Shorewall and Aliased Interfaces

Background

The traditional net-tools contain a program called ifconfig which is used to configure network devices. ifconfig introduced the concept of aliased or virtial 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:
[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. The ip utility does provide for interaction with ifconfig in that it allows addresses to be labeled and labels may take the form of ipconfig virtual interfaces.

Example:

[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]#
Note that 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.

So how do I handle more than one address on an interface?

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.

Separate Rules

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.

Example (allow SSH from net to eth0:0 above):

ACTION
 SOURCE
 DESTINATION
 PROTOCOL
 PORT(S)
 SOURCE PORT(S)
 ORIGINAL DESTINATION
ACCEPT
 net
 fw:206.124.146.178
 tcp
 22


DNAT

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
 DESTINATION
 PROTOCOL
 PORT(S)
 SOURCE PORT(S)
 ORIGINAL DESTINATION
DNAT
 net
 loc:192.168.1.3
 tcp
 80
 -
 206.124.146.178

SNAT

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:
INTERFACE
 SUBNET
 ADDRESS
eth0:0
 eth1
 206.124.146.178

STATIC NAT

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:

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:

EXTERNAL
 INTERFACE
 INTERNAL
 ALL INTERFACES
 LOCAL
206.124.146.178
 eth0:0
 192.168.1.3
 no
 no

In either case, to create rules that pertain only to this NAT pair, you simply qualify the local zone with the internal IP address.

Example: You want to allow SSH from the net to 206.124.146.178 a.k.a. 192.168.1.3.

ACTION
 SOURCE
 DESTINATION
 PROTOCOL
 PORT(S)
 SOURCE PORT(S)
 ORIGINAL DESTINATION
ACCEPT
 net
 loc:192.168.1.3
 tcp
 22


MULTIPLE SUBNETS

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 1:  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.

If you are running Shorewall 1.4.1 or Later

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
loc
 eth1:192.168.20.0/24

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.

If you are running Shorewall 1.4.0 or earlier

In /etc/shorewall/interfaces:

ZONE
 INTERFACE
 BROADCAST
 OPTIONS
loc
 eth1
 192.168.1.255,192.168.20.255
 Note 1:

Note 1: If you are running Shorewall 1.3.10 or earlier then you must specify the multi option.

In /etc/shorewall/policy:

SOURCE
 DESTINATION
 POLICY
 LOG LEVEL
 BURST:LIMIT
loc
 loc
 ACCEPT


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).

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
 Note 1:

Note 1: If you are running Shorewall 1.3.10 or earlier then you must specify the multi option.

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.

Last Updated 5/8/2003 A - Tom Eastep

Copyright © 2001, 2002, 2003 Thomas M. Eastep.