Re: Difference between dummy and loopback interfaces

Re: Difference between dummy and loopback interfaces

[Peter T. Breuer]

In article <e414a6f2.0212082318.71702ff9@posting.google.com> you wrote:
> 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.

It's very different. RH's parctice of configuring the loopback device
as something other than (in addition to) localhost (no domain!) is just
plain broken, as umpteen administrators will tell you. localhost is
in the root domain, and nowhere else, netwise.

> 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).

The dummy device is conventionally used to provide a separate interface
that can be used to bind the hostname to when there is no real nic in
the box to bind it to (binding it to loopback being a no no). One
ususally does if-down ppp0, for example, with a few lines in that
brings up the hostname on dummy0. When ppp comes up, the hostname may
be bound to the ppp0 connection, but personally I prefer t leave it
stable.

"a stable binding point" for the hostname is another use for dummy0,
when the configuration is dynamic, such as in a failover cluster.

Peter

Re: Difference between dummy and loopback interfaces

[John Bradford]

> > 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).
> 
> The dummy device is conventionally used to provide a separate interface
> that can be used to bind the hostname to when there is no real nic in
> the box to bind it to (binding it to loopback being a no no).

Slackware binds the hostname to 127.0.0.1 by default.  As pointed out
in comments in the /etc/hosts file, it is technically incorrect, but
it does work, and it's fine on a non-networked machine.

John.

Re: Difference between dummy and loopback interfaces

[Rob Landley]

On Tuesday 10 December 2002 10:31, John Bradford wrote:
> > > 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).
> >
> > The dummy device is conventionally used to provide a separate interface
> > that can be used to bind the hostname to when there is no real nic in
> > the box to bind it to (binding it to loopback being a no no).
>
> Slackware binds the hostname to 127.0.0.1 by default.  As pointed out
> in comments in the /etc/hosts file, it is technically incorrect, but
> it does work, and it's fine on a non-networked machine.
>
> John.

Um, random aside:

You can attach firewall rules to a dummy0 interface that you can't attach to 
and alias of lo.  I don't remember exactly what failed (it was a while ago), 
but when I tried to "ifconfig lo:1 10.0.0.1 netmask 255.255.255.0" and then 
attach a boatload of firewall rules to it, it got confused.  (It had 
something against -j DNAT in the OUTPUT table, if I recall.  I was also 
having trouble getting packets originating from the loopback interface to 
route outside of the box.  But again, this was a while ago, so I don't 
remember exactly what was wrong.  It was a roll-your-own VPN solution that 
was designed for a machine with 2 network cards acting as a gateway, but 
needed to run on a box that had just one network card yet wanted to 
participate in the VPN...)

Moving over to the dummy interface instead of loopback made it all work.  
Loopback really isn't designed to do anything but bounce packets off of 
127.0.0.1 for local delivery.  It's optimized for that.  The dummy interface 
is more generic.

Rob

(On the other hand, "ifconfig dummy0 down" doesn't actually remove its ip from 
the routing table under 2.4, last I checked.  Annoying, that.  You've got to 
ifconfig it to something else to make it stop receiving packets, even though 
it's down!  I hit that a LOT in testing, sshing to my own box when I didn't 
mean to, and then wondering what the heck was going wrong...)

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