Client Server 1

Client Server and Protocols

Servers and Clients

A “server” is just a computer running a piece of software that provides resources to clients

A client is something that requests something from a server

Servers

Www.nps.navy.mil

MyLaptop

Requests

Responses

Client/Server

The client machine requests something from the server, in this case a web page. The server looks up the web page and returns it.

Sometimes the server machine is dedicated to only that process, but most of the time it can also do other things

You can run a web server on an NT or Unix box and also do other things with it.

Protocols

Requests are made to the server in a specific format--a protocol.

This is called an application-layer protocol. It runs on top of TCP/IP

It’s often just very simple text commands

HTTP Protocol

HTTP is a protocol for requesting web pages from a server. It’s just a series of text commands that is transmitted via TCP.

Example: telnet <webserver> 80– This connects you to the web server software

running on the destination machine. You can issue commands directly to the server software

– This is actually what your web browser is doing behind the scenes!

HTTP Protocol

pinafore 55# telnet azure 80Trying 131.120.178.4...Connected to azure.stl.nps.navy.mil.Escape character is '^]'.GET /index.html<!DOCTYPE HTML PUBLIC….<html><head> <title>STL</title></head>

Connecting to a port

The “telnet 80” command tells the software to directly connect to port 80 on the destination machine. By convention this is usually where http servers listen for requests. This allows us to pass commands directly to the server software

Other servers listen on different ports

SMTP protocol

Kids, don’t do this at home You can also directly connect to SMTP

mail servers and issue commands to them You can send very bogus fake mail that

won’t fool anyone but a novice Some admins get very upset when people

do things like this

SMTP

Telnet mailserver.nps.navy.mil 25HELO pentagon.navy.mil MAIL FROM:[email protected] RCPT TO:[email protected] DATA From: [email protected] (The admiral in charge) To: [email protected] Subject: Next assignment Reply-To: [email protected] Report immediately to Adak island. . QUIT

Protocols

SMTP has a simple command structure that runs on top of TCP/IP

You can hook up to the mail server software and issue it commands

You probably shouldn’t believe everything your read in your mailbox

FTP

File Transfer Protocol is used to move files from one machine to another

You’ll often use this to transfer HTML files from your PC to an HTML server

ws_ftp is one GUI ftp program; see web searches for others

Clients, Servers, and Protocols

Clients talk to servers to request data They communicate using a protocol These are often very simple text commands

that you can simulate yourself

Web Servers

A web server can be almost any machine, named anything.

The thing that makes it a web server is a program running on that machine that understands the http protocol

The machine can also run other things

Web Servers

There are many web server programs out there– Commercial: Netscape Enterprise Server,

Microsoft IIS– Free: Apache

They can run on almost any OS– Sun, Microsoft, Apple, Linux

Web Servers

They just accept requests and return data Serious servers need to be well connected,

since they take up a lot of bandwidth Serious servers should also be secure

Servers

Firewall

Web Server

Internal NetworkTheInternet

Web Servers

Web servers are often kept outside the firewall because they’re prone to subversion and attack. A compromised web server outside the firewall won’t have really bad consequences for machines inside the firewall

Documents

Client Server 1