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Adding an A (Address) record early?

Posted on 2011-05-09
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Last Modified: 2012-05-11
I am changing my public IP for my mail server this week and was wondering if I could go ahead and put in the new A record pointing to the new IP which of course points to the MX record? I would still leave the existing A record until the official switch, I just want to avoid a long propagation time.
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Question by:j_crow1
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by:Randy Downs
ID: 35720979
I would change the expires to the smallest allowed interval to force DNS servers to check back with you.

http://technet.microsoft.com/en-us/library/cc737753(WS.10).aspx
By default, the expire interval for each zone is set to 1 day. The expire interval is used by other DNS servers configured to load and host the zone to determine when zone data expires if not renewed.

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by:MinoDC
ID: 35720988
you can add a A record , next you can add a MX record that point to the new A record with metric lower than to the actual MX record
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by:j_crow1
ID: 35721051
MinoDC:

The MX record will match the existing MX record, is this okay?
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by:Chris Dent
ID: 35721275
> was wondering if I could go ahead and put in the new A record pointing to the new IP

Sure, no harm in doing that. The name you're using is brand new?

Propagation... this one is a bit of a tricky one since a very large number of people insist on quoting 24 - 48 (or 72) hours.

Record propagation when discussing changes to existing records is based on the TTL of the existing record. That is, if I ask your DNS server for your MX record and the TTL is 2 days then I will stick it in memory for that long. I will continue using that cached copy until the TTL runs out.

The key point here is that propagation is based on the TTL, nothing else. If you decrement the TTL on your servers you reduce propagation time.

> The MX record will match the existing MX record, is this okay?

But the server you point at is different, right?

There's no real problem adding more MX records, but you *must* be aware that mail will only ever be delivered to one system. If both are up it will preferentially deliver to the lowest numeric preference, but you should not rely on that.

Chris
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Author Comment

by:j_crow1
ID: 35721636
Chris:

The name I am using for the MX record is not new, and it will be pointing to the same server. Example:

Current A record:
0.0.0.1 - mail.example.com

Current MX record
mail.example.com

I am wanting to do the following so it has time to propagate before the switch:

I want the A record to be: 0.0.0.2 - mail.example.com

Existing MX record:
mail.example.com

The only thing I am wanting to do is put the A record in early so that it has time to propagate. I was just wonder if this is possible, and if it is going to create any mail flow issues between here and then. If it is going to interrupt mail flow I will just wait until the switch, if there is a way to get it in early though I would like to do that.
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by:Chris Dent
ID: 35721704

Then no, you cannot do that early.

If you go ahead with that you will find some mail attempts to deliver to the old system, some to the new. Neither will be guaranteed and you will not be able to control while it chooses. If one of those IPs is offline mail may eventually bounce.

I suggest you reduce the TTL on the Host (A) record, mail.example.com, to around 300 or even 900 (5 or 15 minutes). Doing so will mean a change to that record takes 5 or 15 minutes to propagate.

If you want more you will need to use a different name for the new server. This is fine provided you can control network access to *both* of the SMTP servers.

e.g.

mail-in.example.com - 0.0.0.2

Then add an MX Record, making the MX record set into this:

example.com.  MX  10  mail.example.com.
example.com.  MX  20  mail-in.example.com.

This way you have two routes in. If you cannot control one or the other you should use the TTL modification. When using the same name, reduce the TTL on the A record. If using a different name, reduce the TTL on the MX record.

Chris
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Author Comment

by:j_crow1
ID: 35721985
Alright, I will just have to change everything over on the morning of. Thanks for your help Chris.
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Expert Comment

by:Chris Dent
ID: 35722421
Lower the TTL on the A record first and you'll minimise propagation, it's well worth doing, but it must be done in advance (sorry if I harp on about it, just want to be sure :)).

Chris
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MinoDC earned 2000 total points
ID: 35727242
j_crow1, you can have an MX record that points to an A record with two different IP, with different metrics (DNS Load Balancing).

 Then you can have:

Current A record:
 0.0.0.1 - mail.example.com

Current MX record
 mail.example.com (metric 20)

Future A record
 0.0.0.2 - mail.example.com

Future MX record
 mail.example.com (metric 30)

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by:MinoDC
ID: 35727274
j_crow1, view here:
(alternate solution)
http://www.zytrax.com/books/dns/ch9/rr.html
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