sendmail, 4th Edition

MX to CNAME Is Illegal

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 or AAAA 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. Because there is only a single MX record, sendmail considers mailhub to be official. Next, mailhub is looked up to find an A or AAAA record (IP address), but instead a CNAME (nfsmast) is returned. Now, sendmail must look up the CNAME nfsmast to find its A or AAAA record. But again a CNAME is returned instead. So, sendmail must again look for an A or AAAA record (this time with hostB). Finally, sendmail succeeds by finding the A record for hostB, but only after far too many lookups.^[155]

The correct way to form the preceding 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 records.

^{[155] *} Most of this happens inside the gethostbyname(3) or getipnodebyname(3) C-library routine.