Incorrect MX record orderering
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Key: JAMES-913
URL: https://issues.apache.org/jira/browse/JAMES-913
Project: JAMES Server
Issue Type: Bug
Components: DNSServer
Affects Versions: 2.3.1
Reporter: Amichai Rothman
Priority: Minor
The SMTP RFC states that given multiple MX record, they must be ordered such
that lower priority records come first, and equal priority records are
randomized for load balancing. The JAMES implementation uses Arrays.sort()
along with an MXRecordComparator, which compares the priorities, and 'flips a
coin' if they are equal.
This precariously breaks the Comparator contract, which can theoretically cause
a sort implementation to loop forever, for example if it is implemented using a
trivial bubble sort which stop when no more swaps are performed. In practice,
it causes a different bug: the Sun implementation of Arrays.sort() uses
insertion sort for N <= 7. So for example, if you have 5 MX records with equal
priority, the last element is compared only in the last iteration of the loop,
with a 1/2 probability of remaining last. On the other hand, it has a 1/16
probability of being swapped 4 times to become first on the list.
So while there is randomness involved, equal-priority records will tend to stay
near their original (relative) position, and not be shuffled with the uniform
probability which the RFC implied for load balancing...
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