Re: tcp/ip filtering

Albert Cahalan (
Thu, 16 May 1996 15:05:01 -0400 (EDT)

From: Coolio <>
> On the subject of implementing a server based tcp/ip packet filtering
> system...
> > Okay, I see the scale of what you are trying to do now.... It would
> > certainly be a neat demonstration!
> Yeah! And definetly a bonus for linux, if it became an easily configured
> option (maybe with its own mailing list & constantly updated database of
> "no-no" words & sites...)
> > > Also, about students encrypting their data to evade the censorship..
> >
> > I don't think it would be that hard. It wouldn't take full encryption;
> > simply toggling bit 3 of every byte would do quite well.
> True, but its high school and very few would take the time to figure out
> how to xor their bytes :)
> > Your idea would stop some of the most blatant searches for encryption.
> > However, there is always going to be a way out. And how can anyone
> > stop people from downloading "nuaghty" pictures, anyway, which is
> > probably the teachers' most serious concern? Hmm, actually I'd probably
> > enjoy being on the development team for Optical Breast Recognition :)
> Yes, I have spent a while thinking of loopholes in this plan, but
> hopefully when it is finished it will block web pages (along with irc &
> email, and anything else that arrives over the network) with bunches of
> nasty pictures, not by analyzing the picture data, but by recognizing an
> unwanted picture name in the html source... It would probably be a happy(?)
> side effect that an incoming web page would have <IMG SRC="breast.gif">,
> and the filterer would notice breast.gif and stop the connection... (Im not
> saying anything with the word "breast" should be filtered, just
> "breast.gif"... maybe there would even be a whole sub category of banned
> picture names... it would certainly have to be more restrictive than
> other triggers, since once the incoming data from the picture arrives, it
> is too late.. and of course, not everything with the word "breast" should
> be filtered :)

AOL got into trouble when women discovered they could not discuss
breast cancer anymore. Prodigy had trouble with vietnamese users,
because they often write "sex" as part of an ASCII transliteration.

> Basically, I think the parser could be broken down to a very simple method:
> 1. Step through incoming data until alphabetic character is encountered
> (A-Z)... Wait for more characters, adding them onto the end of a string
> until a non-alphabetic character is encountered..

User complaint: "I can't FTP the Linux kernel!"

> 2. Do an optimized comparison of the uppercased string with all of the
> pre-uppercased "no-no" words.. If any matches, terminate connection
> immediately.. (maybe later it could be made to spit out "No-no word
> found, connection terminated" or something more informative before
> cutting the connection)

> Yes, but like I said, my teachers are very lazy... In fact I think they
> enjoy spending their time obstructing productivity more than
> encouraging it... which is why I think a good piece of code in the kernel
> would do a WAY better job at restricting access (hmm maybe even a bad
> piece of code.. hehe)

N f s Y w n a 1 A p a l
o u h o i e 5 I a t e
c i u l e M r a
k t l d B s s
i . e t
n r .

> There are some companies out there that want to do what you suggested for
> B, but they have a lot more funding (I have none) and time than I have to
> browse the web and rate each page (besides, whats the use of having
> everything automated up to this point to just revert back to requiring
> human intervention?)... it would be a lot more time effective (and neat) to
> build a little filterer... no matter how ineffective the first
> version may be, it would provide a starting point...

Think you can block this?


One night when his charge was pretty high, Micro Farad decided to
get a cute little coil to let him discharge. He picked up Millie Amp and
took her for a ride on his megacycle. They rode across the Wheatstone Bridge,
around the sine wave, and into a magnetic field next to a flowing current.

Micro Farad, attracted by Millie Amp's characteristic curve, soon
had her field fully excited and he couldn't resistor. He laid her on the
ground potential, raised her frequency, lowered her resistance, and pulled
out his high voltage probe. He inserted it in parallel and began to short
circuit her shunt.

Fully excited, Millie Amp cried, "Mho, Mho. Give me Mho!"

"Ohmigod, this is good," shouted Micro. With his tube at maximum
output and her coil vibrating from current flow, her shunt soon reached
maximum heat.

The excess heat had gotten her shunt pretty hot and Micro's
capacitance was rapidly discharging, ... draining off every electron.
They fluxed all night, trying various connections and sockets until Micro's
bar magnet had lost all of it's field strength.

Afterward, Millie tried self-inductance and damaged her solenoid.
But it didn't phasor. With his battery fully discharged, Micro Farad was
unable to excite his transformer. So they ended up by reversing polarity,
and blowing each other's fuses.