MIME Types 101 (was Re: [TriLUG] how to list users connected to PopTop VPN?)
lovelace at wayfarer.org
Mon Feb 23 11:22:32 EST 2004
William Sutton said the following on 2/23/04 1:47 AM:
> Mind explaining why a script (which is basically a text attachment) gets
> filtered out as NOT being a text attachment? Seems peculiar to me....
It all has to do with MIME types (The Robot, The Wall, Walking Your Dog
... um, no wait, wrong Mime! ;-)
MIME = "Multipupose Internet Mail Extensions. A set of extensions to
SMTP designed for transmitting nontext e-mail message attachments."
Basically, when you attach something to your mail message, the MIME
type is a hint of what kind of file it is. It always consists of
two categories, the first one more general than the second.
Common types include "text/plain", "text/html", "application/pgp"
"application/exe", etc... There are a ton of different mime types.
(Google for them if you're interested.).
What I believe happened is that when chris attached a perl script
to his message, his mail program looked at it and said something like,
"Hmm... it's got a .pl extension. I bet that's a perl script! Ok,
I'll set the mime type to something like application/perl or
application/script." Whatever it did, it almost certainly didn't
set it to "text/plain" because otherwise it should have made it to
As to why it chose something other than text/plain, well, how is
the mail program supposed to know it's text plain? You could
try renaming the script something like perlscript.pl.txt and see
if that fools it into thinking it's a text file, but in the
abscense of that it has to do the best it can about guessing.
It's not an exact science.
Note, btw, that MIME types, although they came about for mail,
are now used a lot of other places too. Have you ever seen
a website where it displays the raw html text instead of the
rendered web page? That's because the web server said the
file being sent to the web browser was text/plain instead of
text/html. Or if your web browser tries to save a PHP file
instead of display it, it's probably setting the mime type
So, how does this relate to why it was filtered out on the
trilug list? Basically, because of how recent viruses act,
we've been basically forced to turn on content filtering
in mailman for the trilug list to ensure that we're not
a delivery mechanism for viruses. Since most viruses
send executables, which actually have correct mime types
(they have to or otherwise Outlook, etc, won't execute them),
we filter on mime type. But, rather than make the list
too small, since the number of legitimate mime types that
can be sent to the list is small, we actually specify a list
of mime types that are ok for the list. That list is this:
If a message comes in with a section that has any other
mime type, mailman will remove that section of the message.
If the message is blank after it does that, it will discard
it, otherwise it will hand it off to normal processing.
What happened in Chris's case was that his message had
several sections: a text/plain section that was the
message itself, an application/pgp-signature section,
and a type that was probably something like
application/script. Mailman removed the application/script
part and sent the message on. (Actually, it's a bit more
complicated even than that, but that's a good high
As a side note this will also do things like remove images
from messages too.
So, William, does that answer your question? I wasn't
sure how much you knew about mime types, so if I tailored
my message at a lower level than you were expecting, I
apologize. But, that's basically what happened.
Tanner Lovelace | Don't move! Or I'll fill ya full of... little
lovelace at wayfarer.org | yellow bolts of light! - Commander John Crichton
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