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I realize similar questions were asked/answered on this, but I have a subtle detail to which I can not find answers anywhere. Let's exemplify with the following DNS entries:

foo.example.com  A     1.1.1.1
bar.example.com  A     1.1.1.2
wee.example.com  CNAME foo.example.com

foo.example.com  MX    foo.example.com.s9a1.psmtp.com
bar.example.com  MX    bar.example.com.s9a1.psmtp.com 

Note the last line. My institution has that kind of MX records, where for each MX record the value has the label prepended. Question 1 is, what is the motivation for this (why not just s9a1.psmtp.com)? Question 2 is more subtle and follows..

My understanding is that an MX record should not contain any aliases for neither label nor the value, i.e. the following would be bad practice:

wee.example.com  MX    wee.example.com.s9a1.psmtp.com

Then, how should the RRs look for the alias wee.example.com?

Thanks!

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2 Answers

up vote 6 down vote accepted

Then, how should an MX record should be for the alias wee.example.com

There would not/should not be an MX record for wee.example.com.

Mail to user@wee.example.com would get handled by the MX record for foo.example.com. A CNAME record says, "for any requests about this resource record, reference this other resource record instead".

Note the last line. My institution has that kind of MX records, where for each MX record the value has the label prepended. Question 1 is, what is the motivation for this (why not just s9a1.psmtp.com)?

For a company that is handling MX services for a number of organizations, having domain-specific DNS entries means that you can relocate MX services for a particular domain without affecting other domains. This could be used for load balancing, tiered services, etc.

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Yeah but, can you answer this in the context of first question? –  sly Jan 14 '11 at 18:44
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You caught me in the middle of an update, which might help, but I don't think the context matters at all. In all cases, if you have a CNAME record, MX lookups will use the target of the CNAME. –  larsks Jan 14 '11 at 18:45
    
thanks for the response. It makes sense, I'll accept this answer but let's give a while for other answers to come in for Q1. –  sly Jan 14 '11 at 20:02
    
Any domain sending email should have handle its own email or have an MX. Different MX records can all point to the same IP address as servers do not check the name the destination server. –  BillThor Jan 14 '11 at 20:51
    
Thank, exactly as you say. It works without any MX for alias wee –  sly Jan 16 '11 at 2:34
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  1. The reason to prepend the label would presumable be a requirement for s9a1.psmtp.com. I'm guessing that rather than receive all the mail at one point and then split it depending on the destination domain they are handling each domain separately by treating them as subdomains.

  2. If there is an A record on the s9a1.psmtp.com system for each of those MX targets then all MX record requirements and recommendations are met.

Your example of

wee.example.com MX wee.example.com.s9a1.psmtp.com

may be or may not be valid depending on whether or not s9a1.psmtp.com has the appropriate DNS record. Remember, this would be completely independent of the CNAME entry at example.com.

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