Cheatsheet: Difference between revisions
→Flows
(→Linux) |
(→Flows) |
||
Line 726:
#ARP Reply will reach PC1, it will add entry to its ARP Table
#Then send a packet destined to PC2 with destintion MAC address as Router's Interface's MAC address received in ARP reply.
= Linux =
== Linux Booting ==
Source: [http://technochords.com/linux-booting-process-6-steps/ technochords.com]
The following are the 6 high level stages of a typical Linux boot process:
#BIOS
#MBR
#GRUB
#Kernel
#Init
#Runlevel programs
;BIOS(Basic Input/Output System) - loads and executes the MBR boot loader.
*Performs some system integrity checks (POST-Power On Self Test)
*Searches, loads, and executes the boot loader program.
*It looks for boot loader in floppy, cd-rom, or hard drive.
*You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.
*Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.
;MBR (Master Boot Record) - loads and executes the GRUB boot loader.
*It is located in the 1st sector of the bootable disk.
*Typically /dev/hda, or /dev/sda
*MBR is less than 512 bytes in size.
*This has three components:
#primary boot loader info in 1st 446 bytes,
#partition table info in next 64 bytes(16,16,16,16) 4 partitions,
#magic numbers as mbr validation check in last 2 bytes.
*It contains information about GRUB (or LILO in old systems).
;GRUB (Grand Unified Bootloader) - loads and executes Kernel and initrd images.
*It is a Multiboot boot loader.
*If you have multiple kernel images installed on your system, you can choose which one to be executed.
*GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.
*GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).
*Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this).
<pre>
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-194.el5PAE)
root (hd0,0)
kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/
initrd /boot/initrd-2.6.18-194.el5PAE.img
</pre>
*As you notice from the above info, it contains kernel and initrd image.
;Kernel
*Once the control is given to kernel which is the central part of all your OS and act as a mediator between hardware and software.
*Kernel once loaded into to RAM it always resides on RAM until the machine is shutdown.
*Once the Kernel starts its operations the first thing it do is executing INIT process.
;Init (initialization)
*Looks at the /etc/inittab file to decide the Linux run level.
*Following are the available run levels
0 – halt
1 – Single user mode
2 – Multiuser, without NFS
3 – Full multiuser mode
4 – unused
5 – X11
6 – reboot
*Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.
*Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level
*Typically you would set the default run level to either 3 or 5.
;Runlevel programs
*When the Linux system is booting up, you might see various services getting started.
*For example, it might say “starting sendmail …. OK”.
*Those are the runlevel programs, executed from the run level directory as defined by your run level.
*Depending on your default init level setting, the system will execute the programs from one of the following directories.
Run level 0 – /etc/rc.d/rc0.d/
Run level 1 – /etc/rc.d/rc1.d/
Run level 2 – /etc/rc.d/rc2.d/
Run level 3 – /etc/rc.d/rc3.d/
Run level 4 – /etc/rc.d/rc4.d/
Run level 5 – /etc/rc.d/rc5.d/
Run level 6 – /etc/rc.d/rc6.d/
*Please note that there are also symbolic links available for these directory under /etc directly.
*So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.
*Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.
#Programs starts with S are used during startup. S for startup.
#Programs starts with K are used during shutdown. K for kill.
#There are numbers right next to S and K in the program names.
#Those are the sequence number in which the programs should be started or killed.
#For example, S12syslog is to start the syslog deamon, which has the sequence number of 12.
#S80sendmail is to start the sendmail daemon, which has the sequence number of 80.
#So, syslog program will be started before sendmail.
== Manually Boot using Grub ==
*Locate where the vmlinuz and initrd.* files are located:
grub> ls
(hd0) (hd0,msdos5) (hd1) (hd1,msdos0)
*Boot the system:
grub> linux (hd1,msdos1)/install/vmlinuz root=/dev/sdb1
grub> initrd (hd1,msdos1)/install/initrd.gz
grub> boot
== File system layout ==
/ – The Root Directory
/bin – Essential command binaries
/boot – Boot loader files
/dev – Device Files
/etc – Configuration Files
/home – Home Directory
/lib – Essential Libraries
/lost+found – Recovering Files
/media – Removable Media Devices
/mnt – Temporarily mounted filesystems
/opt – Optional software packages
/proc – Kernel & Process Information
/root – Root Home Directory
/sbin – System binaries
/selinux – Security-Enhanced Linux
/srv – Service Data
/sys – virtual filesystem
/tmp – Temporary files
/usr – binaries, documentation, source code, libraries
/var – Variable Files
== ProcFS ==
*Procfs or /proc is a special FS under Linux used to present process information and kernel processes.
*Much of the information for kernel level of 2.6 & above have been moved to "sysfs" generally mounted under /sys.
*/proc is stored in memory.
*On multi-core CPUs, /proc/cpuinfo contains the fields for "siblings" and "cpu cores":
"siblings" = (HT per CPU package) * (# of cores per CPU package)
"cpu cores" = (# of cores per CPU package)
*A CPU package means physical CPU which can have multiple cores (single core for one, dual core for two, quad core for four).
*This allows a distinction between hyper-threading and dual-core, i.e. the number of hyper-threads per CPU package can be calculated by siblings / CPU cores.
*If both values for a CPU package are the same, then hyper-threading is not supported.
*For instance, a CPU package with siblings=2 and "cpu cores"=2 is a dual-core CPU but does not support hyper-threading.
/proc/cmdline – Kernel command line information.
/proc/consoles – Information about current consoles including tty.
/proc/crypto – list of available cryptographic modules
/proc/devices – Device drivers currently configured for the running kernel.
/proc/diskstats –
/proc/dma – Info about current DMA channels.
/proc/fb – Framebuffer devices.
/proc/filesystems – Current filesystems supported by the kernel.
/proc/iomem – Current system memory map for devices.
/proc/ioports – Registered port regions for input output communication with device.
/proc/kmsg – holding messages output by the kernel
/proc/loadavg – System load average.
/proc/locks – Files currently locked by kernel.
/proc/meminfo – Summary of how the kernel is managing its memory.
/proc/misc – Miscellaneous drivers registered for miscellaneous major device.
/proc/modules – Currently loaded kernel modules.
/proc/mounts – List of all mounts in use by system.
/proc/partitions – Detailed info about partitions available to the system.
/proc/pci – Information about every PCI device.
/proc/scsi – Information about any devices connected via a SCSI or RAID controller
/proc/stat – Record or various statistics kept from last reboot.
/proc/swap – Information about swap space.
/proc/tty – Information about the current terminals
/proc/uptime – Uptime information (in seconds).
/proc/version – Kernel version, gcc version, and Linux distribution installed.
/proc/PID/cmdline – Command line arguments.
/proc/PID/cpu – Current and last cpu in which it was executed.
/proc/PID/cwd – Link to the current working directory.
/proc/PID/environ – Values of environment variables.
/proc/PID/exe – Link to the executable of this process.
/proc/PID/fd – Directory, which contains all file descriptors.
/proc/PID/maps – Memory maps to executables and library files.
/proc/PID/mem – Memory held by this process.
/proc/PID/root – Link to the root directory of this process.
/proc/PID/stat – Process status.
/proc/PID/statm – Process memory status information.
/proc/PID/status – Process status in human readable form (eg: GID, UID, etc)
/proc/PID/limits – Contains information about the limits of the process
Usage:
ls -l /proc/$(pgrep -n python)/exe
== Inode Number ==
Source: [https://linoxide.com/linux-command/linux-inode/ linoxide.com]
* Inode is entry in inode table containing metadata about a regular file and directory.
* An inode is a data structure on a traditional Unix-style file system such as ext3 or ext4.
* Stores all the information about a file except its name and its actual data.
* Linux extended filesystems such as ext2 or ext3 maintain an array of these inodes: the inode table.
* This table contains list of all files in that filesystem.
* The individual inodes in inode table have a unique number (unique to that filesystem) - the inode number.
* There are some data about files, such as their size, ownership, permissions, timestamp etc.
* This meta-data about a file is managed with a data structure known as an inode (index node).
* There is no entry for file name in the Inode, file name is kept as a separate entry parallel to Inode number.
* This is for maintaining hard-links to files.
* Copy file: cp allocates a free inode number and placing a new entry in inode table.
* Move or Rename a file: if destination is same filesystem as the source, Has no impact on inode number, it only changes the time stamps in inode table.
* Delete a file: Deleting a file in Linux decrements the link count and freeing the inode number to be reused.
* A Directory cannot hold two files with same name because it cannot map one name with two different inode numbers.
* The inode number of / directory is fixed, and is always 2.
* There exists an algorithm which is used to create number of Inodes in a file system.
* This algorithm takes into consideration the size of the file system and average file size.
* The user can tweak the number of Inodes while creating the file system.
*Inode number (or index number) consists following attributes:
File type: Regular file, directory, pipe etc.
Permissions: Read, write, execute
Link count: The number of hard link relative to an inode
User ID: Owner of file
Group ID: Group owner
Size of file: or major/minor number in case of some special files
Time stamp: Access time, modification time and (inode) change time
Attributes: Immutable' for example
Access control list: Permissions for special users/groups
Link to location of file
Other metadata about the file
*Check info:
df -i ==> Inodes on Filesystem
df -i /dev/vda1 ==> Inodes on Filesystem
ls -il myfile.txt ==> Show inode no of file
find /home/rahul -inum 1150561 ==> Find file using inode no
stat unetbootin.bin ==> Show all details of file
stat --format=%i unetbootin.bin ==> Shows only inode no
* Manipulate the filesystem meta data
List the contents of the filesystem superblock
tune2fs -l /dev/sda6 | grep inode
Make sure files on the file system are not being accessed:
mount -o remount /yourfilesystem
debugfs /dev/sda1 ==> Manipulate FS here
You can use debugfs to undelete a file by using its inode and indicating a file
* Free Inodes on Filesystem
In the case of inodes are full, You need to remove unused files from the filesystem to make Inode free.
There is no option to increase/decrease inodes on disk.
Its only created during the creation of filesystem on any disk.
== Sort links vs Hard link ==
;Links and index number in Linux
* In the output of ls -l, the column following the permissions and before owner is the link count.
drwxr-xr-x '''6''' aman aman 4096 Mar 30 11:50 Documents
drwxr-xr-x '''3''' aman aman 4096 Sep 15 19:11 Downloads
^
* Link count is the number of Hard Links to a file.
* A link is a pointer to another file.
* There are two types of links:
<br />
;Symbolic links (or Soft Links)
* A separate file whose contents point to the linked-to file.
* When creating a Sym link, first refer to the name of the original file and then to the name of the link:
ln -s /home/bob/sync.sh filesync
* Editing Sym link is like directly edit the original file.
* If we delete or move the original file, the link will be broken and our filesync file will not be longer available.
* The ls -l command shows that the resulting file is a symbolic link:
ls -l filesync
lrwxrwxrwx 1 root root 20 Apr 7 06:08 filesync -> /home/bobbin/sync.sh
* The contents of a symbolic link are the name of target file only.
* The permissions on the symbolic link are completely open.
* This is because the permissions are not managed
* The original file is just a name that is connected directly to the inode, and the symbolic link refers to the name.
* The size of the symbolic link is the number of bytes in the name of the file it refers to, because no other information is available in the symbolic link.
<br />
*Find Sym Links
find . -type l -ls
ls -la | grep "\->"
;Hard links
* The identity of a file is its inode number, not its name.
* A hard link is a name that references an inode.
* It means that if file1 has a hard link named file2, then both of these files refer to same inode.
* So, when you create a hard link for a file, all you really do is add a new name to an inode.
*there is no difference between the original file and the link: they are just two names connected to the same inode.
* Create a Hard link:
ln /home/bob/sync.sh synchro
*Compare:
ls -il /home/bob/sync.sh synchro
517333 -rw-r----- 2 root root 5 Apr 7 06:09 /home/bob/sync.sh
517333 -rw-r----- 2 root root 5 Apr 7 06:09 synchro
*The directories cannot be hard linked as Linux does not permit this to maintain the acyclic tree structure of directories.
*A hard link cannot be created across filesystems. Both the files must be on the same filesystems, because different filesystems have different independent inode tables (two files on different filesystems, but with same inode number will be different).
*How to find hard link in Linux
# find / -inum 517333
/home/bob/sync.sh
/root/synchro
;Remove files
* When rm command is issued, first it checks the link count of the file.
* If the link count is greater than 1, then it removes that directory entry and decreases the link count.
* Still, data is present, nor is the inode affected.
* And when link count is 1, the inode is deleted from the inode table, inode number becomes free, and the data blocks that this file was occupying are added to the free data block list.
== Hosts file ==
* All operating systems with network support have a hosts file in order to translate hostnames to IP addresses.
* The file /etc/hosts started in the old days of DARPA as the resolution file for all the hosts connected to the internet (before DNS existed).
* It has the maximum priority ahead of any other name system
* Order of name resolution is actually defined in /etc/nsswitch.conf, which usually has this entry:
hosts: files dns
* This means "try files (/etc/hosts); and if it fails, try DNS."
* i.e. If the host name is not found there, then consult the remote DNS name servers identified by the /etc/resolv.conf file.
* This order could be changed or expanded.
* As a single file, it doesn't scale well: the size of the file becomes too big very soon.
* That is why the DNS system was developed, a hierarchical distributed name system.
* It allows any host to find the numerical address of some other host efficiently.
* On Linux and Mac OS it is located here: /etc/hosts
* On Windows it is under: Windows\System32\drivers\etc\
* The hosts file contains lines of text consisting of an IP address field followed by One or More Host names.
* Each field is separated by white space – tabs or spaces.
* Comment lines are indicated by an octothorpe (#) in the first position.
* Entirely blank lines in the file are ignored.
* One name may resolve to several addresses (192.168.0.8 10.0.0.27).
* However which one is used depends on the routes (and their priorities) set for the computer.
* By editing the hosts files, you can achieve:
Block a website
Handle an attack or resolve a prank
Create an alias for locations on your local server
Override addresses that your DNS server provides
Control access to network traffic
* IP-to-hostname conversion usually display only the first name found:
192.168.10.12 server.example.com myftp.example.com myhost myftp
$ ping myftp
PING myhost.example.com (192.168.10.12) 56(84) bytes of data.
64 bytes from myhost.example.com (192.168.10.12): icmp_seq=1 ttl=64 time=0.023 ms
64 bytes from myhost.example.com (192.168.10.12): icmp_seq=2 ttl=64 time=0.028 ms
Note that we pinged myftp but results come from host myhost. This is a reliable hint that you are addressing an alias, not the actual host.
== Adding Vlan in Linux ==
== File permission ==
;Linux File Permission Basics
* The first character represents the type of file.
* The remaining nine bits in groups of three represent the permissions for the user, group, and global respectively.
File Type User Group Global
d Directory rwx r-x r-x
- Regular file rw- r-- r--
l Symbolic Link rwx rwx rwx
* Permissions Meaning
Permission On a file On a directory
r (read) read file content (cat) read directory content (ls)
w (write) change file content (vi) create file in directory (touch)
x (execute) execute the file enter the directory (cd)
* Targeted Users:
Who (Letter) Meaning
u user
g group
o others
a all
*Permissions Table:
Binary Octal Permission
000 0 —
001 1 –x
010 2 -w-
011 3 -wx
100 4 r–
101 5 r-x
110 6 rw-
111 7 rwx
;chmod Command Syntax and Options
chmod [who][+,-,=][permissions] filename
*Example:
chmod g+w ~/group-project.txt
* The + operator grants permissions whereas the - operator takes away permissions.
* Copying permissions is also possible:
chmod g=u ~/group-project.txt
* The parameter g=u means grant group permissions to be same as the user’s.
* Multiple permissions can be specified by separating them with a comma, as in the following example:
chmod g+w,o-rw,a+x ~/group-project-files/
* Owner of the file is referred to as the user (e.g. u+x).
* The -R option applies the modification to the permissions recursively to the directory specified:
chmod -R +w,g=rw,o-rw, ~/group-project-files/
* Restrict File Access: Remove all Group and World PermissionsPermalink
chmod 600 .msmtprc
chmod g-rwx,o-rwx .fetchmail
;Octal Notation for File Permissions:
* The permissions to be set for file:
chmod u=rwx,g=rx,o= group-project.txt
chmod 750 group-project.txt
* Disregarding the first bit, each bit that is occupied with a - can be replaced with a 0 while r, w, or x is represented by a 1:
111 101 000
- rwx r-x ---
* This is called octal notation because the binary numbers are converted to base-8 by using the digits 0 to 7
* Typical default permission: 744
Allows R,W,X permissions for the owner
R permissions for the group and “world” users
* Other default permissions are 600 or 644
* For executable files, the equivalent settings would be 700 and 755
;umask
* Known as User Mask or User File creation MASK.
* While creating a file or directory, by default a set of permissions are applied.
* These default permissions are viewed by umask command.
* For safety reasons all Unix systems doesn't provide execution permission to newly created files.
* The 'mkdir -m' command can be used to set the mode.
mkdir -m 777 dir1
mkdir -m 000 dir2
* Preserves the permissions and time stamps from source file:
cp -p list dupli.txt
== Commands ==
=== CPU ===
; CPU Info
lscpu
lshw -C CPU
hardinfo ==> sudo apt install hardinfo
nproc
sudo dmidecode -t 4
cpuid
cat /proc/cpuinfo
cat /proc/cpuinfo | grep processor | wc -l
* The number of processors shown by /proc/cpuinfo might not be the actual number of cores on the processor.
* For example a processor with 2 cores and hyperthreading would be reported as a processor with 4 cores.
* If there are 4 different core ids, this indicates that there are 4 actual cores.
# cat /proc/cpuinfo | grep 'core id'
core id : 0
core id : 2
core id : 1
core id : 3
; CPU Usage
top -o %CPU
htop
vmstat
sar 1 3 ==> yum install sysstat
iostat ==> yum install sysstat
; Top Command
<pre>
top - 01:07:37 up 2:40, 1 user, load average: 0.37, 0.37, 0.39
Tasks: 286 total, 1 running, 285 sleeping, 0 stopped, 0 zombie
%Cpu(s): 4.7 us, 1.6 sy, 0.0 ni, 93.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 15935.7 total, 9403.3 free, 3045.2 used, 3487.1 buff/cache
MiB Swap: 4100.0 total, 4100.0 free, 0.0 used. 11720.3 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
6865 aman 20 0 982620 85280 53716 S 6.2 0.5 2:52.77 Xorg
10082 aman 20 0 3537624 285448 118848 S 6.2 1.7 5:45.24 gnome-shell
</pre>
CPU Section
us user cpu time % CPU time spent in user space
sy system cpu time % CPU time spent in kernel space
ni user nice cpu time % CPU time spent on low priority processes
id idle cpu time % CPU time spent idle
wa io wait cpu time % CPU time spent in wait (on disk)
hi hardware irq % CPU time spent servicing/handling hardware interrupts
si software irq % CPU time spent servicing/handling software interrupts
st steal time % CPU time stolen from a virtual machine
Main Section:
%MEM directly related to RES, percentage use of total physical memory by the process.
VIRT total memory that this process has access to shared memory, mapped pages, swapped out pages, etc.
RES total physical memory used shared or private that the process has access to.
SHR total physical shared memory that the process has access to.
RES is most close to the memory used by the process in memory, excluding what’s swapped out.
This includes the SHR (shared physical memory) which mean it could have been used by some other process as well.
;Obtain the PID:
pgrep -n python
pidof chrome - return all PIDs
pidof -s chrome - return only 1 PID
ps -C chrome -o pid= - C = CMD
=== Memory ===
;Info
dmidecode -t 17
;Usage
cat /proc/meminfo ==> egrep --color 'Mem|Cache|Swap' /proc/meminfo
top -o %MEM
free -m
total used free shared buff/cache available
Mem: 15935 3046 9470 767 3418 11787
Swap: 4099 0 4099
vmstat
vmstat -s ==> More detailed
htop
;Per Process usage check
ps -o pid,user,%mem,command ax | sort -b -k3 -r
sudo pmap 917 ==> Libraries, other files, etc usage of memory
sudo pmap 917 | tail -n 1 ==> Total used by this process
=== HDD ===
du -h ==> space by dir including all subdir in dir tree
du -sh /etc/ ==> total disk space used by dir and suppress subdir
du -ah /etc/ ==> see all files, not just directories:
df -h
Filesystem Type Size Used Avail Use% Mounted on
/dev/sda4 ext4 77G 51G 22G 71% /
df -T -h ==> List Filesystem type as well
df -t ext4 ==> Only see ext4 file system
df -a ==> List all filesystems that have a size of zero blocks as well
df -i ==> Display File System Inodes
lsblk ==> Lists out all the storage blocks, which includes disk partitions and optical drives
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 1.8T 0 disk
├─sda1 8:1 0 500M 0 part /boot/efi
├─sda2 8:2 0 128M 0 part
sudo fdisk -l ==> Partition & FS Type details
parted ==> List out partitions and modify them
=== IP ===
*Show info
ip addr show (ip a)
ifconfig
hostname -I
ip route get 8.8.8.8 | head -1 | awk '{print $7}'
ip route get 8.8.8.8 | head -1 | cut -d' ' -f7
*Assign IP CentOS:
nano /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
IPADDR=192.168.1.2
NETMASK=255.255.255.0
ONBOOT=yes
*Ubuntu
sudo nano /etc/network/interfaces
auto ens31
iface ens31 inet dhcp
auto ens33
iface ens33 inet static
address 192.168.0.2
netmask 255.255.255.0
network 192.168.0.0
broadcst 192.168.0.255
gateway 192.168.0.1
dns-nameservers 192.168.0.3
*Other methods:
ifconfig eth0 10.10.10.45 netmask 255.255.255.0 up
ip addr add 10.20.30.176/24 dev eth0
*IP Alias:
ifconfig eth0:100 192.168.66.1
*VLAN:
vconfig add eth0 700
ifconfig eth0.700 192.168.66.1
*VLAN & Alias:
auto eth0.10
iface eth0.10 inet static
address 192.168.1.61
netmask 255.255.255.0
gateway 192.168.1.11
auto eth0.10:1
iface eth0.10:1 inet static
address 10.20.100.100
netmask 255.255.255.0
auto eth0.10:2
iface eth0.10:2 inet static
address 10.20.100.200
netmask 255.255.255.0
==== Route ====
netstat -nr (n => Numerical not hostname)
ip route
route
route -n
U route is up
H target is a host
G use gateway
R reinstate route for dynamic routing
D dynamically installed by daemon or redirect
M modified from routing daemon or redirect
A installed by addrconf
C cache entry
! reject route
=== DNS ===
==== Config Info ====
cat /etc/resolv.conf
nmcli dev show | grep DNS
systemd-resolve --status
resolvectl status | grep -1 'DNS Server'
==== Resolution ====
===== Host Command =====
host google.com
host -t a google.com
host -t mx google.com
host -t soa google.com
host -t cname files.google.com
host -t txt google.com
host google.com ns2.google.com ==> Query a particular host
host -t any google.com
===== DIG Command =====
dig google.com a
dig google.com mx
dig google.com ns
dig google.com txt
dig @ns1.google.com a
dig @4.2.2.2 google.com soa ==> SOA record
dig +nssearch google.com ==> SOA record
dig +short google.com ==> only IP address
dig +noall +answer google.com ==> Just answer line
dig +noall +answer google.com any ==> Just answers for all records
===== NSLOOKUP =====
nslookup yahoo.com ==> Find A Record
nslookup 209.191.122.70 ==> Reverse Domain Lookup
nslookup -query=mx www.yahoo.com ==> Query MX (Mail Exchange) record
nslookup -query=ns www.yahoo.com ==> NS(Name Server) record
nslookup -query=any yahoo.com ==> query all Available DNS records
nslookup -debug yahoo.com ==> verbose information like TTL, etc
=== TCP Parameters ===
;MSL
cat /proc/sys/net/ipv4/tcp_fin_timeout
To change it:
echo 5 > /proc/sys/net/ipv4/tcp_fin_timeout
;WSF
cat /proc/sys/net/ipv4/tcp_window_scaling
=== CURL ===
curl -I http://domain.com Get HTTP header information
curl -i http://domain.com Get HTTP header + Body information
curl -L http://domain.com Handle URL redirects
curl -v http://domain.com Debug level details
curl -x proxy.sr.com:3128 http://domain.com Using proxy to download a file
curl -k https://domain.com Ignoring the ssl certificate warning
curl -A "Mozilla/5.0" http://domain.com Spoofing user agent:
curl -L -H "user-agent: Mozilla/5.0" https://aman.info.tm Custom Headers
curl smtp://example.com:2525
curl ftp://example.com
curl example.com:21
curl example.com:7822 Troubleshooting SSH: SSH-2.0-OpenSSH_5.3
time curl google.com
curl -i https://site1.lab.com --cert /root/ca/domains/ubnsrv01-cert.pem --key /root/ca/domains/ubnsrv01-key.pem
curl -v -X OPTIONS https://site3.lab.com
curl -v -X TRACE https://site3.lab.com
curl --sslv2 https://yoururl.com
curl --tlsv1 https://yoururl.com
curl -H 'X-My-Custom-Header: 123' https://httpbin.org/get Using httpbin tool; shows header info
curl -e google.com yoururl.com Referrer
curl --data "name=bool&last=word" https://httpbin.org/post Post data
curl -X POST https://httpbin.org/post Empty Post Request
curl -H 'Host: aman.info.tm' 128.199.139.216 If Server using Virtual Hosting
Post Json Data
curl --data '{"email":"test@example.com", "name": ["Boolean", "World"]}' -H 'Content-Type: application/json' https://httpbin.org/post
Time Breakdown
curl https://www.booleanworld.com/ -sSo /dev/null -w 'namelookup:\t%{time_namelookup}\nconnect:\t%{time_connect}\nappconnect:\t%{time_appconnect}\npretransfer:\t%{time_pretransfer}\nredirect:\t%{time_redirect}\nstarttransfer:\t%{time_starttransfer}\ntotal:\t\t%{time_total}\n'
=== IPtables ===
iptables -L ==> List rules
iptables -F ==> Stop iptables
iptables -nvL ==> Check Stats
iptables --flush MYCHAIN ==> Flush Chain
iptables -X MYCHAIN ==> Delete Empty Chain
iptables -A INPUT -p tcp --dport ssh -j ACCEPT ==> Allow SSH
iptables -A INPUT -p tcp --dport 80 -j ACCEPT ==> Allow incoming web traffic
iptables -A INPUT -j DROP ==> Blocking Traffic
iptables -A INPUT -i ens160 -s 10.140.198.7 -j DROP ==> Blocking Traffic
iptables -I INPUT 1 -i lo -j ACCEPT ==> Allow loopback
iptables -I INPUT 5 -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7 ==> Logging
=== TCPDump ===
sudo tcpdump -s 0 -i ens160 host 10.1.1.1 -v -w /tmp/packet_capture.cap
sudo tcpdump -s 0 -i ens160 host 10.1.1.1 and port 22 -v -w /tmp/packet_capture.cap
sudo tcpdump -s 0 -i ens160 host 10.1.1.1 and port not 22 and port not 80 -v -w /tmp/packet_capture.cap
sudo tcpdump -s 0 -i ens160 host 10.1.1.1 and tcp port not 22 and tcp port not 80 -v -w /tmp/packet_capture.cap
for i in `find . -type f | egrep "All.pcap"`; do echo $i; tcpdump -r $i '((host 1.1.1.1 or host 2.2.2.2) and host 3.3.3.3) and port 445' ; echo -e "\n"; done
=== MTR ===
Provides the functionality of both the ping and traceroute commands.
Prints information about the entire route.
mtr google.com
mtr -g google.com Display Numeric IP addresses
mtr -b google.com Both hostnames and numeric IP addresses
mtr --tcp google.com Use TCP SYN packets
mtr --udp google.com UDP datagrams
=== Traceroute ===
traceroute 4.2.2.2 ==> Uses UDP
traceroute -n 4.2.2.2 ==> Do not resolve hostnames
sudo traceroute -nI 4.2.2.2 ==> Use ICMP Packets
sudo traceroute -nT 4.2.2.2 ==> Use TCP Syn (Port 80)
=== Netstat ===
netstat -s
netstat -a Listing all ports (both TCP and UDP)
netstat -at Listing TCP Ports connections
netstat -au Listing UDP Ports connections
netstat -l Listing all LISTENING Connections
netstat -lt Listing all TCP Listening Ports
netstat -s Showing Statistics by Protocol
netstat -st Showing Statistics by TCP Protocol
netstat -tp Displaying Service name with PID
netstat -r Displaying Kernel IP routing
netstat -anp
netstat -ant
=== PS ===
ps -aux Display all processes in BSD format
ps -eo pid,ppid,user,cmd
ps -e --forest Print Process Tree
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu | head
=== LS ===
Append a character to each file name indicating the file type:
ls -F or ls --classify
* Executable files
/ Directories
@ Symbolic links
<nowiki>|</nowiki> FIFOs
= Sockets
> Doors
Nothing for Regular Files
List Symoblic Links:
ls -la
<pre>
lrwxrwxrwx 1 root root 11 Sep 13 14:57 mounts -> self/mounts
dr-xr-xr-x 3 root root 0 Sep 13 14:57 mpt
-rw-r--r-- 1 root root 0 Sep 13 14:57 mtrr
</pre>
=== Redirect Stderr ===
0 stdin – Use to get input (keyboard)
1 stdout – Use to write information (screen) 1> >
2 stderr – Use to write error message (screen) 2>
Redirect Stderr into Stdout:
2>&1
ls > file.log 2>&1 OR ls &> file.log
ls > file.log 2> /dev/null
== Troubleshooting ==
Source: [https://scoutapm.com/blog/slow_server_flow_chart scoutapm.com]
1: Check I/O wait and CPU Idletime
Look for "wa" (I/O wait) and "id" (CPU idletime)
I/O Wait represents the amount of time the CPU waiting for disk or network I/O.
Anything above 10% I/O wait should be considered high.
CPU idle time is a metric you WANT to be high
If your idle time is consistently above 25%, consider it "high enough"
2: IO Wait is low and idle time is low: check CPU user time
Look for the %us column (first column), then look for a process or processes that is doing the damage.
If %usertime is high, see which program is monopolizing the CPU
Be default, top sorts the process list by %CPU, so you can just look at the top process or processes.
If situation seems anomalous: kill/restart the offending processes.
If situation seems typical given history: upgrade server or add more servers.
3: IO wait is low and idle time is high
Your slowness isn't due to CPU or IO problems, so it's likely an application-specific issue.
Slowness might be caused by another server in your cluster or by an external services like DB
If you suspect another server in your cluster use - Strace and Lsof
Strace will show you which file descriptors are being read or written to.
Lsof can give you a mapping of those file descriptors to network connections.
4: IO Wait is high: check your swap usage
Use top or free -m
Cache swaps will monopolize the disk
Processes with legitimate IO needs will be starved for disk access.
In other words, checking disk swap separates "real" IO wait problems from what are actually RAM problems that "look like" IO Wait problems.
5: Swap usage is high
High swap usage means that you are actually out of RAM.
6: Swap usage is low
Low swap means you have a "real" IO wait problem
iotop is an awesome tool for identifying io offenders.
7: Check memory usage
Once top is running, press the M key - this will sort applications by the memory used.
Important: don't look at the "free" memory -- it's misleading.
To get the actual memory available, subtract the "cached" memory from the "used" memory.
This is because Linux caches things liberally, and often the memory can be freed up when it's needed.
A memory leak can be satisfactorily addressed by a one-time or periodic restart of the process.
If memory usage seems anomalous: kill the offending processes.
If memory usage seems business-as-usual: add RAM to the server, or split high-memory using services to other servers.
= Sorting Algorithms =
| |||