Date: Mon, 20 May 1996 19:01:21 -0700 (PDT) From: Jim Dennis <jimd@mistery.mcafee.com> To: gpalmer@freebsd.org (Gary Palmer) Cc: msmith@atrad.adelaide.edu.au, slagos@net1plus.com, questions@freebsd.org Subject: Re: <no subject> Message-ID: <199605210201.TAA06598@mistery.mcafee.com> In-Reply-To: <20527.832557068@palmer.demon.co.uk> from "Gary Palmer" at May 20, 96 02:51:08 am
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> > Michael Smith wrote in message ID > <199605200139.LAA20337@genesis.atrad.adelaide.edu.au>: > > Scott A. Lagos stands accused of saying: > > > How many IP addresses can be supported on a single Ethernet card when > > > running under the latest version of FreeBSD. > > Thousands. I got bored at about the 5,000 mark, but nothing popped up to > > stop me. (20 class C aliases). > *ONLY* 5000? Anyone got a registered class B or A address space they > want to volunteer for testing? :-) (just kidding) > > Gary Speaking of IP aliasing (again) I was thinking.... I currently have two FreeBSD boxes mirrored as ftp sites. I'm using DNS round-robin to balance the load. The question has come up: "What happens if one of them goes down?" I was thinking I could use IP aliases to add some fault tolerance to the soup. Basically if a machine fails now then every other request to start an ftp session will fail. This is better than having them *all* fail -- but not much better from the tech support rep's point of view. What I was thinking would work something like this: BIND the first IP address (as I'm doing already) to the primary interface on each machine. (Call these the ftp/rr addresses) BIND a second IP address (alias) to the ether interface on each machine. (Call these the "control" addresses) Write a script that polls the ftp/rr address on each of the other machines in the ring periodically. If the poll fails (past a set threshold) then IP alias the failed machine's port to the local ether's interface. Write another script and add it to the rc.local of each of these machines. This script does an rsh (or a custom finger or whatever) to the "control" address on the machine that's aliased "my" primary address (I have to bring up my "control" IP to talk at all but I can send my "I'm back up message" to the "my ftp/rr address"). When the machine that took over for a failed member of the round robin team get this remote message (rsh or custom finger) it drops the alias (for the failed system). Meanwhile the system that had failed, and is now back on line polls "its" ftp/rr address until it fails -- then it can finally BIND it back. There might be some race conditions if I set this up for more than two machines. I could add some additional checks for that or I could just limit myself to using machine pairs. I don't expect to have to scale past two ftp hosts (currently 400 ftp connections each and probably 800 - 1000 with the next RAM upgrade in each) and two www hosts (currently only one -- a Sparc 20 that's massively *underloaded*) for at least six to nine months. The idea is that machines in a round robin ring could "cover" for one another using simple scripts and IP aliasing. There's no kernels hacks, no dynamic DNS hacks, and no applications layer hacks necessary. I would naturally lose any connections during the initial failure -- and I'd probably kill any connections that had been established before the "dead" system got resurrected. I might have to add logic to have the "dead" system check it's own messages log and leave itself out of the ring if it rebooted more than three times in an hour or something (I could also use some sort of control file to keep a count). This all seems too easy -- yet I haven't heard of anyone using it. Of course I also haven't heard of any sites using round robin DNS either -- and that was easy, too. So... what am I missing? Jim Dennis, System Administrator, McAfee Associates
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