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 CNAMEthere.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 ...
Get sendmail, 4th Edition now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.