From: Herbert Poetzl (herbert_at_13thfloor.at)
Date: Tue 28 Sep 2004 - 23:41:11 BST
On Tue, Sep 28, 2004 at 04:55:01PM +0200, Christian Mayrhuber wrote:
> On Tuesday 28 September 2004 16:06, Gilles wrote:
> > Thanks for the pictorial description; I start to figure it out...
> > Nevertheless, taking up from "lo as 2 doors", wouldn't it have
> > been clearer to have an alias for "lo", instead of a new
> > interface "dummy0" for the sole purpose of assigning it a
> > different IP address?
> > I.e. is it possible to have e.g. a "lo:dummy0" alias?
> > But maybe "dummy0" has yet other properties (?) that are
> > not apparent from the above explanation.
> > Gilles
> lo is no real interface, lo simply repeats all network traffic
> to everyone who is listening on lo, no firewall rules, no two
> doors, nada. Lo is a big darn door where everything which
> wents out comes back in again.
> The dummy interface is the only "real" virtual network interface,
> able to do things like routing, firewall rules, etc.
whoa! not so fast ...
> Lo cannot do these things, it's fast, but no full fledged network
okay, both interfaces (lo and dummy) _are_ special
in the way they handle packets sent _through_ them ...
here is a (not so much) simplified version of the
'transmit' function of them ...
static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
stats->rx_bytes += skb->len;
stats->tx_bytes += skb->len;
static int dummy_xmit(struct sk_buff *skb, struct net_device *dev)
> In short you can't make da door with lo, you need a real network
> interface card or the dummy network interface for that purpose.
local traffic is always sent via lo, which explains
(not only for my networking examples) why a ping to
a local ip address fails if lo is not configured ...
local or remote traffic is never sent via dummy, because
it just wouldn't return (it's basically /dev/null for
hope that helps a little,
> lg, Chris
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