How to disable reverse DNS lookup in SSH
SSH server is by default configured to perform a reverse DNS lookup of your IP address every time you try to log in to the server. It could improve logging and security but would significantly increase login time.
The login prompt could take a while to appear when reverse DNS lookup is enabled on the server, as the server will perform a reverse DNS lookup on the incoming connection's IP address before presenting the login prompt.
You can choose to enable or disable reverse DNS lookup in SSH by configuring UseDNS directive in the SSH server from the terminal.
UseDNS
Specifies whether sshd(8) should look up the remote host name
and check that the resolved host name for the remote IP address
maps back to the very same IP address. The default is “yes”.
Related: How to speed up SSH authentication
Steps to disable reverse DNS lookup in SSH:
- Launch terminal application.
- Open SSHd configuration file using a text editor.
$ sudo vi /etc/ssh/sshd_config [sudo] password for user:
- Search for UseDNS directive and set the value to no.
UseDNS no
Add the line if it doesn't already exist and remove # at the beginning of the line if the existing line starts with #.
- Reload or restart SSH server service for the changes to take effect.
$ sudo systemctl restart sshd
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