Make sure you have the line
mdns off
in /etc/host.conf on your incoming SMTP servers.
One of my colleagues in our network engineering team discovered today that
ppswitch was spewing multicast packets, much to our surprise. It turns out
that recent versions of glibc have quietly added support for multicast DNS
to the resolver. Multicast DNS is part of Apple's zeroconf networking
system (aka Bonjour, previously known as Rendezvous), and it takes over
responsibility for host names ending in .local. See it in action by typing
strace ping foo.local and observe it sending a DNS query to the class D
multicast address 224.0.0.251.
A significant number of the DNS lookups that our MXs do (10,000 per day)
are names ending in .local which trigger mdns lookups. These names (mostly
HELO domains) do not come from machines named via zeroconf: they are
mostly Small Business Server installations which have followed Microsoft's
recommendations for choosing a domain name.
See http://www.zeroconf.org/ and http://support.microsoft.com/kb/296250
Tony.
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