Content filtering
Mailman can filter the content of messages posted to a mailing list by stripping MIME subparts, and possibly reorganizing the MIME structure of a message.
>>> from mailman.app.lifecycle import create_list
>>> mlist = create_list('test@example.com')
Several mailing list options control content filtering. First, the feature
must be enabled, then there are two options that control which MIME types get
filtered and which get passed. Finally, there is an option to control whether
text/html
parts will get converted to plain text. Let’s set up some
defaults for these variables, then we’ll explain them in more detail below.
>>> mlist.filter_content = True
>>> mlist.filter_types = []
>>> mlist.pass_types = []
>>> mlist.filter_extensions = []
>>> mlist.pass_extensions = []
>>> mlist.convert_html_to_plaintext = False
Filtering the outer content type
A simple filtering setting will just search the content types of the messages parts, discarding all parts with a matching MIME type. If the message’s outer content type matches the filter, the entire message will be discarded. However, if we turn off content filtering altogether, then the handler short-circuits.
>>> from mailman.interfaces.mime import FilterAction
>>> mlist.filter_types = ['image/jpeg']
>>> mlist.filter_action = FilterAction.discard
>>> from mailman.testing.helpers import (specialized_message_from_string
... as message_from_string)
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: image/jpeg
... MIME-Version: 1.0
...
... xxxxx
... """)
>>> from mailman.config import config
>>> process = config.handlers['mime-delete'].process
>>> mlist.filter_content = False
>>> msgdata = {}
>>> process(mlist, msg, msgdata)
>>> print(msg.as_string())
From: aperson@example.com
Content-Type: image/jpeg
MIME-Version: 1.0
xxxxx
>>> msgdata
{}
Similarly, no content filtering is performed on digest messages, which are crafted internally by Mailman.
>>> mlist.filter_content = True
>>> msgdata = {'isdigest': True}
>>> process(mlist, msg, msgdata)
>>> print(msg.as_string())
From: aperson@example.com
Content-Type: image/jpeg
MIME-Version: 1.0
xxxxx
>>> from mailman.testing.documentation import dump_msgdata
>>> dump_msgdata(msgdata)
isdigest: True
Simple multipart filtering
If one of the subparts in a multipart
message matches the filter type,
then just that subpart will be stripped. If that leaves just a single subpart,
the multipart
will be replaced by the subpart.
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=BOUNDARY
... MIME-Version: 1.0
...
... --BOUNDARY
... Content-Type: image/jpeg
... MIME-Version: 1.0
...
... xxx
...
... --BOUNDARY
... Content-Type: image/gif
... MIME-Version: 1.0
...
... yyy
... --BOUNDARY--
... """)
>>> process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: image/gif
X-Content-Filtered-By: Mailman/MimeDel ...
yyy
Collapsing multipart/alternative messages
When content filtering encounters a multipart/alternative
part, and the
results of filtering leave only one of the subparts, then the
multipart/alternative
may be collapsed. For example, in the following
message, the outer content type is a multipart/mixed
. Inside this part is
just a single subpart that has a content type of multipart/alternative
.
This inner multipart has two subparts, a jpeg and a gif.
Content filtering will remove the jpeg part, leaving the
multipart/alternative
with only a single gif subpart. Because there’s
only one subpart left, the MIME structure of the message will be reorganized,
removing the inner multipart/alternative
so that the outer
multipart/mixed
has just a single gif subpart, and then the multipart is
recast as just the subpart.
>>> mlist.collapse_alternatives = True
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=BOUNDARY
... MIME-Version: 1.0
...
... --BOUNDARY
... Content-Type: multipart/alternative; boundary=BOUND2
... MIME-Version: 1.0
...
... --BOUND2
... Content-Type: image/jpeg
... MIME-Version: 1.0
...
... xxx
...
... --BOUND2
... Content-Type: image/gif
... MIME-Version: 1.0
...
... yyy
... --BOUND2--
...
... --BOUNDARY--
... """)
>>> process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: image/gif
X-Content-Filtered-By: Mailman/MimeDel ...
yyy
When the outer part is a multipart/alternative
and filtering leaves this
outer part with just one subpart, the entire message is converted to the left
over part’s content type. In other words, the left over inner part is
promoted to being the outer part.
>>> mlist.filter_types = ['image/jpeg', 'text/html']
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/alternative; boundary=AAA
...
... --AAA
... Content-Type: text/html
...
... <b>This is some html</b>
... --AAA
... Content-Type: text/plain
...
... This is plain text
... --AAA--
... """)
>>> process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: text/plain
X-Content-Filtered-By: Mailman/MimeDel ...
This is plain text
Clean up.
>>> mlist.filter_types = ['image/jpeg']
Conversion to plain text
Some mailing lists prohibit HTML email, and in fact, such email can be a
phishing or spam vector. However, many mail readers will send HTML email by
default because users think it looks pretty. One approach to handling this
would be to filter out text/html
parts and rely on
multipart/alternative
collapsing to leave just a plain text part. This
works because many mail readers that send HTML email actually send a plain
text part in the second subpart of such multipart/alternatives
.
While this is a good suggestion for plain text-only mailing lists, often a
mail reader will send only a text/html
part with no plain text
alternative. in this case, the site administer can enable text/html
to
text/plain
conversion by defining a conversion command. A list
administrator still needs to enable such conversion for their list though.
>>> mlist.convert_html_to_plaintext = True
By default, Mailman sends the message through lynx, but since this program is not guaranteed to exist, we’ll craft a simple, but stupid script to simulate the conversion process. The script expects a single argument, which is the name of the file containing the message payload to filter.
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: text/html
... MIME-Version: 1.0
...
... <html><head></head>
... <body></body></html>
... """)
>>> from mailman.handlers.tests.test_mimedel import dummy_script
>>> with dummy_script():
... process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
X-Content-Filtered-By: Mailman/MimeDel ...
Converted text/html to text/plain
Filename: ...
Discarding empty parts
Similarly, if after filtering a multipart section ends up empty, then the
entire multipart is discarded. For example, here’s a message where an inner
multipart/mixed
contains two jpeg subparts. Both jpegs are filtered out,
so the entire inner multipart/mixed
is discarded.
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=AAA
...
... --AAA
... Content-Type: multipart/mixed; boundary=BBB
...
... --BBB
... Content-Type: image/jpeg
...
... xxx
... --BBB
... Content-Type: image/jpeg
...
... yyy
... --BBB---
... --AAA
... Content-Type: multipart/alternative; boundary=CCC
...
... --CCC
... Content-Type: text/html
...
... <h2>This is a header</h2>
...
... --CCC
... Content-Type: text/plain
...
... A different message
... --CCC--
... --AAA
... Content-Type: image/gif
...
... zzz
... --AAA
... Content-Type: image/gif
...
... aaa
... --AAA--
... """)
>>> with dummy_script():
... process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
Content-Type: multipart/mixed; boundary=AAA
X-Content-Filtered-By: Mailman/MimeDel ...
--AAA
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Converted text/html to text/plain
Filename: ...
--AAA
Content-Type: image/gif
zzz
--AAA
Content-Type: image/gif
aaa
--AAA--
Passing MIME types
All of the above examples set only filter_types
which will cause parts
with matching MIME types to be removed. There is also a pass_types
setting which if non-empty, causes parts with non-matching MIME types to be
removed. I.e., it lists only those MIME types that will be kept. Also, note
that if we want to keep elemental parts including those that might be
sub-parts of some multipart
or message/rfc822
part, we also need to
include those types in pass_types
. Here we use the example above to
illustrate pass_types
instead. Note that we want to keep both
multipart/mixed
and multipart/alternative
to be able to keep some of
their sub-parts so we just keep all multipart
types. Likewise, we’ll
keep all text
parts even though in this case the original text/plain
part will be removed by collapse_alternatives
. Finally, we’ll keep only
the image/gif
image parts.
>>> mlist.filter_types = []
>>> mlist.pass_types = ['multipart', 'text', 'image/gif']
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=AAA
...
... --AAA
... Content-Type: multipart/mixed; boundary=BBB
...
... --BBB
... Content-Type: image/jpeg
...
... xxx
... --BBB
... Content-Type: image/jpeg
...
... yyy
... --BBB---
... --AAA
... Content-Type: multipart/alternative; boundary=CCC
...
... --CCC
... Content-Type: text/html
...
... <h2>This is a header</h2>
...
... --CCC
... Content-Type: text/plain
...
... A different message
... --CCC--
... --AAA
... Content-Type: image/gif
...
... zzz
... --AAA
... Content-Type: image/gif
...
... aaa
... --AAA--
... """)
>>> with dummy_script():
... process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
Content-Type: multipart/mixed; boundary=AAA
X-Content-Filtered-By: Mailman/MimeDel ...
--AAA
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Converted text/html to text/plain
Filename: ...
--AAA
Content-Type: image/gif
zzz
--AAA
Content-Type: image/gif
aaa
--AAA--
Interaction between passing and filtering
If both pass_types
and filter_types
are non-empty, any parts with types
in filter_types
are removed and any remaining parts with types not in
pass_types
are removed as well. Here’s an example.
>>> mlist.filter_types = ['text/rtf']
>>> mlist.pass_types = ['multipart/mixed', 'text']
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=AAA
...
... --AAA
... Content-Type: text/plain
...
... plain text
... --AAA
... Content-Type: text/html
...
...
... <h2>A header</h2>
...
... --AAA
... Content-Type: text/rtf
...
... will be removed
... --AAA
... Content-Type: text/plain
...
... another plain text
... --AAA
... Content-Type: image/gif
...
... image will be removed
... --AAA
... Content-Type: multipart/alternative; boundary="BBB"
...
... --BBB
... Content-Type: text/plain
...
... plain text
... --BBB
... Content-Type: text/html
...
... <h2>Another header</h2>
...
... --BBB--
... --AAA--
...
... """)
>>> with dummy_script():
... process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
Content-Type: multipart/mixed; boundary=AAA
X-Content-Filtered-By: Mailman/MimeDel ...
--AAA
Content-Type: text/plain
plain text
--AAA
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Converted text/html to text/plain
Filename: ...
--AAA
Content-Type: text/plain
another plain text
--AAA--
Note that the html is still converted to plain text because we didn’t change
that, the text/rtf
part is removed due to filter_types
and the image
and the entire multipart/alternative
parts are removed as they aren’t in
pass_types
Passing and filtering extensions
pass_extensions
and filter_extensions
work in exactly the same way.
These filters are applied following filter_types
and pass_types
and
filter based on the filename extension of any part that has an associated
filename with extension. Parts that don’t have an associated filename with
an extension are not affected. Here’s a simple example.
>>> mlist.filter_types = []
>>> mlist.pass_types = []
>>> mlist.filter_extensions = ['pdf']
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=AAA
...
... --AAA
... Content-Type: text/plain
...
... plain text
... --AAA
... Content-Type: application/pdf; name="file.pdf"
... Content-Disposition: attachment; filename="file.pdf"
...
... pdf to remove
... --AAA--
... """)
>>> with dummy_script():
... process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: text/plain
X-Content-Filtered-By: Mailman/MimeDel ...
plain text
Reporting Option
Finally, there is a global mailman.cfg setting to append a report of what was filtered to the message. Here’s a simple example.
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=AAA
...
... --AAA
... Content-Type: text/plain
...
... plain text
... --AAA
... Content-Type: application/pdf; name="file.pdf"
... Content-Disposition: attachment; filename="file.pdf"
...
... pdf to remove
... --AAA--
... """)
>>> with dummy_script('report'):
... process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
MIME-Version: 1.0
X-Content-Filtered-By: Mailman/MimeDel ...
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
plain text
___________________________________________
Mailman's content filtering has removed the
following MIME parts from this message.
Content-Type: application/pdf
Name: file.pdf
And here’s a more complex one. Note that if the filtered message is multipart, the report is attached as a separate MIME part.
>>> mlist.filter_types = ['text/rtf']
>>> mlist.pass_types = ['multipart/mixed', 'text']
>>> msg = message_from_string("""\
... From: aperson@example.com
... Content-Type: multipart/mixed; boundary=AAA
...
... --AAA
... Content-Type: text/plain
...
... plain text
... --AAA
... Content-Type: text/html
...
...
... <h2>A header</h2>
...
... --AAA
... Content-Type: text/rtf
...
... will be removed
... --AAA
... Content-Type: text/plain
...
... another plain text
... --AAA
... Content-Type: image/gif
...
... image will be removed
... --AAA
... Content-Type: multipart/alternative; boundary="BBB"
...
... --BBB
... Content-Type: text/plain
...
... plain text
... --BBB
... Content-Type: text/html
...
... <h2>Another header</h2>
...
... --BBB--
... --AAA--
...
... """)
>>> with dummy_script('report'):
... process(mlist, msg, {})
>>> print(msg.as_string())
From: aperson@example.com
Content-Type: multipart/mixed; boundary=AAA
X-Content-Filtered-By: Mailman/MimeDel ...
--AAA
Content-Type: text/plain
plain text
--AAA
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Converted text/html to text/plain
Filename: ...
--AAA
Content-Type: text/plain
another plain text
--AAA
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
___________________________________________
Mailman's content filtering has removed the
following MIME parts from this message.
Content-Type: text/rtf
Content-Type: image/gif
Content-Type: multipart/alternative
--AAA--