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Debian GNU/Linux Reference Guide
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10.6.2 High level network configuration using ifupdown logical interface definitions

In the following it will be important for the reader to understand the difference between a physical interface and a logical interface. [ 59] A physical interface is what we have been calling "the interface", the thing that the kernel names eth0, eth1, ppp0, or what have you. A logical interface is a set of values that can be assigned to the variable parameters of a physical interface. If you find that confusing, replace the expression "configured as logical interface X" with the expression "configured with interface profile X" as you read.

The iface definitions in /etc/network/interfaces are actually definitions of logical interfaces, not of physical interfaces. [60] If you never want to reconfigure your interfaces then you can ignore this fact since the physical interface foo will by default be configured as logical interface foo.

However, suppose your computer is a laptop that you transport between home and work. When you connect the computer to the corporate network or to your home LAN you need to configure eth0 accordingly.

First define two logical interfaces home and work (instead of eth0 as we did earlier) which describe how the interface should be configured for the home network and the work network, respectively.

     iface home inet static
             address 192.168.0.123
             netmask 255.255.255.0
             gateway 192.168.0.1
     
     iface work inet static
             address 81.201.3.123
             netmask 255.255.0.0
             gateway 81.201.1.1

Then physical interface eth0 can be brought up for the home network with the appropriate configuration by specifying it on the command line:

     # ifup eth0=home

To reconfigure eth0 for the work network issue the commands:

     # ifdown eth0
     # ifup eth0=work

Note that with the interfaces file written as above it will no longer be possible to bring up eth0 by doing ifup eth0 alone. The reason is that ifup uses the physical interface name as the default logical interface name and now in our example no eth0 logical interface is defined.


Debian GNU/Linux Reference Guide
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