Tele2 is an European connectivity provider at a very low price. Let’s say that the service is proportional to the price.

I know it’s difficult to keep something up 24/7, but it’s their due anyway. As I happily learned, Tele2 stops outgoing port 25 to prevent some spam problems (not enough resources to put some server which picks random mails in transit and check if they’re spam or not?), and this already made me angry. but, anyway.

At irregular intervals, a new IP address is given to users. Tipically, giving some disservice in doing so (I don’t know if the disservice provokes a re-connection trial and thus a new address, or vice versa, but none of the options are very nice). Last time, this cause some problem, and no internet was reacheable from this host after the first hop (the one after the peer). I tried a couple of reconnections, at the third trial it all started working again. The point is: I’m usually not where the server is located, and since I can’t hope to make my mother learn the misteries of terminal and the like, I’m better dealing with the situation.

So, I had to write some scripts which do the stuff on my behalf. Since I can’t give 100% guilt to Tele2 (it could also be that I have very strangely bugged modem drivers, even if it sounds strange to me that the two events - “flawless” address change, for once, and connection drops, were simultaneous), I suppose that both a reconnection trial (poff/pon couple) is to be tried, and reboot if nothing else works. Reboot is not that bad: the host is unreacheable anyway in this case. So, in attachment, you find a cron script and the defaults file which I wrote to check everything’s alright. It checks connection is up, gateway is reacheable and dns servers are too (in turns, with tele2 connection I’ve had each of this problem occouring separately from the others - connection down; connection up and gateway unreacheable; connection up, gateway reacheable but dns unreacheable…). Hope this way downtimes are reduced to the bare minimum!

Posted by mattia as boot, cron, dns, smtp, tele2 at 5:41 PM CEST

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Talking about DNS and reverse DNS the other day at lesson, one of the guys asked me about what reverse DNS is good for. In fact, the only useful example I could come up with, was the reverse resolution of the SMTP HELO command used to avoid easy spammers. Is there anything else it is really useful for? I bet it is, but I can’t think for what!

Another thing. Discussions about incredibly-expressive grammars and fast parsers is always on. E.g., we have context-free grammars, but LL(1), LR(1), LALR(1) parsers (and so on) which manage to rapidly parse them. Ok, but: is it my impression or the cost of parsing is a minor factor in the overall equation of compilation, and most of the time is passed into optimization, parsing tree traversals, bytecode interpretation, and so on? In that case, why we don’t use more powerful parsers (like Earley-likes, whose modifications run almost always in linear time too and can parse any context-free grammar - even ambiguous ones - without any modification)? I suppose it’s worthier to write grammars easily and correctly rather than trying to adapt that C grammar to LALR(1) or stuff to gain that small percentage of time…

Posted by mattia as dns, grammars, smtp, spam at 10:45 PM CEST

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