Answer

Issue

You may come across the following error message in the Aspera transfer log:

SSH connection failure: Server unexpectedly closed network connection

One of the most common causes of this error is the MaxStartups setting in the server's sshd_config file:

• Linux: /etc/ssh/sshd_config
• Windows: C:\Program Files[(x86)]\Aspera\[Aspera Product]\etc\sshd_config

Linux example:

#MaxStartups 10

Windows example:

#MaxStartups 10:30:60

(The # in the sshd_config signifies the default).

These limit the number of unauthenticated SSH connections attempts to 10 (in the case of Windows they start failing 30% of the time with 10 increasing to 100% at 60). In high concurrency environments these values may be too low (especially if SSH is using TCP port 22 instead of the recommended TCP port 33001).

Solution

We recommend changing theMaxStartups entry to 100:

MaxStartups 100

This would be the recommended setting on both Linux and Windows. (Notice no # sign). Some customers in extremely high concurrency environments have increased this value to 500.

After this change the SSHD process must be restarted for it to take effect:

Linux

Linux (RedHat)
sudo service sshd restart

Linux (Debian)
sudo /etc/init.d/ssh restart

Windows

Use the steps described in this article (Restarting SSH on Windows).