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I am running OpenSSH via Cygwin on a Windows 2008 server. This system acts as a file server for a pool of 30 build systems. At build time, each build system copies files as necessary from the file server, builds, and sends the results back to the file server.

When I start all of the builds in parallel, a number of them conk out due to scp dropping the connection (with the cryptic Lost connection error message). To make matters even more convoluted, each build system connects to the server using the exact same public/private key pair.

Is there a limit to how many boxes can connect to an OpenSSH server at the same time? From the sshd_config man page, I see that there's a MaxSessions parameter:

MaxSessions

Specifies the maximum number of open sessions permitted per network connection. The default is 10.

Is the MaxSessions limit what I'm running into here? If so, what's a reasonable alternative value (is 50 too high)?

Or is it perhaps some internal limitation with the keys in use? What exactly constitutes a "network connection" in terms of this setting; the socket being used or the physical port in play? Any help is appreciated.

Update 1: When I limit my traffic to this file server (letting 5 build systems through every 5 seconds), all of them succeed. So I seem to be hitting a limit somewhere. Perhaps Windows 2008 has a connection limit of some kind?

  • Each client opens its own connection, so it can't be that limit. Can you watch that wireshark? Can you wrap the scp into for-loops with a sleep after each try? – ott-- May 15 '15 at 21:20

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