Wednesday 31 December 2014

strace & system call tracing

I had written this as few of my colleagues had requested to know how strace works, hope below articles helps out.

Today being the last day of 2014, I thought to share publicly and wishing all readers HAPPY  NEW  YEAR - 2015 " 

The strace tool is one of the most powerful problem determination tools available for Linux. It traces the thin layer (the system calls) between a process and the Linux kernel. System call tracing is particularly useful as a first investigation tool or for problems that involve a call to the operating system.

A system call is a special type of function that is run inside the kernel. It provides fair and secure access to system resources such as disk, network, and memory. System calls also provide access to kernel services such as inter-process communication and system information.

When to use ?

The strace tool should be used as a first investigation tool or for problems that are related or involved  at the operating system level. the system call trace will clearly show the cause of problem. Experienced users might use strace either way until they narrow down the scope of a problem.

The following example uses a simple program to show how to use strace, I would try to open an file which doesn't exist.

root@localhost]# cat main.c 


int main()

int fd;
int i=0;

fd = open( "/tmp/foo", O_RDONLY);

if (fd < 0)

return i;

# gcc main.c -o ./main
# strace -o main.strace ./main
# cat -n ./main.strace 
     1 execve("./main", ["./main"], [/* 21 vars */]) = 0
     2 brk(0)                                  = 0x8697000
     3 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7774000
     4 access("/etc/", R_OK)      = -1 ENOENT (No such file or directory)
     5 open("/etc/", O_RDONLY)      = 3
     6 fstat64(3, {st_mode=S_IFREG|0644, st_size=28116, ...}) = 0
     7 mmap2(NULL, 28116, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb776d000
     8 close(3)                                = 0
     9 open("/lib/", O_RDONLY)        = 3
    10 read(3, "\177ELF\1\1\1\3\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\220\356\300\0004\0\0\0"..., 512) = 512
    11 fstat64(3, {st_mode=S_IFREG|0755, st_size=1906308, ...}) = 0
    12 mmap2(0xbf8000, 1661356, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xbf8000
    13 mmap2(0xd88000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x190) = 0xd88000
    14 mmap2(0xd8b000, 10668, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xd8b000
    15 close(3)                                = 0
    16 mprotect(0xd88000, 8192, PROT_READ)     = 0
    17 mprotect(0xbe6000, 4096, PROT_READ)     = 0
    18 munmap(0xb776d000, 28116)               = 0
    19 open("/tmp/foo", O_RDONLY)              = -1 ENOENT (No such file or directory)
    20 exit_group(5)                           = ?

In this strace output, the vast majority of the system calls are actually for process initialization. In fact, the only system call (on line 19) from the actual program code is open("/tmp/foo", O _ RDONLY ). Also notice that there are no system calls from the if statement or any other code in the program because the if statement does not invoke a system call.

Below would be detail how above code works line by line: 

Line #1:  The execve system call (or one of the exec system calls) is always the first system call in the strace output if strace is used to trace a program off the command line. The strace tool forks, executes the program, and the exec system call actually returns as the first system call in the new process.

Line #2: The brk system call is called with an argument of zero to find the current "break point." This is the beginning of memory management for the process.

Line #3: The mmap call is used to create an anonymous 4KB page. The address of this page is at 0xb7774000

Line #4: This line attempts to open the file. This file contains a list of ELF shared libraries that are to be pre-loaded before a program is able to run.

Line #5-#9: These lines involve finding and loading the libc library.

Line #10: Loads in the ELF header for the libc library.

Line #11: Gets more information (including size) for the libc library file.

Line #12:  This line actually loads ( mmaps ) the contents of libc into memory at address at 0xbf8000

Line #13: This line loads the data section at address 0xd88000 for 12288 bytes, from the beginning of memory segment (0x00d88000). According to the ELF layout of, the data section starts at 0x00d881c8 , but that section must be aligned on 0x1000 boundaries (hence the offset of 0x00d88000 )

# readelf -l /lib/

Elf file type is DYN (Shared object file) Entry point 0xc0ee90 There are 10 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x00bf8034 0x00bf8034 0x00140 0x00140 R E 0x4
  INTERP         0x15c2c8 0x00d542c8 0x00d542c8 0x00013 0x00013 R   0x1
      [Requesting program interpreter: /lib/]
  LOAD           0x000000 0x00bf8000 0x00bf8000 0x18ff98 0x18ff98 R E 0x1000
  LOAD           0x1901c8 0x00d881c8 0x00d881c8 0x027d4 0x057e4 RW  0x1000
  DYNAMIC        0x191d7c 0x00d89d7c 0x00d89d7c 0x000f8 0x000f8 RW  0x4

Line #14: Creates an anonymous memory segment for the bss section This is a special section of a loaded executable or shared library for uninitialized data. Because the data is not initialized, the storage for it is not included in an ELF object like a shared library (there are no real data values to store). Instead, memory is allocated(0xd8b000)for the bss section when the library is loaded.

Line #15: Closes the file descriptor for libc.

Line #16-#17: This removes any protection for a region of memory at 0xd88000

Line #18: unmap files or devices into memory

Line #19: The only system call from the actual program code.

Line #20: Exits the process with a return code of 5.

It can also be useful to time both the difference between system call entries and the time spent in the system calls. With this information, it is possible to get the time spent in the user code between the system calls.

# strace -Tr ./main
     0.000000 execve("./main", ["./main"], [/* 21 vars */]) = 0 <0.000169>
     0.000406 brk(0)                    = 0x8b26000 <0.000014>
     0.000194 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb77e3000 <0.000028>
     0.000111 access("/etc/", R_OK) = -1 ENOENT (No such file or directory) <0.000022>
     0.000117 open("/etc/", O_RDONLY) = 3 <0.000022>
     0.000068 fstat64(3, {st_mode=S_IFREG|0644, st_size=28116, ...}) = 0 <0.000000>

Another useful way to time system calls is with the -c switch. This switch summarizes the output in tabular form:

# strace -c ./main
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  -nan    0.000000           0         1           read
  -nan    0.000000           0         3         1 open
  -nan    0.000000           0         2           close
  -nan    0.000000           0         1           execve
  -nan    0.000000           0         1         1 access

Sometimes it is necessary to trace an existing process that is running, such as a Web daemon (such as apache) or xinetd. The strace tool provides a simple way to attach to running processes with the -p switch:

# strace -p <PID>

I would conclude this article, by letting everyone know that strace when used effectively can narrow the scope of the problem analysis.  

Tuesday 23 December 2014

automate kickstart installations in spacewalk #Redhat 6 / CentOS 6

In my previous article ( ), I had written on how to configure and install spacewalk. Here, I would let you know on how to automate the installations using spacewalk.

My Environment:
Hostname: spacewalk
Environment: CentOS 6.6 x86_64
Spacewalk version: 2.2​

For automating the installation of a Linux system a method called kickstart can be used. First, we have to setup a directory structure on the spacewalk server, copy the directory of the below from your CentOS/Redhat DVD to /var/distro-trees/CentOS6.6-x86_64.

- images
- isolinux
- repodata

Next, open the spacewalk console and navigate to the following location:

systems -> kickstart -> distributions -> new distribution.

Next step is to create a kickstart profile for the channel and distribution. Open the spacewalk console and navigate to the following location:

systems -> kickstart -> create new kickstart profile

Enter the following parameters for the new kickstart profile:

Label: CentOS66-minimal
Base channel: CentOS 6.6 Base - x86_64
Kickstartable tree: CentOS6.6-x86_64
Virtualization type: none

Also, have a look at the other tabs to have an idea of the configuration options which are available, possible interesting areas are provided as below snap where it is self-explanatory reader could work themselves.

We are now creating a new virtual machine, so make sure that your spacewalk server is able to resolve the name resolution. I would leave this to reader to configure DNS.

Next, build an ISO image as described below, on which your ISO image(generated.iso) from where you executed the command.

[root@spacewalk ~]# cobbler buildiso
task started: 2014-12-23_112833_buildiso
task started (id=Build Iso, time=Tue Dec 23 11:28:33 2014)
using/creating buildisodir: /var/cache/cobbler/buildiso
building tree for isolinux
copying miscellaneous files
copying kernels and initrds for profiles
generating a isolinux.cfg
generating profile list
done writing config
running: mkisofs -o /root/generated.iso -r -b isolinux/isolinux.bin -c isolinux/ -no-emul-boot -boot-load-size 4 -boot-info-table -V Cobbler\ Install -R -J -T /var/cache/cobbler/buildiso
received on stdout: 
received on stderr: I: -input-charset not specified, using utf-8 (detected in locale settings)
Size of boot image is 4 sectors -> No emulation
 26.00% done, estimate finish Tue Dec 23 11:28:34 2014
 51.90% done, estimate finish Tue Dec 23 11:28:34 2014
 77.89% done, estimate finish Tue Dec 23 11:28:34 2014
Total translation table size: 4029
Total rockridge attributes bytes: 1320
Total directory bytes: 4700
Path table size(bytes): 40
Max brk space used 1b000
19272 extents written (37 MB)

ISO build complete
You may wish to delete: /var/cache/cobbler/buildiso
The output file is: /root/generated.iso
[root@spacewalk ~]#

[root@spacewalk ~]# ls -l generated.iso
-rw-r--r-- 1 root root 39469056 Dec 23 11:28 generated.iso
[root@spacewalk ~]# 

On your host, create a new virtual machine and provide it with the generated.iso file to boot from. Upon boot you will see a menu allowing you to specify the Centos66-minimal setup to be installed.

Select this entry and the setup will install a base 64 bit CentOS 6.6 Linux system. If all goes well, this will happen completely automated, without any user intervention whatsoever.

I, would configure new server as a client to the spacewalk server in coming articles.

Tuesday 2 December 2014

Install/Configure spacewalk - CentOS 6.6

Spacewalk is an open source Linux systems management solution.Spacewalk is the upstream community project from which the Red Hat Satellite product is derived​.

Differences between Spacewalk and Red Hat Satellite​:

What Can Spacewalk Do?​

- Inventory your systems (hardware and software information)
- Install and update software on your systems
- Collect and distribute your custom software packages into manageable groups
- Provision (kickstart) your systems
- Manage and deploy configuration files to your systems
- Monitor your systems
- Provision and start/stop/configure virtual guests
- Distribute content across multiple geographical sites in an efficient manner.


- Outbound open ports 80, 443, 4545 (only if you want to enable monitoring)
- Inbound open ports 80, 443, 5222 (only if you want to push actions to client machines) and 5269 (only for push actions to a Spacewalk Proxy), 69 udp if you want to use tftp
- Storage for database: 250 KiB per client system + 500 KiB per channel + 230 KiB per package in -channel (i.e. 1.1GiB for channel with 5000 packages)
- Storage for packages (default /var/satellite): Depends on what you're storing; Red Hat recommend 6GB per channel for their channels
- 2GB RAM minimum, 4GB recommended
- Make sure your underlying OS is fully up-to-date.​

​My Environment :
Hostname: spacewalk
Environment: CentOS 6.6 x86_64
Spacewalk version : 2.2​

Setting up Spacewalk repo:
[root@spacewalk ~]# rpm -Uvh
warning: /var/tmp/rpm-tmp.rexVMa: Header V4 DSA/SHA1 Signature, key ID 863a853d: NOKEY
Preparing...                ########################################### [100%]
   1:spacewalk-repo         ########################################### [100%]

[root@spacewalk ~]# 

Jpackage repository:

For Spacewalk on EL 6, a couple of additional dependencies are needed from jpackage configure the following yum repository before beginning your Spacewalk installation
root@spacewalk:~]# cat > /etc/yum.repos.d/jpackage-generic.repo << EOF
name=JPackage generic


Spacewalk requires a Java Virtual Machine with version 1.6.0 or greater. ​EPEL - Extra Packages for Enterprise Linux contains a version of the openjdk that works with Spacewalk. Other dependencies can get installed from EPEL as well. To get packages from EPEL just install this RPM:

warning: /var/tmp/rpm-tmp.1MCWEN: Header V3 RSA/SHA256 Signature, key ID 0608b895: NOKEY
Preparing...                ########################################### [100%]
   1:epel-release           ########################################### [100%]

​​[root@spacewalk ~]# 

Database server:
You can let Spacewalk setup the PostgreSQL server on your machine without any manual intervention.​
[root@spacewalk ~]# yum install spacewalk-setup-postgresql
Loaded plugins: fastestmirror, security
Setting up Install Process
Dependency Installed:
  postgresql-pltcl.x86_64 0:8.4.20-1.el6_5                            postgresql-server.x86_64 0:8.4.20-1.el6_5                            tcl.x86_64 1:8.5.7-6.el6                           

[root@spacewalk ~]# 

Installing Spacewalk:

yum to install the necessary packages. This will pull down and install the set of RPMs required to get Spacewalk to run
[root@spacewalk ~]# yum install spacewalk-postgresql
Loaded plugins: fastestmirror, security
Setting up Install Process
Loading mirror speeds from cached hostfile
Transaction Summary
Install     323 Package(s)
Upgrade       2 Package(s)

Total download size: 241 M
Is this ok [y/N]:y
warning: rpmts_HdrFromFdno: Header V4 DSA/SHA1 Signature, key ID 863a853d: NOKEY
Retrieving key from
Importing GPG key 0x863A853D:
 Userid: "Spacewalk <>"
 From  :
Is this ok [y/N]: y
warning: rpmts_HdrFromFdno: Header V3 DSA/SHA1 Signature, key ID c431416d: NOKEY
Retrieving key from
Importing GPG key 0xC431416D:
 Userid: "JPackage Project (JPP Official Keys) <>"
 From  :
Is this ok [y/N]: y
warning: rpmts_HdrFromFdno: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
Importing GPG key 0xC105B9DE:
 Userid : CentOS-6 Key (CentOS 6 Official Signing Key) <>
 Package: centos-release-6-6.el6.centos.12.2.x86_64 (@anaconda-CentOS-201410241409.x86_64/6.6)
 From   : /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
Is this ok [y/N]: y
warning: rpmts_HdrFromFdno: Header V3 RSA/SHA256 Signature, key ID 0608b895: NOKEY
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
Importing GPG key 0x0608B895:
 Userid : EPEL (6) <>
 Package: epel-release-6-8.noarch (installed)
 From   : /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
Is this ok [y/N]: y
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
Warning: RPMDB altered outside of yum.
  Installing : libgcj-4.4.7-11.el6.x86_64                                                                                                                                              1/327 
  Updating   : 1:java-1.6.0-openjdk-        
Dependency Updated:
  java-1.6.0-openjdk.x86_64 1:                                              policycoreutils.x86_64 0:2.0.83-19.47.el6_6.1                                             

[root@spacewalk ~]#

Configuring Spacewalk:
[root@spacewalk ~]# spacewalk-setup --disconnected
** Database: Setting up database connection for PostgreSQL backend.
** Database: Installing the database:
** Database: This is a long process that is logged in:
** Database:   /var/log/rhn/install_db.log
*** Progress: #
** Database: Installation complete.
** Database: Populating database.
*** Progress: #####################################
* Setting up users and groups.
** GPG: Initializing GPG and importing key.
** GPG: Creating /root/.gnupg directory
You must enter an email address.
Admin Email Address? root@localhost
* Performing initial configuration.
* Activating Spacewalk.
** Loading Spacewalk Certificate.
** Verifying certificate locally.
** Activating Spacewalk.
* Enabling Monitoring.
* Configuring apache SSL virtual host.
Should setup configure apache's default ssl server for you (saves original ssl.conf) [Y]? y
** /etc/httpd/conf.d/ssl.conf has been backed up to ssl.conf-swsave
* Configuring tomcat.
* Configuring jabberd.
* Creating SSL certificates.
CA certificate password? 
Re-enter CA certificate password? 
Organization Unit [spacewalk]?                   
Email Address [root@localhost]? 
City? bancross
State? KA
Country code (Examples: "US", "JP", "IN", or type "?" to see a list)? IN
** SSL: Generating CA certificate.
** SSL: Deploying CA certificate.
** SSL: Generating server certificate.
** SSL: Storing SSL certificates.
* Deploying configuration files.
* Update configuration in database.
* Setting up Cobbler..
Processing /etc/cobbler/modules.conf
`/etc/cobbler/modules.conf' -> `/etc/cobbler/modules.conf-swsave'
Processing /etc/cobbler/settings
`/etc/cobbler/settings' -> `/etc/cobbler/settings-swsave'
Cobbler requires tftp and xinetd services be turned on for PXE provisioning functionality. Enable these services [Y]? y
* Restarting services.
Installation complete.
Visit https://spacewalk to create the Spacewalk administrator account.
[root@spacewalk ~]# 

​Post Installation:​

[root@spacewalk ~]#  spacewalk-service status
postmaster (pid 7077) is running...
router (pid 7104) is running...
sm (pid 7114) is running...
c2s (pid 7124) is running...
s2s (pid 7134) is running...
tomcat6 (pid 7207) is running... [ OK ]
httpd (pid 7335) is running...
osa-dispatcher (pid 7352) is running...
rhn-search is running (7406).
cobblerd (pid 7437) is running...
RHN Taskomatic is running (7464).
[root@spacewalk ~]#  

Login to the web ​interface ​and create first user login.

spacewalk was installed successfully.

Sunday 30 November 2014

Oracle Database 12 Installation on Redhat 6.6/CentOS 6.6

Oracle database (Oracle DB) is a relational database management system (RDBMS) from the Oracle Corporation.The system is built around a relational database framework in which data objects may be directly accessed by users (or an application front end) through structured query language (SQL). Oracle is a fully scalable relational database architecture and is often used by global enterprises, which manage and process data across wide and local area networks. The Oracle database has its own network component to allow communications across networks. Oracle DB is also known as Oracle RDBMS and, sometimes, just Oracle.

Minimum requirements:

- Recommended minimum RAM needed for Oracle is 2GB or more.
- Swap must be enabled double the size of RAM
- Disk space must be more than 8GB and its edition
- /tmp directory must have free space more than 1GB

    Below, was my system requirements on which Oracle 12 will be installed:

    Environment: CentOS 6.6/Redhat 6.6 (x86_64)
    Kernel version : 2.6.32-504.el6
    Hostname: oracle12c
    Memory: 3.5GB
    HDD: 40GB
    Firewall: Disabled.
    SELinux: Disabled

    Make an entry in /etc/hosts file.

    [root@oracle12c ~]# tail -1 /etc/hosts
    <IP address> oracle12c
    [root@oracle12c ~]# 

    Make sure that your system is upto date and download the below packages.

    #yum clean all
    #yum install -y binutils.x86_64 compat-libcap1.x86_64 compat-libstdc++-33.x86_64 compat-libstdc++-33.i686 compat-gcc-44 compat-gcc-44-c++ gcc.x86_64 gcc-c++.x86_64 glibc.i686 glibc.x86_64 glibc-devel.i686 glibc-devel.x86_64 ksh.x86_64 libgcc.i686 libgcc.x86_64 libstdc++.i686 libstdc++.x86_64 libstdc++-devel.i686 libstdc++-devel.x86_64 libaio.i686 libaio.x86_64 libaio-devel.i686 libaio-devel.x86_64 libXext.i686 libXext.x86_64 libXtst.i686 libXtst.x86_64 libX11.x86_64 libX11.i686 libXau.x86_64 libXau.i686 libxcb.i686 libxcb.x86_64 libXi.i686 libXi.x86_64 make.x86_64 unixODBC unixODBC-devel sysstat.x86_64

    Change your kernel parameters as per below snap provided and edit in /etc/sysctl.conf

    [root@oracle12c ~]# tail -12 /etc/sysctl.conf
    #oracle installations kernel parameters settings

    kernel.shmmax = 3698180096
    kernel.shmall = 1805752
    fs.aio-max-nr = 1048576
    fs.file-max = 6815744
    kernel.shmmni = 4096
    kernel.sem = 250 32000 100 128
    net.ipv4.ip_local_port_range = 9000 65500
    net.core.rmem_default = 262144
    net.core.rmem_max = 4194304
    net.core.wmem_default = 262144
    net.core.wmem_max = 1048576
    [root@oracle12c ~]# 

    Once you have added above values, issue below command to take effect.

    [root@oracle12c ~]# sysctl -p
    net.ipv4.ip_forward = 0
    net.ipv4.conf.default.rp_filter = 1
    net.ipv4.conf.default.accept_source_route = 0
    kernel.sysrq = 0
    kernel.core_uses_pid = 1
    net.ipv4.tcp_syncookies = 1
    error: "net.bridge.bridge-nf-call-ip6tables" is an unknown key
    error: "net.bridge.bridge-nf-call-iptables" is an unknown key
    error: "net.bridge.bridge-nf-call-arptables" is an unknown key
    kernel.msgmnb = 65536
    kernel.msgmax = 65536
    kernel.shmmax = 3698180096
    kernel.shmall = 1805752
    fs.aio-max-nr = 1048576
    fs.file-max = 6815744
    kernel.shmmni = 4096
    kernel.sem = 250 32000 100 128
    net.ipv4.ip_local_port_range = 9000 65500
    net.core.rmem_default = 262144
    net.core.rmem_max = 4194304
    net.core.wmem_default = 262144
    net.core.wmem_max = 1048576
    [root@oracle12c ~]# 

    Create user and groups for Oracle installations

    Create the new groups Oracle inventory, OSDBA and OSOPER for Oracle installation.
    [root@oracle12c ~]# groupadd -g 5000 oracle
    [root@oracle12c ~]# groupadd -g 5001 dba
    [root@oracle12c ~]# groupadd -g 5002 opr

    Create the new users and add them to the above group.
    [root@oracle12c ~]# useradd -u 5000 -g oracle -G dba,opr oracle
    [root@oracle12c ~]# usermod -a -G wheel oracle
    [root@oracle12c ~]# passwd oracle
    Changing password for user oracle.
    New password: 
    Retype new password: 
    passwd: all authentication tokens updated successfully.
    [root@oracle12c ~]# 

    Create directory for installing Oracle and change the ownership and grand permission.
    [root@oracle12c ~]#mkdir -p /u01/app/oracle/product/12.1/db_1
    [root@oracle12c ~]#chmod -R 775 /u01
    [root@oracle12c ~]#chown -R /u01
    [root@oracle12c ~]# ls -ld /u01/
    drwxrwxr-x 4 oracle oracle 4096 Nov 29 12:02 /u01/
    [root@oracle12c ~]# 

    Add the environment variable for oracle user by editing the profile file of oracle user and append the oracle environment entries.

    [root@oracle12c ~]#vim /home/oracle/.bash_profile
    ## Oracle Env Settings 
    export TMP=/tmp
    export TMPDIR=$TMP
    export ORACLE_UNQNAME=orcl
    export ORACLE_BASE=/u01/app/oracle
    export ORACLE_HOME=$ORACLE_BASE/product/12.1/db_1
    export ORACLE_SID=orcl
    export PATH=/usr/sbin:$PATH
    export PATH=$ORACLE_HOME/bin:$PATH
    export LD_LIBRARY_PATH=$ORACLE_HOME/lib:/lib:/usr/lib
    export CLASSPATH=$ORACLE_HOME/jlib:$ORACLE_HOME/rdbms/jlib
    [root@oracle12c ~]# 

    Setting shell limits of an oracle user.

    To improve the performance of the software, you must increase the shell limits for the oracle user. Add the following lines to the /etc/security/limits.conf file:

    [root@oracle12c ~]# tail -6 /etc/security/limits.conf
    oracle              soft    nproc   2047
    oracle              hard    nproc   16384
    oracle              soft    nofile  4096
    oracle              hard    nofile  65536
    oracle              soft    stack   10240
    oracle              hard    stack   10240
    [root@oracle12c ~]#

    try to download the oracle packages from the official repository, Click here to download
    I had downloaded the two below files, extracted to database directory, install oracle by running runInstaller script.

    [oracle@oracle12c ~]$ unzip
    [oracle@oracle12c ~]$ unzip

    Note: Make sure X11 forwading is enabled, I would leave to reader to configure if isn't set.
    Reboot the box once to come clean before starting oracle installations.

    [oracle@oracle12c database]$ ./runInstaller
    Starting Oracle Universal Installer...

    Checking Temp space: must be greater than 500 MB.   Actual 1367 MB    Passed
    Checking swap space: must be greater than 150 MB.   Actual 2999 MB    Passed
    Checking monitor: must be configured to display at least 256 colors.    Actual 16777216    Passed
    Preparing to launch Oracle Universal Installer from /tmp/OraInstall2014-11-29_04-29-06PM. Please wait ...

    Step 1:
    Configure security update window would be popped up, where in it would require your email, I would be skipping it by pressing next, thereby providing another window for confirmation press YES

    Step 2: Create and configure a database, press by clicking Next

    Step 3: Installing server class

    Step 4: Single instance database installation, Press Next to continue

    Step 5: Choose advance installation

    Step 6: Default English chosen, press Next to continue.

    Step 7: Choose for DB edition.
    We need more than 6.5 GB space for Enterprise installation because database may grow soon/increase, make sure you have enough space to install.

    Step 8: Installation location

    Step 9: Inventory creation

    every Inventory files will be created under '/u01/app/oralnventory' directory. We have created the group oracle for installation. So now the oracle group has permission to access Inventory Directory

    Step 10: Configuration type 
    select 'General purpose/Transaction processing' and proceed with Next

    Step 11: Database identifiers
    Specify the Global Database name for uniquely identified and un-check the Create as Container database, as here we are not going to create multiple databases.

    Step 12: Configuration options
    I have assigned 3.5GB of memory but this is not enough for Oracle. Here we need to Enable allocate memory automatically for the use of system global Area.

    Step 13: Database storage

    Step 14: Management options
    As, I don't have Cloud control manager credentials from oracle, so I have to skip this step by clicking Next

    Step 15: Recovery options
    This must be enabled in production environment also this would be in a separate file system, hence I choose to un-check(skip) the option and proceed by clicking Next.

    Step 16: Schema password
    We need to define the password for starter database which is all pre-loaded while the installations, as well as weblogin as well.

    Step 17: Operation System groups
    We need to provide system privileges to create database for that we need to choose the oracle group. Choose oracle for every options.

    Step 18: Preform Pre-requisites check.
    this would check for the minimum requirements, and take necessary actions.
    Increase your swap/tmp if not met the required space. Once your pre-requisite check is successful below would be the pic.

    Step 19: Install product
    Installation started to Preparation and copying files. This will take long time to complete according to our Hardware Resource.

    During setup process, it will ask to run two scripts as a root user as shown in the below picture. execute as root user and once done, press OK

    Once above tasks completed successfully, we will receive the Database Configuration Assistant window with the all the details and it will show you the EM Database Express URL. Click OK to move forward.

    Finally Oracle Database installation was successfully completed. Click on Close to quit the Oracle Installer.

    Reboot the system to take necessary effect

    Post verification:

    Start listener:

    [oracle@oracle12c ~]$ lsnrctl start
    LSNRCTL for Linux: Version - Production on 30-NOV-2014 07:26:13
    Copyright (c) 1991, 2014, Oracle.  All rights reserved.
    Starting /u01/app/oracle/product/12.1/db_1/bin/tnslsnr: please wait...

    TNSLSNR for Linux: Version - Production
    System parameter file is /u01/app/oracle/product/12.1/db_1/network/admin/listener.ora
    Log messages written to /u01/app/oracle/diag/tnslsnr/oracle12c/listener/alert/log.xml
    Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(

    Alias                     LISTENER
    Version                   TNSLSNR for Linux: Version - Production
    Start Date                30-NOV-2014 07:26:15
    Uptime                    0 days 0 hr. 0 min. 0 sec
    Trace Level               off
    Security                  ON: Local OS Authentication
    SNMP                      OFF
    Listener Parameter File   /u01/app/oracle/product/12.1/db_1/network/admin/listener.ora
    Listener Log File         /u01/app/oracle/diag/tnslsnr/oracle12c/listener/alert/log.xml
    Listening Endpoints Summary...
    The listener supports no services
    The command completed successfully
    [oracle@oracle12c ~]$ 

    [oracle@oracle12c ~]$ sqlplus / as sysdba
    SQL*Plus: Release Production on Sun Nov 30 07:58:51 2014
    Copyright (c) 1982, 2014, Oracle.  All rights reserved.
    Connected to an idle instance.

    SQL> startup
    ORACLE instance started.

    Total System Global Area 1493172224 bytes
    Fixed Size    2924592 bytes
    Variable Size  973082576 bytes
    Database Buffers  503316480 bytes
    Redo Buffers   13848576 bytes
    Database mounted.
    Database opened.

    you can also connect via web interface which was prompted earlier by Database configuration Assistant, in which you have EM Database Express URL

    If you need to reset the password for the account, below explained.

    SQL> passw system
    Changing password for system
    New password: 
    Retype new password: 
    Password changed

    Finally, Oracle 12c installation has been completed successfully on CentOS 6.6.