At my job, we store student data in a program called PowerSchool. One of PowerSchool's features is AutoSend. AutoSend can make a text file full of data and send it to another computer over SFTP. This is very useful, as it allows student data to be entered once (in PowerSchool) but appear in many systems.
Recently, I replaced and updated the FreeBSD system that runs our SFTP server. After the upgrade, PowerSchool couldn't send data to the FreeBSD SFTP server. Other SFTP programs, such as FileZilla, were able. This issue only seemed to affect PowerSchool's AutoSend. I couldn't figure it out at first. The FreeBSD community couldn't. Our tech support couldn't. They escalated the issue over and over until it reached "engineering" and I never heard back from engineering.
After working on this on-and-off through the summer and eventually found a way to make it work. Since it took me so long and confused so many other people, I wanted to put this out there for others to benefit.
The short version is this: I had to change the default settings of the SSH server.
This may sound strange to some, but SFTP on many Unix systems -- such as FreeBSD and Linux -- is based on OpenSSH. OpenSSH is a system that is all about making encrypted connections and preventing the bad guys from seeing what you're transmitting. However, its default settings moved to a more strict standard at some point and this was why I saw a difference between the old FreeBSD server and the new one.
The setting in question deals with how OpenSSH handles authentication. In plainer language, it's all about the login process. It seems that "keyboard-interactive" is the mode used by programs that interact with humans, such as FileZilla. However, "password authentication" is the kind used by automated systems, such as PowerSchool. Personally, I found the name "password authentication" to be confusing for a while, but that is what it is called.
So I edited /etc/ssh/sshd_config to allow the password authentication system. However, the OpenSSH developers disabled it for a reason. They know more about computer security than I do, so I struck a compromise. I changed the settings so that password authentication was valid if and only if the connection came from my PowerSchool server.
If you need to do this, just find your system's copy of sshd_config and add the following lines to the bottom of the file. If you have any lines beginning with "Match", this should go immediately above those.
# Turn off PasswordAuthentication in general, i.e. without a Match statement to change it.
# Allow only the IP address of the PowerSchool server to use the
# PasswordAuthentication directive. Note the "PasswordAuthentication no"
# above all Match statements. That is part of this configuration, but
# must be before any Match blocks.
Match Address 999.999.999.999/32
You should change "999.999.999.999" to the IP address of your PowerSchool server. Everything else should be fine exactly as I wrote it above.