Cheatsheet

= ARP vs MAC Table vs CAM Table =

= Fragmentation =


 * Before fragmentation:


 * After fragmentation:

= Headers =


 * AD 	Authentic Data
 * CD 	Checking Disabled


 * GARP

Code Checksum Rest of Header
 * ICMP 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 =

MSS (default is 536) WSF SACK Permitted
 * Parameters determined during Handshake:


 * 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.

- 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 Retransmission

- -
 * Fast Recovery:


 * Congestion Control

- 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
 * Slow Start - Exponential Increase


 * 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

- If RTO occured, TCP Reacts Strongly - Reduces cwnd back to 1 Segment, starts the slow start phase again
 * Tahoe

- If 3 Duplicate ACKs are received, TCP has a Weaker Reaction - Starts the Congestion Avoidance phase - This is called fast transmission and fast recovery
 * Reno


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


 * 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 =

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

ID,Proposal List,DH Key,Nonce ID,Accepted Proposal,DH Key,Nonce,ID Hash ID Hash
 * Phase 1 - Aggressive 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
 * Phase 2 - Quick Mode

= HTTP =


 * HTTP Error Codes


 * 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

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/2 Supports Page load speed improvements through:

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.
 * HTTP Request Methods

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

Cookie
Other uses
 * Session cookie
 * Persistent cookie
 * Secure cookie
 * Http-only cookie
 * Same-site cookie
 * Third-party cookie
 * Supercookie
 * Zombie cookie

HTTP Headers
= 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.

SSL 2.0 - Deprecated SSL 3.0 - Deprecated TLS 1.0 - Deprecated TLS 1.1 - Deprecated TLS 1.2 - TLS 1.3 -
 * SSLv1 vs TLS 1.0 vs TLS1.3

= NetScaler =

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 =
 * LB 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
 * Persistence Methods:


 * 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 =

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
 * States

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
 * LSA Type

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

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


 * 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

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

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

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.
 * Directions

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 =

=SRX Architecture= Screens Static NAT | Dest NAT Route ==> Forwarding Lookup Zones Policy Reverse Static NAT | Source NAT Service ALG Session
 * First Path:

Screens TCP NAT Service ALG
 * Fast Path:

= ScreenOS = Sanity Check Screening Session lookup Route Lookup Policy lookup Session creation ARP lookup
 * ScreenOS Flow order

Policy Based Routing Source Interface Based Routing Source Routing Destination Routing Mapped IP Virtual IP  Policy Based NAT (NAT-Src & NAT-Dst) Interface Based NAT
 * Route preference order
 * NAT Preference order

= 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.

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

= 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
grub> ls (hd0) (hd0,msdos5) (hd1) (hd1,msdos0)
 * Locate where the vmlinuz and initrd.* files are located:

grub> linux (hd1,msdos1)/install/vmlinuz root=/dev/sdb1 grub> initrd (hd1,msdos1)/install/initrd.gz grub> boot
 * Boot the system:

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
= Sorting Algorithms =

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.
 * Quicksort

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.
 * Heapsort

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.
 * Merge 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. 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.
 * Radix sort
 * Counting sort


 * Which sorting algorithm has best asymptotic run time complexity?

= Python =

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


 * Lists


 * Dictionary

Using Open: f = open('/etc/passwd') f.read(5) f.close
 * File operations

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: