Raw Sockets

What are Raw Sockets?

• Allows you to bypass the TCP/UDP layers.

• Send/receive your own packets, with your own headers.

• You need to do all protocol processing at user-level.

Typical Uses• ICMP messages

– ping generates ICMP echo requests and received ICMP echo replies.

• Routing protocols– gated implements OSPF routing protocol.– Uses IP packets with protocol ID 89 – not supported by

kernel.

• Hacking – Generating your own TCP/UDP packets with spoofed

headers

Raw socket creation

• Only root can open a raw socket.

sockfd = socket(AF_INET, SOCK_RAW, proto)

where proto is IPPROTO_RAW, IPPROTO_ICMP etc.

Raw socket output

• As usual – sendto(), sendmsg() etc.

• IP_HDRINCL option – Specifies whether the process or the kernel

builds the IP header.

/* allow process to build IP header */int on=1; setsockopt( sockfd, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on))

Raw socket input• Normally using recvfrom()• Conditions for a packet to match raw socket

– If protocol parameter was specified, only packets with that protocol value are delivered.

– If bind() was called on raw socket, only packets destined to bound IP address are delivered.

– If connect() was called, only packets from connected address are delivered.

Which protocol types are delivered?

• TCP and UDP never reach raw sockets– Kernel IP stack handles these– Linux implementation is an exception.

• All ICMP except– ICMP echo request– Timestamp request– Mask request

• All IGMP

• All other protocols that kernel doesn't understand– Such as OSPF

Exercise

Write a simple “Hello” exchange program using raw sockets

that implements your own protocol

on top of IP.

Java

Reference Book -

“Thinking in Java”

by Bruce Eckel

Book is online at

http://www.mindview.net/books/TIJ

Levels of abstraction

Machine

Assembly language

Imperative languages(C, FORTRAN, ALGOL)

Object oriented languages(C++, Java)

Low-levelmachine details

Abstracting the machine model

Abstracting the problem as object interactions

Object

• An object has – State (internal data)– Behavior (methods that operate on data)– Identity (object can be addressed uniquely)

Objects…

• Class defines properties of a set of similar objects.– Think of it as type of the object

• Protection boundaries define what object state is visible to others.

• Inheritance allows a new class to inherit properties of an earlier class without re-defining them.

• Interface defines what requests others can make to a particular object

Parent class

Interface

Child classes

Primitive Data types

• boolean - false• char - ‘\u0000’ (null)• byte - (byte)0• short - (short)0• int - 0• long - 0L• float - 0.0f• double -0.0d

Creating a class

class Circle {

public :Circle(int x, int y, float r) {…}void draw(){…}void erase(){…}void move(){…}

private :int x_;int y_;float radius_;

}

Methods

Fields

Creating an object

Circle c = new Circle(10,20,5);

c.draw();

c.move();

c.erase();

Inheritanceabstract class Shape {

public :

Shape(int x, int y) {

x_ = x;

y_ = y;

}abstract void draw();

abstract void erase();

abstract void move();

protected :

int x_;

int y_;

}

Inheritance…class Circle extends Shape

{

Circle(int x,int y,float r){

super(x,y);

radius_ = r;

}void draw() {…}void erase(){…}void move(){…}

protected :

float radius_;

}

Inheritance…class Square extends Shape

{

Square(int x,int y,float w){

super(x,y);

width_ = r;

}void draw() {…}void erase(){…}void move(){…}

protected :

float width_;

}

void doStuff(Shape s) {

s.erase(); // ... s.draw();

}

Circle c = new Circle(); Square s = new Square(); Line l = new Line();

doStuff(c); doStuff(s); doStuff(l);

Interfaces

interface DrawObject {// automatically public

void draw();

void erase();

void move();

// Compile-time constant:

int SOME_CONST = 5; // static & final

}

class Shape {public :

Shape(int x, int y) {

x_ = x;

y_ = y;

}

protected :

int x_;

int y_;

}

class Circle extends Shape implements DrawObject

{

Circle(int x,int y,float r){

super(x,y);

radius_ = r;

}void draw() {…}void erase(){…}void move(){…}

protected :

float radius_;

}

class Square extends Shape implements DrawObject

{

Square(int x,int y,float w){

super(x,y);

width_ = r;

}void draw() {…}void erase(){…}void move(){…}

protected :

float width_;

}

static members• static fields or methods have one instance across

all object instancesclass X {

static int i = 0;

……

}

x1 = new X();

x2 = new X();

x1.i++;

x2.i++;

System.out.println(“Result = “ + x2.i);

Result = 2

final member/class• final class X {

…}Final classes cannot be extended.

• class X {final int m();

}Final methods cannot be overridden.

• int mthd(final MyClass mc) {…}– Final arguments cannot be modified.

• final int MY_CONST = 10;– Final fields/references are constants

Things you should learn from TIJ

• Packaging your classes - Chapter 5

• Java I/O system - Chapter 11

• Error handling with exceptions - Chapter 10

• Applets - Chapter 14

Using Standard Java Packages

java import java.lang.*;

public class HelloDate {public static void main(String[] args) {

System.out.println("Hello, it's: ");System.out.println(new Date());

} }

Some other packages…

• java.net

• java.rmi

• java.io

• java.applet

• java.awt

• java.math