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In message <WONcR2eTFwSNFAMT at perry.co.uk>, Roland Perry writes:
> In article <5A055785-D55E-47A3-87B0-58B0DE81F60E at delong.com>, Owen 
> DeLong <owen at delong.com> writes
> >>> NAT provides a solution to, lets call it, enterprise multihoming.
> >>> Remote office with a local Internet connection, but failover through
> >>> the corporate network.
> >>
> >> And for home (/homeworker) networks ... eg I have a NAT box with a
> >>default connection to my ADSL provider and an automatic failover to
> >>3G (completely separate supplier).
> >>
> >> Almost everything inside my network doesn't notice when it switches over.
> >>
> >> Now, if only I could get it to automatically revert to ADSL when
> >>it reappears - I wouldn't have to worry so much about the 3G bill.
> >
> >In this case in IPv6, the better choice is to have addresses on each
> >host from both providers. When a provider goes away, the router should
> >invalidate the prefix in the RAs. If the hosts have proper address
> >selection policies, they will actually go back to the ADSL prefix as
> >soon as it reappears.
> Which in turn implies that I'd have to start getting involved in DNS for 
> the hosts inside my network. At the moment I can ignore that and just 
> enter their rfc1918 address into various applications.

No, you can enter their ULA if you don't want to use the DNS.

For external client you enter both their external addresses in the
DNS.  Clients don't need to be stupid about connecting to a multi-homed
server.  It's just that the client developers have ignored RFC
1123's suggestions for 20+ years and there hasn't been a lot of
multi-homed servers.  See the following for C code that works well
even when the network layer fails to report connection errors to
the application.


If a application you depend apon doesn't do something like this
complain to its developers.

UDP is harder but not impossible.  DNS is a classic example.  DNS
servers deal with UDP over dead network paths and has done for the
last 20 years.

> [This is all under Windows, of course, the sort of user I'm playing at 
> being doesn't use anything more sophisticated.]
> In any event, two of my applications are not IPv6 compatible, and would 
> require significant upgrading. And will my ADSL provider and my 3G 
> provider both switch to IPv6 at about the same time?

You shouldn't have to care.  Properly written clients will connect
over whatever is available without significant delay and since you
are multi-homed you really do want your clients to be properly
written.  If they are not complain to your vendor as they are not
meeting the RFC 1123 requirements.

> Unfortunately this all sounds like a lot of work, but am I a rare kind 
> of user?
> -- 
> Roland Perry
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org