From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 9 Dec 2002 03:41:29 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 9 Dec 2002 03:41:29 -0500 Received: from smtp01.uc3m.es ([163.117.136.121]:48133 "HELO smtp.uc3m.es") by vger.kernel.org with SMTP id ; Mon, 9 Dec 2002 03:41:28 -0500 Date: Mon, 9 Dec 2002 09:49:02 +0100 Message-Id: <200212090849.gB98n2319698@oboe.it.uc3m.es> From: "Peter T. Breuer" To: ahtraps@yahoo.com (Andy Trap) Subject: Re: Difference between dummy and loopback interfaces X-Newsgroups: comp.os.linux.networking In-Reply-To: Cc: linux-kernel@vger.kernel.org User-Agent: tin/1.4.4-20000803 ("Vet for the Insane") (UNIX) (Linux/2.2.15 (i686)) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org In article 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