Setting up SMTP auth

If you're on the road a lot, there's an obvious way to setup email on a laptop. Have a local SMTP MTA to which you can send email locally so it can queue your emails for you until you connect to some Wifi somewhere. Use an MTA on the Internet as a relay for your laptop, ideally use your company's MX.

The complication is setting up your mail relaying so that this works in a secure way. You don't want to make an open relay anywhere. Postfix can authenticate when sending mail to a relay - that's an ideal way to work.

By the way, Postfix is a great solution to mail. Personally I gave up on Exim after some nutter in the Debian project made the config so confusing that you need a masters degree in debconf before you can change anything. The Postfix package has never suffered that level of idiocy, fortunately, so it's my preference for email everywhere. It helps that it's also awesome fast and easy to setup.

Doing auth with the server

The first thing you need is to be able to test MTA auth on the relay. Here's how to use Emacs to test:

(let* ((username "testy")
       (password "testytest")
       (smtp-server "")
       ;; make a stream
       (tcp (open-network-stream 
               "smtptest" "*smtptest*" 
                smtp-server 25)))
  ;; Clear the buffer and then send auth
  (with-current-buffer (process-buffer tcp) (erase-buffer))
  (switch-to-buffer-other-window (process-buffer tcp))
  (accept-process-output tcp)
   tcp (format "AUTH PLAIN %s\r\n"
               (base64-encode-string (format "\0%s\0%s" username password))))
  (accept-process-output tcp)
  (delete-process tcp))

I'm sure there are other options, being an Elisp nut this one works for me.

Setting up the relay MTA

Packages you'll need on your relay:

There's quite a good guide here but I found lot of problems so I'll document the whole thing.

Setup saslauthd

Setting up saslauthd is a bit of a pain. This guide gives us this:

rm -r /var/run/saslauthd/
mkdir -p /var/spool/postfix/var/run/saslauthd
ln -s /var/spool/postfix/var/run/saslauthd /var/run
chgrp sasl /var/spool/postfix/var/run/saslauthd
adduser postfix sasl

Then restart postfix and saslauthd

Setup postfix for auth

Postfix needs:

smtpd_sasl_path = smtpd
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous


Also in you need to alter the smtpd_recipient_restrictions. You presumably have this setup already, so you just have to add the ability to allow SASL authorized connections, something like:

smtpd_recipient_restrictions =

You also need to setup how Postfix will talk to authsasld, on my system I made a /etc/postfix/sasl/smtpd.conf:

pwcheck_method: saslauthd
mech_list: PLAIN LOGIN

Now restart Postfix and you should be able to test the auth locally using a PAM user:

testsaslauthd -u somelocaluser -p passwordforthat

You should also be able to connect to the SMTP server and send an AUTH request. The ELisp above is one way of trying that.

the postfix SMTP client

On your mobile computer you need to setup the postfix client. I added the following lines to

relayhost = [MTA-name]
smtp_sasl_auth_enable = yes
smtp_sasl_mechanism_filter = plain, login
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

Where MTA-name is the name of my actual, Internet connected, MTA.

This sets up the SMTP engine to send auth to servers. You then have to add your login details for the MTA to /etc/postfix/sasl_passwd; make a file /etc/postfix/sasl_passwd something like this:

[MTA-name]   username:password

and then run the postmap commnd to make the hashed version:

postmap hash:/etc/postfix/sasl_passwd

you shouldn't have to worry about leaving the password on your laptop. But it's maybe safer to make the file only readable by root:

chmod g=,o= /etc/postfix/sasl_passwd*

Now you should be able to restart Postfix and it will auth to your relay MTA whenever you send anything.

An orthogonal aside

This is all an example of a pattern of computing I'm very interested in, agent computing. You ask your laptop to get something done, send an email for example, and it doesn't actually do it itself, but works with a bunch of other software elsewhere to get it done. Rather than the arguments about centralized or peer to peer Internets I think this is the way to go.