What is a system administrator?
-System Admin Tasks and Duties:
-Install systems (including clients/servers): hardware, software and
o.s.
-Upgrade systems: hardware, software and o.s.
-Backups
-Start/stop system (reboot)
Other duties:
- Create accounts (add/delete), account management
- Job scheduling
- Security
- Performance monitoring and tuning
- Disk space management (formatting, partitions, quotas)
- Writing/modifying scripts (perl, shell, C, etc..)
- Running specialized services/servers (email server, web server, DNS
etc..)
- Problem resolution
Last duties:
•
Training (others and oneself)
•
Fixing Bugs
•
Automate Tasks
•
Maintain system files
• Analysis of logs/systems, collect stats, reports
•
Planning and Recommendations
•
Work with vendors, customers
•
Research New Technologies
Task of Linux Admin:
Task of Linux Admin:
-
managing hardware
-
managing the kernel
-
managing filesystems
-
managing user account
-
managing network and security
-
managing backup/installation/archiving
File System in Linux:
- Built-in
hierarchical file structure
- Common directories
- /bin : essential command binaries which may
be used by the system administrator and by ordinary users, required for system
boot
-
/boot : kernel image and configuration
files used by boot loader
-
/dev : device files
-
/etc : host-specific configuration files
-
/home: user home directories
-
/lib: essential shared lib and kernel
modules
-
/mnt : mount point for temporarily mounting
files system such as those on a CDROM or floppy disk
-
/opt : add-on application software packages
-
/root : the root user’s home directory
-
/sbin : system binaries, essential for
system administration, but not for system boot
-
/tmp : location of temporary files
-
/usr : secondary hierarchy, intended as
sharable, read-only data
-
/var : variable data such as spool
directories & log file
- in unix, a filesystem is some device that is formatted to store
files. It can be found on hard drives, floppies, CD-ROMs and others
- the exact format and means the files are stored are not important;
- based on second extended filesystem, ext2fs
- provide a common interface for all filesystem types that it
recognizes
- In Linux 7.2, ext2 is the default file system
-
Require
check for consistency (e2fsck) when system is not properly shut down
-
Ext =>
ext2 =>ext3
-
Support 256
char filenames, 4 Tbyte max filesize
-
Others
MS-Dos, FAT32, NTFS, ISO9660
- Starting from Linux 7.3, ext3 is the default file system
-
Provide
stronger data integrity in event of unclean shutdown
-
Check for
consistency is needed only on rare hardware failure
-
Higher
throughput, as it is optimizes hard drive head motion
-
Provide easy
transition from ext2 to ext3
-
other common
filesystem are MS-DOS (FAT32, NTFS, ISO9660)
Creating user account:
w
manage users
and group accounts and related system files
w passwd and group
ü
user account
information is stored in /etc/passwd
-
each line in
/etc/passwd contain a username, password, UID, GID, user’s name, home directory
and default shell
ü
group
information is stored in /etc/group
-
each line in
/etc/group contain a group name, group password, GID and group member list
w
to prevent
users from obtaining encrypted passwords from passwd and group, shadow files
are implemented
w
encrypted
passwords are moved to a new file, which is readable only by root
w
the shadow
file for /etc/passwd is /etc/shadow
w
the shadow
file for /etc/group is /etc/gshadow
w Three methods
ü
modify the
files directly, not recommended
-
Login in as
root
w
Create a
record for the user in /etc/passwd
w Set the user’s password
w
Specify a
login shell for the user
w
Create a
home directory for the user
ü Use command line
-
useradd [-D]
[-g default_group] [-b default_home] [-s default_shell]
-
Donald:x:503:503:unka
Donald: home/donald:/bin/bash
ü
Alternatively,
use GUI (most recommended)
Configuration files:
w
bash uses a
number of configuration to set its operating environment when it starts
w /etc/profile:
-
system wide
initiation files, execute during log in, contains environment variables, such
as initial PATH, and startup program
w /etc/bashrc:
-
system wide
initiation files for a user, contains alias
Who use the files?
w
There are 3
sets of permissions for every file or directory -- owner, group, and others
-
The owner
permissions are for the owner of the file or directory
-
The group
permissions are for everyone in the group
-
Others: The
global permissions are for anyone
w
What is the
command to see the permission and who own the files?
w answer: ls -l
w r -- file can be read
w
w -- file
can be written and modified
w
x -- file
can be executed (if it is a program)
w
chmod is a standard
command that allows you to change the permissions of a file or directory.
w
There are
two arguments for chmod:
-
the
permissions : based on numbers
w 1 stands for execute.
2 stands for write.
4 stands for read.
2 stands for write.
4 stands for read.
-
the
file/directory name. The permission argument for chmod.
w
chmod 777
lab2file will allow EVERYBODY full access to read, write and
execute.
w
chmod 644
lab2file will all the owner (you) read and write access, the
group read access, and everybody else (world) read access too.
w
chmod 700
lab2file will give yourself full access while giving everybody
Setting up a secure system:
w
There are
some very basic things that you have to do in order to secure your system
w Shutting down the redundant services
-
You have to
disable all network daemons (services) that are not needed by the system
-
Any network
port that is listening for connections can be vulnerable to attacks due to
probable exploits against running daemon
-
To find out
the ports that are opened type: # netstat
-an
w
Looking in /etc/services
or by passing -p to netstat we can tell which service is running per port
w
Check each
port that looks like unnecessary
w Examples vulnerable services:
-
telnetd, sendmail,
ftpd: Send clear passwords through the web. Instead of
telnet use ssh
w
Shutting
down services involves editing the appropriate files on your system
w
On RedHat
based systems daemons are started by scripts in the /etc/rc.d/init.d directory
w
Depending on
the runlevel each daemon/services in linked to the appropriate rcX.d directory
where 0<X<6
w
What to have
in mind all the time:
-
Never use
simple passwords. Try to make them complex by mixing letters,symbols and
numbers
-
Do NOT work
on the root account unless absolutely necessary
-
Do not
ignore the log files
-
Update your
system in a regular basis
Tcp wrapper configuration:
w
A simple and
effective way to protect the system
w
TCP Wrappers
“wrap” a service access (e.g. apache web server)monitoring the connections to
it and refusing unauthorised sites
w
It is used
in conjunction with inetd and xinetd
w
It's a good
way to control the access to services that do not provide any native access
control mechanism
w
TCP Wrapper
is the first thing encounter when a connection is established with a service
protected by the wrapper
w
TCP Wrapper
is responsible for determining whether the connection comes from a source host
that it is allowed to do so
w
Depending on
whether you are using TCP Wrappers with inetd or xinetd there are two
different approaches
w
If the
system is using the inetd daemon you have to edit the /etc/inetd.conf
file to use the TCP wrapper
w
Using TCP wrappers
requires just a small change to /etc/inetd.conf
w
E.g. for the
finger daemon
finger
stream tcp nowait root /usr/sbin/in.fingerd in.fingerd
has to be
changed to:
finger
stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.fingerd
This cause
the tcpd command, representing the TCP wrapper, to be executed instead of the in.fingerd
and protect the daemon
w
xinetd is the replacement of inetd adopted by some distros
w
In most
cases xinetd has built-in support for TCP wrappers
w
You need to
modify the TCP wrapper configuration files (/etc/hosts.allow, /etc/hosts.deny)
w
/etc/hosts.allow
and /etc/hosts.deny specify the access rules that are applied
in daemon protection
w
When a TCP wrapper
is invoked it obtains the IP address of the connecting host and its hostname
w
If the IP of
the host is specified in the /etc/hosts.allow then access is permitted
to the daemon/service
w
If no match
is found, the /etc/hosts.deny is consulted. If the IP is described there
then the connection is closed
w
If no much
exists both of the files then access is granted
w
The syntax
of those two files is simple
w
Each file
contains a set of rules
w General rule form:
daemon_list
: client_list : shell_command
where daemon_list
is comma separated list of daemons to which the rule applies, the client_list
is comma separated list of the hostnames or IP addresses where the rule applies
and shell_command is optional, specifying the command to be executed
when rule matches
w Example rules:
1. /etc/hosts.deny
ALL:ALL # Deny everything from everywhere
In case that
nothing is specified in the /etc/hosts.allow then this rule will refuse
connection to any service by anyone
2. /etc/hosts.deny
ALL: ALL
EXCEPT localhost
3. /etc/hosts.allow
in.fingerd:
ALL
