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Path: euryale.cc.adfa.oz.au!newshost.anu.edu.au!harbinger.cc.monash.edu.au!news.rmit.EDU.AU!news.unimelb.EDU.AU!munnari.OZ.AU!news.mel.connect.com.au!news.mira.net.au!inquo!in-news.erinet.com!bug.rahul.net!rahul.net!a2i!samba.rahul.net!rahul.net!a2i!dhesi.a2i!dhesi From: Rahul Dhesi <dhesi@rahul.net> Newsgroups: comp.unix.bsd.freebsd.misc Subject: Re: 'iijppp -auto' and Dynamic IP Address Allocation Date: 11 Jun 1996 09:28:27 GMT Organization: a2i network Lines: 42 Message-ID: <4pje7r$cf1@samba.rahul.net> References: <31BBC0E9.41C67EA6@dial.pipex.com> <4phq9f$4n0@uriah.heep.sax.de> NNTP-Posting-Host: bolero.rahul.net NNTP-Posting-User: dhesi In <4phq9f$4n0@uriah.heep.sax.de> j@uriah.heep.sax.de (J Wunsch) writes: [ how to do dial on demand when IP addresses are unknown ] >It's impossible. Demand-dialing requires a fixed IP address to be >assigned for the remote end. If you think about how routing works, >you will know why it must be this way. (You're running into a >chicken-and-egg problem otherwise.) Actually I believe this is an implementation issue. If the 'default' entry in your routing table must point to a specific IP address, then you have the above problem. But what if you add a static 'default' route pointing to the point-to-point device? If your kernel is appropriately coded, such a default route does not need a gateway IP address. It simply means 'packets destined for unknown IP addresses should be sent this way'. There is only one device at the other end of the point-to-point link, so we don't care what its IP address is. Once you implement this, automatic dial-out becomes easy. Just begin dialing if (a) the point-to-point link is not up and (b) we receive a packet that needs to go out tht way. Here, for example, is part of an actual routing table from an Onramp router: Dest Len Interface Gateway Metric P Timer Hits Use 204.095.070.183 32 en0 204.095.064.002 2 239 2085 1465 default 0 syn0 1 0 23490132 22454384 Notice that one route (pointing towards the Ethernet port) specifies a gateway -- because on a LAN one needs a MAC address to send the packet to, and we get this by first knowing the IP address of the gateway. However, the default route points to the syn0 device, which is connected to a point-to-point link. No gateway IP address is needed, because we have no coice: there can only be one device at the other end of the point-to-point link. A LAN is like a highway with many exits. You need to know which one to take. A point-to-point link is like a well down which you jump. You have not much choice, once you have jumped, about where to go... -- Rahul Dhesi <dhesi@rahul.net> "please ignore Dhesi" -- Mark Crispin <mrc@CAC.Washington.EDU>