanted to use eth0:0 as the IP address for outbound
connections from your local zone (eth1), then in
<filename>/etc/shorewall/masq</filename>:</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 192.168.1.0/24 206.124.146.178</programlisting>
2010-11-22 21:04:20 +01:00
<para>Similarly, you want SMTP traffic from local system 192.168.1.22 to
2010-11-22 20:59:59 +01:00
have source IP 206.124.146.178:<programlisting>#INTERFACE SUBNET ADDRESS PROTO DEST PORT(S)
2010-11-22 21:04:20 +01:00
eth0 192.168.1.22 206.124.146.178 tcp 25</programlisting></para>
2010-11-22 20:59:59 +01:00
<para>Shorewall can create the alias (additional address) for you if you
set ADD_SNAT_ALIASES=Yes in
<filename>/etc/shorewall/shorewall.con</filename>f.</para>
<warning>
<para>Addresses added by ADD_SNAT_ALIASES=Yes are deleted and re-added
during <command>shorewall restart</command>. As a consequence,
connections using those addresses may be severed.</para>
</warning>
<para>Shorewall can create the <quote>label</quote> (virtual interface)
so that you can see the created address using ifconfig. In addition to
setting ADD_SNAT_ALIASES=Yes, you specify the virtual interface name in
the INTERFACE column as follows.</para>
<para><filename>/etc/shorewall/masq</filename><programlisting>#INTERFACE SUBNET ADDRESS
eth0:0 192.168.1.0/24 206.124.146.178</programlisting>Shorewall
can also set up SNAT to round-robin over a range of IP addresses. To do
that, you specify a range of IP addresses in the ADDRESS column. If you
specify a label in the INTERFACE column, Shorewall will use that label
for the first address of the range and will increment the label by one
for each subsequent label.</para>
<para><filename>/etc/shorewall/masq</filename><programlisting>#INTERFACE SUBNET ADDRESS
eth0:0 192.168.1.0/24 206.124.146.178-206.124.146.180</programlisting></para>
<para>The above would create three IP addresses:</para>
<programlisting>eth0:0 = 206.124.146.178
eth0:1 = 206.124.146.179
eth0:2 = 206.124.146.180</programlisting>
</section>
<section id="NAT">
<title>One-to-one NAT</title>
<para>If you wanted to use one-to-one NAT to link <filename
class="devicefile">eth0:0</filename> with local address 192.168.1.3, you
would have the following in
<filename>/etc/shorewall/nat</filename>:</para>
<programlisting>#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL
206.124.146.178 eth0 192.168.1.3 no no</programlisting>
<para>Shorewall can create the alias (additional address) for you if you
set ADD_IP_ALIASES=Yes in /etc/shorewall/shorewall.conf.</para>
<warning>
<para>Addresses added by ADD_IP_ALIASES=Yes are deleted and re-added
during <command>shorewall restart</command>. As a consequence,
connections using those addresses may be severed.</para>
</warning>
<para>Shorewall can create the <quote>label</quote> (virtual interface)
so that you can see the created address using ifconfig. In addition to
setting ADD_IP_ALIASES=Yes, you specify the virtual interface name in
the INTERFACE column as follows.</para>
<para><filename>/etc/shorewall/nat</filename><programlisting>#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL
206.124.146.178 eth0:0 192.168.1.3 no no</programlisting></para>
<para>In either case, to create rules in
<filename>/etc/shorewall/rules</filename> that pertain only to this NAT
pair, you simply qualify the local zone with the internal IP
address.</para>
<example id="SSH1">
<title>You want to allow SSH from the net to 206.124.146.178 a.k.a.
192.168.1.3.</title>
<para><programlisting>#ACTION SOURCE DEST PROTO DEST PORT(S)
ACCEPT net loc:192.168.1.3 tcp 22</programlisting></para>
</example>
</section>
<section id="Subnets">
<title>MULTIPLE SUBNETS</title>
<para>Sometimes multiple IP addresses are used because there are
multiple subnetworks configured on a LAN segment. This technique does
not provide for any security between the subnetworks if the users of the
systems have administrative privileges because in that case, the users
can simply manipulate their system's routing table to bypass your
firewall/router. Nevertheless, there are cases where you simply want to
consider the LAN segment itself as a zone and allow your firewall/router
to route between the two subnetworks.</para>
<example id="subnets">
<title>Local interface eth1 interfaces to 192.168.1.0/24 and
192.168.20.0/24. The primary IP address of eth1 is 192.168.1.254 and
eth1:0 is 192.168.20.254. You simply want your firewall to route
between these two subnetworks.</title>
<para>In <filename>/etc/shorewall/zones</filename>:</para>
<programlisting>#ZONE TYPE OPTIONS
loc ipv4</programlisting>
<para>In <filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
loc eth1 - <emphasis role="bold">routeback</emphasis> </programlisting>
<para>In <filename>/etc/shorewall/rules</filename>, simply specify
ACCEPT rules for the traffic that you want to permit.</para>
</example>
<example id="subnets1">
<title>Local interface eth1 interfaces to 192.168.1.0/24 and
192.168.20.0/24. The primary IP address of eth1 is 192.168.1.254 and
eth1:0 is 192.168.20.254. You want to make these subnetworks into
separate zones and control the access between them (the users of the
systems do not have administrative privileges).</title>
<para>In <filename>/etc/shorewall/zones</filename>:</para>
<programlisting>#ZONE TYPE OPTIONS
loc ipv4
loc2 ipv4</programlisting>
<para>In <filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
- eth1 - </programlisting>
<para>In <filename>/etc/shorewall/hosts</filename>:</para>
<programlisting>#ZONE HOSTS OPTIONS
loc eth1:192.168.1.0/24
loc2 eth1:192.168.20.0/24</programlisting>
<para>In <filename>/etc/shorewall/rules</filename>, simply specify
ACCEPT rules for the traffic that you want to permit.</para>
<para>For more information on handling multiple networks through a
single interface, see <ulink
url="Multiple_Zones.html"><emphasis>Routing on One
Interface</emphasis></ulink>.</para>
</example>
</section>
<section>
<title>Defining a Zone-per-Address</title>
<para><ulink url="Vserver.html">Shorewall's support for Linux
Vservers</ulink> can (mis-)used to create a separate zone per alias.
Note that this results in a <emphasis>partitioning of the firewall
zone</emphasis>. In this usage, you probably want to define an ACCEPT
policy between your vserver zones and the firewall zone.</para>
</section>
</section>
</article>