rendmail

rendmail

rend (transitive verb): To separate into parts with force or sudden violence; to split; to burst

rendmail is a command-line program that reads an email message from stdin and writes it (possibly with modifications) to stdout. It can be executed by a message delivery agent like procmail or fdm to remove binary attachments from messages before they are delivered.

rendmail aims to be fast and minimally intrusive (contrary to its name). Messages are read into memory one line at a time and are not written to disk, and the original bytes are unmodified to the full extent possible.

Background

I periodically download messages from a webmail service for local backup. However, I don't want to include bulky images or other binary attachments in backups; using the webmail service to preserve those is good enough for me. I used to manually delete these attachments using mutt, but I found doing so to be tedious. I couldn't find much in the way of software for deleting attachments from messages, so I decided to write my own.

Usage

rendmail can be compiled and installed by first installing Go and then running go install from the root of this repository.

The rendmail executable accepts various command-line flags:

Usage: rendmail [flag]...
Reads an email message from stdin and rewrites it to stdout.

  -backup-dir string
        Directory to which original, unmodified message will be saved
  -decode-subject
        Write X-Rendmail-Subject for RFC-2047-encoded Subject
  -delete-binary
        Delete common binary attachments from message
  -delete-types string
        Comma-separated globs of attachment media types to delete
  -fake-now string
        Hardcoded RFC 3339 time (only used for testing)
  -keep-types string
        Comma-separated glob overrides for -delete-types
  -strict
        Exit with status 1 for malformed message
  -verbose
        Write informative logging to stderr

Rewriting email is scary. You may want to initially pass -backup-dir /some/path to rendmail to save unmodified copies of messages to a temporary location in case something goes wrong.

The -decode-subject flag can be passed to instruct rendmail to insert a new X-Rendmail-Subject header field containing an ASCII representation of any RFC 2047-encoded Subject field that it finds. This can be helpful if you want to filter messages with non-ASCII subjects but your MDA doesn't automatically decode headers for you. For example, instead of giving your MDA a regular expression like ^Subject: =\?UTF-8\?Q\?Confirmaci=C3=B3n_de_Pago\?= (to match the Quoted-Printable subject Confirmación de Pago), you can write ^X-Rendmail-Subject: Confirmacion de Pago. Note that diacritics are removed, but any characters that can't easily be converted to ASCII are omitted from X-Rendmail-Subject.

Please file an issue if you encounter messages that rendmail has trouble processing.

procmail

For procmail, a recipe like the following can be added near the top of .procmailrc to pipe all messages through rendmail to remove binary attachments before other recipes are evaluated:

# h: Process message header
# b: Process message body
# f: Recipe is a filter (messages remain in input stream)
# w: Wait for program to complete and check its exit code
:0 hbfw
| rendmail -delete-binary

If the rendmail executable isn't already in procmail's default PATH, you'll probably need to supply an absolute path or set PATH within .procmailrc.

fdm

For fdm, the following rule can be added near the top of .fdm.conf to pipe all messages through rendmail to remove binary attachments:

match all
      action rewrite "rendmail -delete-binary"
      continue

Further reading

The following RFCs are relevant to rendmail:

• RFC 5322: Internet Message Format (supersedes RFC 2822 and RFC 822)
• RFC 2045: MIME Part One: Format of Internet Message Bodies (supersedes RFC 1521 and RFC 1341)
• RFC 2046: MIME Part Two: Media Types
• RFC 2047: MIME Part Three: Message Header Extensions for Non-ASCII Text

I would've liked to use Go's net/mail and mime/multipart packages to do all of the hard work and call it a day, but doing so would've required messages to be completely rewritten (i.e. with headers reordered and bodies reencoded). This Go issue requested making the net/textproto package's ReadMIMEHeader function preserve header field order, but it was closed.

The copy_delete_attach() function in mutt's copy.c file looks like it's responsible for deleting attachments from multipart messages. rendmail follows the approach used there:

• Replace the original Content-Type header field with one with media type message/external-body.
• Write a blank line to terminate the part's header.
• Write the remaining header fields as the part's body.

Here are some related programs that I came across:

• MIMEDefang uses the milter protocol to filter or modify email on behalf of the Sendmail MTA. It's written in C and Perl.
• Mailmunge is the successor to MIMEDefang, and also uses milter to interact with Sendmail and is written in C and Perl.
• Email Sanitizer was designed to integrate with procmail and appears to support removing attachments from messages based on MIME type (along with many other operations). I think it's a mix of procmail rules and Perl, and it looks like development stopped around 2006.
• Stripmime is a small Perl script that strips HTML and binary parts from messages that are sent to mailing lists. I think development stopped around 2005.
• Demime looks like it was another Perl script for convert messages into plain text for mailing lists. It's unsupported, as far as I can tell (its webpage no longer exists).
• alterMIME is a C program "used to alter your mime-encoded mailpacks as typically received by Inflex, Xamime and AMaViS". I have to admit that I still don't know what a mailpack is after multiple web searches. It looks like the last release was in 2008.

I found Enrico Zini's Migrating from procmail to sieve post to be interesting. It led me to Anarcat's procmail considered harmful page, which pointed me to Nathan Willis's Reports of procmail's death are not terribly exaggerated LWN article from 2010.