Copyright © 2001-2004 Thomas M. Eastep
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 “GNU Free Documentation License”.
2004-02-15
Table of Contents
The traditional net-tools contain a program called ifconfig which is used to configure network devices. ifconfig introduced the concept of aliased or virtual interfaces. These virtual interfaces have names of the form interface:integer (e.g., eth0:0) and ifconfig treats them more or less like real interfaces.
Example 1. ifconfig
[root@gateway root]# ifconfig eth0:0
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]# |
The ifconfig utility is being gradually phased out in favor of the ip utility which is part of the iproute 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 labeled where these labels take the form of ipconfig virtual interfaces.
Example 2. ip
[root@gateway root]# ip addr show dev eth0
2: eth0: <BROADCAST,MULTICAST,UP> 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]# |
One cannot type “ip addr show dev eth0:0” because “eth0:0” is a label for a particular address rather than a device name.
[root@gateway root]# ip addr show dev eth0:0
Device "eth0:0" does not exist.
[root@gateway root]# |
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 or anywhere else except as described in the discussion below.
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.
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 ip utility. The above alias was added using:
ip addr add 206.124.146.178/24 brd 206.124.146.255 dev eth0 label eth0:0 |
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:
#!/bin/sh
case $1 in
eth0)
/sbin/ip addr add 206.124.146.177 dev eth0 label eth0:0
;;
esac |
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).
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.
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.
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:
#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 |
If you wanted to use eth0:0 as the IP address for outbound connections from your local zone (eth1), then in /etc/shorewall/masq:
#INTERFACE SUBNET ADDRESS eth0 eth1 206.124.146.178 |
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.
/etc/shorewall/masq
#INTERFACE SUBNET ADDRESS eth0:0 eth1 206.124.146.178 |
Shorewall can also set up SNAT to round-robin over a range of IP addresses. Do 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.
/etc/shorewall/masq
#INTERFACE SUBNET ADDRESS eth0:0 eth1 206.124.146.178-102.124.146.180 |
The above would create three IP addresses:
eth0:0 = 206.124.146.178 eth0:1 = 206.124.146.179 eth0:2 = 206.124.146.180 |
If you wanted to use one-to-one NAT to link eth0:0 with local address 192.168.1.3, you would have the following in /etc/shorewall/nat:
#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL 206.124.146.178 eth0 192.168.1.3 no no |
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.
/etc/shorewall/nat
#EXTERNAL INTERFACE INTERNAL ALL INTERFACES LOCAL 206.124.146.178 eth0:0 192.168.1.3 no no |
In either case, to create rules in /etc/shorewall/rules that pertain only to this NAT pair, you simply qualify the local zone with the internal IP address.
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.
Example 5. 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.
This example applies to Shorewall 1.4.2 and later.
In /etc/shorewall/zones:
#ZONE DISPLAY DESCRIPTION loc Local Local Zone |
In /etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
log eth1 192.168.1.255,192.168.20.255 routeback |
In /etc/shorewall/rules, simply specify ACCEPT rules for the traffic that you want to permit.
Example 6. 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).
This example applies to Shorewall 1.4.2 and later.
In /etc/shorewall/zones:
#ZONE DISPLAY DESCRIPTION loc Local Local Zone 1 loc2 Local2 Local Zone 2 |
In /etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS - eth1 192.168.1.255,192.168.20.255 |
In /etc/shorewall/hosts:
#ZONE HOSTS OPTIONS loc eth1:192.168.1.0/24 loc2 eth1:192.168.20.0/24 |
In /etc/shorewall/rules, simply specify ACCEPT rules for the traffic that you want to permit.