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ARP vs MAC Table

ARP Table MAC Table (or CAM Table)
Layer3 address to Layer2 address resolution Layer2 address to Interface binding
Matches IP addresses to MAC addresses Maps Ports to MAC addresses
Needed to forward packets at layer 3 Used to Switch frames to the right output interface
Kept by L3 devices Kept only by L2 devices
No entry for dest IP address, machine will send ARP request If no entry, switch will flood the frame
Default timeout is 4 hours Default timeout is 5 minutes
Filled by each ARP reply Filled by source MAC of each frame passing through switch


Before fragmentation
Original IP Datagram
Sequence Identifier Total Length DF Flag MF Flag Fragment offset
0 345 5140 0 0 0
After fragmentation
IP Fragments(ethernet)
Sequence Identifier Total Length DF Flag MF Flag Fragment offset
0-0 345 1500 0 1 0
0-1 345 1500 0 1 185
0-2 345 1500 0 1 370
0-3 345 700 0 0 555


IPv4 Header Format
Version HLEN DSCP ECN Total Length
Identification Flags(DF,MF) Fragment Offset
Time To Live Protocol Header Checksum
Source IP Address
Destination IP Address
Options (if HLEN > 5)

TCP Header
Source port Destination port
Sequence number
Acknowledgment number (if ACK set)
Data offset Reserved
0 0 0
Window Size
Checksum Urgent pointer (if URG set)
Options (if data offset > 5. Padded at the end with "0" bytes if necessary.)

UDP Header
Source port Destination port
Length Checksum
  • ARP Header
Hardware type
Protocol type
Hardware address length
Protocol address length
Source MAC
Source IP
Dest MAC
Dest IP

  • ICMP Header
Rest of Header 


Record Types
A 	Address record 	 	 	 	Returns a 32-bit IPv4 address,
AAAA 	IPv6 address record 	
CNAME 	Canonical name record 	 	 	Alias of one name to another, DNS lookup will continue by retrying the lookup with the new name.
LOC 	Location record 	 	 	Specifies a geographical location associated with a domain name
MX 	Mail exchange record 	 	 	Maps a domain name to a list of message transfer agents for that domain
NS 	Name server record 	 	 	Delegates a DNS zone to use the given authoritative name servers
PTR 	Pointer record 	 	 	 	Pointer to a canonical name. Unlike a CNAME, DNS processing stops and just the name is returned. The most common use is for implementing reverse 
                                                DNS lookups.
SOA 	Start of [a zone of] authority record 	Specifies authoritative information about a DNS zone, including the primary name server, the email of the domain administrator, the domain serial
SRV 	Service locator 	 	 	Generalized service location record, used for newer protocols instead of creating protocol-specific records such as MX.
TXT 	Text record 	 	 	 	Originally for arbitrary human-readable text in a DNS record. Now more often carries machine-readable data, opportunistic encryption, Sender Policy
                                                Framework, etc.
* 	All cached records 	 	 	Returns all cached records of all types known to the name server. If the name server does not have any information on the name, the request will be 
                                                forwarded on.
AXFR 	Authoritative Zone Transfer 	 	Transfer entire zone file from the master name server to secondary name servers.
IXFR 	Incremental Zone Transfer 	 	Requests a zone transfer of the given zone but only differences from a previous serial number.

Glue Record
  • A glue record is a term for a record that's served by a DNS server that's not authoritative for the zone, to avoid a condition of impossible dependencies for a DNS zone.
  • What glue records do is to allow the TLD's servers to send extra information in their response to the query for the zone - to send the IP address that's configured for the name servers.
  • It's not authoritative, but it's a pointer to the authoritative servers, allowing for the loop to be resolved.


  • Parameters determined during Handshake:
SACK Permitted

  • MTU vs MSS
Mtu mss.png
  • Congestion Control
Slow Start - Exponential Increase
- Sender starts with cwnd = 1 MSS, Size increases 1 MSS each time one Ack arrives, Increases the rate exponentially(1,2,4,8....) until a threshold is reached
Congestion Avoidance - Additive Increase
- Increases the cwnd Additively, When a “window” is Ack cwnd is increased by 1, Window = No of segments transmitted during RTT
- The increase is based on RTT, not on the number of arrived ACKs, Congestion window increases additively until congestion is detected
Congestion Detection - Multiplicative Decrease
- If congestion occurs, Window size must be decreased, Sender knows about congestion via RTO or 3 Dup Acks received, Size of Threshold is dropped to half
  • Tahoe
- If RTO occured, TCP Reacts Strongly
- Reduces cwnd back to 1 Segment, starts the slow start phase again
  • Reno
- If 3 Duplicate ACKs are received, TCP has a Weaker Reaction
- Starts the Congestion Avoidance phase
- This is called fast transmission and fast recovery
  • Silly Window Syndrome: Sender creates data slowly or Receiver consumes slowly or both.

Syndrome due to Sender:

- Nagle’s Algorithm: Send data initially, accumulate data in output buffer, Wait for Ack or till 1 MSS Data in Buffer

Syndrome due to Receiver:

- Clark’s Solution: Announce window size 0 till 1) enough space for 1 MSS in Buffer or Half Receive buffer is empty
- Delayed Acknowledgment: Segment not acknowledged immediately, Sender TCP does not slide its window, reduces traffic, sender may unnecessarily retransmit, Not delay more than 500 ms.
  • Fast Retransmission
- If RTO has a larger value
- If sender receives four acknowledgments with same value (three duplicates)
- Segment expected by all of these Ack is resent immediately
  • Persistence Timer
- Issue of Deadlock created by Lost Ack, used to reset Window size 0 advertized earlier, is resolved by this timer
- Sending TCP sends a special segment(1 byte of new data) called Probe, causes the receiving TCP to resend Ack
- If no reply, another probe is sent and value of persistence timer is doubled and reset 
- Sender continues sending probes, doubling, resetting value of persistence timer until it reaches a threshold(generally 60s)
- After that the sender sends one probe segment every 60s until the window is reopened

VPN Messages

  • Phase 1 - Main Mode
Cookie,Proposal List
Cookie,Accepted Proposal
DH Key,Nonce
DH Key,Nonce
ID,ID Hash
ID,ID Hash
  • Phase 1 - Aggressive Mode
ID,Proposal List,DH Key,Nonce
ID,Accepted Proposal,DH Key,Nonce,ID Hash
ID Hash

  • Phase 2 - Quick Mode
Ph1 Hash,Message ID,Proposal List,Nonce, DH Key,Proxy-ID 
Ph1 Hash,Message ID,Accepted Proposal,Nonce,DH Key,Proxy-ID 
Ph1 Hash,Message ID,Nonce 


HTTP Error Codes
Category Type Code
1XX Informational 100 = Continue
2XX Successful 200 = OK
201 = Created (URL)
202 = Accepted (request accepted but not acted upon immediately)
203 = Non-authoritative Information(info in header is from local or third-party copy, not from original server)
204 = No Content (in body)
3XX Re-directional 301 = Moved Permanently
302 = Found (temporary redirect)
304 = Not Modified
305 = Use Proxy (URL must be accessed through the proxy mentioned in the Location header)
307 = Temporary Redirect (requested page has moved temporarily to a new url)
4XX Client Error 400 = Bad Request
401 = Unauthorized
402 = Payment Required
403 = Forbidden
404 = Not Found
405 = Method Not Allowed
5XX Server Error 500 = Internal Server Error
501 = Not Implememted
502 = Bad Gateway or Proxy
503 = Service Unavailable
504 = Gateway or Proxy Timeout
505 = HTTP Version Not Supported
HTTP1.0 vs HTTP1.1


  • Uses a new connection for each request/response exchange
  • Closed connections after every request.
  • Supports GET, POST, HEAD request methods


  • Connection may be used for one or more request/response exchanges
  • Uses persistent connections, save bandwidth & reduces latency as it does not require to do TCP Handshake again for every file download (like images, css, etc.)
  • HTTP Pipeline feature in which client sends multiple requests before waiting for each response.
  • Supports OPTIONS, PUT, DELETE, TRACE, CONNECT request methods

HTTP Request Methods
GET:       Retrieve Data
HEAD:      Header only without Response Body
POST:      Submits Data to DB, web forum, etc
PUT:       Replaces target resource with the uploaded content
DELETE:    Removes target resource given by URI
CONNECT:   Used when the client wants to establish a transparent connection to a remote host, usually to facilitate SSL-encrypted communication (HTTPS) through an HTTP proxy
OPTIONS:   Returns the HTTP methods that the server supports for the specified URL
TRACE:     Performs a message loop back test to see what (if any) changes or additions have been made by intermediate servers
PATCH:     Applies partial modifications to a resource.
PUT method only allows a complete replacement of a document. 
PATCH is used to make changes to part of the resource at a location.


Active-Passive FTP.JPG

SSL Handshake

SSL Handshake.png


  • LB Methods:
Least Connection     = Service with fewest active connections
Round Robin          = Rotates a list of services
Least Response time  = Fewest active connections & lowest average response time
Least Bandwidth      = Service serving least amount of traffic measured in mbps
Least Packets        = Service that received fewest packets
Source IP Hash       =
Destination IP Hash  =
  • Persistence Methods:
SOURCE IP      =
COOKIE Insert  = Connections having same HTTP Cookie inserted by Set-Cookie directive from server belong to same persistence session.
SSL Session    = Connections having same SSL session ID
RULE           = All connection matching a user defined rule
URL Passive    = requests having same server ID(Hexadecimal of Server IP & Port) of service to which request is to be fwded
Dest IP        =
CALL ID        = Same Caller ID in SIP Header
  • What is Stateful & Stateless Persistence? Which one is more scalable/Efficient?
Stateless Session Persistence: Cookie inserted by ADC is more efficient because no need to create a table, NS will insert cookie & forget, with reply, it will read cookie value, decrypt it & fwd request.
State-full Session Persistence: Server will insert cookie, NS will hash it & fwd based on Hash value but will need to keep a table in memory with all hashes & IP Addresses.
Same is true for Source IP based Persistence, Also inefficient behind NAT
Using Set-cookie-header = by Server - insert Name & Value Fields
Client sends cookie in Cookie Header
Who ever generates cookie, will be able to read it


  • States
Init      Hello sent out all int
2-Way     Hello rcvd cont own RID in ngbr list
ExStart   Determine master slave
Exchange  Master sends DBD first, then Slave
Loading   Comp DBDs, send LSR for missing LSAs
Full      LSDB of ngbr are fully syncd
  • LSA Type
Type 1 - Router LSAs 
Type 2 - Network LSAs 
Type 3 - Network Summary LSA 
Type 4 - ASBR summary LSA 
Type 5 - AS external LSA 
Type 7 - NSSA External LSA 

  • Packet Types
Type 1 - Hello 
Type 2 - Database Description (DBD) 
Type 3 - Link-State request (LSR) 
Type 4 - LSU 
Type 5 - LSAck

  • Neighbor Requirements
Same area
Same authentication config
Same subnet
Same hello/dead interval
Matching stub flags

OSFF LSA 2.png
  • OSPF path selection: O > O*IA > O*E1 > O*E2.
  • “area range” summarize type 3 LSA’.
  • “summary-address” summarize type 5 & 7 LSA’s.
  • Auto-cost reference BW (Default = 100mb), formula = 100000000/Int-Bw.


  • Route Selection Criteria
Attribute Which is better
Next Hop reachable Route cannot be used if next hop is unreachable
Weight Bigger
Local Preference Bigger
Locally Injected Locally injected is better than iBGP/eBGP learned
AS Path Length Smaller
Origin Prefer I over E & E over Unknown
MED Smaller
Neighbor Type Prefer eBGP over iBGP
IGP Metric to Next Hop Smaller

  • BGP States
Active         Attempting to connect
Connect        TCP session established
OpenSent       Open message sent
OpenConfirm    Response received
Established    Adjacency established
  • BGP Messages
Keepalive       Sent every 60 seconds
Notification    Always indicate something is wrong

VPN Monitor vs DPD vs IKE Heartbeat

VPN Monitor DPD IKE Heartbeat
Juniper Proprietary RFC Standard Juniper Proprietary
Work with Non Juniper Work with Non Juniper Cannot work with Non Juniper
Uses ICMP Uses ICMP(encrypted IKE Phase 1 message(R-U-THERE)) --
Goes inside the Phase 2 Tunnel Goes through Phase 1 Tunnel --
Implies VPN is UP Implies peer is up and responding Enhancement to detect tunnel availability
Works if supported by one peer only -- Both ends must support
Configured in Phase 2 Configured in Phase 1 Configured in Phase 1

SRX Architecture

First Path
Static NAT | Dest NAT
Route ==> Forwarding Lookup
Reverse Static NAT | Source NAT
Service ALG
Fast Path
Service ALG


  • ScreenOS Flow order
Sanity Check 
Session lookup 
Route Lookup 
Policy lookup
Session creation 
ARP lookup 
  • Route preference order
Policy Based Routing 
Source Interface Based Routing 
Source Routing 
Destination Routing 

  • NAT Preference order
Mapped IP 
Virtual IP 
Policy Based NAT (NAT-Src & NAT-Dst) 
Interface Based NAT 

SYN Flood Protection

Threshold = Proxy connections above this limit
If Syn-cookie is enabled, no sessions established between client & firewall or firewall & server directly
Alarm Threshold = Alarm/Alert (to log)
Queue Size = The number of proxied connections held in queue
After this the firewall starts rejecting new connection requests
Timeout Value is maximum time before a half-completed connection is dropped from the queue
The range is 0–50s; default is 20s


Linux Booting


The following are the 6 high level stages of a typical Linux boot process:

  1. BIOS
  2. MBR
  3. GRUB
  4. Kernel
  5. Init
  6. 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:
  1. primary boot loader info in 1st 446 bytes,
  2. partition table info in next 64 bytes(16,16,16,16) 4 partitions,
  3. 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).
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
  • As you notice from the above info, it contains kernel and initrd image.
  • 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.
  1. Programs starts with S are used during startup. S for startup.
  2. Programs starts with K are used during shutdown. K for kill.
  3. There are numbers right next to S and K in the program names.
  4. Those are the sequence number in which the programs should be started or killed.
  5. For example, S12syslog is to start the syslog deamon, which has the sequence number of 12.
  6. S80sendmail is to start the sendmail daemon, which has the sequence number of 80.
  7. 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 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


ls -l /proc/$(pgrep -n python)/exe

Inode Number


  • 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.
  • 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:

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/ 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/
  • 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.

  • 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/ synchro
  • Compare:
ls -il /home/bob/ synchro 
517333 -rw-r----- 2 root root 5 Apr 7 06:09 /home/bob/
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
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 (
  • 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: myhost myftp
$ ping myftp
PING ( 56(84) bytes of data.
64 bytes from ( icmp_seq=1 ttl=64 time=0.023 ms
64 bytes from ( 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
  • 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



CPU Info
lshw -C CPU
hardinfo            ==>  sudo apt install hardinfo
sudo dmidecode -t 4
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
sar 1 3        ==>  yum install sysstat
iostat         ==>  yum install sysstat
Top Command
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 

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


dmidecode -t 17
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 -s   ==> More detailed 
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


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
   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 addr show   (ip a)
hostname -I
ip route get | head -1 | awk '{print $7}'
ip route get | head -1 | cut -d' ' -f7

DNS Config Info

cat /etc/resolv.conf 
nmcli dev show | grep DNS
systemd-resolve --status
resolvectl status | grep -1 'DNS Server'

DNS of Domains

Host Command
host -t a
host -t mx
host -t soa
host -t cname
host -t txt
host      ==> Query a particular host
host -t any
DIG Command
dig a
dig mx
dig ns 
dig txt
dig  a
dig @ soa         ==> SOA record
dig +nssearch            ==> SOA record
dig +short               ==> only IP address
dig +noall +answer       ==> Just answer line
dig +noall +answer any   ==> Just answers for all records
nslookup                  ==> Find A Record
nslookup             ==> Reverse Domain Lookup
nslookup -query=mx    ==> Query MX (Mail Exchange) record
nslookup -query=ns    ==> NS(Name Server) record
nslookup -query=any       ==> query all Available DNS records
nslookup -debug           ==> verbose information like TTL, etc


curl -I                                   Get HTTP header information
curl -i                                   Get HTTP header + Body information
curl -L                                   Handle URL redirects
curl -v                                   Debug level details 		 		
curl -x                 Using proxy to download a file  		
curl -k                                  Ignoring the ssl certificate warning   		
curl -A "Mozilla/5.0"                     Spoofing user agent:
curl -L -H "user-agent: Mozilla/5.0"   Custom Headers
curl smtp://
curl                                         Troubleshooting SSH:   SSH-2.0-OpenSSH_5.3
time curl
curl -i --cert /root/ca/domains/ubnsrv01-cert.pem --key /root/ca/domains/ubnsrv01-key.pem 
curl -v -X OPTIONS
curl -v -X TRACE
curl --sslv2
curl --tlsv1
curl -H 'X-My-Custom-Header: 123'   Using httpbin tool; shows header info
curl -e                                Referrer
curl --data "name=bool&last=word"  Post data
curl -X POST                       Empty Post Request
curl -H 'Host:'                If Server using Virtual Hosting

Post Json Data

curl --data '{"email":"", "name": ["Boolean", "World"]}' -H 'Content-Type: application/json'

Time Breakdown

curl -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 -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  -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


sudo tcpdump -s 0 -i ens160 host -v -w /tmp/packet_capture.cap
sudo tcpdump -s 0 -i ens160 host and port 22 -v -w /tmp/packet_capture.cap
sudo tcpdump -s 0 -i ens160 host and port not 22 and port not 80 -v -w /tmp/packet_capture.cap
sudo tcpdump -s 0 -i ens160 host 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 or host and host and port 445' ; echo -e "\n"; done


Provides the functionality of both the ping and traceroute commands.
Prints information about the entire route.
mtr -g           Display Numeric IP addresses
mtr -b           Both hostnames and numeric IP addresses
mtr --tcp        Use TCP SYN packets
mtr --udp        UDP datagrams


traceroute             ==> Uses UDP 
traceroute -n          ==> Do not resolve hostnames  
sudo traceroute -nI    ==> Use ICMP Packets
sudo traceroute -nT    ==> Use TCP Syn (Port 80)


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 -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


Append a character to each file name indicating the file type:

ls -F or ls --classify
   *   Executable files
   /   Directories
   @   Symbolic links
   |   FIFOs
   =   Sockets
   >   Doors
   Nothing for Regular Files

List Symoblic Links:

ls -la
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



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.


  • Complete Flow of PC opening a Website:
  1. Check NW config
  2. DHCP if not configured
  3. Check Domain name in Browser Cache
  4. Check Domain name in OS Cache
  5. Check if an entry exists in Hosts File
  6. If not Found in any cache, Prepare to send UDP DNS query to DNS Server
  7. If DNS Server configured is in same Network Check MAC address in ARP Table
  8. If not found, send ARP for MAC Address
  9. Forward DNS Query to DNS Server and wait for reply containing IP address of Website
  10. If DNS server configured is not in same subnet, check Gateway config(IP & MAC address)
  11. If MAC address not found in ARP Table, send ARP request
  12. After getting reply, fwd the DNS query to gateway
  13. After getting DNS response, start TCP 3-way handshake S-SA-A.
  14. Start SSL Handshake if SSL/TLS configured
  15. Send GET Request
  16. Client sends ACK & Body containing HTML Data
  17. If HTTP 1.0, Server sends FIN & CLoses connection
  18. Client send FIN-ACK
  19. Server sends Ack

  • Complete Flow of DNS Traffic
  1. Check NW config
  2. DHCP if not configured
  3. Check Domain name in Browser Cache
  4. Check Domain name in OS Cache
  5. Check if an entry exists in Hosts File
  6. If not Found in any cache, Prepare to send UDP DNS query to DNS Server
  7. If DNS Server configured is in same Network Check MAC address in ARP Table
  8. If not found, send ARP for MAC Address
  9. Forward DNS Query to DNS Server and wait for reply containing IP address of Website
  10. If DNS server configured is not in same subnet, check Gateway config(IP & MAC address)
  11. If MAC address not found in ARP Table, send ARP request
  12. After getting reply, fwd the DNS query to gateway
  13. DNS Server ??
  14. DNS Server ?? Iterative? Recursive? TLD? Authoritative
  15. DNS Server ??
  16. After getting DNS response, start TCP 3-way handshake S-SA-A.

  • Complete Flow of Traffic passing through below scenario:
  1. Check NW config
  2. DHCP if not configured
  3. Check if PC2 in same Subnet(not in this scenario as routers present)
  4. If in Same Subnet, check if MAC address is there in ARP Table
  5. Else send ARP Request
  6. Once MAC address is known, directly send Packet to PC2
  7. If PC2 is in Different Subnet(True for above scenario), Check Gateway IP address & MAC address
  8. If MAC address is not known, send an ARP request.
  9. Hub is directly connected, will receive & Flood packet on all Ports.
  10. Switch will receive packet and check its CAM Table for the MAC to Port bindings
  11. If MAC entry is not found in CAM table, Switch will Flood the ARP packet on all ports.
  12. Other destinations will drop the ARP Request packet as they do not have the IP address requested in ARP Header.
  13. Only Router will accept the packet as it has the requested IP address matching its own MAC address.
  14. It will reply with an ARP Reply message.
  15. Switch will add an entry of this MAC address & port number in its CAM Table once the reply packet pass through it.
  16. Hub will flood the packet through all ports.
  17. ARP Reply will reach PC1, it will add entry to its ARP Table
  18. Then send a packet destined to PC2 with destintion MAC address as Router's Interface's MAC address received in ARP reply.