User Management. René Serral-Gracià Xavier Martorell-Bofill 1. May 26, Universitat Politècnica de Catalunya (UPC)
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1 User Management René Serral-Gracià Xavier Martorell-Bofill 1 1 Universitat Politècnica de Catalunya (UPC) May 26, 2014
2 Lectures 1 System administration introduction 2 Operating System installation 3 User management 4 Application management 5 System monitoring 6 Filesystem Maintenance 7 Local services 8 Network services 9 Security and Protection 10 Virtualization R. Serral-Gracià, et. al Users 2
3 Outline 1 Introduction Goals 2 System Databases 3 User disabling and deletion 4 Login process 5 Permissions and protections R. Serral-Gracià, et. al Users 3
4 Goals Coneixements Knowledge about the system databases File and Directory permissions and protections SetUID/SetGID bits Abilities User management tasks User creation Group creation and user assignment User disabling and creation Commands and Files chmod, chown, id, useradd, userdel, umask /etc/passwd, /etc/group, /etc/shadow R. Serral-Gracià, et. al Users 4
5 Outline 1 Introduction 2 System Databases 3 User disabling and deletion 4 Login process 5 Permissions and protections R. Serral-Gracià, et. al Users 5
6 System Databases /etc/passwd /etc/group /etc/shadow /etc/aliases Mail Alias N:M 1:1 Users Passwords N:M Groups R. Serral-Gracià, et. al Users 6
7 /etc/passwd Must be readable by all the users Format username:passwd:uid:gid:real_name:homedir:shell root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/spool/mail:/sbin/nologin news:x:9:13:news:/etc/news: nobody:x:99:99:nobody:/:/sbin/nologin sshd:x:74:74:privilege-separated SSH:/var/empty/sshd:/sbin/nologin aramirez:x:500:500:alex Ramirez, C6117, 54040:/home/aramirez:/bin/bash R. Serral-Gracià, et. al Users 7
8 More about users Special users root UID 0 (the username does not matter) ftp Anonymous FTP access (without password) nobody Special user for NFS and other services System users Used to run services without superuser privileges Without shell neither password Set of privileges to allow performing the tasks R. Serral-Gracià, et. al Users 8
9 /etc/group A group may have lots of users Each user has a main group (/etc/passwd) Each group has a member list Format groupname:passwd:gid:username,username,... root:x:0:root bin:x:1:root,bin,daemon daemon:x:2:root,bin,daemon sys:x:3:root,bin,adm adm:x:4:root,adm,daemon tty:x:5: disk:x:6:root lp:x:7:daemon,lp mem:x:8: kmem:x:9: wheel:x:10:root Mail:x:12:mail news:x:13:news uucp:x:14:uucp man:x:15: games:x:20: ftp:x:50: nobody:x:99: users:x:100:aramirez aramirez:x:500: R. Serral-Gracià, et. al Users 9
10 More about groups Groups with special meaning configuration dependent wheel User groups with administration privileges nobody Special group for NFS and other services users All users belong to it R. Serral-Gracià, et. al Users 10
11 /etc/shadow Only accessible by root Encrypted Password Password expiration policy Format username:passwd:password expiration policy passwd: change user s password chage: allows to change password expiration policy Max/Min time between password changes Account expiration date root:$1$ivkd84gq$iv7vhg0chdiggnynns00e/:12260:0:99999:7::: bin:*:12260:0:99999:7::: daemon:*:12260:0:99999:7:::... aramirez:$1$jgmk47hy$6lkk.qyrmi67qpqvhtcds.:12262::99999:::: R. Serral-Gracià, et. al Users 11
12 /etc/aliases alias data base Allows redirection For the pseudo-users to administrator to programs to the outside # Basic system aliases -- these MUST be present. mailer-daemon: postmaster postmaster: root # General redirections for pseudo accounts. bin: root webmaster: root support: postmaster # Person who should get root root: aduran, xavim@ac.upc.edu R. Serral-Gracià, et. al Users 12
13 Exercise Individually Detail the user creation process Modification of the data bases Directory creation Default files... In group Gather the notes and discuss Make the pseudo-code for the useradd command R. Serral-Gracià, et. al Users 13
14 User Management Basic commands User Management useradd (adduser) userdel usermod To modify all the fields except the username passwd newusers vipw Group Management groupadd groupdel groupmod gpasswd (passwd -g) newgrp, sg vigr R. Serral-Gracià, et. al Users 14
15 Outline 1 Introduction 2 System Databases 3 User disabling and deletion Disabling User deletion User management policies 4 Login process 5 Permissions and protections R. Serral-Gracià, et. al Users 15
16 Disabling Temporarily disable an user We must avoid the user access to the system 1 Password invalidation Insert an invalid character (*) It allows to recover the original password afterward 2 Invalidate the shell Change it with another one (/bin/false, /bin/nologin) Informs the user it has been disabled If the user tries to login the administrator is informed R. Serral-Gracià, et. al Users 16
17 User deletion Once we are sure the user account is not needed anymore... 1 Disable the account (Password invalidation) 2 Check that the user is not working on the system 3 Backup the user s data 4 Delete the user s data 5 Delete the user from the system databases /etc/shadow /etc/passwd /etc/group 6 Add redirection /etc/aliases R. Serral-Gracià, et. al Users 17
18 User management policies UIDs Assignment Do NOT recycle UIDs username Assignment Store additional information, Office and phone number Home organization /home Flat All the users located at (/home/... ) Hierarchical, creating different directory levels Based on departments... floors... offices... (/home/ac/user)... in several disks R. Serral-Gracià, et. al Users 18
19 Outline 1 Introduction 2 System Databases 3 User disabling and deletion 4 Login process 5 Permissions and protections R. Serral-Gracià, et. al Users 19
20 Login process init fork password login uid/gid additional groups setuid/setgid /etc/passwd /etc/group /etc/shadow login exec root process user process shell fork program access? fitxer R. Serral-Gracià, et. al Users 20
21 Privilege escalation Performed through SetUID/SetGID calls Working as root is dangerous and mostly unneeded It s better to have an admin user and escalate privileges when needed su [user] [-c command] Allows changing the user (by default root) sudo [command] Allows executing a command as another user Admin can restrict which commands can be executed by each user R. Serral-Gracià, et. al Users 21
22 Outline 1 Introduction 2 System Databases 3 User disabling and deletion 4 Login process 5 Permissions and protections R. Serral-Gracià, et. al Users 22
23 Permissions and protections (-,d) rwx rwx rwx owner group 3 types of permissions Read, write and execution (rwx) Regular files... Directories... 3 areas of application Owner, group, others (ugo) Commands: chown: to change a file owner chgrp: to change a file group chmod: to change permissions Set-UID/Set-GID Bits(s) Sticky Bit (t) only directories R. Serral-Gracià, et. al Users 23
24 Permissions and protections Files Directories r Read the contents List the contents w Write/Modify file contents Create/Delete files x Run Access the directory SetUID Runs with owner s UID No effects SetGID Runs with owner s GID File creation with the same group as the directory owner Sticky Bit No effects Only the file owners can erase them R. Serral-Gracià, et. al Users 24
25 Exercise In group Assign the directory and file protections for the file... $ ls -l./dirdades/dades.txt -rw-rw-r-- 1 aso01 aso Nov 28 18:10./dirdades/dades.txt Can only be modified by the owner Readable only by its group Only deletable by its owner Only the owner can run ls in the directory R. Serral-Gracià, et. al Users 25
26 Exercise In group Assign the directory and file protections for the file... $ ls -l./dirdades/dades.txt -rw-rw-r-- 1 aso01 aso Nov 28 18:10./dirdades/dades.txt Can only be modified by the owner -w Readable only by its group --- r dir x --- Only deletable by its owner dir -w Only the owner can run ls in the directory dir rw $ ls -la./dirdades/dades.txt drwx--x--- 1 aso01 aso Nov 28 18:11. -rw-r aso01 aso Nov 28 18:11./dirdades/dades.txt R. Serral-Gracià, et. al Users 25
27 Default permissions During file/directory creation... Owner is determined by current user and group id informs about current user/group newgrp allows changing the current group Permissions are determined by umask: user mask Indicates which permissions DO NOT belong by default to the file or directory 022: rwx r-x r-x 027: rwx r-x --- R. Serral-Gracià, et. al Users 26
28 Homework Application installation mechanisms Software distribution formats tar, gz, rpm, deb, zip... R. Serral-Gracià, et. al Users 27
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