Transcript Network Programming

Network Programming
Chapter 5: Raw Socket
Programming
Agenda
• The ICMP Protocol
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–
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–
–
ICMP Packet Format
ICMP Packet Types
Using Raw Sockets
Creating an ICMP Class – Formatting ICMP Packets
Using ICMP to Create a Tracing Application
• SNMP
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Impending SNMP
SNMP Commands
Community Names
Common Management Information Base (MIB)
SNMP Packets
SMTP Class
• Summary
Introduction
• Cover the basics of
– Internet Control Message Protocol (ICMP)
– Simple Network Management Protocol (SNMP)
• ICMP
– Used by some network utility applications to
communicate with the remote host on the network
– Examples: ping and traceroute
• SNMP
– Used for network administration
– Used to query and control network devices from a
central management station
The ICMP Protocol
• Network Working Group’s RFC 792
• http://www.faqs.org/rfcs/rfc792.html
• Allow network devices to report errors in
datagram processing (original purpose)
• Today – communicates errors and network
information between hosts
• ICMP uses Internet Protocol (IP) to
communicate across the network
• ICMP is an indispensable part of IP and is
implemented by every IP module
The ICMP Protocol
• ICMP Packet Format
– Recognised by the IP protocol type 1.
– The whole ICMP packet is then contained
within the data section of the IP packet
ICMP Packet in an IP packet
8
IP Header
Version
Hlen
16
Service
Type
Total IP Packet Length
Identification
Time to Live
32
19
Flags
Protocol
Fragment
Offset
IP Header Checksum
Source IP Address
ICMP
Packet
Destination IP Address
Type
Code
Message
ICMP Checksum
IP Packet
0
ICMP Packet Fields
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Type: The 1-byte type element helps to define the kind of ICMP message
that is present in the packet. Many types of ICMP packets are used to send
the control request messages to the remote hosts. Each message type has
its own format and data requirements.
Code: An assorted ICMP message type requires specific control and data
options. These options are defined in the single byte Code field.
Checksum: A Checksum element is used to ensure that the ICMP packet
reaches the destination without corruption or tampering. The checksum is
computed on only the ICMP portion of the packet, using a specific algorithm
defined in RFC 792. While computing the checksum value, the Checksum
field is set to zero. The length of the checksum is 2 bytes.
Message: The Message element holds diverse data elements that are
unique to each ICMP message type. The Message data field is used to
hold information that is sent to and from the remote host. The first two fields
in the Message element are defined as an Identifier and Sequence number.
These fields are used to uniquely identify the ICMP packet to the hosts.
ICMP Packet Types
• Commonly used ICMP packet types
– Echo request and Echo Reply packets
– Destination Unreachable packets
• Echo request and Echo Reply packets
– ICMP type 8 with code value of 0 (zero)
– Message data holds the following three elements:
• An identifier of 1 byte that uniquely identifies the Echo Request packet
• A Sequence number of 1 byte that provides additional identification for the
ICMP packet in a stream
• A multibyte data element containing the data that should be returned by the
receiving host
– When a device receives an Echo Request packet, it should respond
with an Echo Reply packet, ICMP type 0. The Echo Reply packet must
contain the same Identifier and Sequence number values as that of the
Echo Request packet to which it is responding. The data element value
must also be the same as that received in the Echo Request packet.
Agenda
• The ICMP Protocol
–
–
–
–
–
ICMP Packet Format
ICMP Packet Types
Using Raw Sockets
Creating an ICMP Class – Formatting ICMP Packets
Using ICMP to Create a Tracing Application
• SNMP
–
–
–
–
–
–
Impending SNMP
SNMP Commands
Community Names
Common Management Information Base (MIB)
SNMP Packets
SMTP Class
• Summary
Using Raw Sockets
• ICMP packets don’t use TCP or UDP
• To use TcpClient and UdpClient, use
the Socket class
• To create a raw socket, the
SocketType.Raw socket is used when
the socket is created.
• There are several protocol type values that
are used to match the raw socket type
itself
Protocol Type Values and Their
Protocol Description
Protocol Value
Protocol Description
Ggp
Gateway To Gateway Protocol.
Icmp
Internet Control Message Protocol.
Idp
Internet Datagram Protocol.
Igmp
Internet Group Management Protocol.
IP
Internet Protocol.
ND
Net Disk Protocol (unofficial).
Raw
Raw IP packet protocol.
Tcp
Transmission Control Protocol.
Udp
User Datagram Protocol.
Unknown
Unknown protocol.
Unspecified
Unspecified protocol.
Using Raw Sockets
• ICMPDemo.cs
• Sending Raw Packets
– ICMP is a connectionless protocol
– Need to bind the socket to a specific local port
to send a packet or use the Connect method
to connect the socket to a specific remote
host.
– ICMP does not use ports – so the port
property of the IPEndPoint is not important
ICMP Demo
using System;
using System.Net;
using System.Net.Sockets;
namespace RawSocket
{
//SimpleICMP.cs
public class SimpleICMP
{
static void Main(string[] args)
{
IPHostEntry host = null;
host = Dns.GetHostEntry("localhost");
Socket tmpS = new Socket(host.AddressList[0].AddressFamily, SocketType.Raw,
ProtocolType.Icmp);
Console.WriteLine("Socket Created.");
Console.WriteLine("Host information");
Console.WriteLine("Host address: {0}", host.AddressList[0]);
Console.WriteLine("Socket details: ");
Console.WriteLine("Address Family: {0}", tmpS.AddressFamily);
Console.WriteLine("Protocol Type: {0}", tmpS.ProtocolType.ToString());
Console.WriteLine("Socket Type: {0}", tmpS.SocketType.ToString());
Console.ReadLine();
}
}
}
Using Raw Sockets
• Receiving data from a raw socket is more
difficult than sending data
• ReceiveFrom method
– Holds the whole of the IP packet contents
– Extract data from raw IP packet to create
ICMP packet
– IP packet data starts at byte 20
– Start reading the byte array at the 20th
position in the received data packet
ICMP Demo
Creating an ICMP Class
• Formatting ICMP Packets
• Raw socket does not do formatting
• Can create an ICMP Class
ICMP Packet
Data Variable
Size
Type
Type
1 byte
Byte
SubCode
1 byte
Byte
Checksum
2 bytes
Unsigned 16-bit integer
Identifier
2 bytes
Unsigned 16-bit integer
SequenceNumber 2 bytes
Unsigned 16-bit integer
Data
Byte array
Multibyte
ICMP Class
public class ICMPPacket
{
//Type of message
public Byte Type;
//Type of subcode
public Byte SubCode;
//One's complement checksum of struct
public UInt16 CheckSum;
//Identifier
public UInt16 Identifier;
//Sequence number
public UInt16 SequenceNumber;
//Data or message
public Byte[] Data;
}//end of class ICMPPacket
Serialize Method
• Code a method named Serialize
• Calculate the total size of the packet from
the packet information passed to it as the
parameter
• Total size is calculated by converting the
packet into a byte array
public static Int32 Serialize(IcmpPacket packet, Byte[] Buffer, Int32 PacketSize, Int32 PingData)
{
Int32 cbReturn = 0;
//Serialize the struct into the array
int Index = 0;
Byte[] b_type = new Byte[1];
b_type[0] = (packet.Type);
Byte[] b_code = new Byte[1];
b_code[0] = (packet.SubCode);
Byte [] b_cksum = BitConverter.GetBytes(packet.CheckSum);
Byte[] b_id = BitConverter.GetBytes(packet.Identifier);
Byte[] b_seq = BitConverter.GetBytes(packet.SequenceNumber);
//serialize type
Array.Copy(b_type, 0, Buffer, Index, b_type.Length);
Index += b_type.Length;
//serialize subcode
Array.Copy(b_code, 0, Buffer, Index, b_code.Length);
Index += b_code.Length;
//serialize cksum
Array.Copy(b_cksum, 0, Buffer, Index, b_cksum.Length);
Index += b_cksum.Length;
//serialize id
Array.Copy(b_id, 0, Buffer, Index, b_id.Length);
Index += b_id.Length;
Array.Copy(b_seq, 0, Buffer, Index, b_seq.Length);
Index += b_seq.Length;
//Copy the data
Array.Copy(packet.Data, 0, Buffer, Index, PingData);
Index += PingData;
if (Index != PacketSize/*sizeof(IcmpPacket)*/)
{
cbReturn = -1;
return cbReturn;
}
cbReturn = Index;
return cbReturn;
} //end of Serialize
Checksum Method
public static UInt16 checksum(UInt16[] buffer, int size)
{
Int32 cksum = 0;
int counter;
counter = 0;
while (size > 0)
{
cksum += Convert.ToInt32(buffer[counter]);
counter += 1;
size -= 1;
}//end of while
cksum = (cksum >> 16) + (cksum & 0xffff);
cksum += (cksum >> 16);
return (UInt16)(~cksum);
}//end of checksum
ICMP Packet Creation
• Pinging another computer
– Create ICMP packet
– Need an echo request packet
• ICMP Type = 8
• SubCode = 0
• Use the Identifier and Sequence elements to track the individual
ping packet and allow any text to be put into the data packet
– The returned ICMP packet creates a new ICMP object
– If the sent packet is the a match for the received packet, the ping
was successful
– You need to check for a timeout – if no ICMP packet has been
received within 10 seconds, then throw an error message
Using ICMP to Create a Tracing
Application
• Need the ICMP header
• Each header has a TTL field (Time To
Live)
• Each time the datagram is processed, TTL
is decremented
• Datagram is processed by the gateway
– When the TTL = 0, the datagram is discarded
– Source host is notified of any discarded
datagrams
Tracing application “algorithm”
1. Start with a datagram with TTL = 1
2. At each machine reached, the gateway
sends a “time-expired” message back
3. Remember machine that sent message
back
4. Send another message with TTL += 1
5. Repeat steps 2 to 4 until an echo reply
has been received successfully
(destination reached)