Connection reset by peer: a client that always worked cannot connect

It happens: a client that has always connected to Syncplify Server! without any issue suddenly cannot, and displays a message similar to this one:

FATAL ERROR: Connection reset by peer (could not connect to server)

INFO

Depending on the client software the message varies, but its meaning is always the same: the client cannot connect to the server.

When that happens, the most likely causes are these, in the order in which to check them.

1. The client's address has been banned

The client violated the protocol or its permissions several times (for example it kept trying to upload into a folder where uploads are forbidden, or failed to sign in a few times with the wrong credentials), and the Shield banned its address. In this case, open the Shield page of the Admin UI, find the client's address on the Bans tab, and lift the ban. The Detail column tells you which rule banned it and why. If the address is a monitor, a load balancer or an integration partner that must never be banned, add it to the safe list.

2. The allow list is active and does not include the client

You added at least one address or network to the allow list, and the client is now connecting from an address that is not on it. Keep in mind that the allow list, when it is not empty, is restrictive: nothing outside of it may connect. The same applies to the per user allow list on the Network access tab of the user editor.

3. Client and server cannot agree on the algorithms

The client and the server may have been unable to negotiate a common set of key exchange, encryption, MAC or public key algorithms; this typically follows an update on either side. In this case, configure the client to use the strong algorithms the server offers. Adding weak algorithms to the server to accommodate an old client is never a good idea. Two pages cover the most common instances: Unsupported public key algorithm ssh-rsa and Unexpected message type 30.

If none of these applies, open a support request as described in How to get support from Syncplify, and attach the virtual site's log from the time of the failed connection.