Difference between dummy and loopback interfaces

[Andy Trap]

Hi,

What is the use of a dummy device? Specifically what benifits does it have
over a loopback device. dummy_xmit() infact drops any packet sent to it,
whereas loopback_xmit() puts it back on the receive queue. In addition,
both the devices don't respond to ARP requests. So isn't it more appropriate
to configure an IP address on a loopback interface instead of a dummy
interface? When a SLIP link is down intra-node client server communication
can still continue. Because dummy_xmit() drops every packet I don't see how
it is of any use when a SLIP link is down.

thx
Andy

[Ricardo García]

In message <e414a6f2.02120...@posting.google.com>, Andy Trap
wrote:

This is from the Linux Network Administrators Guide at
http://www.tldp.org, that you may want to read (this is part of an
example and you should read it completelly):

5.7.7. The Dummy Interface

The dummy interface is a little exotic, but rather useful nevertheless.
Its main benefit is with standalone hosts and machines whose only IP
network connection is a dialup link. In fact, the latter are standalone
hosts most of the time, too.

The dilemma with standalone hosts is that they only have a single
network device active, the loopback device, which is usually assigned
the address 127.0.0.1. On some occasions, however, you must send data
to the ?official? IP address of the local host. For instance, consider
the laptop vlite, which was disconnected from a network for the
duration of this example. An application on vlite may now want to send
data to another application on the same host. Looking up vlite in
/etc/hosts yields an IP address of 172.16.1.65, so the application
tries to send to this address. As the loopback interface is currently
the only active interface on the machine, the kernel has no idea that
172.16.1.65 actually refers to itself?! Consequently, the kernel
discards the datagram and returns an error to the application.

This is where the dummy device steps in. It solves the dilemma by
simply serving as the alter ego of the loopback interface. In the case
of vlite, you simply give it the address 172.16.1.65 and add a host
route pointing to it. Every datagram for 172.16.1.65 is then delivered
locally. The proper invocation is:[5]

# ifconfig dummy vlite
# route add vlite

[Andy Trap]

I have read this. But the description leaves the following questions
unanswered:

1. How this is any different than configuring the IP address of vlite
(72.16.1.65) on the loopback interface.

2. The implementation of dummy_xmit() unconditionally drops every packet
sent to it which suggests that intra-node communication can never
occur over a dummy interface. The loopback interface on the other
hand queues packets sent to it on the receive queue (allowing intra-
node communication even when the dialup line is down)

I can't think of a condition where a dummy device is useful (other than
for simulating a blackhole device which sucks every packet sent to it).

Andy

Ricardo García <cago...@deleteme.yahoo.es> wrote in message news:<3df3a...@news.arrakis.es>...