Reducing SSH Login Delay

Setting UseDNS no in the VM’s /etc/ssh/sshd_config or /etc/sshd_config has made it much faster to ssh into VMs.

centos@128.31.24.158 is a m1.small VM in Kaizen with a public IP.

With UseDNS yes (the default in Ubuntu 16.04), ssh -A centos@128.31.24.158 'exit' (ssh into the VM and then immediately exit) takes 5.8 seconds.

With UseDNS no, ssh -A centos@128.31.24.158 'exit' takes 248 ms. So changing this option shaves 5.5 seconds off the time it takes me to ssh into a VM.

More info about the UseDNS option to see if you actually need it enabled:

• What’s the point of usedns option?
• OpenSSH Bug Report

To configure:

1. sudo vim /etc/ssh/sshd_config or sudo vim /etc/sshd_config
2. Change UseDNS yes to UseDNS no or add UseDNS no to the file, then quit vim
3. Restart ssh sudo service sshd restart