UDP Port allocation

[Alessandro Quagliaroli]

Hello all,
I'd like to know how the operating system works when an udp server socket is created leaving the O.S. to assign the port to use.
Does it look in /etc/services first?
If so also for ports in the unregistered range?

Thanks
Alessandro

[A. Clay Stephenson · Accepted solution]

Entries in the services file or (NIS) map mean nothing other than port to service name mapping and nothing is "reserved" by such an entry.

Essentially you need to execute:
ndd -get /dev/udp udp_largest_anon_port
ndd -get /dev/udp udp_smallest_anon_port

and the first free port in that range wins. NOTE: Not all releases display the udp_smallest_anon_port.

[Matti_Kurkela]

As Clay said.

/etc/services is used only if your application uses either getservbyname() or getservbyport() function.

For a good overview how various operating systems (including HP-UX) allocate ports when nothing is specified, see:
http://www.ncftp.com/ncftpd/doc/misc/ephemeral_ports.html

MK

[Alessandro Quagliaroli]

Thanks both.
AQ

[Alessandro Quagliaroli]

The info provided cleared my doubts.