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SMTP from Exim-equipped roaming notebook (SSH smarthost)

February 13th, 2014 Leave a comment Go to comments

I don’t send email from my notebook often, dealing with my correspondence on my server machine via ssh. When I need to do it, it’s usually when I’m sending Git patches or something like that. I didn’t meet much trouble with sending it directly, but SMTP servers of Debian-involved people are some of the most picky one can meet and I decided it’ll be best if I switch the exim4 on my notebook to smarthost mode where all mail is relayed via my main server.

So that should be trivial to do, right? Wrong, apparently. I figured I’d use SMTP auth, but it just seems mind-bogglingly complicated to configure if you don’t want to spend an evening on it. The client part is fairly easy (probably both on exim4 and postfix), but setting up postfix server to do SMTP auth (for just a single person) is really silly stuff. Maybe not so crazy if you use PAM / shadow for authentication, but that means that on my notebook, I’d have to store (in plaintext) my server password anyone could use to log in – no way. It seems I could switch to Dovecot and somehow pass it a simple password to use, but at that point my patience ran out and I just backed off a litle.

Why not just use ssh for smarthost SMTP transport? Authentication via ssh is something everyone understands nowadays, it does the best job there, no silly passwords involved and you can just pipe SMTP through it. You wouldn’t do that at in a company setting with Windows notebooks, but for a single geek, it seems ideal.

Someone already did set up ssh as exim transport, but that’s for exim3. So here follows a super-quick HOWTO to do this with exim4:

  • Set up ssh key on client:
    sudo -u Debian-exim /bin/bash
    ssh-keygen # go with the default, and empty password, this will be used in an automated way
    ssh me@server.example.org # to fill up known_hosts; it will fail yet
    cat ~/.ssh/id_rsa.pub # this is my public key
    exit # ..the sudo
  • Set up ssh key on server – paste the public key printed by the cat above to ~me/.ssh/authorized_keys and prepend command="nc -w1 localhost smtp",no-agent-forwarding,no-port-forwarding,no-X11-forwarding to the key line. This key can now be used only for mail relaying.
  • Do dpkg-reconfigure exim4-config and configure smarthost mode. Also use it to find out whether you are using split or big configuration. You will also probably want to enable “mailname hiding”, otherwise your return-path will contain an unroutable address.
  • Set up ssh transport in exim4 – add the following to the config file:
      debug_print = "T: ssh_pipe for smarthost delivery"
      driver = pipe
      path = "/bin:/usr/bin:/usr/local/bin"
      command = "ssh me@server.example.org nc -w1 localhost smtp"
      message_prefix = "HELO mynotebook.example.org\r\n"

    (it would be nicer if we used the actual smarthost configuration option value and our notebook’s hostname instead of hardcoded strings, I guess).

  • In the smarthost: section of the configuration file, replace transport = remote_smtp_smarthost with transport = ssh_pipe.
  • /etc/init.d/exim4 reload and voilá, sending mail from anywhere should work now!

I *wish* setting up roaming SMTP nodes would be way easier nowadays and I wouldn’t have to eventually spend about 90 minutes on this stuff…

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