Ubuntu : max connections on port limited to 4036

Hi,

We are seeing a strange behavior when trying to make connections to a port on one of the Ubuntu boxes. We are on Ubuntu - 12.04 LTS (we are already working on upgrades , but until that happens we are on this version).

We have a messaging server running on that port .

Once the connections on the port reaches a count of 4036 connections - any new connections attempts see one of the following two behaviors

(1) An existing connection is dropped and the new connection succeeds
(OR)
(2) New connection hangs and timeout.

I have checked the following and cannot find any limitations that may prevent the port from making more than 4036 connections.

(1) ulimit setting - soft and hard both set 65 K
(2) looked for the max open files allowed on the OS .-->
cat /proc/sys/fs/file-max -- 601850
(3) Looked at the count of open file descriptors using --> lsof |wc -l ( was about 2k)
(4) sysctl -A -- to check if there is any limit that may be imposed here.


Can someone please point in the direction that can help me find where this limit may be coming from and how I can up this limit ?

Thanks

if i take the ubuntu out of the equation, i get a lot of hits:
https://www.startpage.com/do/dsearch...imited+to+4036
i don't know what you already tried and what not. please be precise in sharing your troubleshootings.
i suspect most actions require a server restart at the very least, many also a reboot (kernel stuff).

Thanks for your response. After hours of google search - this finally helped us.

https://blog.james-carr.org/crouchin...g-891723a5eca0