Secondary DNS IP not actually used in SM?
Problem reported by Scarab - October 2 at 11:58 AM
Submitted
SM Enterprise Build 7188
 
Just had our Primary DNS IP fail briefly for about 10 minutes and everything in SM hard-bounced with a "600 Failed to connect to the recipient's mail server.  No DNS information was found for the 'gmail.com' domain. Status: 544 5.4.4 Host not found (not in DNS)." Our Secondary DNS IP in SM SETTINGS > GENERAL SETTINGS was still up and running and serving lookup requests during this time, yet SM did not query the Secondary DNS IP even once. Surely this isn't working as intended?

Not that such an occurrence should occur hardly ever, but SM should be falling back to Secondary DNS if Primary DNS is unable to resolve a query. Otherwise what is the purpose of having a Secondary DNS option?

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Netmate Replied
We faced similar issue couple of weeks back when our primary DNS server was unreachable. I think it's a bug and needs to be fixed asap. 
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Kyle Kerst Replied
Employee Post
Hello and good afternoon. I tested this here but was not able to reproduce the issues you've reported. I disabled DNS caching completely, then set my primary DNS server address to an unroutable address and sent a test message. I see in the logs that SM was still able to find a valid MX record for the destination domain. Can you confirm your secondary DNS server is reachable from the SM server?
Kyle Kerst
Technical Support Specialist
SmarterTools Inc.
(877) 357-6278
www.smartertools.com
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Scarab Replied
Kyle,

Yes, both are reachable from the server and SM, however I see what occurred better now (and had it a little backwards in my OP):

  1. Primary DNS IP is rebooted, so as it was temporarily unreachable for @20 seconds SM rolled over to the Secondary DNS IP successfully. This is obviously Working as Intended and is consistent with your tests as well.

  2. Once the Primary DNS IP was serving requests again the Secondary DNS IP was rebooted (and failed to serve DNS requests when it came back up due to a zonefile containing a hidden character...took me 10 minutes to find that little bugger!). SM did not return to using the Primary DNS IP and instead served the "600 Failed to connect to the recipient's mail server.  No DNS information was found for the 'gmail.com' domain. Status: 544 5.4.4 Host not found (not in DNS)." response for the 10 minute duration that the Secondary DNS IP was not responding.

So, it does appear that SM will roll over from Primary DNS IP to Secondary DNS IP successfully. However, if the Secondary DNS IP then becomes unreachable it does not appear to retry the Primary DNS IP. 

I'm guessing there is a timed mechanism to determine when it will retry a DNS IP after it has previously failed (since it doesn't seem to do it on a per lookup basis)? Knowing what the retry interval is could allow us to stagger DNS reboots accordingly and avoid having this happen in the future.
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Kyle Kerst Replied
Employee Post
Interesting findings here, thanks! I'm betting you're right about the retry period being expired. I'll work on confirming this here and will then get it reported to development. 
Kyle Kerst
Technical Support Specialist
SmarterTools Inc.
(877) 357-6278
www.smartertools.com
0
Kyle Kerst Replied
Employee Post
I simulated (a couple of different ways) having both my primary and secondary DNS servers go down, but found SmarterMail begins reaching out to the previously down DNS servers on the next send attempt:

[2019.10.14] 16:44:00.990 [A]:hostedsmartermail.com | Time: 11003, Exception: No connection could be established to any of the following name servers: 8.8.8.8:53 (Udp: 512), 8.8.4.4:53 (Udp: 512).
[2019.10.14] 16:44:11.996 [MX]:hostedsmartermail.com | Time: 11004, Exception: No connection could be established to any of the following name servers: 8.8.8.8:53 (Udp: 512), 8.8.4.4:53 (Udp: 512).
[2019.10.14] 16:45:25.516 [MX]:hostedsmartermail.com | Time: 11003, Exception: No connection could be established to any of the following name servers: 8.8.8.8:53 (Udp: 512), 8.8.4.4:53 (Udp: 512).
[2019.10.14] 16:45:25.551 [A]:hostedsmartermail.com | Time: 35, Result: 70.167.205.116
[2019.10.14] 16:45:31.089 [MX]:hostedsmartermail.com | Time: 5536, Result: mail.hostedsmartermail.com
[2019.10.14] 16:45:50.578 [MX]:hostedsmartermail.com | Time: 0, Result: mail.hostedsmartermail.com
[2019.10.14] 16:45:50.579 [A]:hostedsmartermail.com | Time: 0, Result: 70.167.205.116


What version are you running in this environment? I see we did a DNSClient update in b7040, and modified DNS lookups again in 7082 to prevent the settings.json being opened/saved repeatedly, perhaps one of these changed how this responds? Let me know what you think when you get a moment. 
Kyle Kerst
Technical Support Specialist
SmarterTools Inc.
(877) 357-6278
www.smartertools.com

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