User, group and file administration

1. User, group and file administration Unit objectives • Create, modify, manage, and delete user and group accounts • Find broken links and orphan files and use file date and time stamps

2. Topic A: Administering users and groups

3. Authentication and user accounts • Authentication • Act of verifying a user’s identity • Compares user name and password to a system database • Database that contains account information typically consists of two files: • /etc/passwd • Contains details of user • /etc/shadow • Contains hash of password and expiration information

4. Administering users and groups • pwconv • Converts to using /etc/shadow file to store encrypted password • pwunconv • Reverts back to /etc/passwd file only continued

5. /etc/passwd file • name:passwd:UID:GID:GECOS:homedirectory:shell • A + before entry marks an account as disabled, unable to log in • * in passwd field marks account as disabled • name • Username • passwd • If /etc/shadow exists • X • If no /etc/shadow • Encrypted password • User Identifier (UID) • Specifies the unique user ID assigned to each user • UID 1 to 100 reserved for system daemons continued

6. /etc/passwd file • name:passwd:UID:GID:GECOS:homedirectory:shell • Group Identifier (GID) • Primary group ID for each user • GECOS • Text description of user-usually blank • homedirectory • Path to home directory • shell • Path to shell continued

7. /etc/shadow file • name:passwd:lastchange:min:max:warn:disable1:disable2 • passwd • Contains encrypted passwd • If ! Then account is lcoked • lastchange • Last password change date • Measured since 1/1/1970 • min • Number of days you must wait to change • Fedora default-0 • max • Number of days you can use a password • Max 99,999 • warn • Number of days before expiration that you get warning • 7 days

8. /etc/group file • name:passwd:GID:members • name • Group name • passwd • Group Password • Usually an x since they are rarely used

9. Creating user accounts • useradd command • Adds a user account to the system • /etc/login.defs default values: • location for e-mail • password expiration • minimum password length • range of UIDs and GIDs • whether to create home directories continued

10. Creating user accounts, continued • /etc/default/useradd default values: • primary group • location of home directory • disable accounts with expired password • date to disable user accounts • shell • skeleton directory • Skeleton directory • Contains the files copied to all new users’ home directories upon creation

11. Options to the useradd command

12. Passwd command • When user is added, password file in /etc/shadow has ! • No password set • Use passwd username command to set • passwd bobg • Sets bobg password • Done as root • Without arguments, user can set own password

13. usermod • Used to change account proprties • Edits details of /etc/passwd, etc/shadow, etc with less risk of making mistake

14. usermod options

15. Modifying user accounts • chage • Modifies password expiry information • Locking an account • usermod –L username • Makes account temporarily unusable by altering the password information • Puts ! In /etc/shadow file password field • Unlock with usermod –U username • Can also change shell to /bin/false

16. Modifying user accounts • chsh • chsh –s /bin/flase bobg • Changes a valid shell to an invalid shell

17. Deleting user accounts • userdel command • Remove a user account from both /etc/passwd and /etc/shadow • To delete files use –r option • When an account is deleted, any files that were previously owned by the user become owned by a number that represents the UID of the deleted user

18. Managing groups • groupadd • Adds a group • groupmod • Modifies the name of a GID • groupdel • Deletes a group • groups • Prints a list of groups the user is a member of

19. Managing groups • newgrp • Temporarily change primary group memebership • New files have group membership according to new setting

20. The graphical User Manager

21. If the /etc/shadow file does not exist when pwconv is run, which of the following occur? A. The system will give an error message. B. /etc/passwd is renamed to /etc/shadow and a new soft link is created. C. Entries in the /etc/passwd file are added to a new /etc/shadow file. The /etc/passwd file is unchanged. D. Password attributes in /etc/passwd are moved to a corresponding entry in a new /etc/shadow file.

22. If the /etc/shadow file does not exist when pwconv is run, which of the following occur? A. The system will give an error message. B. /etc/passwd is renamed to /etc/shadow and a new soft link is created. C. Entries in the /etc/passwd file are added to a new /etc/shadow file. The /etc/passwd file is unchanged. D. Password attributes in /etc/passwd are moved to a corresponding entry in a new /etc/shadow file. Answer: D

23. A system administrator wants to disable shell access for a user. Which of the following is the appropriate shell to set? A. /bin/sh B. /bin/false C. /bin/passwd D. /bin/disable

24. A system administrator wants to disable shell access for a user. Which of the following is the appropriate shell to set? A. /bin/sh B. /bin/false C. /bin/passwd D. /bin/disable Answer: B

25. Examine the following /etc/passwd file: root:X:11423:0:99999:7::: dgringold:X:11423:0:99999:7::: jjones:!!:11432:0:99999:7::11688: tgold:!!:11342:11231:11678:7::: Which of the following is the password field? A. The first field B. The second field C. The third field D. The fourth field

26. Examine the following /etc/passwd file: root:X:11423:0:99999:7::: dgringold:X:11423:0:99999:7::: jjones:!!:11432:0:99999:7::11688: tgold:!!:11342:11231:11678:7::: Which of the following is the password field? A. The first field B. The second field C. The third field D. The fourth field Answer: B

27. Which of the following can be used to view username, UID (User ID), GID (Group ID), full name, home directory, and default shell information? A. cat /etc/group B. cat /etc/services C. cat /etc/shadow D. cat /etc/passwd

28. Which of the following can be used to view username, UID (User ID), GID (Group ID), full name, home directory, and default shell information? A. cat /etc/group B. cat /etc/services C. cat /etc/shadow D. cat /etc/passwd Answer: D

29. Topic B: Administering user files

30. Broken links and files with no owners • Find and fix files with no owners or symbolic links that no longer point to a file • find / -nouser • find / -nouser –ok rm “{}” “;” • Symlinks can find “dangling” symbolic links

31. File date and time stamps • Creation time (ctime) • At creation and when contents, permissions, owner or attributes change • ls –l –time=ctime • Access time (atime) • File is opened • ls –l –time=atime • Modification time (mtime) • When file is written and closed

32. Unit summary • Created, modified, managed, and deleted user and group accounts using command line utilities and the User Manager • useradd • /etc/defaults/useradd • /etc/login.defs • usermod • chage • Found broken links and orphan files, used file date and time stamps • find / -nouser • symlinks –r / | grep dangling

33. Which of the following commands will locate files that have been modified in the past two days? A. locate ­t2 B. find / ­mtime 2 C. whereis ­time 2 D. grep / ­ctime 2

34. Which of the following commands will locate files that have been modified in the past two days? A. locate ­t2 B. find / ­mtime 2 C. whereis ­time 2 D. grep / ­ctime 2 Answer: B

35. Any Questions?