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Top Forums UNIX for Dummies Questions & Answers How the /etc/passwd file is written when user does not have permission Post 302223666 by buffoonix on Monday 11th of August 2008 04:25:18 AM
Old 08-11-2008
Does this really smell like homework?
I would have taken it for a legitimate newbie question,
especially since it was posted in the Dummies section.

Have a look at the user interface's binary to change one's password
which is the command passwd
Code:
$ ls -l /usr/bin/passwd 
-rwsr-xr-x 1 root root 22960 Jul 17  2006 /usr/bin/passwd

As you can see from the owner's (i.e. root) perm bits instead of the usual x
for execute there appears an s, the so called suid bit (that's what era referred to).
This means that although most of the times executed by ordinary users,
the command actually runs under the effective uid of root who is the only one on the system (normally) who is allowed to write to the file /etc/passwd.

If you look at the perms of the ping command you will find another suid binary
because ordinary users are not allowed to emit ICMP echo request packets.

Because of the security implications those elevated privileges impose
you generally will find only very few suid enabled executables on most Unix systems.
And their number should be observed in e.g. security audits or when hardening a system.
 

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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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