Hello,

I'm a newbie to postfix and I have a question about a DNS change on our
network. We have received word that we need to update the DNS records
on all our servers on the network. Could anyone tell me what impact
this will have on postfix? Will we need to force postfix to use the new
DNS entries or do we have to restart it? What things should I do to
postfix to ensure that it works correctly when the new DNS settings
become active?

Many thanks in advance.

@lex wrote:
> Hello,
>
> I'm a newbie to postfix and I have a question about a DNS change on our
> network. We have received word that we need to update the DNS records
> on all our servers on the network. Could anyone tell me what impact
> this will have on postfix? Will we need to force postfix to use the new
> DNS entries or do we have to restart it? What things should I do to
> postfix to ensure that it works correctly when the new DNS settings
> become active?
>
> Many thanks in advance.


Postfix does not cache name service lookups.

The problems normally occur from caching on the DNS server used by
the Postfix systems, or from the upstream DNS servers being followed.
Those will cache the old data until their expiry dates are reached.

As to what types of preventive measures; I suppose that depends on the
nature of the DNS work being done. For example, in some cases if you are
changing domain names, you might be able to install secondary MX records
in advance for the new names. Or if changing network numbering, you might
be able to add multiple IP addresses in advance.

Perhaps you can make a test change in DNS in advance, to determine if there will
be any propagation issues.
--
Greg

Hi Greg,

Thanks for the reply. I should have been a little more specific in my
description. What we're actually wanting to do is change the DNS server
IP addresses in the resolv.conf file so that we are using a completely
new set of DNS servers.

So the question is, does postfix cache the list of DNS servers it
should use on startup or does it always look back to the resolv.conf
file?

Thanks again.


Greg Hackney wrote:

> @lex wrote:
> > Hello,
> >
> > I'm a newbie to postfix and I have a question about a DNS change on our
> > network. We have received word that we need to update the DNS records
> > on all our servers on the network. Could anyone tell me what impact
> > this will have on postfix? Will we need to force postfix to use the new
> > DNS entries or do we have to restart it? What things should I do to
> > postfix to ensure that it works correctly when the new DNS settings
> > become active?
> >
> > Many thanks in advance.
>
>
> Postfix does not cache name service lookups.
>
> The problems normally occur from caching on the DNS server used by
> the Postfix systems, or from the upstream DNS servers being followed.
> Those will cache the old data until their expiry dates are reached.
>
> As to what types of preventive measures; I suppose that depends on the
> nature of the DNS work being done. For example, in some cases if you are
> changing domain names, you might be able to install secondary MX records
> in advance for the new names. Or if changing network numbering, you might
> be able to add multiple IP addresses in advance.
>
> Perhaps you can make a test change in DNS in advance, to determine if there will
> be any propagation issues.
> --
> Greg

@lex wrote:
> What we're actually wanting to do is change the DNS server
> IP addresses in the resolv.conf file so that we are using a completely
> new set of DNS servers.
>
> So the question is, does postfix cache the list of DNS servers it
> should use on startup or does it always look back to the resolv.conf
> file?

No Postfix caching. The changes will be immediate to Postfix, just like
they should be to using the local "nslookup" command.

As a side note, if you have a very busy email server, Postfix performance will
improve if you run a local caching DNS service.

--
Greg

Many thanks Greg.


Greg Hackney wrote:
> @lex wrote:
> > What we're actually wanting to do is change the DNS server
> > IP addresses in the resolv.conf file so that we are using a completely
> > new set of DNS servers.
> >
> > So the question is, does postfix cache the list of DNS servers it
> > should use on startup or does it always look back to the resolv.conf
> > file?
>
> No Postfix caching. The changes will be immediate to Postfix, just like
> they should be to using the local "nslookup" command.
>
> As a side note, if you have a very busy email server, Postfix performance will
> improve if you run a local caching DNS service.
>
> --
> Greg