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Linux / UNIX: Kill User Session

How do I kill all users session under Linux or UNIX operating system using command prompt?
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Sysadmin because even developers need heroes!!!

Iptables Drop IP Address

How do I block particular IP addresses or host with the iptables command under Linux?
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resolvconf: Error: /etc/resolv.conf must be a symlink Error and Solution

When I try to restart my BIND 9 server under Ubuntu Linux using service bind9 restart or service bind9 start command, I get the following error:
resolvconf: Error: /etc/resolv.conf must be a symlink

How do I fix this problem?
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iptables: Read a List of IP Address From File And Block

How do I read a list of ip address (subnets) using a text file and block all of them using Linux iptables command?
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Verify: Keepalived IP Failover Working Or Not With tcpdump Command

I‘ve installed keepalived IP failover software as described here. But how do I verify it is working or not?
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HowTo: Display / Echo Path Settings In Linux / UNIX / *BSD

How do I display my current path settings under Linux / FreeBSD / OpenBSD / NetBSD / Apple OS X / UNIX like operating systems using bash or ksh or csh / tcsh shell?
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Existing lock /var/run/yum.pid: another copy is running as pid 523.

I‘m running the command yum update -y and getting the following error:

Loaded plugins: fastestmirror, priorities
Existing lock /var/run/yum.pid: another copy is running as pid 523.
Another app is currently holding the yum lock; waiting for it to exit…

The other application is: yum-updatesd-he
Memory : 14 M RSS ( 26 MB VSZ)
Started: Tue Feb 9 08:19:02 2010 – 14 day(s) 11:47:39 ago
State : Sleeping, pid: 523
Another app is currently holding the yum lock; waiting for it to exit…
The other application is: yum-updatesd-he
Memory : 14 M RSS ( 26 MB VSZ)
Started: Tue Feb 9 08:19:02 2010 – 14 day(s) 11:47:41 ago
State : Sleeping, pid: 523

How do I fix this problem?
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Find subnet mask on UNIX

Q. How do I find out subnet mask on UNIX operating system? Can you tell us the name of command and configuration file to store subnetmask under UNIX?

A. A subnetwork/ subnet is a range of logical addresses within the address space that is assigned to an organization. Subnetting is a hierarchical partitioning of the network address space of an organization (and of the network nodes of an autonomous system) into several subnets. Routers constitute borders between subnets. Communication to and from a subnet is mediated by one specific port of one specific router, at least momentarily.

Under UNIX / Linux, you need to use the ifconfig utility to assign an address to a network interface
and/or configure network interface parameters. Same command can display subnet. Just type command ifconfig:
$ ifconfig
OR
$ /sbin/ifconfig
OR
$ /sbin/ifconfig interface-name

lnc0: flags=108843 mtu 1500
        inet 72.122.61.51 netmask 0xfffffff8 broadcast 72.122.61.55
        ether 00:0c:29:32:8a:8c
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 

Look for inet line you will see an IP address, followed by netmask.

HowTo: Find Out Router Mac Address

How do I display or find out the MAC address for my small home / business router?
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