[Bug 1906970] Re: dpkg hook hostname error

[Ian Johnson]

This happened to me when upgrading from Groovy to Hirsute just now, with
the same symptoms as above - I had `search .` in resolv.conf, but
nothing creating that entry in /etc/netplan or in
/etc/systemd/resolved.conf.

I fixed the broken package by fixing the erroneous entry
/etc/postfix/main.cf (removing the ".." at the end of the hostname) and
then running:

sudo dpkg-reconfigure --force postfix
sudo systemctl reload postfix

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https://bugs.launchpad.net/bugs/1906970

Title:
  dpkg hook hostname error

Status in postfix package in Ubuntu:
  New
Status in sbuild package in Ubuntu:
  New

Bug description:
  I tried to install postfix (3.4.13-0ubuntu1) within a Dockerfile
  (Ubuntu 20.04.01 LTS) and it failed because there is an error in the
  dpkg info file.

  I'm attaching the log of the 2nd time "apt install -y postfix" is
  executed, as it is way shorter and still shows the error message.

  ```
  root at cc1a572af0dc:/etc# apt install -y postfix
  Reading package lists... Done
  Building dependency tree
  Reading state information... Done
  postfix is already the newest version (3.4.13-0ubuntu1).
  0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
  1 not fully installed or removed.
  After this operation, 0 B of additional disk space will be used.
  Setting up postfix (3.4.13-0ubuntu1) ...

  Postfix (main.cf) configuration was untouched.  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 'systemctl reload postfix'.

  Running newaliases
  newaliases: warning: valid_hostname: misplaced delimiter: cc1a572af0dc..
  newaliases: fatal: file /etc/postfix/main.cf: parameter myhostname: bad parameter value: cc1a572af0dc..
  dpkg: error processing package postfix (--configure):
   installed postfix package post-installation script subprocess returned error exit status 75
  Processing triggers for libc-bin (2.31-0ubuntu9.1) ...
  Errors were encountered while processing:
   postfix
  E: Sub-process /usr/bin/dpkg returned an error code (1)
  ```

  I then looked into the /etc/postfix/main.cf file and tried to manually
  change the hostname, but it automatically gets overwritten by dpkg
  when apt install is ran.

  After that I was able to track the issue into the script
  "/var/lib/dpkg/info/postfix.postinst" more exactly the sed command in
  line 52. It reads the content of the search parameter from
  /etc/resolv.conf, and concatenates it with a "." as delimiter with the
  hostname, but it doesn't honor the special case of "search ." and
  therefore constructs an incorrect myhostname parameter for postfix of
  "cc1a572af0dc..".

  I'd propose to remove this resolve.conf lookup entirely so that the package installation doesn't fail within docker containers...
  It is a bad practice in my opinion to overwrite existing config files upon package installation.
  Also I'd suggest not executing postfix after the configuration is written, so that in case of unforeseen circumstances (like this one) a user can just simply run a sed against the config file after installing the package.

  And the Dockerfile for reference:
  ```
  FROM ubuntu:latest

  ENV DEBIAN_FRONTEND=noninteractive
  #ENV DEBIAN_PRIORITY=low

  RUN apt update \
      && echo "postfix postfix/main_mailer_type select Internet Site" | debconf-set-selections \
      && echo "postfix postfix/mailname string mysubdomain.mydomain.com" | debconf-set-selections \
      && apt install -y postfix
  ```

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