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MX record and A record in dns

Posted on 2011-09-21
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Last Modified: 2012-06-27
Dear Experts:

We have outsourced the mail services to the mail service provider and service provider has placed one linux box in our LAN for fetching. In the domain control panel details are set as below:

1. MX is pointing to the mail service provider
2. A record is pointing to the mail.mydomain.com # mail.mydomain.com is placed in our LAN.

All the users email client is configured with “A” record details that is hosted in the LAN, also all the mobile black berry users are also configured with the A record details that is hosted in the LAN.
Now due to the change in the ISP service type, so for using IP address has to be changed but this change will affect our mail services for the email client users also for the blackberry users we have around 80 blackberry users.
Hence I have scheduled this activity of changing this IP address in the router or firewall on the weekend, Following have to be done:

1.Login to the domain control panel and replace the existing IP address to the new IP address.
2.Configure the router/firewall with the new IP address, In the router/firewall NAT one of the public ip to the private ip address of the linux mail box (mail.mydomain.com)
3.Public IP in the A record for mail.mydomain.com and in the router/firewall natted IP aaddress should be same.

When did nslookup or dig TTL values are 86400 please give me is there any best practice can done so that mail services down time will be very much minimal, we cannot afford to the long time for the dns replication due to this mail services getting affected. Please help me in this with the best possible practice step by step. Thanks in advance.
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Question by:D_wathi
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Accepted Solution

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Papertrip earned 500 total points
ID: 36574358
Set the TTL for the A record to 60.  Since your current TTL is 1 day, this needs to be done at least 1 day before you change the A record.  Once you are done, change the TTL back to something reasonable like 3600.
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Author Comment

by:D_wathi
ID: 36574500
Sir, Thanks for the reply, setting A record to 60 # please explain by setting to 60 what exactly happens , please
2. logged into the domain control panel i do not see the option to edit TTL from 86400 to 60 .I think should contact the name server administrator and request to do this.
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LVL 21

Expert Comment

by:Papertrip
ID: 36574510
TTL defines the maximum time a record can stay in cache.  This means that when you change to the new IP, the maximum amount of time that anyone would still see the old IP is 1 minute.
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Author Comment

by:D_wathi
ID: 36574954
Thank you very much. should i do one day before and once changed to the new ip address and allowing it for complete one day then change the TTL back. Is this correct please suggest.
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Expert Comment

by:Papertrip
ID: 36574960
Once you change to the new IP, you can change the TTL back at the same time.
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Author Comment

by:D_wathi
ID: 36575016
Iam sorry little confusion.

1. Should change the TTL value to 60 one day before (24 hours before the actual change of ip address)
2. Then after 24 hours change the IP address and at that time itself change the TTL back to the orginal.

Please suggest me. Thanks in advance.
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Expert Comment

by:Papertrip
ID: 36575065
That is correct.
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Author Comment

by:D_wathi
ID: 36579514
Sir, Thanks for the reply, Is it possible to set the TTL for only the A record or it gets affected to even MX record, please suggest. Thanks in advance.
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LVL 21

Expert Comment

by:Papertrip
ID: 36580008
TTL values are per label, so only the A record in this case.
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Author Comment

by:D_wathi
ID: 36580270
Thank you very sir. Finally one doubt please do not mind. Request you to please tell me what exactly happens when set the TTL value of A record to 60 before 24 hours and after 24 hours change the IP address and set TTL value back to 3600.

2. when we do as per the above how much downtime is expected for the mail services.

Please help.
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LVL 21

Expert Comment

by:Papertrip
ID: 36581881
OK I've already answered those questions, but I will try to make this easier to understand since there seems to be some communication breakdown.

Plan:
24 hours before changing A record to new IP, set TTL of A record to 60.
Once that 24 hours is up, change A record to new IP, set TTL of A record to 3600.
done.

That will mean a maximum of 60 seconds that any caching resolver will have the old IP.

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Author Comment

by:D_wathi
ID: 36582608
Sir, Thanks i understood and planned to do as per your suggestion was wanting know how the actual process happens in the global dns. sorry for my repeated requests.
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LVL 21

Expert Comment

by:Papertrip
ID: 36582611
It's ok I know you are just trying to be as safe as you can be.
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Author Comment

by:D_wathi
ID: 36582716
Thank you very much for the support.
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Author Closing Comment

by:D_wathi
ID: 36582746
Great support. Thanks.
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