74 lines
2.8 KiB
Plaintext
74 lines
2.8 KiB
Plaintext
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==== SSH session that runs sendmail instead of an interactive shell ====
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=== What? ===
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A wrapper for your local mail transfer agent (MTA) that initiates an SSH
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session when your pubnix email address appears in the "From:" header.
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=== Why? ===
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You want to send email from your pubnix address without running the risk
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that the receiving server will reject it due to a mismatch between the
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"From" header and your ISP's IP block, but ...
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* the low volume of your outgoing mail cannot justify the cost of an
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upgraded membership, or
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* your Internet connection has too high a latency to be running a
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full-screen editor within an SSH session.
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=== Where? ===
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a *BSD, Linux, or WSL environment with a client that generates
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MIME-formatted mail (as expected by sendmail-compatible MTAs such as
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msmtp)
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=== How? ===
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- Set up [[ssh public key authentication|SSH public key authentication]],
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if you have not already done so. This step can be omitted if you
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prefer to type your password for each outgoing mail.
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- Save the wrapper script below somewhere in your $PATH. Change the
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value of $MTA to the mail transport agent that you would otherwise be
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using for outgoing mail, the value of $PATH if any of the commands live in
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non-standard locations, and of course all the hard-coded email addresses
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in the script.
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- Configure your mail user agent (MUA) to call the wrapper script,
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rather than calling your MTA directly.
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<code>
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#!/usr/bin/env bash
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#
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# msmtp.wrap - bypass local in favor of remote sendmail,
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# if certain headers are found
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PATH=/bin:/usr/bin:/usr/local/bin
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Account0=me@pubnix-0.org
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Account1=me@other-pubnix.net
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MTA=msmtp
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draft="$(mktemp -t sendmail.XXXXXX)"
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cat /dev/stdin > "$draft"
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sender="$(grep '^From:' "$draft" | head -n 1 | cut -d: -f2)"
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case "$sender" in
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*$Account0*)
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sendmail="ssh $Account0 sendmail -t" ;;
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*$Account1*)
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sendmail="ssh $Account1 sendmail -t" ;;
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*)
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sendmail="$MTA $*" ;;
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esac
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< "$draft" $sendmail; status=$?
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rm -f "$draft"; exit $status
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</code>
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=== What Next? ===
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The possibilities for customization of this wrapper are only limited by
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your imagination. You can parse the outgoing messages to look for
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particular recipients, subject lines, etc., and define a custom $sendmail
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command to handle each case. On the pubnix side, the arguments following
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''ssh $Account0'' will be passed to your usual login shell after the
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''-c'' option(( see the sshd(8) man-page, [[ https://man.openbsd.org/sshd ]] )),
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so you will enjoy all the environment variables defined in
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''$HOME/.bash_login'' if you use bash as your login shell. Save your
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custom scripts in a directory that your pubnix login shell considers part of
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$PATH, and you can then perform all kinds of tasks (like updating your
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website) without leaving your mail client.
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