thanks to vda

[David Collier]

I thought I's explain why I'm blitzing this mailing list, or apologise,
whichever seems most appropriate :-)

I have read 

http://www.busybox.net/~vda/no_ifup.txt

and as it happens we do things close to the way it is recommended
already!

That file hints at stuff which pretty much covers the scope of the task I
have set myself, which is:

optionally configure eth0 from dhcp, otherwise optionally run on
user-fixed address or set interface quiescent. Run the dhcp forever and
correctly handle any future changes of details, or NAKs to renewals.

optionally run a PPP link over a gprs link, especially on units with no
land-line. Deal with all the wonderful stuff that happens on such a
connection like "gprs unavailable" status.

optionally run a VPN, which should persistently use whatever route it can
grab to connect to a fixed server.

optionally run this lot in fail-over mode, so the WAN connection is used
when present, but the GPRS takes over if it isn't, provided a GPRS link
an be made. All the while monitoring the WAN, and switching back if it
reappears.

send SMS messages using the modem which runs GPRS, even if that has only
one port, so involves temporarily taking the GPRS link up and down.  

----------------------------------
It brings up all sorts of interesting stuff like when you get a new dhcp
lease, you must implement ip and mask NOW, but only set DNS and gateway
if the current default route is over eth0, or that later becomes the
default.

what I'm not doing properly is semaphoring the process of changing
routing tables etc - so there will be moments when a new dhcp lease DNS
would get lost because the PPP s/w has just that moment decided it has
just got a connection.... that's a job for later, see below.

I'm also trying to set up 3 shepherd scripts which will notice, and panic
if any of the PPP, dhcp, or VPN processes dies, and re-launch them.

Plus logging every event that happens on dhcp, PPP and VPN so we can
bug-hunt when it all goes belly-up in the real world.

Much of this is scripted, though there is a c proggie in the background
handing stuff like SMS sending.

==============================================

There is a question here :-) -

Suppose I try to move on to a semaphore-locked system so that changes to
routine and ip address are mutexed and changes can't get lost... what
mechanism would people advise me to use?

Would you use flock on a key file? Or is there something better?

D