Azure Data Factory cannot connect to Serv-U SFTP after upgrading Serv-U ("Client not connected")

After Serv-U is upgraded, ADF pipelines that use the SFTP connector fail, including pipelines that run through a self-hosted Integration Runtime. Symptoms:

• ADF activities such as Get Metadata, Copy Data or Lookup fail when they connect to the Serv-U SFTP server.

• The ADF error contains Renci.SshNet.Common.SshConnectionException with the message Client not connected.

• Other SFTP clients, such as FileZilla, WinSCP or newer SSH.NET-based applications, can still connect and log in with the same accounts.

• In the Serv-U domain log, connections from the ADF Integration Runtime IP open and close in the same second, with no login and no SFTP commands.

• ADF pipelines connected normally before the upgrade.

From Serv-U 15.5.1, the SSH-RSA (SHA-1) key algorithm is deprecated and disabled by default for SSH host key algorithms and user public key algorithms.

When Serv-U is upgraded from an earlier version, or SSH-RSA is otherwise disabled, clients that can only use SSH-RSA with the server's RSA host key can no longer agree on a host key algorithm with Serv-U. Serv-U closes the connection right after connecting, and ADF reports Client not connected.

Newer SFTP clients such as FileZilla, WinSCP, paramiko and current SSH.NET versions still work because they support the newer RSA signatures (rsa-sha2-256/rsa-sha2-512) or other algorithms. This is why the issue appears to affect only ADF.

1. Go to Limits & Settings > Encryption > SSH Key Algorithms, at the level (Global or Domain) that applies to the affected domain.

2. Enable SSH-RSA.

3. Save the changes.