FreeBSD: Disaster-tolerant fallover(?) Server switching software for FreeBSD / Linux?

Mark Blackman mark at
Tue Oct 16 21:47:33 BST 2001

Describes a load "balancing" name server. The basic principle
is that the DNS for that URL is delegated to a special nameserver 
which keeps track of the "status" of a given set of machines
that would normally answer an HTTP request. Depending on
the feedback algorithm one IP is selected and returned
to the client requester on the basis of its loading and availability.

Your fall-over scenario is a special case where one machine 
should be always treated as more loaded than the other and so
the "lesser" loaded machine is always used until it fails.

Anyway the tricky part is getting the feedback about
the state of the servers back to the nameserver in a 
reliable and robust way.

Note that you set the TTL to zero to guarantee that
nobody ever caches any DNS result.

Depending on how demanding your fail-over requirements
are and how likely somebody is to be around, it can be
effective to implement this as a *manual* process. i.e.
some script that flips the (TTL=0) A record over to the
standby server and reloads the zone file. We have
a 24-hour NOC and we do this. Remember TTL=0, so no
caching and you have immediate effect.

Note this doesn't cover the case of nameserver failure. :)

if you want a full-on commercial solution, then  is worth looking at. about
$1000 for a single machine I believe (localcluster/understudy).

- Mark

> This is not specifically a FreeBSD query but it would most
> likely be involving FreeBSD systems somewhere in the chain
> of things.  (and am not sure of the correct terminology here)
> For getting a server system (Web, FTP and Rsync) going where there 
> is a physically separate redundant "fallover" server - 
>   Is there software available that will detect if the primary 
>   server has gone down (or failing) and auto-switch the
>   secondary server to take over transparently.
>   I guess this would also involve software on the DNS server as well
>   to redefine IP addresses and aliases?
> -------
> Any clues or links to intelligent ways of doing this with FreeBSD/Linux.
> (network group here is primarily composed of Linux fans at the moment)
> Cheers,
> Lachlan.
> -- 
> -----------------------
> Lachlan M. D. Cranswick
> Collaborative Computational Project No 14 (CCP14)
>     for Single Crystal and Powder Diffraction
>   Birkbeck University of London and Daresbury Laboratory 
> Postal Address: CCP14 - School of Crystallography,
>                 Birkbeck College,
>                 Malet Street, Bloomsbury,
>                 WC1E 7HX, London,  UK
> Tel: (+44) 020 7631 6849   Fax: (+44) 020 7631 6803
> E-mail: l.m.d.cranswick at
> WWW:
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