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<title>Shorewall and Aliased Interfaces</title>
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<meta name="author" content="Tom Eastep">
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<h1 align="center"><font color="#ffffff">Shorewall and Aliased Interfaces</font></h1>
</td>
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</table>
<br>
<h2>Background</h2>
The traditional net-tools contain a program called <i>ifconfig</i>
which is used to configure network devices. ifconfig introduced the concept
of <i>aliased </i>or <i>virtial </i>interfaces. These virtual interfaces
have names of the form <i>interface</i>:<i>integer </i>(e.g., eth0:0) and
ifconfig treats them more or less like real interfaces.<br>
<br>
Example:<br>
<pre>[root@gateway root]# ifconfig eth0:0<br>eth0:0 Link encap:Ethernet HWaddr 02:00:08:3:FA:55<br> inet addr:206.124.146.178 Bcast:206.124.146.255 Mask:255.255.255.0<br> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1<br> Interrupt:11 Base address:0x2000<br>[root@gateway root]# <br></pre>
The ifconfig utility is being gradually phased out in favor of the <i>ip</i>
utility which is part of the <i>iproute </i>package. The ip utility does
not use the concept of aliases or virtual interfaces but rather treats
additional addresses on an interface as objects. The ip utility does provide
for interaction with ifconfig in that it allows addresses to be <i>labeled
</i>and labels may take the form of ipconfig virtual interfaces.<br>
<br>
Example:<br>
<br>
<pre>[root@gateway root]# ip addr show dev eth0<br>2: eth0: &lt;BROADCAST,MULTICAST,UP&gt; mtu 1500 qdisc htb qlen 100<br> link/ether 02:00:08:e3:fa:55 brd ff:ff:ff:ff:ff:ff<br> inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0<br> inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0:0<br>[root@gateway root]# <br></pre>
Note that one <u>cannot</u> type "ip addr show dev eth0:0" because
"eth0:0" is a label for a particular address rather than a device name.<br>
<pre>[root@gateway root]# ip addr show dev eth0:0<br>Device "eth0:0" does not exist.<br>[root@gateway root]#<br></pre>
The iptables program doesn't support virtual interfaces in either it's
"-i" or "-o" command options; as a consequence, Shorewall does not allow
them to be used in the /etc/shorewall/interfaces file.<br>
<br>
<h2>So how do I handle more than one address on an interface?</h2>
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.<br>
<h3>Separate Rules</h3>
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.<br>
<br>
Example (allow SSH from net to eth0:0 above):<br>
<br>
<blockquote>
<table cellpadding="2" border="1" cellspacing="0">
<tbody>
<tr>
<td valign="top"><b>ACTION<br>
</b></td>
<td valign="top"><b>SOURCE<br>
</b></td>
<td valign="top"><b>DESTINATION<br>
</b></td>
<td valign="top"><b>PROTOCOL<br>
</b></td>
<td valign="top"><b>PORT(S)<br>
</b></td>
<td valign="top"><b>SOURCE PORT(S)<br>
</b></td>
<td valign="top"><b>ORIGINAL DESTINATION<br>
</b></td>
</tr>
<tr>
<td valign="top">ACCEPT<br>
</td>
<td valign="top">net<br>
</td>
<td valign="top">fw:206.124.146.178<br>
</td>
<td valign="top">tcp<br>
</td>
<td valign="top">22<br>
</td>
<td valign="top"><br>
</td>
<td valign="top"><br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
<h3>DNAT</h3>
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 accomplised by a single rule in the /etc/shorewall/rules
file:<br>
<br>
<blockquote>
<table cellpadding="2" border="1" cellspacing="0">
<tbody>
<tr>
<td valign="top"><b>ACTION<br>
</b></td>
<td valign="top"><b>SOURCE<br>
</b></td>
<td valign="top"><b>DESTINATION<br>
</b></td>
<td valign="top"><b>PROTOCOL<br>
</b></td>
<td valign="top"><b>PORT(S)<br>
</b></td>
<td valign="top"><b>SOURCE PORT(S)<br>
</b></td>
<td valign="top"><b>ORIGINAL DESTINATION<br>
</b></td>
</tr>
<tr>
<td valign="top">DNAT<br>
</td>
<td valign="top">net<br>
</td>
<td valign="top">loc:192.168.1.3<br>
</td>
<td valign="top">tcp<br>
</td>
<td valign="top">80<br>
</td>
<td valign="top">-<br>
</td>
<td valign="top">206.124.146.178<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
<h3>SNAT</h3>
If you wanted to use eth0:0 as the IP address for outbound connections
from your local zone (eth1), then in /etc/shorewall/masq:<br>
<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>INTERFACE<br>
</b></td>
<td valign="top"><b>SUBNET<br>
</b></td>
<td valign="top"><b>ADDRESS<br>
</b></td>
</tr>
<tr>
<td valign="top">eth0<br>
</td>
<td valign="top">eth1<br>
</td>
<td valign="top">206.124.146.178<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
Shorewall can create the alias (additional address) for you if you
set ADD_SNAT_ALIASES=Yes in /etc/shorewall/shorewall.conf. Beginning with
Shorewall 1.3.14, Shorewall can actually create the "label" (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:<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>INTERFACE<br>
</b></td>
<td valign="top"><b>SUBNET<br>
</b></td>
<td valign="top"><b>ADDRESS<br>
</b></td>
</tr>
<tr>
<td valign="top">eth0:0<br>
</td>
<td valign="top">eth1<br>
</td>
<td valign="top">206.124.146.178<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
<h3>STATIC NAT</h3>
If you wanted to use static NAT to link eth0:0 with local address 192.168.1.3,
you would have the following in /etc/shorewall/nat:<br>
<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>EXTERNAL<br>
</b></td>
<td valign="top"><b>INTERFACE<br>
</b></td>
<td valign="top"><b>INTERNAL<br>
</b></td>
<td valign="top"><b>ALL INTERFACES<br>
</b></td>
<td valign="top"><b>LOCAL<br>
</b></td>
</tr>
<tr>
<td valign="top">206.124.146.178<br>
</td>
<td valign="top">eth0<br>
</td>
<td valign="top">192.168.1.3<br>
</td>
<td valign="top">no<br>
</td>
<td valign="top">no<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
Shorewall can create the alias (additional address) for you if you
set ADD_IP_ALIASES=Yes in /etc/shorewall/shorewall.conf. Beginning with
Shorewall 1.3.14, Shorewall can actually create the "label" (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:<br>
<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>EXTERNAL<br>
</b></td>
<td valign="top"><b>INTERFACE<br>
</b></td>
<td valign="top"><b>INTERNAL<br>
</b></td>
<td valign="top"><b>ALL INTERFACES<br>
</b></td>
<td valign="top"><b>LOCAL<br>
</b></td>
</tr>
<tr>
<td valign="top">206.124.146.178<br>
</td>
<td valign="top">eth0:0<br>
</td>
<td valign="top">192.168.1.3<br>
</td>
<td valign="top">no<br>
</td>
<td valign="top">no<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
In either case, to create rules that pertain only to this NAT pair,
you simply qualify the local zone with the internal IP address.<br>
<br>
Example: You want to allow SSH from the net to 206.124.146.178 a.k.a.
192.168.1.3.<br>
<br>
<blockquote>
<table cellpadding="2" border="1" cellspacing="0">
<tbody>
<tr>
<td valign="top"><b>ACTION<br>
</b></td>
<td valign="top"><b>SOURCE<br>
</b></td>
<td valign="top"><b>DESTINATION<br>
</b></td>
<td valign="top"><b>PROTOCOL<br>
</b></td>
<td valign="top"><b>PORT(S)<br>
</b></td>
<td valign="top"><b>SOURCE PORT(S)<br>
</b></td>
<td valign="top"><b>ORIGINAL DESTINATION<br>
</b></td>
</tr>
<tr>
<td valign="top">ACCEPT<br>
</td>
<td valign="top">net<br>
</td>
<td valign="top">loc:192.168.1.3<br>
</td>
<td valign="top">tcp<br>
</td>
<td valign="top">22<br>
</td>
<td valign="top"><br>
</td>
<td valign="top"><br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
<h3>MULTIPLE SUBNETS</h3>
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.<br>
<br>
Example 1: &nbsp;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 simply route all requests between
the two subnetworks.<br>
<h4>If you are running Shorewall 1.4.1 or Later</h4>
In /etc/shorewall/interfaces:<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>ZONE<br>
</b></td>
<td valign="top"><b>INTERFACE<br>
</b></td>
<td valign="top"><b>BROADCAST<br>
</b></td>
<td valign="top"><b>OPTIONS<br>
</b></td>
</tr>
<tr>
<td valign="top">-<br>
</td>
<td valign="top">eth1<br>
</td>
<td valign="top">192.168.1.255,192.168.20.255<br>
</td>
<td valign="top"><br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
In /etc/shorewall/hosts:<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>ZONE<br>
</b></td>
<td valign="top"><b>HOSTS<br>
</b></td>
<td valign="top"><b>OPTIONS<br>
</b></td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">eth1:192.168.1.0/24<br>
</td>
<td valign="top"><br>
</td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">eth1:192.168.20.0/24<br>
</td>
<td valign="top"><br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
Note that you do NOT need any entry in /etc/shorewall/policy as Shorewall
1.4.1 and later releases default to allowing intra-zone traffic.<br>
<h4>If you are running Shorewall 1.4.0 or earlier<br>
</h4>
In /etc/shorewall/interfaces:<br>
<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>ZONE<br>
</b></td>
<td valign="top"><b>INTERFACE<br>
</b></td>
<td valign="top"><b>BROADCAST<br>
</b></td>
<td valign="top"><b>OPTIONS<br>
</b></td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">eth1<br>
</td>
<td valign="top">192.168.1.255,192.168.20.255<br>
</td>
<td valign="top">Note 1:<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
Note 1: If you are running Shorewall 1.3.10 or earlier then you must
specify the <b>multi</b> option.<br>
<br>
In /etc/shorewall/policy:<br>
<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>SOURCE<br>
</b></td>
<td valign="top"><b>DESTINATION<br>
</b></td>
<td valign="top"><b>POLICY<br>
</b></td>
<td valign="top"><b>LOG LEVEL<br>
</b></td>
<td valign="top"><b>BURST:LIMIT<br>
</b></td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">loc<br>
</td>
<td valign="top">ACCEPT<br>
</td>
<td valign="top"><br>
</td>
<td valign="top"><br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
Example 2: 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).<br>
<br>
In /etc/shorewall/zones:<br>
<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>ZONE<br>
</b></td>
<td valign="top"><b>DISPLAY<br>
</b></td>
<td valign="top"><b>DESCRIPTION<br>
</b></td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">Local<br>
</td>
<td valign="top">Local Zone 1<br>
</td>
</tr>
<tr>
<td valign="top">loc2<br>
</td>
<td valign="top">Local2<br>
</td>
<td valign="top">Local Zone 2<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
In /etc/shorewall/interfaces:<br>
<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>ZONE<br>
</b></td>
<td valign="top"><b>INTERFACE<br>
</b></td>
<td valign="top"><b>BROADCAST<br>
</b></td>
<td valign="top"><b>OPTIONS<br>
</b></td>
</tr>
<tr>
<td valign="top">-<br>
</td>
<td valign="top">eth1<br>
</td>
<td valign="top">192.168.1.255,192.168.20.255<br>
</td>
<td valign="top">Note 1:<br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
Note 1: If you are running Shorewall 1.3.10 or earlier then you must
specify the <b>multi</b> option.<br>
<br>
In /etc/shorewall/hosts:<br>
<blockquote>
<table cellpadding="2" cellspacing="0" border="1">
<tbody>
<tr>
<td valign="top"><b>ZONE<br>
</b></td>
<td valign="top"><b>HOSTS<br>
</b></td>
<td valign="top"><b>OPTIONS<br>
</b></td>
</tr>
<tr>
<td valign="top">loc<br>
</td>
<td valign="top">eth1:192.168.1.0/24<br>
</td>
<td valign="top"><br>
</td>
</tr>
<tr>
<td valign="top">loc2<br>
</td>
<td valign="top">eth1:192.168.20.0/24<br>
</td>
<td valign="top"><br>
</td>
</tr>
</tbody>
</table>
<br>
</blockquote>
In /etc/shorewall/rules, simply specify ACCEPT rules for the traffic
that you want to permit.<br>
<br>
<p align="left"><font size="2">Last Updated 5/8/2003 A - <a
href="support.htm">Tom Eastep</a></font></p>
<p><a href="copyright.htm"><font size="2">Copyright</font> &copy;
<font size="2">2001, 2002, 2003 Thomas M. Eastep.</font></a><br>
<br>
</p>
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