Re: Transport: Multiple routes to internal domain

[Jose Ildefonso Camargo Tolosa]

Hi!

I would answer you with another question:

In the event of a server failure, how are users directed to the other server?

I mean, there are several ways of doing that, and knowing your current
one would be helpful.

Sincerely,

Ildefonso Camargo.

On Mon, Feb 6, 2012 at 6:57 PM, Greg Wilson <gr...@actionforms.com.au> wrote:
> I'm using Postfix\Amavis\Spamassassin\Clamav to scan incoming emails for
> virus and spam and forward to our internal MS Exchange email system.
> Postifx shares the Internet domain with MS Exchange,  each
> forwarding messages to unknown  accounts to the other system.
>
> MS Exchange uses clustering to duplicate it's mailbox databases to
> 2 different servers. How do I setup Postfix to automatically forward
> messages to one of the MS Exchange servers if the other one  goes
> offline?
>
> I currently use an IP number in square brackets in
> Postfix's transport map to route to one of the MS exchange servers.
>
> Thanks,
>
> Greg.

[Wietse Venema]

Greg Wilson:

> 2 different servers. How do I setup Postfix to
> automatically forward
> messages to one of the MS Exchange servers if
> the other one  goes
> offline?

Two options:

A) Assuming that you use the "relay" transport in master.cf
(which you should if relaying mail from outside):

/etc/postfix/master.cf:
relay unix - - n - - smtp
-o smtp_fallback_relay=[1.2.3.4]

B) In the transport map, specify a hostname that resolves to TWO
IP addresses. Make up your own DNS or /etc/hosts entry if you
need to.

Wietse

[Greg Wilson]

Thanks for the advice.

I don't use relay maps for the domain as the Internet doamin is shared betweent he MS Exchange server and the Postfix server. i.e I use MySql address lookups onthe Postfix server so it's easy to create addresses to forward email to different or multiple accounts. e.g sa...@xxx.com

I don't think that multiple DNS records will work. (Although I may be wrong.)

I use this technique, DNS round robin to evenly spread rdp connections to our terminal servers. My understanding is that a device does a DNS lookup and the server hands out each different IP address sequentially. Each device uses (caches) the first IP address it recieves until it's rebooted.

i.e When the primary MS Exchange server went offline, Postifx wouldn't necessarally do another DNS looup and find the second record.

Is it possible to setup multiple transport records wth different costs to a domain? This may be a solution.

Cheers,

Greg.

[Greg Wilson]

I've been testing Postifx for some solutions.......

One attempt was to make 2 entries with the same host name in /etc/hosts

e.g
10.222.100.1 exchange.mydomain.local exchange
10.333.200.2 exchange.mydomain.local exchange

Then changed the transport map to

mydomain.local smtp:[exchange.mydomain.local]

My info is that the square brackets stop Postifix doing mx record lookups.

This didn't work and I don't know why. It works fine with an IP address inthe square brackets and the Linux box can resolve exchange.mydoamin.local to an IP.

Any suggestions?

[Lorens Kockum]

On Tue, Feb 07, 2012 at 11:42:37AM +1100, Greg Wilson wrote:
> I use this
> technique, DNS round robin to evenly spread rdp connections to our
> terminal servers. My understanding is that a device does a DNS lookup and
> the server hands out each different IP address sequentially. Each device
> uses (caches) the first IP address it recieves until it's rebooted.

That might be true for rdp connections to your terminal servers,
but (luckily!) it's a bit more complicated.

The DNS server sends a reply containing all the IP
addresses. The order varies. If the code asking the question
only wants one address, then it takes only one, and the next
time it asks the question it might get another answer. Postfix,
however, understands that there are multiple addresses and
tries them all, one after the other in a random order until
one replies or until it has tried them all and decides to wait
before trying again. After waiting Postfix will make another DNS
lookup.

The response to that lookup may be cached since the previous
lookup, but the cache is valid only until the record's time to
live (TTL) expires. The TTL is set in the DNS server. It can
be anything from zero ("do not cache") to a week or more. A
common default value is 24 hours. Of course, in the case of
a long-lived TCP connection you may have the impression that
the IP address is used until the device is restarted, and you
might even be correct (for an RDP client running on Windows, for
example), but that is not the case when sending mails.

> Is it possible to setup multiple transport records wth different
> costs to a domain? This may be a solution.

You can do unequal load balancing by creating multiple records
with the same IP. That could work using /etc/hosts, you'd have
to test.

You can do priorities (first try X and if it doesn't work try
Y then Z) by using different-priority MX records. This needs
access to your DNS server, and cannot be done with /etc/hosts.

Hope this helps.

[Andreas Berton]

As an alternative, it guess it would suffice to put an MX with lower
priority in your dns to the backup server, so if mastern in your cluster
is down postfix would then try next one. But what Loren wrote before
creating loadbalancing with round-robin is probably what you want.

[/dev/rob0]

On Tue, Feb 07, 2012 at 04:32:14PM +1100, Greg Wilson wrote:
> I've been testing Postifx for some solutions.......
>
> One attempt was to make 2 entries with the same host name
> in /etc/hosts
>
> e.g
> 10.222.100.1 exchange.mydomain.local exchange
> 10.333.200.2 exchange.mydomain.local exchange

IPv5? I haven't seen many of those quads>255. :)

> Then changed the transport map to
>
> mydomain.local smtp:[exchange.mydomain.local]
>
> My info is that the square brackets stop Postifix doing mx
> record lookups.

And instead, do a DNS "A" lookup. To use hosts(5) resolution
specify smtp_host_lookup to include "native":
http://www.postfix.org/postconf.5.html#smtp_host_lookup

> This didn't work and I don't know why. It works fine with

> an IP address in the square brackets and the Linux box can

> resolve exchange.mydoamin.local to an IP.

By default smtp_host_lookup is "dns" only.

> Any suggestions?

Also, please don't top-post your replies. Your MUA badly mangled
quoting in the rest of the thread so I didn't bother to try to fix
that. Thanks.
--
http://rob0.nodns4.us/ -- system administration and consulting
Offlist GMX mail is seen only if "/dev/rob0" is in the Subject:

[Wietse Venema]

Greg Wilson:

> One attempt
> was to make 2 entries with the same host name in /etc/hosts
>
> e.g
> 10.222.100.1 exchange.mydomain.local exchange
> 10.333.200.2
> exchange.mydomain.local exchange
>

> Then changed the transport map
> to
>
> mydomain.local smtp:[exchange.mydomain.local]
>
> My
> info is that the square brackets stop Postifix doing mx record lookups.
>
>

> This didn't work and I don't know why. It works fine with an

That 's because LINUX does not support multiple /etc/hosts records
per name.

Use a better OS, use DNS, or use my smtp_fallback_relay solution.

Wietse

[James Day]

Hi Wietse,

When you say a better OS, I'd be interested to know what your preference is.

Kind regards,

James Day
(IT Engineer)

[Jose Ildefonso Camargo Tolosa]

On Tue, Feb 7, 2012 at 9:03 AM, Wietse Venema <wie...@porcupine.org> wrote:
> Greg Wilson:
>> One attempt
>> was to make 2 entries with the same host name in /etc/hosts
>>
>> e.g
>> 10.222.100.1 exchange.mydomain.local exchange
>> 10.333.200.2
>> exchange.mydomain.local exchange
>>
>> Then changed the transport map
>> to
>>
>> mydomain.local smtp:[exchange.mydomain.local]
>>
>> My
>> info is that the square brackets stop Postifix doing mx record lookups.
>>
>>
>> This didn't work and I don't know why. It works fine with an
>
> That 's because LINUX does not support multiple /etc/hosts records
> per name.
>
> Use a better OS, use DNS, or use my smtp_fallback_relay solution.
>
>        Wietse

Eh, Linux *does* support that, from man host.conf (/etc/host.conf):

multi Valid values are on and off. If set to on, the resolv+
library will return all valid addresses for a host that appears in
the /etc/hosts file, instead of only the first. This is off by
default, as it may cause a substantial performance loss at sites with
large hosts files.

[Greg Wilson]

Lorens,

Thank you for the detailed reply. I'm interested to hear that some of the DNS smarts reside in postfix itself.

As I had no luck with multiple records in the /etc/hosts file, I've gone to the DNS option and setup multiple A records for exchange.xxx.local, using that name in the Postfix's transport file. I can now see that mail is coming in via our backup MS Exchange server, which is fine.

As long as Postfix is smart enough to send email to the other IP address for this record if one of the Exchange servers is down, all is well.

Thanks again,

Greg.