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(N.B. In this question, a leading # indicates a root prompt, not a comment. Also, I have replaced the actual hostname with <myhostname>.)

As the debconf (7) man-page states, dpkg-reconfigure can be invoked with any one of several front-ends, including the default ("dialog") interactive front-end, and the "noninteractive" frontend.

Am I right in thinking that running

# dpkg-reconfigure postfix

to invoke the dialog front-end, and pressing Enter in response to each question, should be equivalent to running the following?

# dpkg-reconfigure -f noninteractive postfix

If so, then I do not understand the following discrepancy.

Discrepancy

On a VPS freshly-populated with Debian 9 "Stretch" from my web host's image (which has postfix already installed), I preseed using debconf-set-selections:

# debconf-set-selections <<< "postfix postfix/destinations string <myhostname>"
# debconf-set-selections <<< "postfix postfix/mailbox_limit string 51200000"              
# debconf-set-selections <<< "postfix postfix/mailname string <myhostname>"    
# debconf-set-selections <<< "postfix postfix/main_mailer_type select Internet Site"      
# debconf-set-selections <<< "postfix postfix/protocols select ipv4"                      
# debconf-set-selections <<< "postfix postfix/root_address string root"

If I then run dpkg-reconfigure interactively, and press Enter for every question, I get:

# dpkg-reconfigure postfix
dpkg-reconfigure postfix
Removing sqlite map entry from /etc/postfix/dynamicmaps.cf
setting synchronous mail queue updates: false
Adding sqlite map entry to /etc/postfix/dynamicmaps.cf
changing /etc/mailname to <myhostname>
setting myorigin
setting destinations: localhost
setting relayhost: 
setting mynetworks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
setting mailbox_size_limit: 51200000
setting recipient_delimiter: +
setting inet_interfaces: all
setting default_transport: smtp
setting relay_transport: smtp
setting inet_protocols: ipv4
WARNING: /etc/aliases exists, but does not have a root alias.

Postfix (main.cf) is now set up with the changes above.  If you need to make 
changes, edit /etc/postfix/main.cf (and others) as needed.  To view Postfix 
configuration values, see postconf(1).

After modifying main.cf, be sure to run 'service postfix reload'.

Running newaliases

Note that this does quite a lot, including setting /etc/mailname. At this point, postfix is ready to send mail.

By contrast, if I use the same VPS, starting again from a fresh image, and run the same commands except for invoking dpkg-reconfigure non-interactively, I get:

# dpkg-reconfigure --frontend noninteractive postfix
Removing sqlite map entry from /etc/postfix/dynamicmaps.cf
Adding sqlite map entry to /etc/postfix/dynamicmaps.cf

Postfix (main.cf) configuration was not changed.  If you need to make changes, 
edit /etc/postfix/main.cf (and others) as needed.  To view Postfix 
configuration values, see postconf(1).

After modifying main.cf, be sure to run 'service postfix reload'.

Running newaliases

Note that this time, /etc/mailname was not set. Moreover, at this point, postfix is not ready to send mail and any attempts to make it do so result in bounces showing up in /var/log/mail.log.

My questions

  1. Was my initial assumption correct?
  2. Is the above discrepancy a bug, or is it expected behaviour? (And if it is expected behaviour, then what was the rationale for this design choice?)

Addendum

If I run rm /etc/postfix/main.cf before running dpkg-reconfigure --frontend noninteractive postfix, then /etc/mailname and /etc/postfix/main.cf both end up being created, and postfix ends up ready to send mail. This seems to be a viable workaround for the above failure, but it certainly violates the Rule of Least Surprise and feels as though it might be undefined behaviour that should not be relied upon.

1 Answers1

3

The postfix config files including /etc/mailname CANNOT be changed using debconf-set-selections and dpkg-reconfigure once the postfix config files have been written out. I burnt way too many hours trying to find out why...

Why?

dpkg-reconfigure -f noninteractive returns value 30 question skipped for all questions (and uses the debconf-set-selections presets), the code in /var/lib/dpkg/info/postfix.config then does not set the debconf settings as changed and /var/lib/dpkg/info/postfix.postinst then does not write any postfix config.

Workarounds?

  1. Good: The workaround as you found is to remove /etc/postfix/main.cf which forces /var/lib/dpkg/info/postfix.postinst to write out a new config for postfix which will include the new debconf-set-selections options.

  2. Better: Alternative use dpkg-reconfigure in an interactive mode and confirm the questions, in which case /var/lib/dpkg/info/postfix.config marks the options as changed and /var/lib/dpkg/info/postfix.postinst writes out the changes.

  3. Best: Alternative switch to managing /etc/postfix/main.cf etc options using postconf. (Likely a good idea to have a echo "postfix postfix/main_mailer_type select No configuration" | debconf-set-selections safety net)

Firefishy
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