How do I run X windows program as normal user?

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Microsoft windows XP has runas command which allows a user to run specific tools and programs with different permissions than the user’s current logon provides. Linux and other UNIX like operating system provides the su or sudo command for same purpose. However, su/sudo command is not so useful when it comes to X program. For example when you logged in as a normal user, you need to run an X window application as root. If I run application as follows:

$ su -
Password
# xeyes

It will bump you back with an error:
(program:15082): Gtk-WARNING **: cannot open display:
OR
** WARNING ** cannot open display

However both KDE and Gnome come with tools to deal with this problem.

Method # 1:
If you are using KDE then use following command at shell prompt:
kdesu command-name

$ kdesu xeyes

Method # 2:
If you are using Gnome then use following command at shell prompt:

$ gksuexec

Or use GUI itself, click on Applications > System tools > Select Run as different user
Method # 3:
Create runas alias as follows:

$ alias runas='su -c $@'

Add above alias to your bash startup script

$ echo "alias runas='su -c $@'" >> .bash_profile

You can now use alias as follows to start any x program

$ runas program-name
$ runas xeyes

Method # 4: The old way
The problem is with two environment variable DISPLAY and XAUTHORITY. You need to setup them correctly to run X windows program as a root user while logged in as a normal user. So how do you fix this problem? Simply set these two variables to point to current logged in users environment variable. Let us assume you are currently login as vivek user.

Step # 1 Become super-use

vivek@debian:~$ su -
debian:~#

Step # 2 Setup variables

# export DISPLAY=0:0
# export XAUTHORITY=/home/vivek/.Xauthority

Step # 3 Execute X program as a root user

# xeyes

How do I find out what network services are running or listing under Linux?

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Q. How do I find out what network service are running under Linux operating system?

A. For security reason it is necessary to find out what services are running. With the help of netstat command, you can print information about the Linux networking subsystem including running services. It can display program name and PID for each socket belongs to. Use netstat as follows:

$ netstat -atup

OR

$ netstat -atup | grep LISTEN

Where,

  • -t : Select all TCP services
  • -u : Select all UDP services
  • -a : Display all listening and non-listening sockets.
  • -p : Display the PID and name of the program to which each socket belongs

Do I need antivirus software for linux?

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To be frank there is no serious viruses found so far for Linux. The main reason is Linux is quite secure as compare to Windows. Also, viruses cannot cause any serious damage if they are not activated by root user (that is why you need to use su or sudo command and always login as normal user). However if you are using any one of the following program then consider getting a good virus scanner:

  1. Windows via Samba
  2. Linux Email server
  3. Linux as a router etc

Most are windows virus that, may affects above program.

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How do I find out the MAC address of my Linux or FreeBSD system?

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Q. Can you tell me how can I find out MAC address under Linux or FreeBSD server?

A. A mac address is acronym for media access control address, is a unique address assigned to almost all-networking hardware such as Ethernet cards, router etc. Most layer 2 network protocols use one of three numbering spaces managed by the IEEE: MAC-48, EUI-48, and EUI-64, which are designed to be globally unique. (see mac address at wikipedia for more information).

Following command work with Linux and other UNIX oses:

ifconfig command

Ifconfig is used to configure the kernel-resident network interfaces.

In order to find out MAC address of system you can use ifconfig command as follows:
$ /sbin/ifconfig | grep HWaddr
Output:

eth0      Link encap:Ethernet  HWaddr 00:0F:EA:91:04:07

OR

$ /sbin/ifconfig
eth0      Link encap:Ethernet  HWaddr 00:0F:EA:91:04:07
        inet addr:192.168.1.2  Bcast:192.168.1.255  Mask:255.255.255.0
        inet6 addr: fe80::20f:eaff:fe91:407/64 Scope:Link
        UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
        RX packets:60400 errors:0 dropped:0 overruns:0 frame:0
        TX packets:109216 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:1000
        RX bytes:69273219 (66.0 MiB)  TX bytes:14285799 (13.6 MiB)
        Interrupt:18 Base address:0xc000

lo        Link encap:Local Loopback
        inet addr:127.0.0.1  Mask:255.0.0.0
        inet6 addr: ::1/128 Scope:Host
        UP LOOPBACK RUNNING  MTU:16436  Metric:1
        RX packets:3869 errors:0 dropped:0 overruns:0 frame:0
        TX packets:3869 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:0
        RX bytes:396498 (387.2 KiB)  TX bytes:396498 (387.2 KiB)

OR as a root user type following command:

# grep eth0 /var/log/dmesg
eth0: RealTek RTL8139 at 0xc000, 00:0f:ea:91:04:07, IRQ 18
eth0:  Identified 8139 chip type 'RTL-8100B/8139D'
eth0: link up, 100Mbps, full-duplex, lpa 0x45E1

FreeBSD example

# ifconfig
Output:

lnc0: flags=108843 mtu 1500
        inet 74.xx.yy.zzz netmask 0xfffffff0 broadcast 74.xx.yyy.zzz
        ether 00:0c:29:b8:92:8b
plip0: flags=108810 mtu 1500
lo0: flags=8049 mtu 16384
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3 
        inet6 ::1 prefixlen 128 
        inet 127.0.0.1 netmask 0xff000000

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Why is it possible to create symbolic links across file system boundaries?

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Symbolic links link by pathname rather than inode number. As you know, each pathname is a unique file on a system. Because of this, it is possible to create symbolic links across file system boundaries. Try to create symbolic links using following command:

$ touch /home/you/file1
# ln -s /home/you/file1 /tmp/file2

Find out inode of both file1 and file2

# ls -i /home/you/file1

1567789

# ls -i /tmp/file2

1567795
As you can see inode number are unique to each file. So it is possible to create symbolic links across file system boundaries. Please note that in above example both /tmp and /home are two different file systems.

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Howto: Prevent root user from being able to log in via SSH service

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Securing root account is one of the main tasks. Most systems have a password assigned to the root account. The first thing you do is assume that the password is always compromised. This does not mean that you should remove the password. The password is almost always necessary for console access to the machine. What it does mean is that you should not make it possible to use the password outside of the console. Direct root logins should only be allowed via the system console.

1) Login as a root user

2) Open /etc/ssh/sshd_config file
# vi /etc/ssh/sshd_config

3) Make changes to ssh server configuration find the following line or edit the line from:
PermitRootLogin yes

Change it to:
PermitRootLogin no

4) Save the changes

5) Restart sshd service
# /etc/init.d/sshd restart

The option PermitRootLogin specifies whether root can log in using ssh.