Tag Archives: linux

How to Install Kerberos 5 KDC Server on Linux for Authentication

Kerberos is a network authentication protocol.

Kerberos provides a strong cryptographic authentication against the devices which lets the client & servers to communicate in a more secured manner. It is designed to address network security problems.

When firewalls acts a solution to address the intrusion from the external networks, Kerberos usually used to address the intrusion and other security problems within the network.

The current version of Kerberos is version 5 which is called as KRB5.

To implement the Kerberos, we need to have the centralized authentication service running on server.

This service is called KEY DISTRIBUTION CENTER (KDC).

A server registered with KDC is trusted by all other computers in the Kerberos realm.

Sample krb5.conf File

Here’s an example krb5.conf file that contains all the REALM and domain to REALM mapping information,

# cat /etc/krb5.conf
 default = FILE:/var/log/krb5libs.log
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log

 default_realm = EXAMPLE.COM
 dns_lookup_realm = false
 dns_lookup_kdc = false
 ticket_lifetime = 24h
 renew_lifetime = 7d
 forwardable = true

  kdc = kerberos.example.com
  admin_server = kerberos.example.com

 .example.com = EXAMPLE.COM
 example.com = EXAMPLE.COM

Install Kerberos KDC server

For security reason, it is recommended to run the Kerberos (KDC) server on a separate server.

Download and install the krb5 server package.

# rpm -ivh krb5-server-1.10.3-10.el6_4.6.x86_64.rpm
Preparing...       ########################################### [100%]
   1:krb5-server   ########################################### [100%]

Verify that the following rpm are installed before configuring KDC:

# rpm -qa | grep -i krb5

Modify /etc/krb5.conf File

Change /etc/krb5.conf to reflect like the below with the appropriate REALM and DOMAIN_REALM mappings.

# cat /etc/krb5.conf
 default = FILE:/var/log/krb5libs.log
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log

 default_realm = MYREALM.COM
 dns_lookup_realm = false
 dns_lookup_kdc = false
 ticket_lifetime = 24h
 renew_lifetime = 7d
 forwardable = true

  kdc = elserver1.example.com
  admin_server = elserver1.example.com

 .myrealm.com = MYREALM.COM
 myrealm.com = MYREALM.COM

Modify kdc.conf File

Also the kdc.conf should be modified as shown below.

# cat /var/kerberos/krb5kdc/kdc.conf
 kdc_ports = 88
 kdc_tcp_ports = 88

  #master_key_type = aes256-cts
  acl_file = /var/kerberos/krb5kdc/kadm5.acl
  dict_file = /usr/share/dict/words
  admin_keytab = /var/kerberos/krb5kdc/kadm5.keytab
  supported_enctypes = aes256-cts:normal aes128-cts:normal des3-hmac-sha1:normal 
  arcfour-hmac:normal des-hmac-sha1:normal des-cbc-md5:normal des-cbc-crc:normal

Create KDC database

Next, create the KDC database using the kdb5_util command as shown below. As this stage, enter the appropriate pasword for the KDC database master key.

# /usr/sbin/kdb5_util create -s
Loading random data
Initializing database '/var/kerberos/krb5kdc/principal' for realm 'MYREALM.COM',
master key name 'K/M@MYREALM.COM'
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.
Enter KDC database master key:
Re-enter KDC database master key to verify:

Assign Administrator Privilege

The users can be granted administrator privileges to the database using the file /var/kerberos/krb5kdc/kadm5.acl.

# cat /var/kerberos/krb5kdc/kadm5.acl
*/admin@MYREALM.COM     *

In the above example, any principal in the MYREALM with an admin instance has all administrator privileges.

Create a Principal

Create the principal using the following command. In this example, I created the principal with the user name “eluser”.

# kadmin.local -q "addprinc eluser/admin"
Authenticating as principal root/admin@MYREALM.COM with password.
WARNING: no policy specified for eluser/admin@MYREALM.COM; defaulting to no policy
Enter password for principal "eluser/admin@MYREALM.COM":
Re-enter password for principal "eluser/admin@MYREALM.COM":
Principal "eluser/admin@MYREALM.COM" created.

Start the Kerberos Service

Start the KDC and kadmin daemons as shown below.

# service krb5kdc start
Starting Kerberos 5 KDC:               [  OK  ]

# service kadmin start
Starting Kerberos 5 Admin Server:      [  OK  ]

Configure Kerberos Client

Configure the Kerberos client to authenticate against the KDC database:

Now let’s see how to configure the krb5 client to authenticate against the Kerberos KDC database we created above.

Step 1: Install the krb5-libs and krb5-workstation packages on the client machine.

Step 2: Copy the /etc/krb5.conf from the KDC server to the client machine.

Step 3: Now we need to create the principal for the client in the KDC/Kerberos database.

You can use the below commands to create the principal for the client machine on the KDC master server. In the below example the I am creating a host principal for the client elserver3.example.com on the master KDC server elserver1.example.com

# kadmin.local -q "addprinc host/elserver3.example.com"
Authenticating as principal root/admin@MYREALM.COM with password.
WARNING: no policy specified for host/elserver1.example.com@MYREALM.COM; defaulting to no policy
Enter password for principal "host/elserver1.example.com@MYREALM.COM":
Re-enter password for principal "host/elserver1.example.com@MYREALM.COM":
Principal "host/elserver1.example.com@MYREALM.COM" created.

Step 4: Extract the krb5.keytab for the client from the KDC master server using the below command:

# kadmin.local -q "ktadd -k /etc/krb5.keytab host/elserver3.example.com"
Authenticating as principal root/admin@MYREALM.COM with password.
Entry for principal host/elserver3.example.com with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/elserver3.example.com with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/elserver3.example.com with kvno 2, encryption type des3-cbc-sha1 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/elserver3.example.com with kvno 2, encryption type arcfour-hmac added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/elserver3.example.com with kvno 2, encryption type des-hmac-sha1 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/elserver3.example.com with kvno 2, encryption type des-cbc-md5 added to keytab WRFILE:/etc/krb5.keytab.

This completes the configuration. You are all done at this stage.

From now on, everytime you establish a SSH, RSH connection the host verifies its identity against the KDC database using keytab file and it establishes secure connection over the Kerberos.

Ktadd is used a generate new keytab or add a principal to an existing keytab from the kadmin command.

Ktremove is used to remove the principal from an existing keytab. The command to remove the principal that we created above is,

kadmin.local -q "ktremove -k /etc/krb5.keytab –q all"

Delete a KDC database

For some reason, if you have to delete a KDC database, use the following command:

# kdb5_util -r MYREALM.COM destroy
kdb5_util: Deleting KDC database stored in /usr/local/var/krb5kdc/principal, you sure
(type yes to confirm)? <== yes
OK, deleting database '/usr/local/var/krb5kdc/principal'...

-f option in the above command forces the deletion of KDC database.

Backup and Restore KDC Database

To backup a KDC database to a file, use krb5_util_dump.

# kdb5_util dump kdcfile

# ls -l kdcfile
-rw-------. 1 root root 5382 Apr 10 07:25 kdcfile

To restore the KDC database from the dump file created in the above step, do the following:

# kdb5_util load kdcfile

Configuring Interface Bonding (Wheezy)

This is a brief article detailing the steps to configure network interface bonding on Debian Wheezy (7.0 stable). The procedure is very different from RHEL/CentOS. I will be configuring active-backup (i.e. failover) mode bonding – there are other modes available, including round-robin load-balanced, LACP aggregation, etc. Read /usr/share/doc/ifenslave-2.6/README.Debian or http://www.kernel.org/doc/Documentation/networking/bonding.txt for further information.

First, verify via ifconfig that your two slave interfaces are available – I’ll be bonding eth0 and eth1 into a bond called bond0:

Install the ifenslave package:

Next, stop networking. As you’re stopping networking, ensure that you’re connected to your host via a console of some form:

Modify /etc/network/interfaces. Remove (or comment out) any existing configuration for your slave interfaces (eth0 and eth1), and configure your new bond0 interface appropriately:

bond_miimon is the MII link monitoring frequency in milliseconds, and bond_{down,up}delay are the time, in milliseconds, to wait before disabling or enabling an interface in the bond (to safeguard against flapping), and should be a multiple of the bond_miimon value. You can adjust these values to suit your needs. These bond_<parameter> directives correlate to the <parameter> directives passed to the bonding module itself.

Once configured, start networking:

There is no need to fiddle with module loading (editing /etc/modules, creating a file under /etc/modprobe.d, etc.) – the ifenslave-2.6 package deposits scripts to do this for us. Take a look at /etc/network/if-pre-up.d/ifenslave to see this being done.

You can see the other scripts installed by ifenslave-2.6 with a dpkg -L:

Running ifconfig -a should now show the correct network configuration:

You can also view the contents of /proc/net/bonding/bond0 to check the status of the bond:

Linux Kernel Upgrade on Exadata(manual way)

Kernel upgrade can be applied node by node on exadata so there will be no service interruption. Kernel upgrades are required when you need new functionality or when you hit bugs on the current kernel version. I had to upgrade kernel of a box. It is a good experience and The following procedure is based on kernel upgrade on Oracle Linux 5.8 with Unbreakable Enterprise Kernel [2.6.32], a compute node of exadata.


==> If you have EM12C the targets on the host will be unavailable for upgrade period. Put them in blackout state so that no false alarms generated from them.

==> Run the upgrade step on X-windows like vnc. This will prevent any disconnection issues from ssh clients.

==> Disable all NFS mounts on the system. check the locations /etc/rc.local , /etc/fstab

==> Is there any asm operations going on the system. Wait for them to finish. Make sure no rebalance job is running on the ASM part. check v$asm_operation.

==> Backup the grup startup file /boot/grub/grub.conf. you might need it for rollback.

==> Shutdown the crs and disable crs auto start. Also shutdown any databases or listeners that are not registered with the csr.
[root@host1 ~]# /u01/app/ disable crs
CRS-4621: Oracle High Availability Services autostart is disabled.
[root@host1 ~]# /u01/app/ stop crs -f

==> Make sure crs is not running
[root@host1 ~]# ps -ef | grep d.bin
root 66664 60395 0 09:55 pts/1 00:00:00 grep d.bin

==> Reboot the system and make sure it is able to restart before any kernel changes 🙂

==> Check the ilom problem page and make sure there is no problem on the server. If there are any like memory problems etc. fix them.

==> Record the current kernel
[root@host1 ~]# uname -r

==> Check the server version and make sure the next kernel is designed for the server.
[root@host1 ~]# dmidecode -s system-product-name

==> Make sure enough space is available
[root@host1 ~]# df -h

==> Shutdown any database or listeners that hasn’t been registered with the crs. check the crs for the last time.
[root@host1 ~]# ps -ef | grep d.bin
root 66664 60395 0 09:55 pts/1 00:00:00 grep d.bin

==> upgrade the kernel

[root@host1 ~]# rpm -ivh kernel-uek-firmware-2.6.32-400.34.1.el5uek.noarch.rpm kernel-uek-2.6.32-400.34.1.el5uek.x86_64.rpm ofa-2.6.32-400.34.1.el5uek-1.5.1-
Preparing… ########################################### [100%]
1:kernel-uek-firmware ########################################### [ 33%]
2:kernel-uek ########################################### [ 67%]
3:ofa-2.6.32-400.34.1.el5########################################### [100%]

==> Reboot the system
[root@host1 ~]# reboot

==> Check ilom for any errors. Check /var/log/messages for any errors.

==> Check the new kernel version
[root@host1 ~]# uname -r

==> Start the crs and enable crs auto start
[root@host1 ~]# /u01/app/ enable crs
CRS-4622: Oracle High Availability Services autostart is enabled.
[root@host1 ~]# /u01/app/ start crs

==> Check if crs is starting

[root@host1 ~]# ps -ef | grep d.bin
root 11852 1 4 10:22 ? 00:00:00 /u01/app/ reboot
oracle 12013 1 0 10:22 ? 00:00:00 /u01/app/
oracle 12025 1 0 10:22 ? 00:00:00 /u01/app/
oracle 12109 1 1 10:22 ? 00:00:00 /u01/app/
root 12119 1 0 10:22 ? 00:00:00 /u01/app/
oracle 12122 1 1 10:22 ? 00:00:00 /u01/app/
root 12137 1 1 10:22 ? 00:00:00 /u01/app/
root 12150 1 0 10:22 ? 00:00:00 /u01/app/
root 12167 1 0 10:22 ? 00:00:00 /u01/app/
oracle 12169 1 1 10:22 ? 00:00:00 /u01/app/ -d -f
oracle 12187 1 2 10:22 ? 00:00:00 /u01/app/
root 12389 10620 0 10:23 pts/0 00:00:00 grep d.bin
[root@host1 ~]#

==> Enable any NFS mount on the system and mount them

==> On EM12c end the blackout period for the targets.

Now you can move on the other server in the cluster.

Partitioning with fdisk

1.1. fdisk usage

fdisk is started by typing (as root) fdisk device at the command prompt. device might be something like /dev/hda or /dev/sda. The basic fdisk commands you need are:

p print the partition table

n create a new partition

d delete a partition

q quit without saving changes

w write the new partition table and exit

Changes you make to the partition table do not take effect until you issue the write (w) command. Here is a sample partition table:

Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
   Device Boot    Start       End    Blocks   Id  System
/dev/hdb1   *         1       184    370912+  83  Linux
/dev/hdb2           185       368    370944   83  Linux
/dev/hdb3           369       552    370944   83  Linux
/dev/hdb4           553       621    139104   82  Linux swap

The first line shows the geometry of your hard drive. It may not be physically accurate, but you can accept it as though it were. The hard drive in this example is made of 32 double-sided platters with one head on each side (probably not true). Each platter has 621 concentric tracks. A 3-dimensional track (the same track on all disks) is called a cylinder. Each track is divided into 63 sectors. Each sector contains 512 bytes of data. Therefore the block size in the partition table is 64 heads * 63 sectors * 512 bytes er…divided by 1024. The start and end values are cylinders.

2.2. Four primary partitions

The overview:

Decide on the size of your swap space and where it ought to go. Divide up the remaining space for the three other partitions.


I start fdisk from the shell prompt:

# fdisk /dev/hdb 

which indicates that I am using the second drive on my IDE controller. When I print the (empty) partition table, I just get configuration information.

Command (m for help): p

Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes

I knew that I had a 1.2Gb drive, but now I really know: 64 * 63 * 512 * 621 = 1281982464 bytes. I decide to reserve 128Mb of that space for swap, leaving 1153982464. If I use one of my primary partitions for swap, that means I have three left for ext2 partitions. Divided equally, that makes for 384Mb per partition. Now I get to work.

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
Partition number (1-4): 1
First cylinder (1-621, default 1):<RETURN>
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-621, default 621): +384M

Next, I set up the partition I want to use for swap:

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
Partition number (1-4): 2
First cylinder (197-621, default 197):<RETURN>
Using default value 197
Last cylinder or +size or +sizeM or +sizeK (197-621, default 621): +128M

Now the partition table looks like this:

   Device Boot    Start       End    Blocks   Id  System
/dev/hdb1             1       196    395104   83  Linux
/dev/hdb2           197       262    133056   83  Linux

I set up the remaining two partitions the same way I did the first. Finally, I make the first partition bootable:

Command (m for help): a
Partition number (1-4): 1

And I make the second partition of type swap:

Command (m for help): t
Partition number (1-4): 2
Hex code (type L to list codes): 82
Changed system type of partition 2 to 82 (Linux swap)      
Command (m for help): p

The end result:

Disk /dev/hdb: 64 heads, 63 sectors, 621 cylinders
Units = cylinders of 4032 * 512 bytes
   Device Boot    Start       End    Blocks   Id  System
/dev/hdb1   *         1       196    395104+  83  Linux
/dev/hdb2           197       262    133056   82  Linux swap
/dev/hdb3           263       458    395136   83  Linux
/dev/hdb4           459       621    328608   83  Linux          

Finally, I issue the write command (w) to write the table on the disk.


1.3. Mixed primary and logical partitions

The overview: create one use one of the primary partitions to house all the extra partitions. Then create logical partitions within it. Create the other primary partitions before or after creating the logical partitions.


I start fdisk from the shell prompt:

# fdisk /dev/sda

which indicates that I am using the first drive on my SCSI chain.

First I figure out how many partitions I want. I know my drive has a 183Gb capacity and I want 26Gb partitions (because I happen to have back-up tapes that are about that size).

183Gb / 26Gb = ~7

so I will need 7 partitions. Even though fdisk accepts partition sizes expressed in Mb and Kb, I decide to calculate the number of cylinders that will end up in each partition because fdisk reports start and stop points in cylinders. I see when I enter fdisk that I have 22800 cylinders.

> The number of cylinders for this disk is set to 22800.  There is
> nothing wrong with that, but this is larger than 1024, and could in
> certain setups cause problems with: 1) software that runs at boot
> time (e.g., LILO) 2) booting and partitioning software from other
> OSs  (e.g., DOS FDISK, OS/2 FDISK)

So, 22800 total cylinders divided by seven partitions is 3258 cylinders. Each partition will be about 3258 cylinders long. I ignore the warning msg because this is not my boot drive.

Since I have 4 primary partitions, 3 of them can be 3258 long. The extended partition will have to be (4 * 3258), or 13032, cylinders long in order to contain the 4 logical partitions.

I enter the following commands to set up the first of the 3 primary partitions (stuff I type is bold ):

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
Partition number (1-4): 1
First cylinder (1-22800, default 1): <RETURN>
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-22800, default 22800): 3258

The last partition is the extended partition:

Partition number (1-4): 4
First cylinder (9775-22800, default 9775): <RETURN>
Using default value 9775
Last cylinder or +size or +sizeM or +sizeK (9775-22800, default 22800): <RETURN>
Using default value 22800

The result, when I issue the print table command is:

/dev/sda1             1      3258  26169853+  83  Linux
/dev/sda2          3259      6516  26169885   83  Linux
/dev/sda3          6517      9774  26169885   83  Linux
/dev/sda4          9775     22800 104631345    5  Extended

Next I segment the extended partition into 4 logical partitions, starting with the first logical partition, into 3258-cylinder segments. The logical partitions automatically start from /dev/sda5.

Command (m for help):  n
First cylinder (9775-22800, default 9775): <RETURN>
Using default value 9775
Last cylinder or +size or +sizeM or +sizeK (9775-22800, default 22800): 13032

The end result is:

   Device Boot    Start       End    Blocks   Id  System
/dev/sda1             1      3258  26169853+  83  Linux
/dev/sda2          3259      6516  26169885   83  Linux
/dev/sda3          6517      9774  26169885   83  Linux
/dev/sda4          9775     22800 104631345    5  Extended
/dev/sda5          9775     13032  26169853+  83  Linux
/dev/sda6         13033     16290  26169853+  83  Linux
/dev/sda7         16291     19584  26459023+  83  Linux
/dev/sda8         19585     22800  25832488+  83  Linux

Finally, I issue the write command (w) to write the table on the disk. To make the partitions usable, I will have to format each partition and then mount it.

1.4. Submitted Examples

I’d like to submit my partition layout, because it works well with any distribution of Linux (even big RPM based ones). I have one hard drive that … is 10 gigs, exactly. Windows can’t see above 9.3 gigs of it, but Linux can see it all, and use it all. It also has much more than 1024 cylenders.

Table 7. Partition layout example

Partition Mount point Size
/dev/hda1 /boot (15 megs)
/dev/hda2 windows XP partition (2 gigs)
/dev/hda3 extended (N/A)
/dev/hda5 swap space (64 megs)
/dev/hda6 /tmp (50 megs)
/dev/hda7 / (150 megs)
/dev/hda8 /usr (1.5 gigs)
/dev/hda9 /home (rest of drive)

I test new kernels for the USB mass storage, so that explains the large /boot partition. I install LILO into the MBR, and by default I boot windows (I’m not the only one to use this computer).


2.1. Formating Partitions

At the shell prompt, I begin making the file systems on my partitions. Continuing with the example in, this is:

# mke2fs /dev/sda1

I need to do this for each of my partitions, but not for /dev/sda4 (my extended partition). Linux supports types of file systems other than ext2. You can find out what kinds your kernel supports by looking in: /usr/src/linux/include/linux/fs.h

The most common file systems can be made with programs in /sbin that start with “mk” like mkfs.msdos and mke2fs.

2.2. Activating Swap Space

To set up a swap partition:

# mkswap -f /dev/hda5

To activate the swap area:

# swapon  /dev/hda5

Normally, the swap area is activated by the initialization scripts at boot time.

2.3. Mounting Partitions

Mounting a partition means attaching it to the linux file system. To mount a linux partition:

# mount -t ext2 /dev/sda1 /opt
-t ext2

File system type. Other types you are likely to use are:

  • ext3 (journaling sile system based on ext2)

  • msdos (DOS)

  • hfs (mac)

  • iso9660 (CDROM)

  • nfs (network file system)


Device name. Other device names you are likely to use:

  • /dev/hdb2 (second partition in second IDE drive)

  • /dev/fd0 (floppy drive A)

  • /dev/cdrom (CDROM)


mount point. This is where you want to “see” your partition. When you type ls /opt, you can see what is in /dev/sda1. If there are already some directories and/or files under /opt, they will be invisible after this mount command.

20 Linux System Monitoring Tools Every SysAdmin Should Know

Need to monitor Linux server performance? Try these built-in commands and a few add-on tools. Most Linux distributions are equipped with tons of monitoring. These tools provide metrics which can be used to get information about system activities. You can use these tools to find the possible causes of a performance problem. The commands discussed below are some of the most basic commands when it comes to system analysis and debugging server issues such as:

  1. Finding out bottlenecks.
  2. Disk (storage) bottlenecks.
  3. CPU and memory bottlenecks.
  4. Network bottlenecks.

#1: top – Process Activity Command

The top program provides a dynamic real-time view of a running system i.e. actual process activity. By default, it displays the most CPU-intensive tasks running on the server and updates the list every five seconds.

Fig.01: Linux top command

Fig.01: Linux top command

Commonly Used Hot Keys

The top command provides several useful hot keys:

Hot Key Usage
t Displays summary information off and on.
m Displays memory information off and on.
A Sorts the display by top consumers of various system resources. Useful for quick identification of performance-hungry tasks on a system.
f Enters an interactive configuration screen for top. Helpful for setting up top for a specific task.
o Enables you to interactively select the ordering within top.
r Issues renice command.
k Issues kill command.
z Turn on or off color/mono

=> Related: How do I Find Out Linux CPU Utilization?

#2: vmstat – System Activity, Hardware and System Information

The command vmstat reports information about processes, memory, paging, block IO, traps, and cpu activity.
# vmstat 3
Sample Outputs:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0      0 2540988 522188 5130400    0    0     2    32    4    2  4  1 96  0  0
 1  0      0 2540988 522188 5130400    0    0     0   720 1199  665  1  0 99  0  0
 0  0      0 2540956 522188 5130400    0    0     0     0 1151 1569  4  1 95  0  0
 0  0      0 2540956 522188 5130500    0    0     0     6 1117  439  1  0 99  0  0
 0  0      0 2540940 522188 5130512    0    0     0   536 1189  932  1  0 98  0  0
 0  0      0 2538444 522188 5130588    0    0     0     0 1187 1417  4  1 96  0  0
 0  0      0 2490060 522188 5130640    0    0     0    18 1253 1123  5  1 94  0  0

Display Memory Utilization Slabinfo

# vmstat -m

Get Information About Active / Inactive Memory Pages

# vmstat -a
=> Related: How do I find out Linux Resource utilization to detect system bottlenecks?

#3: w – Find Out Who Is Logged on And What They Are Doing

w command displays information about the users currently on the machine, and their processes.
# w username
# w vivek

Sample Outputs:

 17:58:47 up 5 days, 20:28,  2 users,  load average: 0.36, 0.26, 0.24
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0       14:55    5.00s  0.04s  0.02s vim /etc/resolv.conf
root     pts/1       17:43    0.00s  0.03s  0.00s w

#4: uptime – Tell How Long The System Has Been Running

The uptime command can be used to see how long the server has been running. The current time, how long the system has been running, how many users are currently logged on, and the system load averages for the past 1, 5, and 15 minutes.
# uptime

 18:02:41 up 41 days, 23:42,  1 user,  load average: 0.00, 0.00, 0.00

1 can be considered as optimal load value. The load can change from system to system. For a single CPU system 1 – 3 and SMP systems 6-10 load value might be acceptable.

#5: ps – Displays The Processes

ps command will report a snapshot of the current processes. To select all processes use the -A or -e option:
# ps -A
Sample Outputs:

  PID TTY          TIME CMD
    1 ?        00:00:02 init
    2 ?        00:00:02 migration/0
    3 ?        00:00:01 ksoftirqd/0
    4 ?        00:00:00 watchdog/0
    5 ?        00:00:00 migration/1
    6 ?        00:00:15 ksoftirqd/1
 4881 ?        00:53:28 java
 4885 tty1     00:00:00 mingetty
 4886 tty2     00:00:00 mingetty
 4887 tty3     00:00:00 mingetty
 4888 tty4     00:00:00 mingetty
 4891 tty5     00:00:00 mingetty
 4892 tty6     00:00:00 mingetty
 4893 ttyS1    00:00:00 agetty
12853 ?        00:00:00 cifsoplockd
12854 ?        00:00:00 cifsdnotifyd
14231 ?        00:10:34 lighttpd
14232 ?        00:00:00 php-cgi
54981 pts/0    00:00:00 vim
55465 ?        00:00:00 php-cgi
55546 ?        00:00:00 bind9-snmp-stat
55704 pts/1    00:00:00 ps

ps is just like top but provides more information.

Show Long Format Output

# ps -Al
To turn on extra full mode (it will show command line arguments passed to process):
# ps -AlF

To See Threads ( LWP and NLWP)

# ps -AlFH

To See Threads After Processes

# ps -AlLm

Print All Process On The Server

# ps ax
# ps axu

Print A Process Tree

# ps -ejH
# ps axjf
# pstree

Print Security Information

# ps -eo euser,ruser,suser,fuser,f,comm,label
# ps axZ
# ps -eM

See Every Process Running As User Vivek

# ps -U vivek -u vivek u

Set Output In a User-Defined Format

# ps -eo pid,tid,class,rtprio,ni,pri,psr,pcpu,stat,wchan:14,comm
# ps axo stat,euid,ruid,tty,tpgid,sess,pgrp,ppid,pid,pcpu,comm
# ps -eopid,tt,user,fname,tmout,f,wchan

Display Only The Process IDs of Lighttpd

# ps -C lighttpd -o pid=
# pgrep lighttpd
# pgrep -u vivek php-cgi

Display The Name of PID 55977

# ps -p 55977 -o comm=

Find Out The Top 10 Memory Consuming Process

# ps -auxf | sort -nr -k 4 | head -10

Find Out top 10 CPU Consuming Process

# ps -auxf | sort -nr -k 3 | head -10

#6: free – Memory Usage

The command free displays the total amount of free and used physical and swap memory in the system, as well as the buffers used by the kernel.
# free
Sample Output:

            total       used       free     shared    buffers     cached
Mem:      12302896    9739664    2563232          0     523124    5154740
-/+ buffers/cache:    4061800    8241096
Swap:      1052248          0    1052248

=> Related: :

  1. Linux Find Out Virtual Memory PAGESIZE
  2. Linux Limit CPU Usage Per Process
  3. How much RAM does my Ubuntu / Fedora Linux desktop PC have?

#7: iostat – Average CPU Load, Disk Activity

The command iostat report Central Processing Unit (CPU) statistics and input/output statistics for devices, partitions and network filesystems (NFS).
# iostat
Sample Outputs:

Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in) 	06/26/2009
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           3.50    0.09    0.51    0.03    0.00   95.86
Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              22.04        31.88       512.03   16193351  260102868
sda1              0.00         0.00         0.00       2166        180
sda2             22.04        31.87       512.03   16189010  260102688
sda3              0.00         0.00         0.00       1615          0

=> Related: : Linux Track NFS Directory / Disk I/O Stats

#8: sar – Collect and Report System Activity

The sar command is used to collect, report, and save system activity information. To see network counter, enter:
# sar -n DEV | more
To display the network counters from the 24th:
# sar -n DEV -f /var/log/sa/sa24 | more
You can also display real time usage using sar:
# sar 4 5
Sample Outputs:

Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in) 		06/26/2009
06:45:12 PM       CPU     %user     %nice   %system   %iowait    %steal     %idle
06:45:16 PM       all      2.00      0.00      0.22      0.00      0.00     97.78
06:45:20 PM       all      2.07      0.00      0.38      0.03      0.00     97.52
06:45:24 PM       all      0.94      0.00      0.28      0.00      0.00     98.78
06:45:28 PM       all      1.56      0.00      0.22      0.00      0.00     98.22
06:45:32 PM       all      3.53      0.00      0.25      0.03      0.00     96.19
Average:          all      2.02      0.00      0.27      0.01      0.00     97.70

=> Related: : How to collect Linux system utilization data into a file

#9: mpstat – Multiprocessor Usage

The mpstat command displays activities for each available processor, processor 0 being the first one. mpstat -P ALL to display average CPU utilization per processor:
# mpstat -P ALL
Sample Output:

Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in)	 	06/26/2009
06:48:11 PM  CPU   %user   %nice    %sys %iowait    %irq   %soft  %steal   %idle    intr/s
06:48:11 PM  all    3.50    0.09    0.34    0.03    0.01    0.17    0.00   95.86   1218.04
06:48:11 PM    0    3.44    0.08    0.31    0.02    0.00    0.12    0.00   96.04   1000.31
06:48:11 PM    1    3.10    0.08    0.32    0.09    0.02    0.11    0.00   96.28     34.93
06:48:11 PM    2    4.16    0.11    0.36    0.02    0.00    0.11    0.00   95.25      0.00
06:48:11 PM    3    3.77    0.11    0.38    0.03    0.01    0.24    0.00   95.46     44.80
06:48:11 PM    4    2.96    0.07    0.29    0.04    0.02    0.10    0.00   96.52     25.91
06:48:11 PM    5    3.26    0.08    0.28    0.03    0.01    0.10    0.00   96.23     14.98
06:48:11 PM    6    4.00    0.10    0.34    0.01    0.00    0.13    0.00   95.42      3.75
06:48:11 PM    7    3.30    0.11    0.39    0.03    0.01    0.46    0.00   95.69     76.89

=> Related: : Linux display each multiple SMP CPU processors utilization individually.

#10: pmap – Process Memory Usage

The command pmap report memory map of a process. Use this command to find out causes of memory bottlenecks.
# pmap -d PID
To display process memory information for pid # 47394, enter:
# pmap -d 47394
Sample Outputs:

47394:   /usr/bin/php-cgi
Address           Kbytes Mode  Offset           Device    Mapping
0000000000400000    2584 r-x-- 0000000000000000 008:00002 php-cgi
0000000000886000     140 rw--- 0000000000286000 008:00002 php-cgi
00000000008a9000      52 rw--- 00000000008a9000 000:00000   [ anon ]
0000000000aa8000      76 rw--- 00000000002a8000 008:00002 php-cgi
000000000f678000    1980 rw--- 000000000f678000 000:00000   [ anon ]
000000314a600000     112 r-x-- 0000000000000000 008:00002 ld-2.5.so
000000314a81b000       4 r---- 000000000001b000 008:00002 ld-2.5.so
000000314a81c000       4 rw--- 000000000001c000 008:00002 ld-2.5.so
000000314aa00000    1328 r-x-- 0000000000000000 008:00002 libc-2.5.so
000000314ab4c000    2048 ----- 000000000014c000 008:00002 libc-2.5.so
00002af8d48fd000       4 rw--- 0000000000006000 008:00002 xsl.so
00002af8d490c000      40 r-x-- 0000000000000000 008:00002 libnss_files-2.5.so
00002af8d4916000    2044 ----- 000000000000a000 008:00002 libnss_files-2.5.so
00002af8d4b15000       4 r---- 0000000000009000 008:00002 libnss_files-2.5.so
00002af8d4b16000       4 rw--- 000000000000a000 008:00002 libnss_files-2.5.so
00002af8d4b17000  768000 rw-s- 0000000000000000 000:00009 zero (deleted)
00007fffc95fe000      84 rw--- 00007ffffffea000 000:00000   [ stack ]
ffffffffff600000    8192 ----- 0000000000000000 000:00000   [ anon ]
mapped: 933712K    writeable/private: 4304K    shared: 768000K

The last line is very important:

  • mapped: 933712K total amount of memory mapped to files
  • writeable/private: 4304K the amount of private address space
  • shared: 768000K the amount of address space this process is sharing with others

=> Related: : Linux find the memory used by a program / process using pmap command

#11 and #12: netstat and ss – Network Statistics

The command netstat displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. ss command is used to dump socket statistics. It allows showing information similar to netstat. See the following resources about ss and netstat commands:

#13: iptraf – Real-time Network Statistics

The iptraf command is interactive colorful IP LAN monitor. It is an ncurses-based IP LAN monitor that generates various network statistics including TCP info, UDP counts, ICMP and OSPF information, Ethernet load info, node stats, IP checksum errors, and others. It can provide the following info in easy to read format:

  • Network traffic statistics by TCP connection
  • IP traffic statistics by network interface
  • Network traffic statistics by protocol
  • Network traffic statistics by TCP/UDP port and by packet size
  • Network traffic statistics by Layer2 address

Fig.02: General interface statistics: IP traffic statistics by network interface

Fig.02: General interface statistics: IP traffic statistics by network interface

Fig.03 Network traffic statistics by TCP connection

Fig.03 Network traffic statistics by TCP connection

#14: tcpdump – Detailed Network Traffic Analysis

The tcpdump is simple command that dump traffic on a network. However, you need good understanding of TCP/IP protocol to utilize this tool. For.e.g to display traffic info about DNS, enter:
# tcpdump -i eth1 'udp port 53'
To display all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not, for example, SYN and FIN packets and ACK-only packets, enter:
# tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
To display all FTP session to, enter:
# tcpdump -i eth1 'dst and (port 21 or 20'
To display all HTTP session to
# tcpdump -ni eth0 'dst and tcp and port http'
Use wireshark to view detailed information about files, enter:
# tcpdump -n -i eth1 -s 0 -w output.txt src or dst port 80

#15: strace – System Calls

Trace system calls and signals. This is useful for debugging webserver and other server problems. See how to use to trace the process and see What it is doing.

#16: /Proc file system – Various Kernel Statistics

/proc file system provides detailed information about various hardware devices and other Linux kernel information. See Linux kernel /proc documentations for further details. Common /proc examples:
# cat /proc/cpuinfo
# cat /proc/meminfo
# cat /proc/zoneinfo
# cat /proc/mounts

17#: Nagios – Server And Network Monitoring

Nagios is a popular open source computer system and network monitoring application software. You can easily monitor all your hosts, network equipment and services. It can send alert when things go wrong and again when they get better. FAN is “Fully Automated Nagios”. FAN goals are to provide a Nagios installation including most tools provided by the Nagios Community. FAN provides a CDRom image in the standard ISO format, making it easy to easilly install a Nagios server. Added to this, a wide bunch of tools are including to the distribution, in order to improve the user experience around Nagios.

18#: Cacti – Web-based Monitoring Tool

Cacti is a complete network graphing solution designed to harness the power of RRDTool’s data storage and graphing functionality. Cacti provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features out of the box. All of this is wrapped in an intuitive, easy to use interface that makes sense for LAN-sized installations up to complex networks with hundreds of devices. It can provide data about network, CPU, memory, logged in users, Apache, DNS servers and much more. See how to install and configure Cacti network graphing tool under CentOS / RHEL.

#19: KDE System Guard – Real-time Systems Reporting and Graphing

KSysguard is a network enabled task and system monitor application for KDE desktop. This tool can be run over ssh session. It provides lots of features such as a client/server architecture that enables monitoring of local and remote hosts. The graphical front end uses so-called sensors to retrieve the information it displays. A sensor can return simple values or more complex information like tables. For each type of information, one or more displays are provided. Displays are organized in worksheets that can be saved and loaded independently from each other. So, KSysguard is not only a simple task manager but also a very powerful tool to control large server farms.

Fig.05 KDE System Guard

Fig.05 KDE System Guard {Image credit: Wikipedia}

See the KSysguard handbook for detailed usage.

#20: Gnome System Monitor – Real-time Systems Reporting and Graphing

The System Monitor application enables you to display basic system information and monitor system processes, usage of system resources, and file systems. You can also use System Monitor to modify the behavior of your system. Although not as powerful as the KDE System Guard, it provides the basic information which may be useful for new users:

  • Displays various basic information about the computer’s hardware and software.
  • Linux Kernel version
  • GNOME version
  • Hardware
  • Installed memory
  • Processors and speeds
  • System Status
  • Currently available disk space
  • Processes
  • Memory and swap space
  • Network usage
  • File Systems
  • Lists all mounted filesystems along with basic information about each.

Fig.06 The Gnome System Monitor application

Fig.06 The Gnome System Monitor application

Bonus: Additional Tools

A few more tools:

  • nmap – scan your server for open ports.
  • lsof – list open files, network connections and much more.
  • ntop web based tool – ntop is the best tool to see network usage in a way similar to what top command does for processes i.e. it is network traffic monitoring software. You can see network status, protocol wise distribution of traffic for UDP, TCP, DNS, HTTP and other protocols.
  • Conky – Another good monitoring tool for the X Window System. It is highly configurable and is able to monitor many system variables including the status of the CPU, memory, swap space, disk storage, temperatures, processes, network interfaces, battery power, system messages, e-mail inboxes etc.
  • GKrellM – It can be used to monitor the status of CPUs, main memory, hard disks, network interfaces, local and remote mailboxes, and many other things.
  • vnstat – vnStat is a console-based network traffic monitor. It keeps a log of hourly, daily and monthly network traffic for the selected interface(s).
  • htop – htop is an enhanced version of top, the interactive process viewer, which can display the list of processes in a tree form.
  • mtr – mtr combines the functionality of the traceroute and ping programs in a single network diagnostic tool.