Internet Protocols & Client Server Programming SWE 344 Internet Protocols & Client Server Programming ICMP, SMTP & HTTP

ICMP The Internet Control Message Protocol (ICMP) is used to communicate with remote hosts on the network. Many popular network utilities, such as ping and traceroute, are based on ICMP. ICMP was defined in RFC 792 to allow network devices to report errors in datagram processing. ICMP is a robust means of communicating errors and network information among hosts. ICMP uses IP to communicate across the network. The entire ICMP packet is then contained within the data section of the IP packet. Figure shows how the ICMP packet fields are placed in an IP packet.

ICMP Packet Format ICMP uses a specific packet format to identify information in the packet. the ICMP packet contains the following fields: Type The 1-byte Type element defines what kind of ICMP message is in the packet. Many types of ICMP packets are used to send control request messages to remote hosts. Each message type has its own format and data requirements. Code The 1-byte Code element further defines the Type field. The various ICMP message types require specific control and data options. These options are defined in the Code field. Checksum The 2-byte Checksum element ensures that the ICMP packet has arrived without corruption or tampering. The checksum is computed on only the ICMP portion of the packet, using a specific algorithm defined in RFC 792. When computing the checksum value, the Checksum field is set to zero. Message The multibyte Message element contains various other data elements that are unique to each ICMP message type. The Message data fields are often used to contain information sent to and from the remote host. Many of the ICMP message types define the first two fields in the Message element as an Identifier and a Sequence number. Both of these fields are used to uniquely identify the ICMP packet to the hosts.

ICMP Packet Types Type Code Description Echo reply 3 The ICMP Packet Types Type Code Description Echo reply 3 Destination unreachable 4 Source quench 5 Redirect 8 Echo request 11 Time exceeded 12 Parameter problem 13 Timestamp request 14 Timestamp reply 15 Information request 16 Information reply

Echo Request and Echo Reply Packets Two of the ICMP packets used most often are the Echo Request and Echo Reply. These packets allow a device to request an ICMP response from a remote device on the network—the core of the ping utility that has become a universal staple for network administration. The Echo Request packet uses ICMP Type 8, with a Code value of 0. The Message data area contains three elements: A 1-byte Identifier that uniquely identifies the Echo Request packet A 1-byte Sequence number providing additional identification for the ICMP packet in a stream of A multibyte data element containing data that should be returned by the receiving host When a device receives an Echo Request packet, it must respond with an Echo Reply packet, ICMP Type 0. The Echo Reply packet must contain the same Identifier and Sequence number values as the Echo Request packet to which it is responding. Also, the data element value must be the same as received in the Echo Request packet

Destination Unreachable Packet The Destination Unreachable ICMP packet (Type 3) is usually returned by a router device after it receives an IP packet that it cannot forward to the appropriate destination. The data portion of the Destination Unreachable packet contains the IP header plus the first 64 bits of the datagram. In this packet, the Code field identifies the reason the packet could not be forwarded by the router Time Exceeded Packet The Time Exceeded (ICMP Type 11) packet has become an important tool that is used for network troubleshooting. It reports that an IP packet has exceeded the time to live (TTL) value defined in the IP header. Each time an IP packet traverses a network router, the TTL value is decreased by 1. If the TTL value reaches 0 before the IP packet reaches the intended destination, the last receiving router must send a Time Exceeded ICMP packet to the sending host. As you will see, this procedure is exploited in the traceroute program.

Creating an ICMP Class in C# The raw socket does not automatically format your ICMP packet, so you must do this yourself. C# is an object-oriented language. It makes sense to create a C# ICMP class that you can use to format an ICMP packet and manipulate the packet contents as necessary. ICMP Class Constructors The ICMP class should define a data variable for each element in the ICMP packet. The ICMP Class Generic Data Elements Data Variable Size Type 1 byte Byte Code Checksum 2 bytes Unsigned 16-bit integer Message multibyte Byte array Here is the format for the ICMP class default constructor: class ICMP { public byte Type; public byte Code; public UInt16 Checksum; public int MessageSize; public byte[] Message = new byte[1024]; public ICMP() { } }

SMTP The System.Web.Mail namespace contains classes that are used for creating and sending e-mail messages to remote hosts. This is done using either the default Windows Simple Mail Transfer Protocol (SMTP) service on Windows 2000 and XP machines, or an external mail server. The SmtpMail Class The SmtpMail class, found in the System.Web.Mail namespace, allows you to send SMTP messages in your C# network programs. The Send() method is overloaded, using two separate formats: Send(MailMessage message) Send(string from, string to, string subject, string body) The first format allows you to send a MailMessage object. The MailMessage class is a self-contained e-mail message, created by populating the properties of the class with information related to the message, and the destination address(es).

From specifies the e-mail address of the sender. The second format allows you to send a raw message, manually specifying the typical e-mail message header fields: From specifies the e-mail address of the sender. To specifies the e-mail address of one or more recipients, separated by commas. Subject specifies the topic of the e-mail message. The final parameter of the Send() method is the actual body of the message. The body can be in either a plain text or HTML format. The sole property of the SmtpMail class is SmtpServer. If you are using a relay mail server, you must set the SmtpServer property before you attempt to send messages: SmtpMail.SmtpServer = "mailsrvr.myisp.net";

This program demonstrates how to create a simple mail message and send it through a remote mail relay to the recipient. using System; using System.Net; using System.Web.Mail; class MailTest { public static void Main() { string from = “myself@myisp.net"; string to = “friend@anotherisp.net"; string subject = "This is a test mail message"; string body ="Hi there, I hope things are going well today."; SmtpMail.SmtpServer = "192.168.1.150"; SmtpMail.Send(from, to, subject, body); }

HTTP (Hyper text transfer protocol) The .NET WebClient class—the easiest way to communicate via HTTP from your C# programs. The WebRequest and WebResponse classes have many features available to help you create full-featured web programs. The WebClient Class The easiest way to communicate with websites from a C# network program is to use the WebClient class, found in the System.Net namespace. This class provides methods to send requests and receive responses from a web server within your program. Downloading Web Data The WebClient class provides three methods that can download information from a web server: DownloadData() Downloads data to a byte array from a specified URI DownloadFile() Downloads data to a local file from a specified URI OpenRead() Opens a read-only stream to download data from a specified URI.

Uploading Web Data The WebClient class provides four ways to upload information to the web server: OpenWrite() Sends a stream of data to the web server UploadData() Sends a byte array of data to the web server UploadFile() Sends a local file to the web server UploadValues() Sends a NameValueCollection of data names and values to the web server The NetworkCredential Class The NetworkCredential class (found in the System.Net namespace) authenticates a client to the web server using a simple username/password combination (and a specified domain name for Windows web servers). The NetworkCredential object is created using one of three constructor formats: NetworkCredential() NetworkCredential(string username, string password) NetworkCredential(string username, string password, string domain)

This program to reads the URL content from webserver. using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Text; using System.Windows.Forms; using System.Net; using System.IO; namespace Lab3c { public partial class Form1 : Form public Form1() InitializeComponent(); } private void button1_Click(object sender, EventArgs e) //to read URL Content from webserver try StreamReader inStream; WebRequest webRequest; WebResponse webresponse; webRequest = WebRequest.Create(textBox1.Text); webresponse = webRequest.GetResponse(); inStream = new StreamReader(webresponse.GetResponseStream()); textBox2.Text = inStream.ReadToEnd(); catch { MessageBox.Show("URL error"); }

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