PROC


 * /proc/cpuinfo   - Information about CPU
 * /proc/interrupts - IRQ usage in your system
 * /proc/ioports - Displays the listining of the registered port regions used for I/O comunication with devices
 * /proc/iomem - Displays the current map of the system's memory for each physical device
 * /proc/mdstat - Displays Status of RAID
 * /proc/meminfo - Displays of memory usage
 * /proc/kcore - Represents the physical memory of the system.
 * /proc/modules - Show the currently loaded kernel modules. - lsmod
 * /proc/buddyinfo - Information stored in this file can be used for diagnosing memory fragmentation issues.
 * /proc/swaps     - Status of swap partitions,volume and or files
 * /proc/cmdline   - Displays parameters passed to the kernel when the kernel started up
 * /proc/version    - Current version of kernel, the machine on which it was compiled, and the date and time of compilation
 * /proc/scsi/* - Information about scsi devices
 * /proc/net/arp - Arp table
 * /proc/net/dev/ - Information about each network device
 * /proc/net/snmp- Information about SNP
 * /proc/net/sockstat - Statistics on network socket utilization
 * /proc/sys/fs/  - Settings for file system utilization by the kernel.
 * /proc/sys/net/core/netdev_max_backlog - When the kernel receives packets from the network faster than it can process them, it places them on special queue. By default, a maximum of 1000 packets is allowed on the queue. Under extraordinary circumstances , you may need to edit this file and change that.
 * /proc/sys/net/ipv4/icmp_echo_ignore_all - Default = 0, meaning that the kernel will respond to ICM echo reply messages. Set 1 to stop replaying
 * /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts - Default is 1 so kernel will no allow to broadcast or multicast
 * /proc/syst/net/ipv4/ip_forward - Default=0, meaning the kernel will not forward packets between network interfaces.
 * /proc/sys/net/ipv4/ip_local_port_range- Range of ports Linux will use when originating a connection default - 32768-61000
 * /proc/sys/net/ipv4/tcp_syncookies- Default= 1. Change to 0 to disable protection for the system against SYN flood attacks

Enabling IP forwarding on CENTOS 7 and Fedora


 * 1) echo '#!/bin/sh -e' > /etc/rc.d/rc.local  && chmod +x /etc/rc.d/rc.local
 * 2) echo '"1" > proc/syst/net/ipv4/ip_forward
 * 3) ln -s /usr/lib/systemd/system/rc-local.service /etc/systemd/system

sysctl for displaying and modifying kernel parameters in real time.

- -p file for loading from some file

- a to show

To persistent configuration, we have to save appropriate entries to /etc/sysctl.conf

Changing sysctl -w fs.file-max=some value provide to changing the maximum number of open files that Linux can support at any one time.