Web Servers

Sockets

To determine the "services" running on a unix system, look at the "/etc/services" file. To find just the www services, use the grep command to filter this file for just the relevant contents.
```
[killbill@reepicheep perlcgiclass]$ grep www /etc/services
www		80/tcp		http		# WorldWideWeb HTTP
www		80/udp				# HyperText Transfer Protocol
[killbill@reepicheep perlcgiclass]$ 
```

This tells us that the www "service" is running on socket number 80 (this is the normal www server socket).

Sockets simply establish a set of rules (a protocol) that allow well behaved systems to communicate with each other.

This is how information moves between systems across the Internet network.

It is possible to talk directly to a socket via the "telnet" program. "Telnet" simply exchanges the specified (probably ASCII) bytes back and forth with the specified host at the specified socket number.

This allows a "stupid Unix Trick" such as the following, where we use the telnet program to connect with and talk directly too a web server listening on socket 80 on the specified host.

[killbill@reepicheep perlcgiclass]$ telnet peepicheep 80
Trying 129.202.180.1...
Connected to peepicheep.mousenet.net.
Escape character is '^]'.
GET HTTP://index.html  This is an illegal command I typed 
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<HTML><HEAD>
<TITLE>404 Not Found</TITLE>
</HEAD><BODY>
<H1>Not Found</H1>
The requested URL HTTP://index.html was not found on this server.<P>
<HR>
<ADDRESS>Apache/1.3.12 Server at peepicheep.mousenet.net Port 80</ADDRESS>
</BODY></HTML>
Connection closed by foreign host.
[killbill@reepicheep perlcgiclass]$

Note that it responded (to our illegal request) with an HTML formatted error message.

Note also that the server really could not care less who connects to it, it just cuts off the conversation if the other system stops following the rules.

This means that any browser can talk to any server.

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