# 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--