Unable to connect via SFTP: Couldn't upload to your SFTP server: Error in the SSH layer

Symptoms

1. Unable to connect via SFTP with an FTP client (for example, Rapid Weaver). The following error is shown:

Couldn't upload to your SFTP server: Error in the SSH layer

2. SSH keys are correct.

3. The following errors could be found in /var/log/messages or /var/log/secure :

shd[26790]: reverse mapping checking getaddrinfo for 203-0-113-2.example.com [203.0.113.2] failed - POSSIBLE BREAK-IN ATTEMPT! 
sshd[26788]: reverse mapping checking getaddrinfo for 203-0-113-2.example.com  [203.0.113.2] failed - POSSIBLE BREAK-IN ATTEMPT! 
sshd[26790]: Accepted publickey for example from 203.0.113.2 port 59806 ssh2: RSA 9d:3a:b6:fc:92:ae:34:2c:7a:22:ba:0d:a7:fc:a0:ce 
sshd[26788]: Accepted publickey for example from 203.0.113.2 port 59805 ssh2: RSA 9d:3a:b6:fc:92:ae:34:2c:7a:22:ba:0d:a7:fc:a0:ce 

Cause

There is no reverse DNS setup for the server hostname that is used.

Resolution

1. Set the UseDNS value to no in the /etc/ssh/sshd_config file:

CONFIG_TEXT: useDNS no

2. Reload the sshd service:

service sshd reload