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Ipmasq User's Manual
Chapter 3 - Dealing with Oddball Networks


An oddball network is a network which fails to meet the assumptions ipmasq makes (please see Assumptions, Section 2.3 for more information).

Should ipmasq incorrectly identify interfaces or misclassify them (with respect to being internal or external), create a file /etc/ipmasq/rules/A01interfaces.rul that defines the variables EXTERNAL to contain the name of all your external interfaces (i.e. EXTERNAL="eth0 eth0:0") and INTERNAL containing the names of all interfaces connected to networks you wish to masquerade. Make sure that the loopback interface (lo) appears in neither EXTERNAL or INTERNAL.

Should ipmasq incorrectly determine the IP address, netmask, or peer IP address of an interface, create a file /etc/ipmasq/rules/A01precompute.rul that defines, for all interfaces in EXTERNAL and INTERNAL, the IP address, netmask, peer IP address, and broadcast address. (If you override the .def to change one value, you must specify all values.) The following shows how to define an interface foo0:

     IPOFIF_foo0=10.1.2.3
     NMOFIF_foo0=255.252.0.0
     PEEROFIF_foo0=192.168.1.1

Aliased (i.e. foo0:0) and vlan (i.e. foo0.0) interfaces are mangled before being made into variable names. The colon in aliased interface names is transformed into an underscore, while the period in vlan interface names is transformed into two underscores.


3.1 Asymmetric Routing

Asymmetric routing occurs when the interface on which external packets enter the system is not the same interface on which packets leave the system. This happens most commonly with a load-balancing setup (i.e. traffic enters the system on ppp0 and ppp1, while it "leaves" the system over eql0).

To correctly indicate the interfaces over which packets enter and leave the system, create a file /etc/ipmasq/rules/A01interfaces.rul that defines the variables EXTERNAL_OUT to contain the name of all your outbound external interfaces (i.e. EXTERNAL_OUT="eql0"), EXTERNAL_IN to contain the name of all your inbound external interfaces (i.e. EXTERNAL_IN="ppp0 ppp1") and INTERNAL containing the names of all interfaces connected to networks you wish to masquerade. Make sure that the loopback interface (lo) does not appear in any of EXTERNAL_OUT, EXTERNAL_IN, or INTERNAL.


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Ipmasq User's Manual

Brian Bassett brianb@debian.org
Osamu Aoki (update) osamu@debian.org