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The PLIP Interface

When using a PLIP link to connect two machines, things are a little different from what you have to do when using an Ethernet. The former are so-called point-to-point links, because they involve only two hosts (``points''), as opposed to broadcast networks.

As an example, we consider the laptop computer of some employee at the Virtual Brewery that is connected to vlager via PLIP. The laptop itself is called vlite, and has only one parallel port. At boot time, this port will be registered as plip1. To activate the link, you have to configure the plip1 interface using the following commands:gif

           # ifconfig plip1 vlite pointopoint vlager
           # route add default gw vlager
The first command configures the interface, telling the kernel that this is a point-to-point link, with the remote side having the address of vlager. The second installs the default route, using vlager as gateway. On vlager, a similar ifconfig command is necessary to activate the link (a route invocation is not needed):
           # ifconfig plip1 vlager pointopoint vlite

The interesting point is that the plip1 interface on vlager does not have to have a separate IP-address, but may also be given the address

Now, we have configured routing from the laptop to the Brewery's network; what's still missing is a way to route from any of the Brewery's hosts to vlite. One particularly cumbersome way is to add a specific route to every host's routing table that names vlager as a gateway to vlite:

           # route add vlite gw vlager

A much better option when faced with temporary routes is to use dynamic routing. One way to do so is to use gated, a routing daemon, which you would have to install on each host in the network in order to distribute routing information dynamically. The easiest way, however, is to use proxy ARP. With proxy ARP, vlager will respond to any ARP query for vlite by sending its own Ethernet address. The effect of this is that all packets for vlite will wind up at vlager, which then forwards them to the laptop. We will come back to proxy ARP in section-gif below.

Future Net-3 releases will contain a tool called plipconfig which will allow you to set the IRQ of the printer port to use. Later, this may even be replaced by a more general ifconfig command.

Next: The SLIP and PPP Up: Interface Configuration for IP Previous: Configuring a Gateway

Andrew Anderson
Thu Mar 7 23:22:06 EST 1996