If SSH login is very slow in CentOS 6 it’s because they have added another security feature, the SSH server will do a reverse DNS again for any incoming connection, this will delay connection up to 13 seconds, to avoid this:

 

  1. Access the server
  2. Edit the ssh config
    nano /etc/ssh/sshd_config
  3. Look for use dns and change it to no
    Ctrl +w, usedns, enter

    Change it from

    #useDNS yes

    to

    useDNS no

  4. Save and close
    Ctrl + X, Y, enter
  5. Restart the sshd service

 

Also

 

When logging in using ssh -vvv user@hostname the debug output shows the delay happens at these lines:

 

debug3: authmethod_is_enabled gssapi-with-mic

debug1: Next authentication method: gssapi-with-mic

debug1: An invalid name was supplied

Cannot determine realm for numeric host address

 

debug1: An invalid name was supplied

Cannot determine realm for numeric host address

 

debug1: An invalid name was supplied

 

The solution appears to be disabling the GSS API authentication method on the SSH server.

 

To disable GSS API, first open/etc/ssh/sshd_config in your favorite editor

vim /etc/ssh/sshd_config

Then find the line that says

GSSAPIAuthentication yes

Change it to

GSSAPIAuthentication no

and save the file. Finally, restart the sshd service.

service sshd restart

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