NOC

# trace {{{
# find command for PID
ps e -p 7707

# system call trace
strace -p 3761
# lib trace
ltrace -p 3761

# log locations
/var/log/inotify.log
/var/log/httpd/error_log
# }}}

# study {{{
1) nginx setup linux host website via domain name
2) setup sql
3) apache setup linux
4) apache web root /var/www/
5) .htaccess file used only in apache not nginx
6) typically apache is the webserver and nginx is reverse proxy or load balancer
7) ssh to server
8) arp

NGINX specs
1) nginx can serve http, https, imap, pop3, smtp
2) reverse proxy
3) load balancer

NGINX install
1) precompiled binaries
2) compile from source

# }}}

# system info{{{

# check hostname
# check kernel version
# release
# architecture
uname -a

# kernel version
uname -v

# kernel release
uname -r

# ip address
ip addr **newer tool recommended**
ip addr wlp6s0
ifconfig

# disk space
# a: all resources
# h: human readable formatting
df -h

# process
# newer recommended method
# status
systemctl status udev

# check status
service udev status

# start
service udev start

# stop
service udev stop

# disk size of directory
du -sh [folder]

# check open ports/listening network sockets/services listening on tcp/udp ports
# 0.0.0.0:80 means all public addresses, on port 80
# 127.0.1.1 is local machine (not accessible outside of machine)
netstat
# filter tcp/udp/listening/program/numeric
netstat -tulpn
# reveal pid with sudo
sudo netstat -tulpn

# cpu usage
# ps aux to find status of process
# S: interruptable sleep state, waiting for event to complete
# D: uninterruptable sleep state
# R: running state
# Z: defunct process, zombie process, completed execution but has process entry
# T: stopped state
# <: process is high priority
# N: low priority
ps aux | grep udev
top
htop

# mount
# check drives
sudo fdisk -l
# standard location to mount '/mnt'
# mount drive sdX to /mnt
mount /dev/sdX /mnt
# unmount device
umount /dev/sdX
# list mounted devices
mount
# automount devices
less /etc/fstab

# list shells
cat /etc/shells

# daemon
# service that runs in background, that has no controlling terminal
# listens and acts on service requests
# daemons typically end with 'd'
# daemons have '?' in tty field
ps -ef

# inspect physical/swap memory usage in megabytes
free -m

# inspect virtual memory
vmstat -a

# check disk
e2fsck -f /dev/sdX

# steps below work only for LVM partitions
# unmount device
umount /dev/sdX

# resize disk to 3 gigs
resize2fs /dev/sdX 3G

# resize logical partition
lvreduce -L 3GB /dev/sdX

# mount
# lvm: logical volume manager
mount /dev/sdX /lvm

# serial ports/comm ports
/dev/ttyS0 (COM 1 in windows)
ls -l /dev/ttyS*

# parallel ports (printer ports)
ls -l /dev/lp*

# permissions
# change permissions of files and directories
chmod

# change owner of files and directories
chown

# change group ownership of files and directories
chgrp

# table
users: 'u'
groups: 'g'
other: 'o'
all: 'a'
add: '+'
remove: '-'
replace: '='

chmod g+wx,o-wx,u=rwx filename
chown username filename

# symbolic links
# point to programs, directories, files but do not contain target data
# symbolic link and target file have different permissions, sizes, names
# soft links can be created for files and directories
# soft links can be linked across filesystems
ln -s /home/xeno/p /home/xeno/sp

# hard links
# point to physical file on disk, not path name
# link persists if original file is renamed or moved
# only for files not directories

# environment variables
# global settings that control behavior of Shell, software and processes
# path where software is installed is stored as environment variable
# used to pass information into processes that are spawned from shell
# system variables
env
# PATH is a system variable that contains path for all binaries

# redirection
# used to pass output of one operation as input to another operation
# w: who is online
w > log
# redirect contents of file1 and file2 to file3.txt (overwrites contents)
cat file1 file2 > file3.txt
# redirect and append contents to file.txt
cat >> file.txt

# grep
# search for string in text file
# supports pattern based searching
grep searchString filename

# return line number of search query
grep -n searchString filename

# count occurences of string
grep -c searchString filename

# process
# pid process id
kill pid

# list all files ending in txt with permissions
ls -lah *.txt

# find string 'edureka' in all files ending in .txt
find / -name *.txt | xargs grep -i edureka

# login as sudo
su -

# run linux program in background when starting linux server
# processes that receive nohup signal will be terminated when logging out of
# program, until then the process runs in the background
nohup

# syslogd
# daemon that tracks system information and saves it to specific log files

# inode
# contents of file are stored in data blocks
# information (meta data) about files are stored in inode
# inode info: file size, mode/permissions, ownerid, groupid, number of
# hardlinks, time last accessed, time last modified, time inode last modified

# nice
# set priority for processor intensive job to execute in less time
# priority range: -20(highest) - 19(lowest)
# 0 is default
# NI column
# inspect priority
ps -fl cat

# set priority of cat > file to 10
nice -10 cat > file
# set priority of cat > file to -10
nice --10 cat > file

# renice if process already in use
renice -n -19 -p

# shadow passwords
# enhanced security
# passwords stored in /etc/passwd
# implementing shadow passwords results in all passwords stored and encrypted
# in a new file /etc/shadow and passwords in original file are replaced with 'x'
ls -l /etc/passwd
ls -l /etc/shadow

# }}}

# networking {{{

# keywords
1) DNS server
2) DHCP
3) default gateway
4) subnet mask

# ping
# establishes basic connectivity:  host is reachable
1) send signal to device
2) if device is active it will respond to sender

# tracert
# trace route (hops) taken to reach a host
1) route packet takes to destination
2) if takes 10 routers to get to google.com, you can say it took 10 hops

# ipconfig/ifconfig
1) find ip configuration of host
2) ip address can be static or dynamic
*) ifconfig -a: view all network configuration & settings
*) ifconfig wlp6s0: network config for specific device
*) ifconfig wlp6s0 up: enable device
*) ifconfig wlp6s0 down: disable device

# nslookup
# DNS used to resolve domain names to ip address: google.com : 74.125.115.147
*) most common networking issues involve DNS
*) inspect hostname from ip address
*) inspect ip address from hostname
2) lookups ip address(es) associated with a domain name

3) if utility is unable to resolve host there is a DNS issue
4) can be used to query specific DNS servers to determine issue with default DNS servers configured on host
5) authoritative answer comes from nameserver that is considered authoritative for the domain which it's returning a record
6) non-authoritative answer comes from everywhere else
7) to find authoritative servers:
> set query=ns
> stackoverflow.com

# gateway
1) route -n
2) 'U': route is up
3) 'G': route is gateway

# netstat
# most useful for finding connections to and from host
*) usage: netstat -lt
1) inspect state of active network connections on host
2) verify status of listening port on host
3) check which remote hosts are connected to a local host on a specific port
4) determine which host services are associated with specific active ports
5) netstat -g: find all multicast groups (network) subscribed by this host
6) netstat -nap | grep port: display process id of application which is using that port
7) netstat -a: display all connections including TCP and UDP
8) netstat -t: display only TCP connections
9) netstat -u: display only UDP connections
10) netstat -l: list only listening ports
11) netstat -a | less: display all, pipe with less

# hostname
1) hostname: machine hostname
2) hostname -d: domain name
3) hostname -f: fully qualified host and domain name
4) hostname -i: displays machine ip address

# ip address
curl 'icanhazip.com'
# }}}

# netstat {{{
# netstat
# info about: network connections, routing tables, interface stats, masquerade
# connections, multicast memberships
# apps that listening to what port and bind to what port

# list all ports
netstat -a | more

# list all tcp ports
netstat -at | more

# list all udp ports
netstat -au | more

# list sockets in listening state
netstat -l | more
netstat -lt | more # tcp
netstat -lu | more # udp

# stats
netstat -s
netstat -sc # stats continuous

# display PID and program names in netstat
# find which PID binding to which port
netstat -lp

# kernel routing info
netstat -r

# find which port a program is running
netstat -ap | grep ssh
netstat -apn | grep ':80'

# list network interfaces
netstat -i
netstat -ie

# list all unix listening ports
netstat -lx

# display promiscous mode
# accept all packets no filter
# updates every 5 sec, only tcp
netstat -ac 5 | grep tcp

# display IPv4 IPv6
# group memberships
netstat -g

# display RAW Network stats
netstat --statistics --raw


# }}}