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Asked: 2026-05-22T17:06:01+00:00

i am currently testing a server with an automatic test client that simulates a

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i am currently testing a server with an automatic test client that simulates a large number of users. Both the server and the client are written in Java. The client opens a tcp/ip connection for every user. Both the server and client run on Ubuntu linux, client runs on 11.04 and server on 10.04.

The testing went good up till 27000 concurrently open connections, after that i decided to jump to 36000 (the servers and clients resources weren’t really all that used up at 27000 so i decided to make a slightly bigger jump). When i tried running the test for 36k i got the following exception on the client side:

  • java.net.BindException: cannot assign requested address

As far as i know at 36k i should still have free ports since not much else is running on either machine and tcp limits the port number at 2^16 which is 65536. Now since it is linux i also set the number of open files for the user to 100k with ulimit -n 100000.
But i am still getting the same exception.

I’m wondering what else could be a possible cause for the mentioned exception, or does linux in some other way limit the number of outgoing connections ?

Thanks in advance,

Danijel

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1 Answer

  1. Editorial Team

    By default, Linux picks dynamically assigned ports from the range 32768..61000. The others are available for static assignment, if you bind to a specific port number. The range can be changed if you want more of the ports to be available for dynamic assignment, but just be careful that you do not include ports that are used for specific services that you need (e.g. 6000 for X11). Also you should not allow ports < 1024 to be dynamically assigned since they are privileged. To check or change the range:

    $ cat /proc/sys/net/ipv4/ip_local_port_range
32768   61000

# echo "16384 65535" > /proc/sys/net/ipv4/ip_local_port_range
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