CPSC 526NETWORK SYSTEMS SECURITY#2 – BASIC NETWORKING

TA: HENRIQUE PEREIRA

SERVER-CLIENT APPLICATIONS

The basic mechanisms of client-server setup are:

1. A client app send a request to a server app.

2. The server app returns a reply.

3. Some of the basic data communications between client and

server are:

• File transfer - sends name and gets a file.

• Web page - sends url and gets a page.

• Echo - sends a message and gets it back.

SOCKET PROGRAMMING

Socket is an interface into which na application process

can both send and/or receive messages to/from another

application process.

PORTS

• A port is a 16-bit number in the range 0-65535

• Managed by the operating system and used by clients to identify servers

• Ports 0-1023 are reserved by the system and used by common network protocols

• Firewalls are commonly configured to differentiate between packets based on their

source or destination port numbers as in port forwarding.

• Ports Database: https://www.speedguide.net/ports.php

SOCKET PROGRAMMINGWITH TCP

• TCP provides a reliable way

to transfer bytes from one

process to another

TCP SOCKET

SIMPLE TCP ECHO SERVER - PYTHON

• http://pages.cpsc.ucalgary.ca/~henrique.pereira/pdfs/TCPServer.py

• Socket Module

• socket.bind – binds a socket to an address ( tuple with hostname/ip and port )

• socket.listen – enable the socket to accept connections

• socket.accept – accepts a connection (returns a new socket and information about the client)

• socket.recv – receives data from the socket as bytes (Py3)

• socket.send – sends data to the socket as bytes (Py3)

• socket.close – closes connection

NMAP

• Most popular port scanner available

• Offers many different scanning techniques:

• Scan for hosts that are up

• TCP ports

• UDP ports

• Other IP Protocols

• Can identify software, version, some configuration details

• nmap -A -T4 127.0.0.1

CONNECTING TO THE *SIMPLE* ECHO SERVER

• Netcat

• Utility for reading/writing to network connections using TCP or UDP.

• Basic client usage: nc [targetHost] [port]

• nc localhost 8888

• nc 127.0.0.1 8888

• Netcat Cheat Sheet

• https://www.sans.org/security-resources/sec560/netcat_cheat_sheet_v1.pdf

PYTHON – TCP CLIENT

• http://pages.cpsc.ucalgary.ca/~henrique.pereira/pdfs/TCPClient.py

• socket.connect – connects to a server

SOCKET PROGRAMMING WITH UDP

• UDP has no reliable “connection” between client and server

• No handshaking

• Sender attaches IP address and port of destination to each packet

• Server must extract that information from the received packet

• Message oriented protocol

• Data may be lost or arrive out of order

SIMPLE PYTHON UDP SERVER

• http://pages.cpsc.ucalgary.ca/~henrique.pereira/pdfs/UDPServer.py

• socket.socket( socket.AF_INET, socket.SOCK_DGRAM )

• SOCK_DGRAM – UDP DataGRAM

• Default for TCP is SOCK_STREAM

• socket.recvfrom – receives a datagram (returns the data and information about the sender)

• socket.sendto – sends data in bytes to an ip, port tuple

CONNECTING TO THE UDP SERVER

• Netcat

• Utility for reading/writing to network connections using TCP or UDP.

• Advanced client usage: nc [options] [targetHost] [port]

• nc –u 0.0.0.0 8889

• Netcat Cheat Sheet

• https://www.sans.org/security-resources/sec560/netcat_cheat_sheet_v1.pdf

SIMPLE PYTHON UDP CLIENT

• http://pages.cpsc.ucalgary.ca/~henrique.pereira/pdfs/UDPClient.py