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False r-DNS with Cisco router, 3 IPs on one Exchange box

Hello all-

I have a question. As of late we've been having a lot of problems with mail getting rejected back to us because of a bad r-DNS.

What happens is that we have 3 companies under one roof, all seperate domains, all seperate IP's.

We have all internet/email traffic coming in through one Cisco box (.162). Our IP's for email are .148,.149,.150.

Receiving email is no problem because we have a static nat route stating that all traffic for these above email IP's are to go to our mail server (LAN x.x.x.220).

However all outbound mail is utilizing the gateway (.162) and thus looks like it is coming from that address rather then .148, .149, or .150 (as appropriate)...

We've been struggling with this issue for a week now and don't know if it's a router issue or if a DNS record needs to be changed with our ISP?

Any help would be much appreciated. Thank you in advance.


Re: False r-DNS with Cisco router, 3 IPs on one Exchange box

Hi, let me understand your problem.

You have one physical email-server (LAN .220) which needs to look like three different servers (.148, .149, .150) from three different domains to your ISP?

This would mean the router has to check the domain in the emails sent to figure out which outbound IP to use. That is not possible afaik.

So you have to look for another solution. You could setup three mail servers in your LAN with one static NAT entry for each box. The router could then replace the LAN IP with the proper WAN IP and you should have no problem. Or you could use three different IP addresses on your mail server for the same purpose. I doubt that this is easily done.

The other - maybe most simple - option is to use one WAN IP for all three domains. Have one static NAT entry (f.e. .150) and let DNS point to that IP for all three domains. This should solve the r-DNS problems you are facing.

Hope this helps


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