User Environments• Objectives

– to provide appropriate environments for different types of users

• Contents– different login programs

– user profiles

– restricted shells

– security issues

• Practicals– to provide special user accounts

• Summary

Login Shell• The password file defines the user's login shell

– full program pathname must be specified

– no parameters allowed

• Standard programs are interactive shells– Bash, Bourne Again Shell (bash)

– Bourne shell (sh)

– C shell (csh)

• Specialist programs can be provided– pppd

• Any executable program can be provided– database front end query programs

– simple menu selection programs

Restricted Shells• A restricted shell stops the user from:

– changing directories with cd

– changing the value of the PATH variable

– specifying command names containing /

– using output redirection > and >>

• Restrictions only apply to interactive session– profiles can still perform all functions

• Restricted shells apply to some standard applications– Sendmail uses /usr/sbin/smrsh

• Create a special bin directory for restricted users– often in /usr/rbin with links to required programs in /usr/bin

Restricted root Access

# cat >/home/admin/bin/shutdown#!/usr/bin/sh# shutdown scripttrap "" 2 3echo "Type number of seconds to shutdown or"echo -n "Ctrl-D to abandon shutdown [180]? "if read N; then [ -z "$N" ] && N=180

echo "Shutting down in $N seconds"/sbin/shutdown -i0 -g$N -y

fi# useradd -o -u 0 -d / -s /home/admin/bin/shutdown shutdown# passwd shutdown

# cat >/home/admin/bin/shutdown#!/usr/bin/sh# shutdown scripttrap "" 2 3echo "Type number of seconds to shutdown or"echo -n "Ctrl-D to abandon shutdown [180]? "if read N; then [ -z "$N" ] && N=180

echo "Shutting down in $N seconds"/sbin/shutdown -i0 -g$N -y

fi# useradd -o -u 0 -d / -s /home/admin/bin/shutdown shutdown# passwd shutdown

• Administrator provided shell scripts are useful for providing restricted root access to the system– user written shell scripts

– shutdown or backup programs

Environment Files• Two environment files are executed at login time when a

Bourne or Korn shell is executed

/etc/profile – contains system commands

– common environment (set of variable definitions) for all users

– may define variable ENV, describing the location of Shell specific environment file, usually set to $HOME/.bashrc

$HOME/.bash_profile – contains user commands and definitions

– run once at login

• Finally, if the variable ENV is defined then the shell executes the file given by its value– use to set up shell environment: aliases, functions, prompts, etc.

• Many definitions are predefined in the /etc/profile file

Environmental Definitions• Most users will need to modify some of the environmental

definitions to suit their requirements

• Good system administrator will make sure that user has a basic set of definitions available when account is created

• These may include:PATH=$PATH:/usr/X/bin:. # include required search directories

EDITOR=vi # define line editting editor (for history mechanism)

TERM=vt100 # must reflect terminal used, otherwise all non-

character-based applications, including vi, will fail

MAIL=/var/mail/$LOGNAME# location for your mail box

SHELL=/bin/bash # shell used for 'escape' from program. eg. when you

type :!cmd within vi, the cmd is executed by the shell

defined here

ENV=$HOME/.bashrc # location of bash shell run command file

• All variable must be exported after they have been set

The umask Command• Used to define default permissions on newly created files

• The user mask (umask) is part of the process environment– used when creating files to remove permissions

– the creating process defines the file permissions

– then the umask value is subtracted

• Each bit set in the octal umask clears the corresponding bit in the permissions

umask plain text files (vi)directories (mkdir)rw-rw-rw- rwxrwxrwx

000 rw-rw-rw- rwxrwxrwx022 rw-r--r-- rwxr-xr-x033 rw-r--r-- rwxr--r--027 rw-r----- rwxr-x---077 rw------- rwx------

umask plain text files (vi)directories (mkdir)rw-rw-rw- rwxrwxrwx

000 rw-rw-rw- rwxrwxrwx022 rw-r--r-- rwxr-xr-x033 rw-r--r-- rwxr--r--027 rw-r----- rwxr-x---077 rw------- rwx------

Message of the day• Profiles often display a message of the day

– traditionally stored in /etc/motd

• Keep message short and up to date– otherwise users will stop reading it

• Include a restricted access notice

# more /etc/motdWelcome to Acme's Development System. Access to this system is restricted to authorised users only. Unauthorised access is prohibited and offenders are liable to prosecution.

System will be down all day on Sun 23 April for preventative maintenance.

# more /etc/motdWelcome to Acme's Development System. Access to this system is restricted to authorised users only. Unauthorised access is prohibited and offenders are liable to prosecution.

System will be down all day on Sun 23 April for preventative maintenance.

Exercise - Example Environment

A typical system profile (/etc/profile) :

trap "" 2 3 ENV=$HOME/.kshrc export ENV ulimit -c 0 umask 022 cat /etc/motd trap 2 3

A typical system profile (/etc/profile) :

trap "" 2 3 ENV=$HOME/.kshrc export ENV ulimit -c 0 umask 022 cat /etc/motd trap 2 3

• Which file is maintained by the administrator?

• What do the two trap commands imply for /etc/profile?

A typical user profile (.bash_profile):

PATH=$PATH:$HOME/bin TERM=vt100

A typical user profile (.bash_profile):

PATH=$PATH:$HOME/bin TERM=vt100

A typical user bashrc file (.bashrc): alias dir=ls lf='ls -FC' export PS1='$PWD[!] ' set -o vi

A typical user bashrc file (.bashrc): alias dir=ls lf='ls -FC' export PS1='$PWD[!] ' set -o vi

what do you think thisline should contain?

Guest Accounts• Many systems define a login called guest

– password null or set to guest

• Disable or remove this account– it is a security risk

• System hackers logging in as guest can:– get a list of login names (/etc/passwd)

– look at encrypted passwords on older unix systems

– use find to look for security holes

– find out about networked machines (/etc/hosts)

– access the network via telnet, rlogin & rsh & rcp

• If someone wants to use your system you should know about them and provide an account accordingly

Shared Group Directories• Users working on a common set of files should belong to the

same group

• Files should be stored in a common directory

• Directory should have:– sticky bit

– set group id (so that all files are owned by the directory group)

$ ls -al /shared drwxrwsr-t 1 trapper staff 1024 Dec 25 09:30 .drwxr-xr-x 1 root sys 512 Dec 25 09:30 ..$ id -auid=318(hawkeye) gid=300(users) groups=100(staff),300(users)$ cp lfile /shared$ ls -l lfile /shared/lfile-rw-r--r-- 1 hawkeye users 232 Jan 18 09:58 lfile-rw-r--r-- 1 hawkeye staff 232 Jan 19 11:42 /shared/lfile

$ ls -al /shared drwxrwsr-t 1 trapper staff 1024 Dec 25 09:30 .drwxr-xr-x 1 root sys 512 Dec 25 09:30 ..$ id -auid=318(hawkeye) gid=300(users) groups=100(staff),300(users)$ cp lfile /shared$ ls -l lfile /shared/lfile-rw-r--r-- 1 hawkeye users 232 Jan 18 09:58 lfile-rw-r--r-- 1 hawkeye staff 232 Jan 19 11:42 /shared/lfile

Summary• Users can be provided any login program not

just an interactive shell

• A standard user environment should be set and maintained by the administrator using /etc/default/login and /etc/profile

• Users may be restricted in their access to the system and what they can do

/usr/lib/rsh & /usr/bin/rksh

• Users can be setup to share directories by using combination of guid and sticky bits

• Guest account without a password is bad news