Troubleshooting SFTP errors on Mule ESB

• Enable DEBUG level for package org.mule.transport.sftp in your application log4j configuration file. Refer to the following knowledge article for further guidance.

How to enable verbose logging for FTP, SFTP and FTPS module

• If using variables to setup the SFTP, consider to log the variables values on Logger component to allow you to inspect the effective value used

<logger message="Log the path if it is stored in a variable so it can be checked --  #[flowVars.path]" level="INFO" doc:name="Logger"/>
<sftp:outbound-endpoint exchange-pattern="one-way" connector-ref="SFTP" host="0.0.0.0" port="22" path="#[flowVars.path]" user="mule" password="mulesoft" responseTimeout="10000" doc:name="SFTP"/>

• Use separate SFTP client to test same operation. Example of SFTP clients would be : FileZilla, WinSCP, command line sftp clients etc. 
• Enable verbose logging on SFTP server side to inspect exact attempt that is failing. 
• Ensure there is connectivity between machines involved (Mule ESB host and SFTP Server host) 

NOTES

List of errors related to issues with SFTP:

java.io.IOException: Error 'No such file' occurred when trying to CDW to '//Users/mule/nonexist'
java.io.IOException: Error during login to mulesoft@0.0.0.0: Auth fail
java.io.IOException: Error during login to mulesoft@0.0.0.0: java.net.ConnectException: Connection refused (Connection refused)
 

SFTP fails to login from time to time with the error verify: false
FTP/SFTP Login error : Error during login to null@null