DNS Issue I Think?

[popeus]

I have 3 dns servers and 2 internet connections, dsl and cable both with static ip addresses. One email server behind router that routes all traffic on ports 25 and 110 to the single mail server, doesn't matter which internet connection the requests comes in on. I'm trying to use round-robin dns, not for load balancing, but for redundancy. The host "mail" has two A records, one to an ip address on internet connection 1 and the other to an ip address on internet connection 2. The mx record is then "mail.mycompany.com"

Clients setup their mail client using "mail.mycompany.com" as both smtp and pop server. We also have webmail that answers on port 80 for "mail.mycompany.com".

When one internet connection goes down, the webmail still works, so the client requesting "mail.mycompany.com" through internet explorer can access, send and receive email. But, their mail client can't find the mail server. Any help would be greatly appreciated.

 

[GlenJohnson]

One email server behind router that routes all traffic on ports 25 and 110 to the single mail server,

Why? This might have something to do with the problem. Also, why are the internet connections going down? That would be my first interest. Just a thought.

Glen A. Johnson
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"Education is the best provision for old age."
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[ShackDaddy]

Your problem probably has to do with how different applications cache address resolutions.

The mail client caches the address that works, and doesn't do another DNS query to get the other address when the connection that worked goes down. Browser's don't work that way, since the web environment is generally more in flux. I'll bet that browsers either re-query when something in the general cache isn't reachable, or they check the cache for multiple addresses and use them. Mail clients probably have their own internal resolver cache or something. Does it make a difference when you close the mail client and reopen it?

When you type 'ipconfig /displaydns' on a client, do you see your mail.mycompany.com host listed there? And are there multiple addresses in the listing, or just one? Round-robin gives out all the addresses, but it changes the priority order of them each time. Could be that a mail client will only use the first address it is given. A lot of 'ifs' here, I know. I'm just thinking it out with you.

One thing for sure, I would set the TTL on those two records to a VERY low number. 60 is the recommended number, which would be sixty seconds.

ShackDaddy