> The basic idea is to allow before you block. Be
> mindful of where things are placed. You want to
> make sure the stuff you want blocked is before
> the stuff you allow. In other words, if you want to
> allow exe files, but only if they are zipped,
> place all the stuff you want blocked before you
> allow anything (but don't include exe type files
> in the files you want blocked yet). Then, allow
> zipped files. Then, place anything that might
> detect a file as an exe, after the point where
> zipped files are allowed.
In short, first match wins, top down on the list.
A banning lookup is performed for each mail part (leaf) independently.
If any is blocked, the whole message is blocked.
A query key for each lookup carries all the information
about the full path (within a structure of a message and archives)
leading to this leaf, e.g. (from your case):
multipart/mixed |
application/x-zip-compressed, .zip, BitTorrent-6.0.2.zip |
.exe, .exe-ms, BitTorrent-6.0.2.exe
So the above query is stepping through each rule in the list:
1.) qr'^\.(exe-ms|exe|dll)$', # block
2.) [ qr'^\.(zip|rar)$'=> 0 ], # allow
at 1): multipart/mixed ? NO
application/x-zip-compressed? NO
.zip ? NO
BitTorrent-6.0.2.zip ? NO
.exe ? YES => block, game over,
never reaches rule 2 for this leaf
If the order of rules is reversed:
1.) [ qr'^\.(zip|rar)$'=> 0 ], # allow
2.) qr'^\.(exe-ms|exe|dll)$', # block
at 1): multipart/mixed ? NO
application/x-zip-compressed? NO
.zip ? YES => allow, game over,
never reaches rule 2 for this leaf
Mark
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