First of all, I thought it might be a good idea to explain what NetExtender and OS X El Capitan are...
NetExtender is an SSL VPN client for Windows, Mac, or Linux users. NetExtender allows remote clients seamless access to resources on your local network.
OS X El Capitan is the twelfth major release of OS X, Apple Inc.'s desktop and server operating system for Macintosh.
The issue at hand is that if you are trying to install NetExtender through the app store/ the internet on OS X El Capitan, you might get stuck and be in a position where you are unable to connect to the VPN server, displaying messages regarding disconnecting etc. If you check the logs, it will display this...
However, if you try to run the following command (Which is displayed within the error log), then this will also fail.
In the update of El Capitan, there is a brand new feature called ‘System integrity Protection’ in OS X El Capitan. This ultimately restricts the root account to perform certain actions.
SIP is a security technology in OS X El Capitan that is designed to prevent malicious software from being able to edit software, folders and files on your Mac. Previous to this update, the root user had no permissions, meaning it was able to access any file/folder/app on that machine. Software progressed and then added the feature where it requires an administrator name and password to help restrict access to folders and files used by different users.
The Steps to Solve ‘disconnecting’ with NetExtender on OS X El Capitan
Please first make sure that NetExtender has been installed prior to the steps below!
1. Restart your Mac in recovery mode (hold CMD+R during restart)
One other way to do this would be Clicking on the Apple logo at the top left of the screen > Select ‘Restart’ > Immediately hold down the command key (⌘) and R key. This will then display the following box. Here, click Utilities.
2. Go to Utilities menu and open Terminal and type in the command ‘csrutil disable’
3. Once done, restart the Mac again and run the command ‘sudo chmod u+s /usr/sbin/pppd’ and then install NetExtender again. Try connecting to VPN and confirm it works this time.
4. Restart Mac again in recovery mode and run command ‘csrutil enable’