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Re: "hostname" question during Debian installation

Tom H wrote:
> >> Bob Proulx wrote:
> >>> Yes. And also to /etc/postfix/main.cf if postfix is installed.  Or to
> >>> other places if other MTAs are installed.
> >>
> >> When you use "dpkg-reconfigure exim4-config" or "dpkg-reconfigure
> >> postfix", "/etc/mailname" is updated; in postfix's case because "my
> >> origin" is set to it in "/etc/postfix/main.cf".
> >
> > I thought it set 'myhostname'.  No?  I should try a pristine
> > installation and look.

And a quite long response from doing this results below.  :-)

> AFAIR, "myhostname" is "/etc/hostname"

Not quite.  For Debian's Postfix configuration the postfix myhostname
variable is set to the fqdn in /etc/postfix/main.cf.  Postfix requires
myhostname to be the fqdn.  The default value for Postfix is to use
the value of `hostname`.  However in Debian the default hostname is
the short hostname without the domain name.  Therefore Postfix's
default of `hostname` isn't suitable and must be supplemented by
setting the FQDN explicitly.

Well...  What I said wasn't quite true.  It is the summary of the
overall result.  What Postfix actually does is somewhat circular.
Here are the official docs and a somewhat longer explanation.

  mydomain (default: see "postconf -d" output)

      The internet domain name of this mail system. The default is to
      use $myhostname minus the first component, or "localdomain"
      (Postfix 2.3 and later). $mydomain is used as a default value for
      many other configuration parameters.

  myhostname (default: see "postconf -d" output)

      The internet hostname of this mail system. The default is to use
      the fully-qualified domain name (FQDN) from gethostname(), or to
      use the non-FQDN result from gethostname() and append
      ".$mydomain". $myhostname is used as a default value for many
      other configuration parameters.

If not set then it sets mydomain from the fqdn obtained from hostname
or to localdomain if that isn't a fqdn.  On Debian with a short
hostname set that means mydomain always defaults to localdomain.

If not set then it sets myhostname from the non-fqdn obtained from
hostname and appends mydomain.  On Debian with a short hostname that
means myhostname always defaults to somename.localdomain.  And if the
hostname was chosen to be localhost then it would default to
localhost.localdomain which is one consistent and desirable strategy
for thin client systems without a network and without a network domain

[Postfix was born on systems where the hostname was normally set to
the fqdn.  In that environment the above makes a lot of sense.  On
Debian with the short hostname it means that myhostname should always
be set to provide that supplemental domain name information.]

Something similar happens with Exim because exim4-config sets up
/etc/exim4/update-exim4.conf.conf with the fqdn too.

> and "myorigin" is "/etc/mailname".

Yes.  If an MTA has been installed.  That file does not exist if no
MTA has been installed.

> What I find somewhat weird is that when you install Debian,
> "/etc/hostname" and "/etc/mailname" are the same.

They are only the same if you choose a fqdn for the hostname.  By your
comment I read that it implies that you do choose a fqdn for your
hostnames?  (And that you always choose either "Standard system" or
otherwise install an MTA?)  That is why they would be the same for
you.  If you choose a short name for the hostname, which is the worded
with recommendation from the debian-installer and so most people
probably choose it, then they are different.  /etc/hostname will be
the short name and /etc/mailname will be the fqdn.

The debian-installer says:

  Please enter the hostname for this system.

  The hostname is a single word that identifies your system to the
  network.  If you don't know what your hostname should be, consult your
  network administrator.  If you are setting up your own home network,
  you can make something up here.

By saying "single word" and defaulting to a single word such as
"dhcp224" for the hostname I think most people wouldn't type in the
fully qualified domain name.  Although typing in a fqdn results in a
perfectly valid system configuration.  And if you do so then it
doesn't ask you for a domain name separately.  I always used to use
that configuration.  In previous releases I recall that the wording
was much more strong that it should not be the fqdn.  I would need to
try previous releases in order to know for sure but I think that
wording has been softened in Squeeze.

Additionally if you choose "Standard system utilities" from the task
selection menu then you get Exim installed since exim4 is graced with
being marked as "Priority: standard".  If you unselect that box then
you don't get any MTA installed (which I do in many configurations)
unless you have preseeded in Postfix or something else.  If you don't
have an MTA installed then /etc/mailname is also not installed.

In which case /etc/mailname is different from /etc/hostname by virtue
of not being there.  A valid system configuration is one without an
MTA in which case /etc/hostname is still needed to provide the system
hostname but no /etc/mailname file is needed since there isn't an MTA
to need that file.

> What I find somewhat weird is that when you install Debian,
> "/etc/hostname" and "/etc/mailname" are the same.
> So if it's "box.company.internal" and bob runs "mail tom", bob's
> address'll be "bob@box.company.internal".

Yes.  Seems reasonable to me.  That is exactly what I want.  It is
traditional BSD/Unix behavior.

> That's fine when emailing on a box or from one box to another within
> "company.internal".

Yes.  Seems reasonable to me.

> There was a debian-devel thread where there was an argument about
> whether "/etc/mailname" should be "box.company.internal" or
> "company.internal".

Oh!  Well...  If you want address masquarading then I think that
should be an explicit choice.  I wouldn't default to address
masquarading by default.  That would be wrong for many environments.
It assumes that an environment has a central mail relay to handle mail
for the entire domain and it routes all mail through that central mail
relay.  But in a simple environment with a few machines (or several
thousand) that wants email to route directly from machine to machine
then that is the wrong configuration.  I have environments both ways.
It is easy enough to set up address masquarading if desired.

Either set the local domain in /etc/mailname for an MTA agnositc
solution OR for postfix explicitly set:

  masquerade_domains = example.com
  masquerade_exceptions = root

It would be perfectly nice if the package installation dialog were to
ask the user if they want to set up address masquerading and to do it
automatically.  But for anyone who wants it the configuration is very
easily accomplished with one of the above.  I prefer to leave
/etc/mailname along and to configure postfix explicitly.  Then I can
control over masquerade_classes and local_header_rewrite_clients too.

> If you you do a regular install or don't use a preseed that installs
> postfix, you'll have exim4 by default.


> You can then change "/etc/hostname"

No.  Changing /etc/hostname has no effect for postfix.  That is only
used by the /etc/init.d/hostname.sh script.  The postfix postinst
script does:

	    mailname=$(hostname --fqdn 2>/dev/null || echo localdomain)

And that uses the hostname command coupled with DNS to do a lookup of
the hostname to get an IP address and then a reverse lookup of that to
get a name associated with it.  I disapprove of that process because
it doesn't work in non-typical configurations such as machines with
multiple network cards and with multiple IP addresses bridging
different domains.  But that is what it does.

> and "/etc/mailname", install postfix, and see what gets pulled in as
> "myhostname" into "/etc/postfix/main.cf".

Changing mailname doesn't really have an effect on the
/etc/postfix/main.cf configuration.  Because there is a Debian
specific patch to in enable reading myorigin from a file.  So it has
an effect only through the patched code that pulls in the value
dynamically when postfix starts from /etc/mailname instead of needing
to specify myorigin in the main.cf file explicitly.

> I'm going to try it too.

Ha!  Beat you to it! :-)


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