DNS DNAME record results in failed message delivery

Products

• Messaging Gateway

Issue/Introduction

 When attempting to deliver messages outbound to the internet, some messages becomes stuck in the delivery queue with a "no MX or A for domain" message even though a lookup of the domain's MX records returns valid mx hosts. Looking up the DNS records shows something similar to the following:

domain.org.        1951  IN    DNAME domain.com.
mx.domain.org.     1951  IN    CNAME mx.domain.com.
mx.domain.com.     1951  IN    MX    10 smg3.domain.com.
mx.domain.com.     1951  IN    MX    10 smg1.domain.com.
mx.domain.com.     1951  IN    MX    10 smg2.domain.com.

Queue message:
454 4.4.4 [internal] no MX or A for domain

mailog

2014 Jul 29 09:42:24 PDT (debug) ecelerity: [8666] LT-00284 inserted 80/00-08666-0FEC7D35 into lookuptable (calling dns_get_MXs)
2014 Jul 29 09:42:24 PDT (debug) ecelerity: [8666] ARES_00320 process dns response 17465 [mx.domain.org]
2014 Jul 29 09:42:24 PDT (info) ecelerity: [8666] smsrouting: DNS reply to MX lookup of \'mx.domain.org\': wrong answer name \'domain.org\'; unroutable messages may be stuck in domain container \'#sms#00000005\'

Environment

Messaging Gateway 10.6

Cause

 Some DNS servers return the DNAME record as part of the response to the MX record lookup. The SMG mail server does not understand the DNAME record and so does not process the DNS response.

Resolution

Some DNS servers do not respond with the DNAME record as part of the MX record response. SMG systems which use Microsoft Exchange as their primary DNS server do not appear to be affected by this issue as MS Exchange leaves the DNAME record out of the DNS response. Use of the DNAME record type in DNS is very limited so this issue is unlikely to much effect on outbound mail delivery.

This issue has been addressed with the Messaging Gateway 10.6.2 release.

Please see the Messaging Gateway Administration Guide for detailed instructions on how to update the SMG software.