Dialup backup to fast connection?

I posted a similar message to this on a different NG, sorry for the dupe.

Does anyone know of a Linux script or other mechanism to allow a PPP
dialup connection act as a backup for an ADSL connection? Hopefully
something that would autodial if the "main" connection failed, would
hangup after some idle time, (this part is easy) and only dial out again
when the main connection was not working. This is for a Linux router
machine.

This *must* have been answered before, but I did not find it. Please
excuse me if this has been answered too many times already.

Brad

In article <esyHd.13169$(E-Mail Removed)> , Brad wrote:

>I posted a similar message to this on a different NG, sorry for the dupe.

Yeah, comp.protocols.ppp - saw it yesterday.

>Does anyone know of a Linux script or other mechanism to allow a PPP
>dialup connection act as a backup for an ADSL connection?

A lot depends on what the two connections are used for. If what is on
this side of the router has public addresses, you get into a routing
situation from the world side of things. Lets say your public server is
on 36.200.62.19 (a non-existent address by the way), the world knows how
to reach that address by making that left turn in Albuquerque. When your
DSL link goes down, _you_ can change to a serial link and send packets
out, but everyone is still trying to reach you via the DSL, and nothing
you do is going to change that.

On the other hand, if you are not offering services to the world, and
the link goes down, you merely establish new connections via the serial
link, and continue surfing. The guy at the web server saw your DSL
connection disappear, and now there is (apparently) some other connection
that is trying to download the same thing. Coincidence? Who cares.

>This *must* have been answered before, but I did not find it.

The normal way this is handled is a dynamic routing daemon, like 'routed'
or 'gated', but these require the cooperation of both ISPs (to know that
to get "here' you now have to go via that pipe, not this one, and vice-
versa).

If you are only dealing with the personal surfing end, you need to
monitor the DSL connection - perhaps with ICMP echos directed out the
DSL connection to perhaps the name server of the ISP, and if that fails,
you bring up the ppp link. You would continue to attempt to ping the
name server via the DSL interface (not via the 'default route'), and
when it again responds, change the default back. But this is a very
individualized setup.

>Please excuse me if this has been answered too many times already.

The question gets asked a lot, but I'll agree there are not that
many answers posted, mainly because of the specialized nature.

Old guy