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Basic Linux Operations FAQ

Part 3 of the Linux Newbie Administrator Guide

3.2.3 I forgot the root password
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Linux Newbie Administrator Guide
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1. Before Installation
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3. Basic Operations FAQ
4. Newbie Admin FAQ
~ 4.1 Lilo
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A. How to Upgrade Kernel?

Even if I never forget any passwords, I would still study this issue in detail because it can give me a hint on how my mother might be reading my ICQ chats history :-)
First method. The easiest way to solve your "forgotten root password" problem is to boot your Linux in the single-user mode, namely at the "lilo"prompt (during bootup) type:

linux single

This will make you "root" without asking for a password. Now, being root, you may change the root password using this command (no knowledge of the old password required):

passwd

If it strikes you as insecure, that's because no computer system is secure if other people have physical access to your hardware. Nevertheless, I did not like the "linux single" hole on my home computer and plugged it by adding the following lines to my /etc/lilo.conf file (at the end of the "image=" section):

password="my_password"
restricted

[This "lilo" password is required when, at the LILO prompt during bootup, somebody enters the word "linux" with any parameter (normal bootup without any parameters will still be possible without a password).] For the changes to /etc/lilo.conf to take effect, I must re-run the command lilo . Since my lilo password is not encrypted, I must make /etc/lilo.conf readable only for root:

chmod 600 /etc/lilo.conf

Second Method.
Another way to solve the "lost-root-password" problem is to boot your computer from the Linux boot diskette or the CD. Then find your Linux root partition on the hard drive, mount it, and edit the file /etc/shadow. (I can do it because after booting from the floppy, I become root without being asked for a password.) In the password file, I erase the encrypted password for root (for example, using the pico editor), so it is empty.
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 just the letter x).

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.

Next > 3.2.4 I forgot my user password

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