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Alias IP and reverse DNS

Posted on 2010-08-29
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Hello Experts,

I have a router I have set up with a static IP and several alias static IP addresses in the usable range provided by the ISP (Comcast). On one of alias IP's, I forwarded port 25 to my Exchange server. I would like to set up a reverse DNS record (mail.domain.com) and tie it to that alias IP.

My question is....can I set up a reverse dns record to an alias IP on a router? When I do go to www.whatismyip.com, it shows the gateway (primary IP) address so my concern is that mail sent from that server would always show as sending from that primary gateway IP address and the reverse DNS record would not help. Any thoughts?
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Question by:JohnnyD74
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6 Comments
 
LVL 2

Expert Comment

by:gilsh
ID: 33554337
What purpose you want to do this?
You can do this with the ISP's DNS
But you run the risk of mail blocked by spam services
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Expert Comment

by:Eyecue
ID: 33554355
The IP that you get from "what is my ip" is the one that is assigned to you by your IP's last router before it gets to your modem. Anything that you set inside "your network" Wont affect this.  Your modem uses NAT and that means that it assigns information tags (headers) (your IP addresss) to your outgoing information. IT is converted it IPV6.
A reverse DNS is of no use to you either as it is a database of domains and assigned IP addresses. to do that in your home would be fruitless.
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Expert Comment

by:hilltop
ID: 33554373
RDNS should be done at the IP holders DNS. Host all the zones you like, but leave this to them.
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rfc1180 earned 1000 total points
ID: 33554380
>My question is....can I set up a reverse dns record to an alias IP on a router?

Yes you can, contact Comcast and have them add a PTR for the IP

When I do go to www.whatismyip.com, it shows the gateway (primary IP) address so my concern is that mail sent from that server would always show as sending from that primary gateway IP address and the reverse DNS record would not help. Any thoughts?

Yes, you did not setup a one to one nat, all you did was forward the port (PAT)

You need a one to one NAT.

Billy
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by:Steve
Steve earned 1000 total points
ID: 33554421
the static IP assigned to your modem/router is the primary IP used for incoming and outgoing traffic.
You need to set the router to make sure the outgoing traffic from the mail server is coming from the alias IP, not the default one.

This can be done easiest by assigning a one-to-one IP assignment in NAT to ensure that anay traffic coming in or going out from that server is routed by a specific IP.

once you have got this setup, contact your ISP and request an rDNS record for the IPs you specify. as they own the IP addresses on the internet you need them to host the rDNS record.

If unsure, I'd set an rDNS record for all your external IPs as a precaution, just in case you have your outgoing setup incorrect.
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Author Closing Comment

by:JohnnyD74
ID: 33554698
Thanks a million guys....just the info I needed!
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