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Linux Authentication and Access Control 4-2 Linux IDs and /etc/passwd Linux Authentication and Each user has a user id (uid) and belongs to (possibly several) groups Access Control each of which has a gid. uid and default gid are stored in /etc/passwd. E.g, in my Fedora VM: 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 Tue/Wed/Thu, September 16/17/18, 2014 lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin Reading: Pollock Article; Hacking Linux Exposed sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt CS342 Computer Security mail:x:8:12:mail:/var/spool/mail:/sbin/nologin news:x:9:13:news:/etc/news: uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin Department of Computer Science ... Wellesley College lynux:x:500:500:Franklyn Turbak:/home/lynux:/bin/bash gdome:x:501:501:Georgia Dome:/home/gdome:/bin/bash cs342:x:502:502:CS342 Account:/home/cs342:/bin/bash Linux Authentication and Access Control 4-2 /etc/group /etc/shadow /etc/group defines groups and Sysadmins can define new groups, lists which users belong to them. /etc/shadow contains hashed passwords: e.g. on puma root:$1$SdMYD5fz$cr120C7tA0wDhXPrQHc3H1:15583:0:99999:7::: root:x:0:root bin:*:13665:0:99999:7::: bin:x:1:root,bin,daemon • faculty (all cs faculty) daemon:*:13665:0:99999:7::: daemon:x:2:root,bin,daemon • adm:*:13665:0:99999:7::: sys:x:3:root,bin,adm cs111sta (members of the … adm:x:4:root,adm,daemon CS111 teaching staff) lynux:$1$mUwRqvWp$2yfe5MJV0HbtdcDad8E7i/:15583:0:99999:7::: tty:x:5: • cs251stu (cs251 students) gdome:$1$d6tpFiwO$rBD5jd1ASAYatV2UuAlXW.:14274:0:99999:7::: disk:x:6:root cs342:$1$kmTSYJoG$0AnYYfykI2e/nWPRXdjwO.:14194:0:99999:7::: lp:x:7:daemon,lp mem:x:8: kmem:x:9: wheel:x:10:root Your Ubuntu VM uses longer salts and hashes: ... lynux:x:500: wendy:$6$FoFIUVCi$/FUkEldzfdJXMefmv/s76m4wRpeZPnHjsVdJ9pO.QgKWuZmV gdome:x:501: cjt5J53lZ8Sifj9Q3Pm6n6ukR9p8A1\43mnE2Q0:15594:0:99999:7::: cs342:x:502: cs342sta:x:503:cs342,lynux cs342stu:x:504:cs342,gdome,lynux Linux Authentication and Access Control 5-3 Linux Authentication and Access Control 5-4 LDAP Checking/Changing Who You Are Our networked dept. machines now use LDAP, a database system for o whoami: name associated with current uid users/groups that does not use /etc/passwd and /etc/group directly. o groups: groups of which current uid is a member (But you will be using /etc/passwd and /etc/group on the machines you o su username : "become" username administer in E125.) o su - username: "become" username, using initialization files On puma, use getent command to get info from the LDAP database: o su: "become" root (su = superuser) [fturbak@puma ~] getent passwd fturbak o sudo command: Execute command as root fturbak:x:708:708:Franklyn Turbak:/home/fturbak:/bin/bash (if youre in /etc/sudoers and you give your password.) [fturbak@puma ~] getent passwd acarnigl acarnigl:x:4568:4569:Allison Carniglia class of 2013:/students/acarnigl:/usr/local/bin/scponly # /usr/local/bin/scponly only allows SCP, not login access. For 110/111 students [fturbak@puma ~] getent group cs242stu cs242stu:x:4678:networks,fturbak,amckenna,choover,clee2,cvaldes,kneugent,ksulliv3,lbell, gwunnava,skim17 http://xkcd.com/688/ Linux Authentication and Access Control 5-5 Linux Authentication and Access Control 5-6 Checking/Changing Who You Are: Examples sudo in Ubuntu [fturbak@puma ~] whoami fturbak [fturbak@puma ~] echo $UID The culture in Ubuntu is to use sudo rather than su to root. 708 [fturbak@puma ~] groups fturbak faculty cvs lumberjacks cs301stu cs301pri cs301sta itr2004 • Reminds you that you’re doing rooty things: less likely to cs230pri cs230sta cs230stu cs251pri cs251stu … accidentally do something bad (e.g. rm –rf * on /) [fturbak@puma ~] su – gdome Password: ********** • Don’t have to remember root password, only your own. [gdome@puma ~] whoami gdome • Can still become root if you want to (red part not echoed) [gdome@puma ~] echo $UID 707 wendy@cs342-ubuntu-1:~$ sudo su – [gdome@puma ~] groups gdome cs301stu cs230stu cs251stu cs235stu cs242stu [sudo] password for wendy: Tr0ub4dor&3 [gdome@puma ~] su – root@cs342-ubuntu-1:~# Password: ********** [root@puma ~] whoami root [root@puma ~] echo $UID 0 [root@puma ~] groups root bin daemon sys adm disk wheel Linux Authentication and Access Control 5-7 Linux Authentication and Access Control 5-8 passwd: Change Password useradd: Create User Account Sudoer wendy changes guest password (red parts not echoed) Sudoer wendy adds user acct1 wendy@cs342-ubuntu-1:~$! sudo passwd guest wendy@cs342-ubuntu-1:~$! sudo useradd -m -s /bin/bash acct1 [sudo] password for wendy: Tr0ub4dor&3 [sudo] password for wendy: Tr0ub4dor&3 Enter new UNIX password: foobar Retype new UNIX password: foobar • By default, puts user in empty group with same name as user passwd: password updated successfully /etc/passwd now has: acct1:x:1101:1101::/home/acct1:/bin/bash! /etc/group now has: acct1:x:1101: • As root, wouldn’t need extra sudo password • Still need to set password (via passwd); ! in /etc/shadow locks user out: acct1:!:15601:0:99999:7::: • Can change anyone’s password as root • -m creates account /home/acct1 • Anyone change her own password w/o being root. • -s specifies shell program for user. • Many more options; see man useradd • Add user to group sudo to make her a sudoer • There are graphical user interfaces for creating accounts, setting passwords, etc. but you will not have access to these in many contexts (like Capture the Flag contests) So best to learn shell commands instead! Linux Authentication and Access Control 5-9 Linux Authentication and Access Control 5-10 File Permissions in Linux What Do Permissions Mean? [lynux@localhost cs342]$ ls -al handouts total 68 o On file: drwxrwx--- 4 lynux lynux 4096 2008-09-12 07:36 . drwxrwxr-x 6 lynux lynux 4096 2008-09-02 03:08 .. r: can read from file drwxrwx--- 2 lynux lynux 4096 2008-09-02 03:15 course-info -rw-rw---- 1 lynux lynux 638 2008-09-09 08:59 linux-commands.txt w: can write to file -rw-rw---- 1 lynux lynux 12335 2008-09-12 07:33 os-security.txt -rw-rw---- 1 lynux lynux 3073 2008-09-11 21:27 os-security.txt~ x: can execute file as a program drwxrwx--- 2 lynux lynux 4096 2008-09-09 05:38 security o On directory: How do you read a permission string (e.g. drwxrwxr-x, -rw-rw----)? r: can list the filenames in the directory. Leftmost chars: Other 9 chars: w: can add new file and delete existing file read (r), write (w), execute (x) (even if don't have any permissions on file!) - normal file permissions for 3 entities: x: can cd to directory and “search” files in directory d directory • 3 chars for owner (u=user); (i.e., get inode information necessary for file contents.) l link • 3 for group (g); s socket • 3 for everyone else (o=other) Linux Authentication and Access Control 5-11 Linux Authentication and Access Control 5-12 Special permissions Permissions as Bits & Octal Numbers There are 12 (not 9) permission bits in Linux. 11 10 9 8 7 6 5 4 3 2 1 0 11 10 9 8 7 6 5 4 3 2 1 0 set set stic- user user user group group group other other other uid gid key r w x r w x r w x set set stic- user user user group group group other other other uid gid key r w x r w x r w x setuid bit permissions octal setuid bit changes user "x" to "s" -rwxrwxrwx 777 • on executable program: change effective user id (more later) and no "x" to "S" -rwxr-x--- 750 setgid bit setgid bit -rw-r—-r-- 644 • on executable program: change effective group (more later) changes group "x" to "s" -rwsr-sr-- 6754 • on directory: files/subdirectories inherit group and its permissions and no "x" to "S" from directory -rwsr-Sr-- 6744 sticky bit -rwsr-xr-- 4754 sticky bit changes other "x" to "t" -rwxr-sr-- 2754 • on directory: only owner can delete files in the dir (used in /tmp) and no "x" to "T -rwxr-xr-t 1755 -rwxr-xr-T 1754 Linux Authentication and Access Control 5-13 Linux Authentication and Access Control 5-14 Changing File Permissions in Linux Changing File Permissions in Linux (Continued) chmod perms file … : changes file permissions [lynux@localhost handouts]$ chmod u+s os-security.txt; ls -al os-security.txt chmod -R perms file … : changes file permissions recursively -rwsr-xr-- 1 lynux lynux 13290 2008-09-12 07:48 os-security.txt [lynux@localhost handouts]$ chmod g+s os-security.txt; ls -al os-security.txt [lynux@localhost handouts]$ ls -al os-security.txt -rwsr-sr-- 1 lynux lynux 13290 2008-09-12 07:48 os-security.txt -rw-rw---- 1 lynux lynux 13290 2008-09-12 07:48 os-security.txt [lynux@localhost handouts]$ chmod 754 os-security.txt; ls -al os-security.txt [lynux@localhost handouts]$ chmod o+rx os-security.txt; ls -al os-security.txt -rwxr-xr-- 1 lynux lynux 13290 2008-09-12 07:48 os-security.txt -rw-rw-r-x 1 lynux lynux 13290 2008-09-12 07:48 os-security.txt [lynux@localhost handouts]$ chmod 4754 os-security.txt; ls -al os-security.txt [lynux@localhost handouts]$ chmod g-w os-security.txt; ls -al os-security.txt -rwsr-xr-- 1 lynux lynux 13290 2008-09-12 07:48 os-security.txt -rw-r--r-x 1 lynux lynux 13290 2008-09-12 07:48 os-security.txt [lynux@localhost handouts]$ ls -al .
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