Currently I encounted an issue of the root account: If I delete the root account in QNX OS and then use a telnet client 
(e.g. Putty) to connect to it, the connection will fail with the error “Connection is closed by the remote host”.
But after I added a new user which is named “root” but with any valid value given to userID  and groupID, the telnet 
then can connect to the QNX OS successfully. 
I don’t know why and could anyone help to explain it, thank you very much. 
BTW, my QNX version is 6.5.0.
 

Below is the detailed steps to repeat the issue:
1.       Prepare for the test environment: a QNX OS which can be telneted successfully by telnet client, here the client
 is PuTTY software. Note that the telnet client and server are on different PC.
2.       Login the QNX OS. 
3.       Create a new user manually by modifying the password files  /etc/passwd, /etc/group and /etc/shadow (e.g. add a
 line in the passwd file like this “wbUser1:x:100:100:wubenUser 1:/home/wbUser1:/bin/sh”) 
4.       Use the chmod command to  change the permission of file passwd, group and shadow to 777 ( this will allow other
 users to modify the password files after the root user is deleted) 
5.       Use PuTTY to telnet the QNX, it is successful.
6.       Delete the “root” user in QNX by modifying the three files passwd, group and shadow, and save when completed
7.       Use PuTTY to telnet again to the QNX, the connection is failed with error message “Connection is closed by 
remote host.”
8.       Then in QNX OS, by local, added a new user named “root”, but with groupID and userID given any value, here I 
added a line like this: “root:x:104:100:wubenRoot-root:/home/wbRoot:/bin/sh”
9.       Use PuTTY to telnet again to the QNX, the connection is successful and the telnet window prompts to input 
username and password.
10.   Remove the “root” user (later added root) again and then use PuTTY to connect, connection failed again.