Install Oracle Linux 6.x with reduced set of packages for Oracle Database 12c, Grid Infrastructure 12c, EM12c etc ...

It's always best to install the operating system with a smaller footprint which automatically make the environment more secure since there are much less to consider to secure the environment.

From the My Oracle Support Note: Linux OS Installation with Reduced Set of Packages for Running Oracle Database Server (Doc ID 728346.1): Limiting the number of installed packages to only those needed provides a smaller footprint for Linux operating system installation and will help to reduce the amount of storage used. A minimal configuration will also help to avoid unnecessary processes and services and result in reduced memory usage, along with a more secure and reliable setup overall.

So, before starting this, it is required to have ANY one of the followings -
-- A local yum repository (Local ULN mirror)
-- Connected with Oracle Public Yum Server
-- Ability to mount the CD/DVD Rom or USB with the Oracle Linux ISO image into the operating system (or copy the ISO into the OS) after the initial installation: ISO based local yum repo.

For my case, I have a local yum repo (Local ULN mirror) running in my environment so it's relatively easy to proceed faster. In fact, I can complete the installation & configuration within an hour max.

So, here are the details -

-- Install Oracle Linux 6.7 using ISO (mounted via CD/DVD Rom, USB, iLOM etc.)
-- During installation, select the "Minimal" as shown in the screenshot

 -- Once the installation completes, install the following set of packages, these are the basic set of required packages  
 # yum install bind-utils ntp nscd sos parted unzip lsof nmap traceroute sysstat strace ltrace mailx openssl-clients perl dmidecode  

 -- Install xWindows packages (This is to run runInstaller, DBCA, DBUA, NETCA etc.)  
 # yum install xorg-x11-xauth xorg-x11-utils livdmx libXxf86vm xterm xorg-x11-apps xauth  
 Note: The bold ones (above) are additional (optional).  

 -- Install required packages for Oracle Database  
 # yum install oracle-rdbms-server-12cR1-preinstall or yum install oracle-database-preinstall-19c  

 -- Enable/disable the required services  
 # chkconfig iptables off  
 # chkconfig ip6tables off  
 # chkconfig --level 35 ntpd on   
 # chkconfig --level 35 nscd on

 -- Configure NTP  
 File: /etc/ntp.conf  
 -- Disable the public NTP server configuration  
    ...  
    #server 0.rhel.pool.ntp.org iburst  
    #server 1.rhel.pool.ntp.org iburst  
    #server 2.rhel.pool.ntp.org iburst  
    #server 3.rhel.pool.ntp.org iburst  
    ...  
 -- Enable the site specific NTP server configuration, for my case
    # NTP Config: NOEL  
    server pxe-yum.example.com prefer iburst burst minpoll 4 maxpoll 4  
    #  

 -- Disable SELinux  
 File: /etc/selinux/config  
 From: SELINUX=enforcing  
 To: SELINUX=disabled  

 -- Personally, I prefer to disable the graphical boot process. Also, it's important to disable transparent huge pages which we don't want when running Oracle Database.  
 File: /etc/grub.conf  
 -- Default Value (Single Line)  
    kernel /vmlinuz-3.8.13-118.el6uek.x86_64 ro root=/dev/mapper/VGOraDB-LVDbSys1 rd_NO_LUKS KEYBOARDTYPE=pc KEYTABLE=us LANG=en_US.UTF-8 rd_LVM_LV=VGOraDB/LVDbSys1 rd_NO_MD rd_LVM_LV=VGOraDB/LVDbSwap1 crashkernel=auto SYSFONT=latarcyrheb-sun16 rd_NO_DM rhgb quiet  
 -- Changed To (Single Line)  
    kernel /vmlinuz-3.8.13-118.el6uek.x86_64 ro root=/dev/mapper/VGOraDB-LVDbSys1 rd_LVM_LV=VGOraDB/LVDbSys1 rd_LVM_LV=VGOraDB/LVDbSwap1 loglevel=7 panic=60 debug pci=noaer log_buf_len=1m nmi_watchdog=0 nomce transparent_hugepage=never rd_NO_PLYMOUTH audit=1 console=tty1 console=ttyS0,115200n8 crashkernel=380M@128M numa=off processor.max_cstate=1 intel_idle.max_cstate=0 SYSFONT=latarcyrheb-sun16 LANG=en_US.UTF-8 KEYTABLE=us  

Important Note:

-- It's important to disable transparent huge pages (It's already disabled by default in UEK kernel) and enable kdump.

-- Transparent huge pages is not the same as normal huge pages. The regular huge pages is highly recommended to enable for Oracle Database in Linux.

-- If the H/W have iLOM, then it's a good idea to redirect the boot sequence output into iLOM console.

Reference MOS Note: ALERT: Disable Transparent HugePages on SLES11, RHEL6, OL6 and UEK2 Kernels (Doc ID 1557478.1)

 -- Edit the network configuration: (Optional)  
 File: /etc/sysconfig/network-scripts/ifcfg-eth0  
    DEVICE=eth0  
    TYPE=Ethernet  
    UUID=7eb525c2-5e33-45e0-8479-97d9a0b2d0d0  
    ONBOOT=yes  
    NM_CONTROLLED=yes  
    BOOTPROTO=none  
    IPADDR=192.168.0.175  
    PREFIX=24  
    GATEWAY=192.168.0.199  
    DNS1=192.168.0.199  
    DEFROUTE=yes  
    IPV4_FAILURE_FATAL=yes  
    IPV6INIT=no  
    NAME="System eth0"  
    HWADDR=08:00:27:37:46:87  
    MTU=1500  

 File: /etc/sysconfig/network  
    NETWORKING=yes  
    NETWORKING_IPV6=no  
    IPV6INIT=no  
    HOSTNAME=srv01.example.com  
    GATEWAY=192.168.0.199  
    NOZEROCONF=yes  
    GATWWAYDEV=eth0  

 -- Add a static route/rule (Optional)  
 File: /etc/sysconfig/network-scripts/rule-eth0  
    from 192.168.0.175 table 210  
    to 192.168.0.175 table 210  
 File: /etc/sysconfig/network-scripts/route-eth0  
    192.168.0.0/24 dev eth0 table 210  
    default via 192.168.0.199 dev eth0 table 210  
 Note: For this environment, I am using only one network interface. It's a way to add the route/rule shown as example.

 -- Add host information in /etc/hosts: Required by Universal Installer for GI  
 # ip addr show eth0  
 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP glen 1000  
   link/ether 08:00:27:37:46:87 brd ff:ff:ff:ff:ff:ff  
   inet 192.168.0.175/24 brd 192.168.0.255 scope global eth0  
   inet6 fe80::a00:27ff:fe37:4687/64 scope link  
     valid_lft forever preferred_ft forever  

 # hostname  
 srv01.example.com   

 # hostname -s  
 srv01  

 # cat /etc/hosts | grep example.com  
 192.168.0.175 srv01.example.com srv01

 It's possible to run 12c GI & Databases with this minimal (only 330 packages) configuration -  
 # ps -ef | grep pmon  
 oracle  3968   1 0 13:48 ?    00:00:00 asm_pmon_+ASM  
 oracle  4327   1 0 13:51 ?    00:00:00 ora_pmon_DB  
 oracle  5107   1 0 13:51 ?    00:00:00 ora_pmon_TEST  
 oracle  6774 4157 0 13:53 pts/0  00:00:00 grep pmon  
 # df -h  
 Filesystem      Size Used Avail Use% Mounted on  
 /dev/mapper/VGOraDB-LVDbSys1  
             15G 1.5G  13G 11% /  
 tmpfs         1.8G 630M 1.2G 36% /dev/shm  
 /dev/sda1       477M  45M 404M 10% /boot  
 /dev/sdb1       30G  22G 6.4G 78% /u01  
 # rpm -qa | wc -l  
 330  

Now, conclude with a reboot of the operating system (shutdown -r now).

Well, it looks like we are pretty much done with the initial configuration of the operating system.

At this point, you can harden the server, install/upgrade driver/firmware etc. and/or move forward with the storage configuration/software installation of Oracle Products (e.g. Grid Infrastructure, Oracle Database, EM etc.).

Important Note: It's highly recommended to use Oracle Unbreakable Enterprise Kernel for RHEL & Oracle Linux which is most optimized to run Oracle Database workload.

For more information refer to -
-- MOS Note: Unbreakable Enterprise Kernel: Frequently Asked Questions (Doc ID 1210995.1)
-- Unbreakable Enterprise Kernel Release 2
-- Unbreakable Enterprise Kernel Release 3

Consider the following links to check out (applicable when installing Oracle Linux in an Oracle Server) -
-- Oracle Hardware Management Pack 2.3.x Documentation
-- Firmware Downloads and Release History for Oracle Systems