Dialup DESCHALL experiences

Xeno (xeno@presence.com)
Thu, 15 May 1997 23:53:47 -0700

I've just got my Linux SMP box at home running DESCHALL clients over
a dialup link in an ISP-friendly fashion, using diald and PPP.
Thought I'd send out a mail to give dialup DESCHALL people some hope,
and to mention a couple pitfalls that others might avoid.

Linux people will want to see the diald home page at
http://www.dna.lth.se/~erics/diald.html. Also the PPP HOWTO at
http://www.linuxhq.com/HOWTO/PPP-HOWTO.html. Setting these up
correctly will let your clients dial up only when necessary - in the
equilibrium state mine connect every forty minutes or so.

My provider (work) only gives me dynamic IP address allocation,
but the above setup mostly works even then. Yucky things go on
behind the scenes, with packets getting dropped because the IP
address changes. But for DNS lookups (which the DESCHALL clients
perform if you're using them correctly), the loss of a couple UDP
packets isn't traumatic.

What was traumatic for me was that my PPP would only function
every other connection. The first call, it would connect and then
hangup, and the second call it would connect and login properly.
The delay was long enough to cause DNS to timeout - stopping my
clients. I was all poised to debug this (having determined that
it was probably my side of the connection), when my X2 modem upgrade
came in. Previously I had a 33.6 Sportster Internal Fax-modem.
After the X2 upgrade, PPP connected cleanly every call. I'm not
100% sure the upgrade fixed it, but I think some nasty ju-ju state
was getting cached in my modem. The "server" side PPP is an
Ascend MAX 200.

So now it connects cleanly, and my clients can chew on numbers all
day, connecting occasionally to report results and get more work.
Of course it would still be better to increase the amount of work
I can check out so I'd be able to connect less frequently, but it's
much better than staying logged in all day (not that my ISP cares,
but my housemate...).

Incidentally, if you go this route, there's a wonderful unofficial
kernel patch that prevents the packet loss incurred by dynamic IP
address allocation and diald. Check out
http://www.linuxhq.com/patch/20-p0468.html. With the patch,
initial TCP connections to IP numbers (instead of hostnames) work
as well. This is useful if applications cache DNS lookup results
(browsers do this), and it reduces the risk of DNS lookup timeouts
due to lost packets.


ps Yes, I know it's a lot of trouble to go through considering
the odds of finding the key with one box, but that's not
the point of it all, is it?