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[datums]

Hello


Does anyone know why you would not have mx records for a cname.

Example

cname.domain.com

cname.domain.com is an alias for domain.com
domain.com mail is handled by 10 mail4.mailserver.com
domain.com mail is handled by 20 mail9.mailserver.com


If so, can you reference an RFC ?


Thanks

[wKkaY]

quote www.dnsreport.com:

If an MX record query returns a CNAME, extra processing is required, and some mail servers may not be able to handle it.

CNAMEs are prohibited in MX records, according to RFC974, RFC1034 3.6.2, RFC1912 2.4, and RFC2181 10.3.

[datums]

Wkkay this I know.

My question is having mx records for a cname.

you have abc.domain.com <-- is a cname

abc.domain.com has mail.whatever.com as their mx

[WB Tech]

MX records just aren't compatible with CNAME records. Ü

[dan_erat]

datums: No, you can't do that.

RFC 1034 3.6.2:

If a CNAME RR is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different. This rule also insures that a cached CNAME can be used without checking with an authoritative server for other RR types.

RFC 1912 2.4:

A CNAME record is not allowed to coexist with any other data. In other words, if suzy.podunk.xx is an alias for sue.podunk.xx, you can't also have an MX record for suzy.podunk.edu, or an A record, or even a TXT record.

[dan_erat]

Well, you can do it, but you shouldn't. Don't use CNAMEs unless you have a really good reason to do so.

[datums]

RFC 1912 2.4:

This is just saying I can't have similar entry, like
A record myalias.domain.com
Cname record myalias.domian.com

RFC 1034 3.6.2
Reading more on this one.

[dan_erat]

datums: Re 1912, no it's not. It says that the CNAME can't coexist "with any other data." They even mention MX records by name.

[datums]

dan_erat, You are 100% correct. I was reading it all wrong.
That RFC is exactly what I needed. My apologize.

Thank You

[Brightadmin]

Hi,

MX to CNAME Causes Extra Lookups

The sendmail program is frequently more forgiving than other MTAs because it accepts an MX record that points to a CNAME record. The presumption is that, eventually, the CNAME will correctly point to an A record. But beware, this kind of indirection can cost additional DNS lookups. Consider this example of an exceptionally bad setup:


hostA IN MX 10 mailhub
mailhub IN CNAME nfsmast
nfsmast IN CNAME hostB
hostB IN A 123.45.67.89

First, sendmail looks up hostA and gets an MX record pointing to mailhub. Since there is only a single MX record, sendmail considers mailhub to be official. Next, mailhub is looked up to find an A record (IP address), but instead a CNAME (nfsmast) is returned. Now sendmail must look up the CNAME nfsmast to find its A record. But again a CNAME is returned instead of an A record. So sendmail must again look for an A record (this time with hostB). Finally, sendmail succeeds by finding the A record for hostB, but only after a few too many lookups.

The correct way to form the above DNS file entries is as follows:


hostA IN MX 10 hostB
mailhub IN CNAME hostB
nfsmast IN CNAME hostB
hostB IN A 123.45.67.89

In general, try to construct DNS records in such a way that the fewest lookups are required to resolve any A or MX records

More information About MX record then Look this
MX to CNAME Causes Extra Lookups
The sendmail program is frequently more forgiving than other MTAs because it accepts an MX record that points to a CNAME record. The presumption is that, eventually, the CNAME will correctly point to an A record. But beware, this kind of indirection can cost additional DNS lookups. Consider this example of an exceptionally bad setup:


hostA IN MX 10 mailhub
mailhub IN CNAME nfsmast
nfsmast IN CNAME hostB
hostB IN A 123.45.67.89

First, sendmail looks up hostA and gets an MX record pointing to mailhub. Since there is only a single MX record, sendmail considers mailhub to be official. Next, mailhub is looked up to find an A record (IP address), but instead a CNAME (nfsmast) is returned. Now sendmail must look up the CNAME nfsmast to find its A record. But again a CNAME is returned instead of an A record. So sendmail must again look for an A record (this time with hostB). Finally, sendmail succeeds by finding the A record for hostB, but only after a few too many lookups.

The correct way to form the above DNS file entries is as follows:


hostA IN MX 10 hostB
mailhub IN CNAME hostB
nfsmast IN CNAME hostB
hostB IN A 123.45.67.89

For more Information.

http://www.busan.edu/~nic/networking...il/ch21_03.htm

Bright
24/7 Technical Support