Return to BSD News archive
Xref: sserve comp.protocols.tcp-ip:27735 comp.unix.bsd:13499 comp.unix.ultrix:21596 Newsgroups: comp.protocols.tcp-ip,comp.unix.bsd,comp.unix.ultrix Path: sserve!newshost.anu.edu.au!munnari.oz.au!bruce.cs.monash.edu.au!harbinger.cc.monash.edu.au!yeshua.marcam.com!news.kei.com!eff!news.umbc.edu!europa.eng.gtefsd.com!howland.reston.ans.net!agate!library.ucla.edu!csulb.edu!csus.edu!netcom.com!netcomsv!calcite!vjs From: vjs@calcite.rhyolite.com (Vernon Schryver) Subject: Re: How do multi-homed hosts choose the interface? Message-ID: <CLrBHq.2t9@calcite.rhyolite.com> Organization: Rhyolite Software Date: Fri, 25 Feb 1994 01:44:14 GMT References: <2khb6f$9fl@breeze.dra.hmg.gb> <2kj4h8$phl@mserv1.dl.ac.uk> Lines: 44 In article <2kj4h8$phl@mserv1.dl.ac.uk> rbr@dl.ac.uk (R.Bradshaw) writes: >Anthony Heading (heading@signal.dra.hmg.gb) wrote: > ... >You need to set up explicit routes. If you are using static routes, >then try: >on Alpha: route add Iue Iuf 1 >on Ultrix: route add Iae Iaf 1 >If you are running dynamic routing, you'll need to do the equivalent >in gated/routed/whatever. Your netstat tables should then show these >as host routes. Static routes are ok if you are already using static routes. If static routes are not being use, then a good solution is to adjust the "interface metric" on the Ethernet interfaces. This will cause traffic to automatic prefer the FDDI interface when it is alive and fall back to the Ethernet otherwise. To see how to use interface metrics, look at the "ifconfig" man page, and whatever configuration file scheme the system uses to do things like setting the netmask. In a BSD style network system, you would set the interface metric in the same places as the netmask. There are good reasons for and against each of the common choices, static routes, router-discovery by snooping on routing protocols, and official router discovery. The most common choice by a large margin is snooping on RIP packets, because that's what happens by default on most UNIX boxes. The most common choice among those who make a conscious choice is static routes, sometimes for very good technical and administrative reasons and sometimes for bad reasons related to the common psychological and socialogical reasons that cause people to want to decide and control things in the first place. There is another scheme that works well for multi-homed servers in a mesh of networks, where you want to prefer the shortest route regardless of which interface on the server carries its cannonical name. For example, you might have an NFS server with 4 Ethernets and a couple of FDDI interfaces, and all 6 of those networks are connected to a few dozen other networks, and you want distant hosts to use the nearest interface on the server and not only the cannoically named interface. Arrange to have the server advertise a point-to-point RIP route to its cannonical address from all of its interfaces. This can be done with gated, by modifying routed, or with the routed standard on at least one UNIX vendor's systems. Vernon Schryver vjs@rhyolite.com