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Linux for dummies - Klimas M.

Klimas M. Linux for dummies - Wiley publishing , 2002. - 169 p.
Download (direct link): linuxfordummies2002.pdf
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Information about a user account is kept in plain-text files: /etc/passwd and /etc/shadow.
The file /etc/passwd contains the "world-readable" information about all accounts on my computer Each line in this file contains information about one account. Each line has 7 colon-delimited fields (this means 8 entries separated by colons): login name, the letter "x", the numerical user ID, the numerical primary group ID for the user, a comment field (for example, the full name of the user), the user's $HOME directory, the name of the shell (meaning the program that is run at login).
The balance of information about accounts on my computer is stored in the file /etc/shadow. This file is more secure because normally only root can read it. In this file, each line describes "shadow" information about one account, and has 9 colon-delimited fields: login name, encrypted password, days since Jan 1 1970 that password was last changed, days before password may be changed, number of days after which the password must be changed, number of days before password expiration to warn the user, number of days after password expiry that account is disabled, number of days since Jan 1 1970 that account is disabled, and a reserved field.
Some (older) UNIX or Linux systems do not contain the file /etc/shadow and store the encrypted user password in the second field of each line of the file /etc/passwd (the field which on newer systems contains
Part 3: Basic Operations FAQ
38
Linux Newbie Guide by Stan, Peter and Marie Klimas just the letter x).
01/08/2003
For example, my /etc/shadow entry for "root" account may look like this:
root:$1$BuPbmLAz$1G7.evIChyqaEI0TlZp0F.:11071:0:99999:7:-1:-1:134540356 and after the password is erased, it looks like this: root::11071:0:99999:7:-1:-1:134540356
Now, the root account has no password, so I can reboot the computer and, at the login prompt, type "root" and for password just press ENTER (empty, no password). After a successful login, I immediately set the password for root using the command:
passwd
Apparently, despite deleting the password from /etc/shadow , the Debian distribution will not let you log in "passwordless"
(enhanced security?). In such a case, what needs to be done is to replace the password in /etc/shadow with an encrypted password from another account, where you know the password. After that, you can login since you know the password.
E-mailing an encrypted password may be also a secure way to set up an account for somebody remote: "I am setting up an ftp account for you on my server. Email me your encrypted password." After you receive the encrypted password, you insert it into the appropriate field in /etc/shadow. Now, the user can log in, since she knows the password, but nobody else can.
To make the "floppy access" to my system a little bit more difficult, I considered running a computer without a floppy drive :-) Unfortunately, Linux CDs are bootable these days. I set up my boot sequence (in the BIOS setup) so that the system boots from the hard drive before floppy and CDROM are tried, and added an "administrative" password on changes to the BIOS settings. Still, I worry that these BIOS passwords are very easily crackable, or that one could remove the small battery that sustains the BIOS setting. One could also remove my harddrive and connect it to another computer for reading :-) . I am thinking about installing an "encrypted file system" which is now available on Linux, but considering all the trouble associated with it, perhaps I will settle on locking my room :-) . If all this sounds paranoid to you, it probably is—it just illustrates the point there is little computer security, even under Linux, if the potential cracker has a physical access to your hardware.
3.2.4 I forgot my user password
If a regular (non-root) user forgets his/her password, this is not a problem since root can change any password. For example (as root): passwd barbara
will prompt for a new password for the user "barbara" (no knowledge of the old password required by root). If a regular user (non-root) wants to change his/her password, s/he will be asked for the old password first. (This is a security feature so nobody changes your password if you have left your terminal unattended.)
3.2.5 Disabling or removing a user account
A user account can be temporarly disabled or permanently removed.
To temporarily disable (lock) a user account, there is no need to change his/her password. Just put an asterisk "*" at the beginning of the second field (before the encrypted password) in the file /etc/shadow . The "*" means that no login is permitted for this account. When you want to restore the account, you just erase the asterisk and the user account is back in operation, with its old password.
Here is an example entry from the file /etc/shadow with the password disabled for user "peter": peter:*$1$narMEFm6$fhA1puOU422HiSL5aggLI/:11193:0:99999:7:-1:-1:134539228 I could also lock a user account with the following command: passwd peter -l and unlock it with passwd peter -u
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