networking

Unix networking stuff.

TCP / IP networking model

layer 0
  physical layer
  standards for voltages, frequencies and other physical properties

layer 1
  link layer
  communication with devices that share a physical connection
  Ethernet, 802.11a/b/g/n, and Token Ring

layer 2
  network layer
  talk to machines without knowing where they are physically located
  IP

layer 3
  transport layer
  determine which program to deliver messages to, usually through port numbers
  TCP, UDP

layer 4
  application layer protocol
  set of protocols for a specific program
  HTTP, SMTP, IMAP, POP3

On top of that there are an infinite amount of layers, most commonly:

layer 5
  security layer
  make sure communication is private
  TLS, SSL

layer 6
  API layer
  talk to a program living within the application layer
  Twitter API, GitHub API

But these layers are not fixed, sometimes protocols switch between layers. OSI, for example, has 7 layers.

quora/http-tcp

UDP

unreliable
little overhead

TCP

reliable (retries)
overhead

nc(1)

nc is the netcat command. It does anything surrounding TCP and UDP / Datagram.

create a direct chat connection

Implement a rudimentary IRC chat using netcat. When a connection is established, messages will be echoed on both sides. terminal 1

$ nc -l 1337  # listen on port 1337

terminal 2

$ nc localhost 1337

Create a proxy

#!/bin/sh
# proxy.sh

url=$2

$ nc -lk 127.0.0.1 80 | ./proxy.sh

Port scanning

Using netcat:

$ nc -z host.example.com 20-30

DNS

DNS is about naming things and pointing them in the right direction. DNS uses "records" to store this relation. There's different types of records:

MX: mail exchange record, specify a mail server that can accept emails
A: return a 32-bit IPv4 address
AAAA: return a 128-bit IPv6 address
CNAME: canonical name record, alias of one name to another
CERT: certificate record, stores PGP, etc.
SRV: service locator, used for newer protocols instead of creating a new record type
TXT: used for human readable data, can be leveraged for protocols
URL: used for redirects

Records are cached by default using a TTL (Time To Live) value. Using dig(1):

;; ANSWER SECTION:
yoshuawuyts.com.        299     IN      A       104.27.133.163
yoshuawuyts.com.        299     IN      A       104.27.132.163

The value here is 299 seconds.

tools

dnsmasq(1) - DNS forwarder and server
dig(1) - DNS lookup utility

DNS Response Policy Zone

ICE

Interactive Connectivity Establishment. Used amongst others for WebRTC handshakes.

https://en.wikipedia.org/wiki/Interactive_Connectivity_Establishment

NAT

Network Address Translation

https://en.wikipedia.org/wiki/Network_address_translation

STUN

Session Traversal Utilities for NAT. Standardized set of methods and a network protocol to allow an end host to discover its public IP address if it is located behind a NAT.

https://en.wikipedia.org/wiki/STUN

TURN

Traversal Using Relays around NAT (TURN) is a protocol that assists in traversal of network address translators (NAT) or firewalls for multimedia applications. It may be used with the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). It is most useful for clients on networks masqueraded by symmetric NAT devices. TURN does not aid in running servers on well known ports in the private network through a NAT; it supports the connection of a user behind a NAT to only a single peer, as in telephony, for example.

https://en.wikipedia.org/wiki/Traversal_Using_Relays_around_NAT

QUIC

moving the web from TCP to UDP

Inspect if port is open

ss(1) is a tool to validate if a port is open.

$ ss -tl4 '( sport = :<port_number> )'
$ ss -tl4 '( sport = :22 )'

Or using netstat(1)

$ netstat -lnt

http://stackoverflow.com/questions/9609130/quick-way-to-find-if-a-port-is-open-on-linux

Change ip of router

$ open http://192.168.1.1/
$ open http://192.168.0.1/

network