Monday, November 06, 2017

Selecting relay smarthosts and using SMTP AUTH on illumos

A problem I looked at recently involved configuring a system to send (relay) email via a customer's own SMTP servers. There are 2 parts to this:

  • Select the relay host depending on some condition
  • Authenticate with the remote relay using SMTP AUTH

Search for SMTP AUTH with sendmail on illumos or Solaris, and you invariably end up with advice on how to build Cyrus SASL and sendmail from scratch.

For example, Andrew has some good instructions.

However, if you look at the sendmail we ship on illumos you'll find that it's already been built with SASLv2 support:

# /usr/lib/sendmail -bt -d0.1 < /dev/null
Version 8.14.4+Sun

And, if you telnet to port 25 and look at the EHLO response it includes:


However, that's not actually the part we want here (but I'll come back to that later). I don't want to authenticate against my own server, I need my system to authenticate against a remote server.

Back to the problem at hand.

The first part - selecting the right smarthost - can be achieved using smarttable. All you need is the smarttable.m4 file, and then build a configuration using it by enabling the smarttable feature.

The second part, SMTP AUTH, should also be very simple. Again, it's all documented, and just involves enabling the authinfo feature. But wait - on illumos, there is no authinfo.m4 file, so that won't work.

In fact, it does. So what you need to do is to download the sendmail source, unpack it, and there in the cf/feature directory you'll find the authinfo.m4 file.

OK, so copy both files - smarttable.m4 and authinfo.m4 - into the /etc/mail/cf/feature directory on a server. Copy and edit the file (i'm going to copy it to /tmp and edit it there) to add the 2 feature lines, like this fragment of the file here:

define(`confFALLBACK_SMARTHOST', `mailhost$?m.$m$.')dnl

Basically, just add the features above the MAILER line. Then compile that:

cd /etc/mail/cf/cf
m4 ../m4/cf.m4 /tmp/ > /tmp/

That's your new ready. It uses 2 databases in /etc/mail, to create these (initially empty):

cd /etc/mail
touch smarttable
touch authinfo
makemap hash smarttable < smarttable
makemap hash authinfo < authinfo

then copy your new into /etc/mail and restart sendmail

cp /tmp/ /etc/mail
svcadm restart sendmail

So far so good, but what should those files look like?

First the smarttable file, which is just a map of sender to relay host. For example, it might just have:

Which means that if I want my home system to send out mail with my address on it, it should route it through gmail's servers rather than trying to deliver it direct (and likely getting marked as spam).

Then the authinfo file, which looks like "U:root" "" "P:mypassword" "M:LOGIN PLAIN" "U:root" "" "P:mypassword" "M:LOGIN PLAIN"
(There are just 2 lines there, starting with Authinfo:, even if the blog shows it wrapped.)

Basically, for gmail, you need to supply your email address as the identifier and your password as, well, the password. (Note: if you've got two-factor authentication set up, you'll need to set up an app key.)

Of course, the authinfo files ought to to readable only by root, otherwise anyone one your system can read your password in the clear.

There are a couple of non-standard tweaks you'll need for gmail to work. First, you need to go to your gmail account settings and allow less secure apps. Second, you will need the "M:LOGIN PLAIN" entry in the authinfo file, else you'll get an "available mechanisms do not fulfill requirements" error back.

Redo the two makemap commands above and you're good to go.

That's SMTP AUTH the one way. At which point you're probably thinking, can we authenticate against an illumos sendmail using SMTP AUTH?

The answer, sadly, is no. At least as far as I can tell. While our sendmail is built correctly against SASLv2, illumos doesn't seem to ship enough supporting bits of the SASL infrastructure to make this work. You should be able to create the file /etc/sasl/Sendmail.conf to configure it. Unfortunately the only pwcheck_method available is auxprop (using shadow, which would allow you to authenticate against local system accounts, isn't available; neither is saslauthd, and there's no saslauthd anyway). Worse, illumos has no auxprop plugins, so the whole thing is rather useless. Note that rebuilding sendmail alone won't fix this, as the problem is in the underlying sasl implementation.

The above notes were developed on Tribblix, but ought to apply to any illumos distribution using the vanilla illumos sendmail+sasl combination.

1 comment:

Andy Fiddaman said...

Thanks Peter, nice write up - I've seen that question asked a few times too.

Note that you can automate the mc -> cf conversion by setting the config/path_to_sendmail_mc property on the service and restart it. That will then regenerate the .cf on service restart if the .mc is newer.

IF you don't want to download and install those two feature files, this works just as well in the .mc. It's weird that most of the glue is in place and perhaps authinfo.m4 should be added to upstream?

define(`SMART_HOST', `')
define(`_AUTHINFO_TABLE_', `')
Kauthinfo hash /etc/mail/authinfo