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https://github.com/abhinavkorpal/linux

Linux is a family of free and open-source software operating systems based on the Linux kernel, an operating system kernel. Linux is typically packaged in a Linux distribution.
https://github.com/abhinavkorpal/linux

linux linux-kernel unix

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Linux is a family of free and open-source software operating systems based on the Linux kernel, an operating system kernel. Linux is typically packaged in a Linux distribution.

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README 

        
Linux Cheat Sheet

===============

### Index

* [Linux kernel](#linux-kernel)

* [Daemon-Monitoring Daemon](#daemon-monitoring-daemon)

* [Installing and Configuring monit](#installing-and-configuring-monit)

* [Email](#email)






# Linux kernel

This repo contain the content of Linux kernel



uname -n kernal name

uname -s computer name

uname -r kernel release

uname -v kernel version

uname -m Machine name

uname -p Processor type

uname -i hardware platform

uname -o operating system



The information that we harvest with the uname command is coming from the file in the procfs /proc/version



/proc/version -- This command uname displays information from the file /proc/version



Trobleshooting enviorment is looking at /proc/cmdline



/proc/cmdline -- To view which options were supplied to the kernel at boot time, we can view the file /proc/cmdline



if you want to open configuration: cat /boot/config-$(uname -r), usefull if we have multiple kernels.



or 



ls /boot



yum install kernel-doc

ls /usr/share/doc/kernel-doc-3.10.0/Documentation/

less filesystem/proc.txt



file /boot/vmlinuz



type of initail ram disk: initrd and initramfs



mount -t sysfs initrd /mnt

umount /mnt



ls /usr/src/kernels/

tar -Jxvf linux-4.15.7.tar.xz -C /usr/src/kernels/



ln -s /usr/src/kernels/linux-4.15.7/ /usr/src/linux



compling a kernel in /usr/src/linux:



make mrproper

make bzimage

make modules_install

make menuconfig

make modules

make install



make clean

make menuconfig

make bzImage

make modules

make install



mkinitrd:



generated the initrd ram disk



kernel is compiled the ram disk may also need to change



dracut:



mkinitrd relies on udev and script to lessen the amount of hard-coded activity in the initramfs



cat /proc/modules



modinfo sr_mod



systemd, provides a standard process for controlling which program start when a linux system boots.

Runlevels are now target units

SysVinit scripts are now service units

PID 1 is now /usr/lib/systemd/systemd



Targets in systemd    Runlevels with init



poweroff.target         0

rescue.target           1

multiuser.target        2

multiuser.target        3

multiuser.target        4

graphical.target        5

reboot.target           6



cat /etc/system-release



systemctl get-default



systemctl set-default multi-user.target



systemctl isolate multi-user.target



systemctl status sshd



/lib/systemd/system



/etc/systemd/system              need to change anything on running script



systemctl show kdump.service



systemctl list-units

systemctl list-unit-files






 ## Daemon-Monitoring Daemon (DMD)



A daemon-monitoring daemon (DMD) is a utility that watches your services for you and automatically attempts to restart them when they fail.






 ## Installing and Configuring monit

 

To install monit, you can either use your Linux system package manager or down- load the tarball from http://www.tildeslash.com/monit. If you’re using the Debian setup, simply enter:

```shell

# apt-get install monit

```

After you’ve installed monit, edit /etc/monit/monitrc. The file created during installa- tion contains lots of examples, and you can find more configuration examples at http://www.tildeslash.com/monit/doc/examples.php. In our case, we want to:



• Enable the monit web interface on port 2812.

• Monitor the proftpd, sshd, mysql, apache, and postfix services.

• Create a Secure Sockets Layer (https) web interface where we can log in with the username admin.

• Tell monit to send email alerts to root@localhost.



Our /etc/monit/monitrc configuration file looks like this:

```shell

     set daemon  60

     set log file syslog facility log_daemon

     set mailserver localhost

     set mail-format { from: [email protected] }

     set alert root@localhost

     set httpd port 2812 and

          SSL ENABLE

          PEMFILE  /var/certs/monit.pem

          allow admin: test

     check process proftpd with pidfile /var/run/proftpd.pid

        start program = "/etc/init.d/proftpd start"

        stop program  = "/etc/init.d/proftpd stop"

        if failed port 21 protocol ftp then restart

        if 5 restarts within 5 cycles then timeout

     check process sshd with pidfile /var/run/sshd.pid

        start program  "/etc/init.d/ssh start"

        stop program  "/etc/init.d/ssh stop"

        if failed port 22 protocol ssh then restart

        if 5 restarts within 5 cycles then timeout

     check process mysql with pidfile /var/run/mysqld/mysqld.pid

        group database

        start program = "/etc/init.d/mysql start"

        stop program = "/etc/init.d/mysql stop"

        if failed host 127.0.0.1 port 3306 then restart

        if 5 restarts within 5 cycles then timeout

     check process apache with pidfile /var/run/apache2.pid

        group www

        start program = "/etc/init.d/apache2 start"

        stop program  = "/etc/init.d/apache2 stop"

        if failed host www.centralsoft.org port 80 protocol http

           and request "/monit/token" then restart

        if cpu is greater than 60% for 2 cycles then alert

        if cpu > 80% for 5 cycles then restart

        if totalmem > 500 MB for 5 cycles then restart

        if children > 250 then restart

        if loadavg(5min) greater than 10 for 8 cycles then stop

        if 3 restarts within 5 cycles then timeout

     check process postfix with pidfile /var/spool/postfix/pid/master.pid

        group mail

        start program = "/etc/init.d/postfix start"

        stop  program = "/etc/init.d/postfix stop"

        if failed port 25 protocol smtp then restart

        if 5 restarts within 5 cycles then timeout

```

Statements and options are described in the monit documentation at http://www. tildeslash.com/monit/doc/manual.php.



In the apache section of the monit configuration, you’ll see this statement:

```shell

if failed host www.xxx.org port 80 protocol http and request "/monit/token" then restart

```

This means that monit tries to connect to www.xxx.org on port 80 and tries to access the file /monit/token. Because our web site’s document root is /var/www/www.xxx.org/web, the filename expands to /var/www/www.xxx.org/web/ monit/token. If monit doesn’t succeed, this means Apache isn’t running, so monit tries to restart it.



Now we must create the file /var/www/www.xxx.org/web/monit/token and write some arbitrary string into it:

```shell

# mkdir /var/www/www.centralsoft.org/web/monit

# echo "hello" > /var/www/www.centralsoft.org/web/monit/token

```

You can follow a similar procedure on your own system. 



Next, create a directory to hold the pem cert file (/var/certs/monit.pem) required for

the SSL-encrypted monit web interface: 

```shell

# mkdir /var/certs

# cd /var/certs

```

You’ll need an OpenSSL configuration file to create the certificate. The resulting /var/

certs/monit.pem file should look like this:

```shell

# create RSA certs - Server

```

Now create the certificate:

```shell

# openssl req -new -x509 -days 365 -nodes -config ./monit.cnf -out \ /var/certs/monit.pem -keyout /var/certs/monit.pem

# openssl gendh 512 >> /var/certs/monit.pem

# openssl x509 -subject -dates -fingerprint -noout -in /var/certs/monit.pem # chmod 700 /var/certs/monit.pem

```

Then edit /etc/default/monit to enable the monit daemon. Change startup to 1 and set CHECK_INTERVALS to the interval in seconds at which you would like to check your sys- tem. We chose 60. The file should now look like this:

```shell

     # Defaults for monit initscript

     # sourced by /etc/init.d/monit

     # installed at /etc/default/monit by maintainer scripts

     # Fredrik Steen 

     # You must set this variable to for monit to start

     startup=1

     # To change the intervals which monit should run uncomment

     # and change this variable.

     CHECK_INTERVALS=60

```

Finally, start monit:

```shell

# /etc/init.d/monit start

```

Now point your browser to https://your_domain:2812/ (make sure port 2812 isn’t blocked by your firewall) and log in with the username admin and password test. You should see the monit web interface.






## Email



Mail User Agent (MUA)



Mail Transfer Agent (MTA)



Post Office Protocol (POP)



Interactive Mail Access Protocol (IMAP)



SimpleAuthenticationandSecurityLayer(SASL)



Unsolicited bulk emailers (UBEs), also knowns as spammers.UBEs use a variety of techniques to hide their real identities, including spoofing IP addresses, forging mail envelopes, and relaying through open SMTP servers.



unsolicited commercial emailers (UCEs)



File locking is a mechanism that enforces access to a computer file by only one user or process at any specific time. The purpose of locking is to prevent conflicting updates.