Hub Transport fails when NIC is not set to register in DNS

Friday, December 22, 2017
In a previous post I wrote that email delivery fails after installing Exchange 2016 CU8. Fellow MCSM and MVP Brian Ricks reminded me of an old bug in Exchange Server that causes Hub Transport services to fail if a server is not configured to register this connection's addresses in DNS.

This is the default setting for all Windows servers, but in some cases, it's not appropriate. For example, Edge Transport servers are not domain-joined and will log event ID 8015 - "The system failed to register host (A or AAAA) resource records (RRs) for network adapter with settings: <NIC details>":

The problem is that the Hub Transport service relies on the NIC being configured to register this connection's address in DNS for it to use the Windows Server settings for name resolution. If you uncheck that box to prevent the DNS Client Events warning, Hub Transport no longer uses the Windows DNS settings and doesn't have any name resolution.

The solution is either to check the box to register DNS (which will fail and cause the warning) or manually configure the Transport Service to use specific DNS settings. You can do that with the following cmdlet on the server experiencing mail delivery issues:
Set-TransportService <edgetransportserver> -ExternalDNSServers <external DNS server> -InternalDNSServers <internal DNS server>
You can configure multiple DNS servers by entering them in comma delimited format. For example:
Set-TransportService mailgate -ExternalDNSServers, -InternalDNSServers,
Once configured (either by selecting the DNS registration checkbox or through the cmdlet) you'll need to restart the Microsoft Exchange Transport Service.
Restart-Service MSExchangeTransportService
I don't believe that the Exchange Transport Service should rely on this checkbox being set, so I've filed a bug on this to have the dependency removed or at least document it.