How to Hide OpenSSH Ubuntu version from Nmap and other scanners

In Ubuntu or Debian, a default OpenSSH server will display OpenSSH version alongside with Ubuntu/Debian distribution banner:

$ telnet repeater.my 172.16.91.20 22 
Trying 172.16.91.20...
Connected to 172.16.91.20.
Escape character is '^]'.
SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2

You can hide the identifying part of Ubuntu-2ubuntu2 from the server banner by editing /etc/ssh/sshd_config file, and adding “DebianBanner no” either at the end of the file, or just under “Port 22” configuration in “/etc/ssh/sshd_config”

#/etc/ssh/sshd_config 
# What ports, IPs and protocols we listen for

Port 22
DebianBanner no

Save and restart OpenSSH server by typing

sudo service ssh restart

Now the response will just be:

Trying 172.16.91.20...
Connected to 172.16.91.20.
Escape character is '^]'.
SSH-2.0-OpenSSH_6.6.1p1

Happy trying!

Further Reading: Ubuntu Server Administrator Reference