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OpenFTPD Manual -- Portrange for dataports
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There are 3 ways OpenFTPD may allocate dataports for you as of now (theese are
used for passive transfers etc and may have to have special values in order
for us to handle firewalls etc).

1. Let the OS determine what port to use.
   This is the default and will be used unless otherwise specified.

2. Use the old cons option PORTRANGE=<lport>:<hport>
   If you define this at compile time OpenFTPD will always use these
   ports nomatter what.

3. Use new cons option PORTRANGE=<num ports>
   If you dont define this cons option you will default to the value
   PORTRANGE=100 (which should be more than sufficient for users who need this
   functionality. Read on to understund how it is different from number 2 and
   how to activate it.

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The difference between number 2 (OLD) and 3 (NEW) is following:
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NEW -- tries to bind socket with SO_REUSEADDR. This means that for a small
       site with a maximum of 10 users a portrange of 10 ports will be
       sufficient. This is not true otherwise due to the fact that a port is
       supposed to timeout for X seconds before it can be reused just incase
       any more data would be recieved on it.

OLD -- does not do like this and will hence need more ports if site is busy.

NEW -- we can define a range wich looks like 5010-5020,5555,5558,5590-6000.

OLD -- the range cannot have holes like above.

NEW -- The range may be reconfigured via the registry on the fly for as long
       as the range doesnt hold more than <num ports> ports and the port
       string isnt longer than 254 characters. (Setting is transfered to ftps
       upon client connection).

OLD -- Range change needs a recompile and restart of ftpd.

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How to use
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If you compiled OpenFTPD with no special options you may define up to 100
different ports via the registry key dataports. if you need more you will have
to recompile.

dataports	5010-5020,5040-5050,5555 (this is 23 different ports)

If you specify to many ports the overflowing ones will not be used.
