I had seen forum articles related this but not found any specific answer.

In my case, it occurs intermittently when trying to connect using putty's command line client 'plink.exe' to a linux machine using ssh. For some testing purposes, a linux server is setup with 8K virtual ip addresses and the testing scripts will connect to these ip addresses through ssh using plink.exe. There will be concurrently 25 ssh sessions launched to the server. While it works for sometimes, it throws 'server unexpectedly closed network connection' error intermittently. If the same ip address is tried again manually, it works. While the ssh server is accepting connections, this error is thrown sometimes and unable to find out the reason from the logs. It is not that the ssh connection is completely blocked. It gives this error message while connecting to some of the virtual ip addresses but works some times.

The hosts.deny, sshd_config are tuned to allow the connections, allow more sessions etc.

Any inputs on resolving this will be helpful.

-- Thanks

You need more details.

Please let me know what details you want.

The linux server is running with fedora core 13 with standard sshd configuration. 8K virtual ip addresses are created on eth0. At any point of time, there will 50 concurrent processes trying to login to any of the 8K ip addresses using ssh. The ssh client used is putty's plink.exe from windows. There is no issue with the network connectivity. For the same ip address, it works for sometime and gives the said error some other time.