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PToN
Premium
join:2001-10-04
Houston, TX
reply to tomdlgns

Re: DNS - A, MX and PTR records - Silly quesiton

Yes, i am trying to achieve connectivity on the event either ISP goes down.

yeah, i see what you are saying. I know that it will use the one with the lowest priority, but if i was to assign the same priority i would still get some timeouts (if the server is down) as some requests would be sent to down server, right?

Thanks.


tomdlgns
Premium
join:2003-03-21
Chicago, IL
kudos:1

said by PToN:

Yes, i am trying to achieve connectivity on the event either ISP goes down.

yeah, i see what you are saying. I know that it will use the one with the lowest priority, but if i was to assign the same priority i would still get some timeouts (if the server is down) as some requests would be sent to down server, right?

Thanks.

»en.wikipedia.org/wiki/MX_record#···ority.3F

Why have priority?
A common misconception about the MX preference ordering is that it is intended to increase the likelihood that mail may be delivered; however, merely having multiple MX records with the same preference provides this benefit (see below). Because the MX preference ordering specifies that some servers should be tried first, it is, if anything, a means of establishing load imbalance. Another common misinterpretation of MX preference ordering is that it is intended to provide a means of "failover" in the case of server overload. While it can be used that way, it is a poor resource management technique because it intentionally creates overload and does not fully utilize the available hardware. Assigning the same preference value to all of the available servers provides the same benefit and may even help avoid overload situations and thereby increase system throughput by decreasing latency.

The SMTP protocol establishes a store-and-forward network, and if a domain's mail servers are all offline, sending servers are required to queue messages destined for that domain to retry later. However, these sending servers have no way of being notified that a previously offline domain's servers are now available. The sending servers will only discover that the domain is available whenever delivery of the delayed messages is next attempted. The delay between when a domain's servers come online and when delayed messages are finally delivered can be anywhere from minutes to days, depending on the retry schedule of the sending servers. This is the problem that backup MX records are uniquely qualified to solve. The idea is that the servers listed as secondary MX servers have some out-of-band way of knowing when the primary servers are back online. Thus, they are a more useful place to queue messages when the primary servers are offline than the original sender's queue.


Steve
I know your IP address
Consultant
join:2001-03-10
Foothill Ranch, CA
kudos:5

said by tomdlgns:

The idea is that the servers listed as secondary MX servers have some out-of-band way of knowing when the primary servers are back online.

ETRN is an in-band way of knowing this.

tomdlgns
Premium
join:2003-03-21
Chicago, IL
kudos:1
reply to tomdlgns

does anyone do email spooling for you? if not, look into it. we have a company that does email spooling along with virus/spam filtering. our MX records point to their servers and they have to records, set at diff priorities.

i dont need to worry about which ISP i am using (my office has two ISP connections) the service we subscribe to looks for port 25 to be open, it doesnt care which ISP connection it sends it to, i just need to make sure i have both static WAN addresses listed in their portal (which i do) those are the only IPs they ping/see if they are online before they attempt to deliver mail.

i recommend keeping your TTL values as low as your provider allows.

i use network solutions and the lowest setting is 3600.


tomdlgns
Premium
join:2003-03-21
Chicago, IL
kudos:1
reply to Steve

said by Steve:

said by tomdlgns:

The idea is that the servers listed as secondary MX servers have some out-of-band way of knowing when the primary servers are back online.

ETRN is an in-band way of knowing this.

sorry, you lost me.


PToN
Premium
join:2001-10-04
Houston, TX
reply to tomdlgns

No, we dont have anyone doing spooling for us. We are switching to DNSMadeEasy and their default TTL is 1800, i may try a lower one once we do the switch and then set it back to their default.


tomdlgns
Premium
join:2003-03-21
Chicago, IL
kudos:1

is this due to cost or never had a need for it?

i would consider this.

for email spooling, email scanning for virus and spam it only costs me 3.00 per user per month.

worth every penny, IMO.

also, alias addresses for users do not count against the total count (i confirmed with my rep, but things might have changed, not sure).



Steve
I know your IP address
Consultant
join:2001-03-10
Foothill Ranch, CA
kudos:5
reply to tomdlgns

said by tomdlgns:

sorry, you lost me.

When the final mailserver is back up, it can phone home to the backup (via port 25/tcp) and use the ETRN command that essentially says "This would be a great time to scan your outgoing queue", at which time it presumably finds that it can consummate the final delivery.

No out-of-band mechanism required.

Steve
--
Stephen J. Friedl | Unix Wizard | Security Consultant | Orange County, California USA | my web site

tomdlgns
Premium
join:2003-03-21
Chicago, IL
kudos:1

ok, but were you correcting something i posted or adding more information?

i just copy/pasted from the website and included a link.