SSH server is by default configured to perform reverse DNS lookup of your IP address every time you try to log in to the server. It could slightly improve logging and security, but would significantly increase login time.

How to speed up SSH authentication

Basically the login prompt could take a while to appear as the server will perform reverse DNS lookup on the incoming IP address prior to presenting the login prompt.

You can fix this issue by disabling the UseDNS directive on 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”.

Steps to disable reverse DNS lookup in SSH:

  1. Launch terminal application.
  2. Open SSHd configuration file using a text editor. 
    $ sudo vi /etc/ssh/sshd_config
[sudo] password for user:
  3. Search for UseDNS directive and set the value to no. 
    UseDNS no

    Add the line if it doesn't already exist and remove the # at the beginning of the line if existing line starts with #.

  4. Reload or restart SSH server service. 
    $ sudo systemctl restart sshd

    How to start, restart and stop SSH service

Guide compatibility:

Operating System
Ubuntu Linux
Debian Linux
Red Hat Enterprise Linux
Fedora Linux
CentOS Linux
openSUSE Linux
SUSE Linux Enterprise Server
FreeBSD
OpenBSD
NetBSD
macOS
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