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Top Forums UNIX for Dummies Questions & Answers shadow file Post 59686 by S.Vishwanath on Wednesday 29th of December 2004 05:12:42 AM
Old 12-29-2004
HI,

Unix Stores passwords in a text file. Usually passwords were
stored in /etc/passwd file. So any body can see the contents of
the passwd file. Also can look into the password string (ofcourse
they are encrypted (cypher text). Which could only be
understood by the passed program.

This gave some space for the world community to start guessing
and re-coining the passwords for some definite kind of password
text.

Hence in order to avoid / give no chance to the password visibility,
a file called /etc/shadow was being provided which has one and only one permission, and passwords are now being stored in this file instead.
i.e. Read permission on the file only for
root/superuser. Hence no body will be able to see the contents
of this /etc/shadow file other than the root user.

Well, now you may feel that, why shouldn't we apply the only root user
read permission for the /etc/passwd file?

Actually the /etc/passwd file will have -r--r--r type of permission (i.e., read only for all).
This read only for all is required because,
when a valid user loggs on successfully to the unix box, he will be
put into some specific directory (home directory), and his required
shell is loaded for him to interect with the Unix World.
So, all the infromation regarding the users Home Dirctory, his/her preferrd shell etc.,
are stored in this file. So, when the user logs
in, login program should be able to read and load all this information from this file,
before user is allowed to work in his way.

For more details type man passwd and go through it.
 

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d_passwd(4)							   File Formats 						       d_passwd(4)

NAME
d_passwd - dial-up password file SYNOPSIS
/etc/d_passwd DESCRIPTION
A dial-up password is an additional password required of users who access the computer through a modem or dial-up port. The correct pass- word must be entered before the user is granted access to the computer. d_passwd is an ASCII file which contains a list of executable programs (typically shells) that require a dial-up password and the associ- ated encrypted passwords. When a user attempts to log in on any of the ports listed in the dialups file (see dialups(4)), the login program looks at the user's login entry stored in the passwd file (see passwd(4)), and compares the login shell field to the entries in d_passwd. These entries determine whether the user will be required to supply a dial-up password. Each entry in d_passwd is a single line of the form: login-shell:password: where login-shell The name of the login program that will require an additional dial-up password. password An encrypted password. Users accessing the computer through a dial-up port or modem using login-shell will be required to enter this password before gaining access to the computer. d_passwd should be owned by the root user and the root group. The file should have read and write permissions for the owner (root) only. If the user's login program in the passwd file is not found in d_passwd or if the login shell field in passwd is empty, the user must sup- ply the default password. The default password is the entry for /usr/bin/sh. If d_passwd has no entry for /usr/bin/sh, then those users whose login shell field in passwd is empty or does not match any entry in d_passwd will not be prompted for a dial-up password. Dial-up logins are disabled if d_passwd has only the following entry: /usr/bin/sh:*: EXAMPLES
Example 1: Sample d_passwd file. Here is a sample d_passwd file: /usr/lib/uucp/uucico:q.mJzTnu8icF0: /usr/bin/csh:6k/7KCFRPNVXg: /usr/bin/ksh:9df/FDf.4jkRt: /usr/bin/sh:41FuGVzGcDJlw: Generating An Encrypted Password The passwd (see passwd(1)) utility can be used to generate the encrypted password for each login program. passwd generates encrypted pass- words for users and places the password in the shadow (see shadow(4)) file. Passwords for the d_passwd file will need to be generated by first adding a temporary user id using useradd (see useradd(1M)), and then using passwd(1) to generate the desired password in the shadow file. Once the encrypted version of the password has been created, it can be copied to the d_passwd file. For example: 1. Type useradd tempuser and press Return. This creates a user named tempuser. 2. Type passwd tempuser and press Return. This creates an encrypted password for tempuser and places it in the shadow file. 3. Find the entry for tempuser in the shadow file and copy the encrypted password to the desired entry in the d_passwd file. 4. Type userdel tempuser and press Return to delete tempuser. These steps must be executed as the root user. FILES
/etc/d_passwd dial-up password file /etc/dialups list of dial-up ports requiring dial-up passwords /etc/passwd password file /etc/shadow shadow password file SEE ALSO
passwd(1), useradd(1M), dialups(4), passwd(4), shadow(4) WARNINGS
When creating a new dial-up password, be sure to remain logged in on at least one terminal while testing the new password. This ensures that there is an available terminal from which you can correct any mistakes that were made when the new password was added. SunOS 5.10 2 Sep 2004 d_passwd(4)
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