Cheatsheet

ARP vs MAC Table vs CAM Table

Need to confirm if MAC Table is same as CAM table

ARP Table	MAC Table (or CAM Table)	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

Fragmentation

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

Headers

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

N S

C W R

E C E

U R G

A C K

P S H

R S T

S Y N

F I N

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

DNS Headers

Identification

QR

Opcode

A A

T C

R D

R A

Z

A D

C D

RCode

Total Questions

Total Answers

Total Authority Resource Records

Total Additional Resource Records

• AD Authentic Data
• CD Checking Disabled

ARP Headers

Hardware type (Ethernet = 1)
Protocol type (IPv4 = 0x0800)
Hardware address length (Ethernet size is 6)								Protocol address length (IPv4 size is 4.)
Operation ( 1 for request; 2 for reply)
Source MAC
Source IP
Dest MAC
Dest IP

GARP

• ICMP Header

Code 
Checksum 
Rest of Header 

DNS

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
                                                number,etc
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 example.com 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.

TCP

• Parameters determined during Handshake:

MSS  (default is 536)
WSF
SACK Permitted

• MTU vs MSS

• RTO: Four ACKs acknowledging the same packet, which are not piggybacked on data and do not change the receiver's advertised window.

• 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

• Fast Recovery:

-
-

• 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

---

• Both consider RTO and Duplicate ACKs as packet loss events.
• Behavior of Tahoe and Reno differ primarily in how they react to duplicate ACKs.

Event	Tahoe	Reno
3 Dup Acks	Performs a fast retransmit Sets the slow start threshold to half of the current congestion window Reduces the congestion window to 1 MSS Resets to slow start state	Perform a fast retransmit Skip the slow start phase by instead halving the congestion window (instead of setting it to 1 MSS like Tahoe) Setting the slow start threshold equal to the new congestion window Enter a phase called fast recovery.
RTO (Ack time out)	Slow start is used Reduce congestion window to 1 MSS	Slow start is used Reduce congestion window to 1 MSS

• 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.

• 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

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

HTTP/1.0:

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

HTTP/1.1:

• 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/1.1 vs HTTP/2

• HTTP/2 Supports Page load speed improvements through:

Compression of request headers
Binary protocol
HTTP/2 Server Push: capability allows the server to send additional cacheable information to the client that isn’t requested but is anticipated in future requests.
Request multiplexing over a single TCP connection
Request pipelining
HOL blocking (Head-of-line) — Package blocking

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 vs PATCH

PUT method only allows a complete replacement of a document. 
PATCH is used to make changes to part of the resource at a location.

Cookie

• Session cookie
• Persistent cookie
• Secure cookie
• Http-only cookie
• Same-site cookie
• Third-party cookie
• Supercookie

Other uses

• Zombie cookie

HTTP Headers

Header	Uses
Set-cookie
Location	1) Used to ask a web browser to load a different web page Client request: GET /index.html HTTP/1.1 Host: www.example.com Server response: HTTP/1.1 302 Found Location: http://www.example.org/index.php. 2) To provide info about location of a newly created resource, the Location header should be sent with an HTTP status code of 201 or 202.
Host	The host Header tells the webserver which virtual host to use if same virtual host is using several aliases
Accept
User-Agent
content-type
content-length
date
expires
Authorization	Basic access authentication is used to provide a user name and password when making a request. Credentials are the base64 encoding of id and password joined by a single colon Base64-encoding of 'Aladdin:OpenSesame' is 'QWxhZGRpbjpPcGVuU2VzYW1l' Authorization: Basic QWxhZGRpbjpPcGVuU2VzYW1l https://Aladdin:OpenSesame@www.example.com/index.html ==> This will not ask for the credentials; deprecated now
Referrer	When a user clicks a hyperlink in a web browser, the browser sends a request to the server holding the destination webpage. The request may include the referer field, which indicates the last page the user was on (the one where they clicked the link).
X-Forwarded-For (XFF)	Used for identifying the originating IP address of a client connecting to a Web Server through an HTTP Proxy or Load Balancer.

FTP

SSL Handshake

--> Client Hello
<-- Server Hello, Certificate, Server Hello Done
--> Client Key Exchange, Change Cipher Spec, Encrypted Handshake Message(Finished)
<-- Change Cipher Spec, Encrypted Handshake Message(Finished)
--> Application Data(GET)
<-- Encrypted Handshake Message(Hello Request)

1. Client sends the supported parameters
2. Server chooses the parameters; Sends the certificate; And first half of the Diffie-Hellman key exchange
3. Client sends the second half of the Diffie-Hellman exchange, Computes the session keys; Switches to encrypted communication
4. Server computes the session keys; Switches to encrypted communication.

SSLv1 vs TLS 1.0 vs TLS1.3

SSL 2.0 - Deprecated
SSL 3.0 - Deprecated
TLS 1.0 - Deprecated
TLS 1.1 - Deprecated
TLS 1.2 - 
TLS 1.3 -

NetScaler

• 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        =
SRC IP DST 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

OSPF

• States

Down  
Attempt
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          Sent from router to other routers in the same area, has info reg router's int in the same area, int IPs, adjacent routers
Type 2 - Network LSAs         Generated by the DR on a multi access segment, similar to LSA Type 1
Type 3 - Network Summary LSA  Generated by ABRs, contain the subnets & costs 
Type 4 - ASBR summary LSA     Same as summary LSA except the destination advertised by ABR is ASBR, ABR in same area as the ASBR will originate the Type 4 LSA.
Type 5 - AS external LSA      Generated by ASBRs, Flooded throughout the AS to advertise a route external to OSPF
Type 7 - NSSA External LSA    Generated by the ASBR in an NSSA area, Converted into a type 5 LSA by the ABR when leaving the area

• Packet Types

Type 1 - Hello 
Type 2 - Database Description (DBD) 
Type 3 - Link-State request (LSR) 
Type 4 - LSU (Contain LSAs)
Type 5 - LSAck

• Neighbor Requirements:

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

• LSA Details

• OSPF path selection: O > O*IA > O*E1 > O*E2 > N1 > N2.
• “area range” summarize type 3 LSA’.
• “summary-address” summarize type 5 & 7 LSA’s.
• Auto-cost reference BW (Default = 100mb), formula = 100000000/Int-Bw.

BGP

• Route Selection Criteria

Attribute	Which is better	Type
Next Hop reachable	Route cannot be used if next hop is unreachable	Well-known Mandatory
Weight	Bigger; value local to the router; Cisco proprietary; default is 0 for all routes not originated by local router
Local Preference	Bigger; used within AS and exchanged bw iBGP routers; default is 100	Well-known discretionary
Locally Injected (Originate)	Prefer path local router originated; Locally injected > iBGP/eBGP learned; In BGP table it will hv next hop 0.0.0.0
AS Path Length	Smaller; e.g: AS path 1 2 3 is preferred over AS path 1 2 3 4 5	Well-known Mandatory
Origin	Prefer IGP(advertised by network cmd - i) > EGP > INCOMPLETE - '?'(reditributed)	Well-known Mandatory
MED(Metric)	Smaller; used to advertise to neighbors how they should enter your AS; propagated to all routers within the neighbor AS but not passed along any other AS	Optional non-transitive
Neighbor Type	Prefer eBGP over iBGP
IGP Metric to Next Hop	Smaller; Prefer the path within the AS with the lowest IGP metric to the BGP next hop
Oldest path	Prefer the path that we received first
Router ID	Prefer the path with the lowest BGP neighbor router ID (Manually conf > Highest Loopback IP address > Highest Interface IP address)
Neighbor IP address	Prefer the path with the lowest neighbor IP address

• BGP States

Idle
Active         Attempting to connect
Connect        TCP session established
OpenSent       Open message sent
OpenConfirm    Response received
Established    Adjacency established

• BGP Messages

Open
Update 
Keepalive       Sent every 60 seconds
Notification    Always indicate something is wrong

• Directions

Aspath prepend:  Applied outwardly.
                 Impacts incoming path.
                 Shorter the as-path length higher the preference
                 As-path prepend is the way to add AS number to the list of subnet u want to advertise. 
                 This is a way to route poisoning. 
                 Tell the outside world not to follow the path.

Local preference:  Applied while the traffic coming inside.  
                   Impacts traffic while going out.  
                   Non transitive. 
                   Propagates within the same as-path.
                   Higher the local preference value higher the preference

MED:  Multiexitdescriptor
      When your router has connection with two other routers with same AS. 
      Let's say you have 2 subnets behind your router.  
      You can use MED value to mention which networks should be accessed through which links. 
      It is advertised outwards. 
      Impacts the incoming traffic. 
      Semi transitive. 
      Propagates to one AS.
      Lower the MED value higher the preference.
      MED should be used carefully as it reduces network resiliency.

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

Screens
Static NAT | Dest NAT
Route ==> Forwarding Lookup
Zones
Policy
Reverse Static NAT | Source NAT
Service ALG
Session

Fast Path

Screens
TCP
NAT
Service ALG

ScreenOS

• ScreenOS Flow order

Sanity Check 
Screening
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

Flows

• 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 [200 OK] & 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:

[PC1]-----[Hub]-----[Switch]-----[Router]------[Router]------[PC2]

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.

Linux

Linux Booting

1. BIOS(Basic Input/Output System) - POST, Loads and executes the MBR boot loader.
2. MBR (Master Boot Record) - Loads and executes the GRUB boot loader.
3. GRUB (Grand Unified Bootloader) - Loads and executes Kernel and Initrd images.
4. Kernel - Heart of OS; Memory, Process mgmt; Executes INIT process.
5. Init (initialization) - Decides the Linux run level; default run level to either 3 or 5.
6. Runlevel programs - Executes programs like sendmail, etc from the run level directory as defined by the run level.

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

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
   |   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

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

System Calls

This section is under construction.

Sorting Algorithms

• Quicksort

It is a good default choice. 
It tends to be fast in practice with some small tweaks its dreaded O(n2)O(n^2)O(n2) worst-case time complexity becomes very unlikely. 
A tried and true favorite.

• Heapsort

It is a good choice if you can't tolerate a worst-case time complexity of O(n2)O(n^2)O(n2) or need low space costs. 
The Linux kernel uses heapsort instead of quicksort for both of those reasons.

• Merge sort

It is a good choice if you want a stable sorting algorithm.
It can easily be extended to handle data sets that can't fit in RAM where the bottleneck cost is reading and writing the input on disk, not comparing and swapping individual items.

• Radix sort

It looks fast, with its O(n)O(n)O(n) worst-case time complexity. 
If you're using it to sort binary numbers, then there's a hidden constant factor that's usually 32 or 64 (depending on how many bits your numbers are).
That's often way bigger than O(lg⁡(n))O(\lg(n))O(lg(n)), meaning radix sort tends to be slow in practice.

• Counting sort

It is a good choice in scenarios where there are small number of distinct values to be sorted. 
This is pretty rare in practice, and counting sort doesn't get much use.

• Which sorting algorithm has best asymptotic run time complexity?

Python

• Regex

re.match() => Matches Beginning
re.search() => Matches Anywhere
re.findall() => All Matching Objects
re.sub('[ES]', 'a', s) => Substitute

• Lists

• Dictionary

• File operations

Using Open:

f = open('/etc/passwd')
f.read(5)
f.close()

Using With Open(better, auto closes the file):

with open('/etc/passwd') as f:
    for line in f:
        print(line)

• Class

• OS Interaction:

import os
os.system("date")

import os
f = os.popen('date')
now = f.read()
print("Today is ", now)

import subprocess
subprocess.call(["ls", "-l", "/etc/resolv.conf"])

import subprocess
p = subprocess.Popen("date", stdout=subprocess.PIPE, shell=True)
(output, err) = p.communicate()
print("Today is", output)

SMTP

HELO or EHLO (Hello)
MAIL FROM
250 OK reply code
RCPT TO (Recipient To)
250 OK reply code
DATA
345 reply code
250 OK code
QUIT
221 code

RSET (Reset)

SMTP errors:

4.X.X Persistent Transient Failure
5.X.X Permanent Error: