sendmail, 4th Edition

DontExpandCnames

Prevent CNAME expansion V8.7 and later

Ordinarily, the $[ and $] operators (Canonicalize Hostname: $[ and $] on page 668) cause the enclosed hostname to be looked up with DNS^[382] and replaced with the canonical address for that host. The canonical address is the A or AAAA DNS record. For example, consider these DNS records:

here.us.edu.    IN     A     123.45.67.89
ftp.us.edu.     IN     CNAME here.us.edu.

But if the address ftp.us.edu is fed to the $[ and $] operators in the RHS of a rule:

R $*      $[ $1 $]

the rewritten result of passing ftp.us.edu as $1 will be the name here.us.edu. This behavior was correct under RFC822 and RFC1123, and with the publication of RFC2821 and RFC2822 this change is now officially correct.

Sometimes it is important for the CNAME to appear in email headers as the canonical name. One example might be that of an FTP service moving from one machine to another during a transition phase. In that instance, outgoing mail should appear to be from ftp.us.edu because the records will change after the move, and the ability to reply to such mail must be maintained:

here.us.edu.    IN     A     123.45.67.89     ← retired and gone
ftp.us.edu.     IN     CNAME there.us.edu.
there.us.edu.   IN     A     123.45.67.90

Another possibility might be that of a mobile host (a workstation that plugs into different networks and thus has different A records over time):

mobile.us.edu.    IN     CNAME monday.dc.gov.
monday.dc.gov.    IN     A     12.34.56.78
tuesday.foo.com.  IN     A     23.45.67.89

Whenever this workstation is plugged in, its CNAME ...