Solution for slow SSH login in CentOS 6

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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