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What's the best way to secure /etc/passwd? I was reading I should remove accounts within passwd I don't need e.g. news, ftp, etc. Now what's the best way to remove it? Should I just comment the line e.g. "#" or remove it completely? Also is it best to backup this file and if so where?
I don't recomend you comment any lines in /etc/passwd. Usually, password are really being stored in /etc/shadow, since this is a more secure way to store them. They are also encrypted, so you don't need to worry.
Nevertheles, it's good to control the access to /etc/shadow since brute force rippers might try to retrieve the shadow file and try to decrypt the passwords in there.
Those accounts (mail, news, ftp, etc.) are jailed, that means they're only given permission to execute some services, programs or to access some ports. They are used by parent processes which uses those 'services'. You don't need to delete them. If you want to do so (it might affect some system services, if you use them) the best way is to issue an "rmuser mail" (i.e.) command as root.
"Jail" was not the term I must had used. I think it's ok to say that they're naturally harmless. Again, you should remove those who correspond to services you don't use. In example, I have removed "mail", "news", "ftp", since I don't run any servers of those.