ISA & time synchronization (external)

Anyone configured ISA to let SNTP traffic thru so the domain controller
can synchronize its time with authoritative time servers outside?  I
used a Protocol Rule that to allow "send receive" UDP port 27 to one of
the US Navy Observatory time servers but it doesn't work.  I know that
Q323621 says to configure packet filters for SNTP access, but I find
those less safe than Protocol Rules.  Is there a way to bind the W32Time
service so it only sends & receives packets on the internal NIC? Thx!
A.-

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