sendmail-shim

We had a long streak of troubles in configuring /usr/sbin/sendmail inside of a Docker container to host a legacy website for a customer. Thus, we created this simple program that mimics some of its behavior.

NOTE: this implementation is explicitly not feature-complete compared to sendmail. It is just enough to send emails, and rewrite the sender and recipient email addresses.

Technical details

This program is written in Go, only using modules from the standard library, and compiles to a single, statically-linked binary for ease of use. Provide environment variables at build-time, to make the makefile link these values into the final binary.

To build the program, simply run make. The easiest strategy for environment is probably to prefix them to this command, like SMTP_SERVER=foo:587 SMTP_USER=me SMTP_PASSWORD=secret LOG_FILE=sendmail-shim.jsonl make.

Supported flags

• -t: get recipients from the message (headers provided on stdin)
• -f: sets the "From" header to postfixed string, f.ex: -fsender@example.com makes sender@example.com the email sender.
• -r: same behavior as -f

Email addresses provided independent of the -f/-r flag are interpreted as recipients.

All other flags are completely ignored. Feel free to submit an issue and a PR to add support for more.

Development

I recommend installing smtp4dev for an easy email test interface.

Prefix with all required arguments and run make --always-make run [appropriate sendmail parameters]. This will put you into a reading prompt. Type out headers and email content, then use ^D (ctrl+d) to quit.

Simple example:

SMTP_SERVER=localhost:25 SMTP_USER=nobody SMTP_PASSWORD=password LOG_FILE=sendmail-shim.jsonl make --always-make run recipient@example.com
To: recipient@example.com
From: sender@example.com
Content-Type: text/plain

Hi! This is my mail message! Sent from the sendmail shim utility!
Bye!
^D

Run go fmt to format code properly before committing.

Possible changes

• Remove evil hacks in flag parsing
• Better error handling, though this is not critical as the program is instantiated once per use
• Switch to a more pass-by-value approach. Multiple functions mutating shared objects scales very poorly