Cheatsheet: Difference between revisions
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= ARP vs MAC Table vs CAM Table =
{{notice|Need to confirm if MAC Table is same as CAM table}}
<center>
{| class="wikitable"
|-
! ARP Table !! MAC Table (or CAM Table) ||CAM Table
|-
| Layer3 address to Layer2 address resolution || Layer2 address to Interface binding
Line 149 ⟶ 152:
|}
<br />
{| class="wikitable" style="margin: 0 auto; text-align:center; width:80%;"
|+DNS Headers
|-
| colspan="16"| Identification || colspan="1"| QR || colspan="4"| Opcode || cellpadding="1"|<tt>A<br>A</tt>|||<tt>T<br>C</tt>|||<tt>R<br>D</tt>|||<tt>R<br>A</tt>|||<tt>Z</tt>|||<tt>A<br>D</tt>|||<tt>C<br>D</tt>|| colspan="4"| RCode
|-
| colspan="16"| Total Questions || colspan="16"| Total Answers
|-
| colspan="16"| Total Authority Resource Records || colspan="16"| Total Additional Resource Records
|}
<br />
*AD Authentic Data
*CD Checking Disabled
<center>
{| class="wikitable" style="text-align: center; width: 30em;" border=1
|+ARP Headers
|-
| colspan="16"| Hardware type <small>(Ethernet = 1</small>)
|-
| colspan="16"| Protocol type <small>(IPv4 = 0x0800)</small>
|-
| colspan="8"| Hardware address length (Ethernet size is 6)
| colspan="8"| Protocol address length (IPv4 size is 4.)
|-
| colspan="16"| Operation ( 1 for request; 2 for reply)
|-
| colspan="16" style="background:#f0fff0"| Source MAC
|-
| colspan="16" style="background:#d0ffd0"| Source IP
|-
| colspan="16" style="background:#f0f0ff"| Dest MAC
|-
| colspan="16" style="background:#d0d0ff"| Dest IP
|}
</center>
;<center>GARP</center>
[[File:GARP.png|center]]
Line 167 ⟶ 201:
Rest of Header
<br />
= 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.
<br />
;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
Line 179 ⟶ 242:
[[File:Mtu mss.png|center]]
* '''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
Line 201 ⟶ 276:
- 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.
<center>
{| class="wikitable"
|-
! Event !! Tahoe !! Reno
|-
| 3 Dup Acks || Performs a fast retransmit<br>Sets the slow start threshold to half of the current congestion window<br>Reduces the congestion window to 1 MSS<br>Resets to slow start state || Perform a fast retransmit<br>Skip the slow start phase by instead halving the congestion window<br>(instead of setting it to 1 MSS like Tahoe)<br>Setting the slow start threshold equal to the new congestion window<br>Enter a phase called fast recovery.
|-
| RTO (Ack time out) || Slow start is used<br>Reduce congestion window to 1 MSS || Slow start is used<br>Reduce congestion window to 1 MSS
|}
</center>
*Silly Window Syndrome: Sender creates data slowly or Receiver consumes slowly or both.
Line 211 ⟶ 302:
- '''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
Line 225 ⟶ 312:
= VPN Messages =
*;Phase 1 - Main Mode
Line 239 ⟶ 325:
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 =
<center>
Line 276 ⟶ 354:
</center>
;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
<br />
;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
Line 285 ⟶ 389:
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 ==
{|class="wikitable"
|-
! Header !! Uses
|-
|Set-cookie ||
|-
|Location ||1) Used to ask a web browser to load a different web page<br/>Client request:<br/>''GET /index.html HTTP/1.1''<br/>''Host: www.example.com''<br/>Server response:<br/>''HTTP/1.1 302 Found''<br/>''Location: http://www.example.org/index.php.''<br/>
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.<br/>Credentials are the base64 encoding of id and password joined by a single colon<br/>Base64-encoding of 'Aladdin:OpenSesame' is 'QWxhZGRpbjpPcGVuU2VzYW1l'<br/>''Authorization: Basic QWxhZGRpbjpPcGVuU2VzYW1l''<br/>''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.<br/>
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 =
[[File:Active-Passive_FTP.JPG|centre]]
= SSL Handshake =
[[File:SSL Handshake.png|center]]
--> 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)
#Client sends the supported parameters
#Server chooses the parameters; Sends the certificate; And first half of the Diffie-Hellman key exchange
#Client sends the second half of the Diffie-Hellman exchange, Computes the session keys; Switches to encrypted communication
#Server computes the session keys; Switches to encrypted communication.
<br>
; 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
Least Bandwidth = Service serving least amount of traffic measured in mbps
Least Packets = Service that received fewest packets
Line 302 ⟶ 482:
*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
Line 311 ⟶ 491:
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 =
*
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
*
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 subnet
Same hello/dead interval
Matching stub flags
* LSA Details
[[File:OSFF LSA 2.png|center]]
* 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
<center>
{| class="wikitable"
|-
! 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) ||
|-
|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
|-
|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 ||
|}
</center>
<br />
*BGP States
Line 395 ⟶ 593:
Notification Always indicate something is wrong
<br />
* 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 =
Line 474 ⟶ 695:
</div>
= SYN Flood Protection =
Threshold = Proxy connections above this limit
If Syn-cookie is enabled, no sessions established between client & firewall or firewall & server directly
Line 482 ⟶ 703:
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:
#Check NW config
#DHCP if not configured
#Check Domain name in Browser Cache
#Check Domain name in OS Cache
#Check if an entry exists in Hosts File
#If not Found in any cache, Prepare to send UDP DNS query to DNS Server
#If DNS Server configured is in same Network Check MAC address in ARP Table
#If not found, send ARP for MAC Address
#Forward DNS Query to DNS Server and wait for reply containing IP address of Website
#If DNS server configured is not in same subnet, check Gateway config(IP & MAC address)
#If MAC address not found in ARP Table, send ARP request
#After getting reply, fwd the DNS query to gateway
#After getting DNS response, start TCP 3-way handshake S-SA-A.
#Start SSL Handshake if SSL/TLS configured
#Send GET Request
#Client sends ACK [200 OK] & Body containing HTML Data
#If HTTP 1.0, Server sends FIN & CLoses connection
#Client send FIN-ACK
#Server sends Ack
*Complete Flow of DNS Traffic
#Check NW config
#DHCP if not configured
#Check Domain name in Browser Cache
#Check Domain name in OS Cache
#Check if an entry exists in Hosts File
#If not Found in any cache, Prepare to send UDP DNS query to DNS Server
#If DNS Server configured is in same Network Check MAC address in ARP Table
#If not found, send ARP for MAC Address
#Forward DNS Query to DNS Server and wait for reply containing IP address of Website
#If DNS server configured is not in same subnet, check Gateway config(IP & MAC address)
#If MAC address not found in ARP Table, send ARP request
#After getting reply, fwd the DNS query to gateway
#DNS Server ??
#DNS Server ?? Iterative? Recursive? TLD? Authoritative
#DNS Server ??
#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]
#Check NW config
#DHCP if not configured
#Check if PC2 in same Subnet(not in this scenario as routers present)
#If in Same Subnet, check if MAC address is there in ARP Table
#Else send ARP Request
#Once MAC address is known, directly send Packet to PC2
#If PC2 is in Different Subnet(True for above scenario), Check Gateway IP address & MAC address
#If MAC address is not known, send an ARP request.
#Hub is directly connected, will receive & Flood packet on all Ports.
#Switch will receive packet and check its CAM Table for the MAC to Port bindings
#If MAC entry is not found in CAM table, Switch will Flood the ARP packet on all ports.
#Other destinations will drop the ARP Request packet as they do not have the IP address requested in ARP Header.
#Only Router will accept the packet as it has the requested IP address matching its own MAC address.
#It will reply with an ARP Reply message.
#Switch will add an entry of this MAC address & port number in its CAM Table once the reply packet pass through it.
#Hub will flood the packet through all ports.
#ARP Reply will reach PC1, it will add entry to its ARP Table
#Then send a packet destined to PC2 with destintion MAC address as Router's Interface's MAC address received in ARP reply.
= Linux =
== Linux Booting ==
#BIOS(Basic Input/Output System) - POST, Loads and executes the MBR boot loader.
#MBR (Master Boot Record) - Loads and executes the GRUB boot loader.
#GRUB (Grand Unified Bootloader) - Loads and executes Kernel and Initrd images.
#Kernel - Heart of OS; Memory, Process mgmt; Executes INIT process.
#Init (initialization) - Decides the Linux run level; default run level to either 3 or 5.
#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
<nowiki>|</nowiki> FIFOs
= Sockets
> Doors
Nothing for Regular Files
List Symoblic Links:
ls -la
<pre>
lrwxrwxrwx 1 root root 11 Sep 13 14:57 mounts -> self/mounts
dr-xr-xr-x 3 root root 0 Sep 13 14:57 mpt
-rw-r--r-- 1 root root 0 Sep 13 14:57 mtrr
</pre>
=== Redirect Stderr ===
0 stdin – Use to get input (keyboard)
1 stdout – Use to write information (screen) 1> >
2 stderr – Use to write error message (screen) 2>
Redirect Stderr into Stdout:
2>&1
ls > file.log 2>&1 OR ls &> file.log
ls > file.log 2> /dev/null
=== System Calls ===
{{UC}}
= 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:
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