Meaning of a blank "mydestination" value

[Erik Thuning]

Hi!

I've inherited a server environment where all the servers have local
postfix agents installed, relaying mail to our central email server. I'm
trying to understand the main.cf settings shared by all our servers, but
there is one setting that I can't understand:

mydestination =

Aka nothing assigned to the mydestination value. To me the documentation
makes it sound like this is an error, but I'm unclear as to the exact
consequences of forcing this setting to be blank. Mail delivery works as
expected (mostly).

If it makes any difference, here is the complete main.cf file that we
deploy:

mydomain = dsv.su.se
myorigin = $mydomain
mydestination =
relayhost = [smtp.su.se]
mynetworks = 127.0.0.0/8 [::1]/128
inet_interfaces = loopback-only
biff = no
append_dot_mydomain = no


Thanks,
Erik Thuning

[Hans van Zijst]

Hi Erik,

If $mydestination is empty, it defaults to $myhostname, which defaults
to the FQDN of the machine:

http://www.postfix.org/postconf.5.html#mydestination

Kind regards,

Hans


On 25-09-2020 16:37, Erik Thuning wrote:

> Hi!
>
> I've inherited a server environment where all the servers have local
> postfix agents installed, relaying mail to our central email server. I'm
> trying to understand the main.cf settings shared by all our servers, but
> there is one setting that I can't understand:
>
> mydestination =
>
> Aka nothing assigned to the mydestination value. To me the documentation
> makes it sound like this is an error, but I'm unclear as to the exact
> consequences of forcing this setting to be blank. Mail delivery works as
> expected (mostly).
>
> If it makes any difference, here is the complete main.cf file that we
> deploy:
>
> mydomain = dsv.su.se
> myorigin = $mydomain
> mydestination =
> relayhost = [smtp.su.se]
> mynetworks = 127.0.0.0/8 [::1]/128
> inet_interfaces = loopback-only
> biff = no
> append_dot_mydomain = no
>
>
> Thanks,
> Erik Thuning

[Erik Thuning]

Hi Hans,

Thanks for the clarification. Just to make sure I understand correctly, setting "myhostname = " is equivalent to not setting the value at all?

The output of 'postconf' vs 'postconf -d' confused me here:

# postconf | grep 'mydestination ='
mydestination =
# postconf -d | grep 'mydestination ='
mydestination = $myhostname, localhost.$mydomain, localhost


/T

On 2020-09-25 16:48, Hans van Zijst wrote:

Hi Erik,

If $mydestination is empty, it defaults to $myhostname, which defaults
to the FQDN of the machine:

http://www.postfix.org/postconf.5.html#mydestination

Kind regards,

Hans


On 25-09-2020 16:37, Erik Thuning wrote:
> Hi!
>
> I've inherited a server environment where all the servers have local
> postfix agents installed, relaying mail to our central email server. I'm
> trying to understand the main.cf settings shared by all our servers, but
> there is one setting that I can't understand:
>
> mydestination =
>
> Aka nothing assigned to the mydestination value. To me the documentation
> makes it sound like this is an error, but I'm unclear as to the exact
> consequences of forcing this setting to be blank. Mail delivery works as
> expected (mostly).
>
> If it makes any difference, here is the complete main.cf file that we
> deploy:
>
> mydomain = dsv.su.se
> myorigin = $mydomain
> mydestination =
> relayhost = [smtp.su.se]
> mynetworks = 127.0.0.0/8 [::1]/128
> inet_interfaces = loopback-only
> biff = no
> append_dot_mydomain = no
>
>
> Thanks,
> Erik Thuning

[Richard Laysell]

On Fri, 2020-09-25 at 16:37 +0200, Erik Thuning wrote:

> Hi!
>
> I've inherited a server environment where all the servers have local
> postfix agents installed, relaying mail to our central email server.
> I'm
> trying to understand the main.cf settings shared by all our servers,
> but
> there is one setting that I can't understand:
>
> mydestination =
>
> Aka nothing assigned to the mydestination value. To me the
> documentation
> makes it sound like this is an error, but I'm unclear as to the
> exact
> consequences of forcing this setting to be blank. Mail delivery works
> as
> expected (mostly).

<snip>

> Thanks,
> Erik Thuning

Hello Erik,

A blank entry for mydestination means 'do not deliver mail to the local
machine' and all mail will be sent to the relayhost.  This is known as
a 'null client'

See
http://www.postfix.org/STANDARD_CONFIGURATION_README.html#null_client
for details.

Regards,

Richard

[Hans van Zijst]

Hi Erik,

I might have been a little quick to react. Explicitly setting it to
empty actually makes it an empty value :)

What $mydestination means is what domains the machine should accept mail
for as its final destination. Explicitly setting it empty would ensure
that no mail whatsoever is ever considered to be local, what would mean
that it ends up in a local mailstore. Where it will probably be
unnoticed forever...

Your predecessor probably did this to make absolutely sure that that
would never happen, and that every message had to be passed on to the
central relaying server.

Kind regards,

Hans



On 25-09-2020 17:07, Erik Thuning wrote:

> Hi Hans,
>
> Thanks for the clarification. Just to make sure I understand correctly,
> setting "myhostname = " is equivalent to not setting the value at all?
>
> The output of 'postconf' vs 'postconf -d' confused me here:
>
> # postconf | grep 'mydestination ='
> mydestination =
> # postconf -d | grep 'mydestination ='
> mydestination = $myhostname, localhost.$mydomain, localhost
>
>
> /T
>
> On 2020-09-25 16:48, Hans van Zijst wrote:
>> Hi Erik,
>>
>> If $mydestination is empty, it defaults to $myhostname, which defaults
>> to the FQDN of the machine:
>>
>> http://www.postfix.org/postconf.5.html#mydestination
>>
>> Kind regards,
>>
>> Hans
>>
>>
>> On 25-09-2020 16:37, Erik Thuning wrote:
>> > Hi!
>> >
>> > I've inherited a server environment where all the servers have local
>> > postfix agents installed, relaying mail to our central email server. I'm
>> > trying to understand the main.cf settings shared by all our servers, but
>> > there is one setting that I can't understand:
>> >
>> > mydestination =
>> >
>> > Aka nothing assigned to the mydestination value. To me the documentation
>> > makes it sound like this is an error, but I'm unclear as to the exact
>> > consequences of forcing this setting to be blank. Mail delivery works as
>> > expected (mostly).
>> >
>> > If it makes any difference, here is the complete main.cf file that we
>> > deploy:
>> >
>> > mydomain = dsv.su.se
>> > myorigin = $mydomain
>> > mydestination =
>> > relayhost = [smtp.su.se]
>> > mynetworks = 127.0.0.0/8 [::1]/128
>> > inet_interfaces = loopback-only
>> > biff = no
>> > append_dot_mydomain = no
>> >
>> >
>> > Thanks,
>> > Erik Thuning
>

[Viktor Dukhovni]

On Fri, Sep 25, 2020 at 05:16:26PM +0200, Hans van Zijst wrote:

> I might have been a little quick to react. Explicitly setting it to
> empty actually makes it an empty value :)

Correct.  More specifically it ensures that no domains are in the
"local" address class.  See ADDRESS_CLASS_README.

> What $mydestination means is what domains the machine should accept mail
> for as its final destination.

Final destinations include and

    - Local domains (listed in $mydestination)
    - Virtual mailbox domains (listed in $virtual_mailbox_domains)
    - Virtual alias domains (listed in $virtual_alias_domains), but
      for these, the address has to then actually rewrite to some
      "real" domain.

Additionally, Postfix will by default also accept inbound email to
domains in the "relay address class, which are then (unsurprisingly)
relayed (ideally closer) to their final destination.

> Explicitly setting it empty would ensure that no mail whatsoever is
> ever considered to be local, what would mean that it ends up in a
> local mailstore. Where it will probably be unnoticed forever...

It largely disables delivery to local accounts listed in /etc/passwd,
and in the local aliases(5) (but not virtual(5) aliases) table.
Delivery to virtual mailbox domains, and virtual alias domains is not
affected, nor is forwarding to any relay domains dependent on a non-
empty $mydestination.

> Your predecessor probably did this to make absolutely sure that that
> would never happen, and that every message had to be passed on to the
> central relaying server.

It ensures that mail to local user accounts is forwarded to the
relayhost for processing.

--
    Viktor.