Client Server and Protocols
Servers and Clients 4 A “server” is just a computer running a piece of software that provides resources to clients 4 A client is something that requests something from a server
Servers MyLaptop Requests Responses
Client/Server 4 The client machine requests something from the server, in this case a web page. The server looks up the web page and returns it. 4 Sometimes the server machine is dedicated to only that process, but most of the time it can also do other things 4 You can run a web server on an NT or Unix box and also do other things with it.
Protocols 4 Requests are made to the server in a specific format--a protocol. 4 This is called an application-layer protocol. It runs on top of TCP/IP 4 It’s often just very simple text commands
HTTP Protocol 4 HTTP is a protocol for requesting web pages from a server. It’s just a series of text commands that is transmitted via TCP. 4 Example: telnet 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 80 Trying Connected to azure.stl.nps.navy.mil. Escape character is '^]'. GET /index.html <!DOCTYPE HTML PUBLIC…. STL
Connecting to a port 4 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 4 Other servers listen on different ports
SMTP protocol 4 Kids, don’t do this at home 4 You can also directly connect to SMTP mail servers and issue commands to them 4 You can send very bogus fake mail that won’t fool anyone but a novice 4 Some admins get very upset when people do things like this
SMTP Telnet mailserver.nps.navy.mil 25 HELO pentagon.navy.mil MAIL RCPT DATA From: (The admiral in charge) To: Subject: Next assignment Reply-To: Report immediately to Adak island.. QUIT
Protocols 4 SMTP has a simple command structure that runs on top of TCP/IP 4 You can hook up to the mail server software and issue it commands 4 You probably shouldn’t believe everything your read in your mailbox
FTP 4 File Transfer Protocol is used to move files from one machine to another 4 You’ll often use this to transfer HTML files from your PC to an HTML server 4 ws_ftp is one GUI ftp program; see web searches for others
Clients, Servers, and Protocols 4 Clients talk to servers to request data 4 They communicate using a protocol 4 These are often very simple text commands that you can simulate yourself
Web Servers 4 A web server can be almost any machine, named anything. 4 The thing that makes it a web server is a program running on that machine that understands the http protocol 4 The machine can also run other things
Web Servers 4 There are many web server programs out there –Commercial: Netscape Enterprise Server, Microsoft IIS –Free: Apache 4 They can run on almost any OS –Sun, Microsoft, Apple, Linux
Web Servers 4 They just accept requests and return data 4 Serious servers need to be well connected, since they take up a lot of bandwidth 4 Serious servers should also be secure
Servers Firewall Web Server Internal NetworkThe Internet
Web Servers 4 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