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Chapter 14. Security

The security chapter is designed to give the user a very basic level of understanding of security within the GNU/Linux operating system. This chapter also has information on the UNIX system style file permissions used on most GNU/Linux machines.

More comprehensive guides can be found at the Linux Documentation Project, such as the Linux Security howto authored by Kevin Fenzi and Dave Wreski.

There are also sites such as Linux Security. If your looking for a program to assist in locking down your operating system you may want to check Bastille Linux that runs on RPM based distributions (Redhat/Mandriva/SuSE).

Changing root's password

This trick works well if you have forgotten your superuser password, type linux single at a LILO/Grub prompt. Then passwd once the system has started and you are at a console.


If you are using grub go to the relevant line (the one with the kernel and various options) then press 'e' for edit and add “single” on to the end of the lines that boot the kernel. Then hit [Enter] and press 'b' (to boot).


If you are using lilo press escape and type “ linux single” and then hit [Enter] to boot.

Security Warning

This is also a basic security hazard if you have others using your computer and security is a concern, you may like to add a password to your LILO or Grub prompt to stop this from being done.


The umask is a value set by the shell. It controls the default permissions of any file created during that shell session. This information is inherited from the shell's parent and is normally set in some configuration file by the root user (in my case /etc/profile).

umask has an unusual way of doing things ...to set the umask you must describe file permissions by saying what will be disabled.

You can do this by doing 777 minus the file permissions you want. Note that umask works with numbers only, for an explanation please see, the Section called File Permissions

For example:

You want the default during a particular shell session to be equivalent to chmod 750 (user has r/w/x, group has r/x and other has no permissions), then the command you would use would be:

umask 027

Some basic Security Tools


Compute an md5 checksum (128-bit) for file “file_name” to verify it's integrity. You normally use the “ md5sum -c” option to check against a given file (often with a “.asc” extention) to check whether the various files are correct, this comes in handy when downloading isos as the checking is automated for you.

Command syntax:

md5sum file_name
mkpasswd -l 10

This command will make a random password of length ten characters. This password generator creates passwords that are designed to be hard to guess. There are similar alternatives to this program scattered around the internet.

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