Introduction

On Monday, we went over the syllabus.
Then we described the TCP/IP protocol for creating a communication connection between a process on one machine and a process on another machine.
To make it easier to remember the names of machines on the internet we said humans use domains names (such as pollett.org) that are mapped to numeric IP addresses (173.13.143.73), and the lookup of this mapping can be done using a domain name server (DNS).
The internet is the collection of machines using the IP protocol, the world wide web is the collection of machines running a particular application layer protocol (HTTP) on top of TCP/IP or QUIC/IP for the purposes of transmitting web pages.
We talked about web browsers (HTTP Clients -- getters of web pages) and web servers (HTTP servers -- senders of web pages).
We said that for the top million most trafficked web-site, Apache was still the most common web server with nginx being a close second.
We start today by looking at some of the configuration options of this web server as they give us insight into how to develop web-applications...

Apache

This is the most common web server.
It has one main configuration file, usually named something like httpd.conf or apache2.conf, which may be self-contained or may load several other additional configuration files.
On Linux variant it can usually be found in the /etc/apache2 folder.
Configuration files consists of a bunch of lines in a pseudo-tagged based language. # is used for comments.
Here is a brief list of common directives:
- ServerName --- names the server system
- ServerRoot --- path to the server root (used when relative path given in over the commands such as LoadModule)
- DocumentRoot --- path to the document root
- LoadModule --- loads a module (like PHP to be used with the server)
- AddModule --- makes the added module available for use
- AddType --- associate a file name extension with a MIME type.
- Alias -- alias a URL path to a physical directory path
- <Directory> </Directory> --- control how the contents of a particular directory can be served. .htaccess files
Even if you don't have access to the httpd.conf file, if the webserver admin allows it, you might be able to use .htaccess files in the directories you control.
An .htaccess file allows you to do configurations similar to what can be done in <Directory></Directory> tags.

Example of an .htaccess File to Password Protect a Directory

AuthUserFile  "/Volumes/Library/WebDocuments/library/.htpasswd"
AuthGroupFile /dev/null
AuthName library
AuthType Basic

<Limit GET POST>
require valid-user
</Limit>

MIME

MIME stands for Multipurpose Internet Mail Extensions.
It was originally developed as a way to send attachments containing non-text documents over the internet.
Nowadays, it provides a way for a browser to determine the format of the documents it receives from the server.
A MIME specification has the format:
```
type/subtype.
```
The most common types are text, image, video.
Some example complete specifications are: text/plain, text/html, image/gif, image/jpeg, video/mpeg.
In addition to the above, you sometimes see types such as application/xhtml+xml, application/xml, or experimental types which begin with an x-. For example, video/x-msvideo

HTTP

The details of HTTP (hypertext transfer protocol) can be found in RFC 1945 (HTTP 1.0), RFC 2616 (HTTP 1.1), RFC 7549 (HTTP/2), and RFC 9114 (HTTP/3). Additional specifications for things like security, links to which can be found at the W3C HTTP Spec page.
HTTP 1.0 and HTTP 1.1 use text based messages which can be sent over the network unencrypted.
HTTP 2 and 3 require encryption. Although many of the messages are the same as HTTP/1.1 they are sent using a more succinct binary format.
HTTP 2 allows persistent connections with multiple streams of data, server pushes, and special headers that can tell a browser related resources that should be pre-fetched.
HTTP 3 is essentially HTTP 2 using the QUIC protocol for the connection rather than TCP. One reason for the switch is that lost frames in TCP could cause all streams in an HTTP 2 connection to stall.
HTTP 3 is, as of Aug 2022, close to being rolled out in the wild on a large-scale, but not there yet. Since it uses QUIC/IP rather than TCP/IP, Apache has been slow to implement it. nginx, on the other hand, does support HTTP/3.

HTTP Basics

On Monday, we introduced HTTP, the protocol above TCP/IP for retrieving hypertext documents. I.e., web pages. We start today by going over in a little more detail how it works, and then switch to talking about HTML, the language that web pages are written in.

As most of the messages used by HTTP 2 and HTTP 3 are the same as HTTP 1.1 and typing text is easier for humans than binary, I will focus the rest of the discussion on HTTP 1.0 and HTTP 1.1.
For these protocols, HTTP consists of two phases, the request and the response.

A request has the format:

HTTP method  path part of URL  HTTP version
Header fields
Blank line
Message Body

For example, at a Unix prompt trying typing:

For example, at a Unix prompt trying typing:
telnet www.cs.sjsu.edu 80  
GET / HTTP/1.0
<blank line>

More HTTP

The most commonly used HTTP requests are:

GET --- request contents of specified document
HEAD --- request header contents of specified document
POST --- Executes the specified document, using the enclosed data
PUT --- replaces the specified document with the  enclosed data
DELETE ---deletes the specified document.

Of these, GET and POST are the most common.

Header Fields

After the first line of the HTTP request one can have any number of header fields.
There are four kinds of headers: general, those used in the request, those used in the response, and entity headers which are used in both.
Two common headers are:
- Accept: says what MIME types the browser can handle
- Host: (required in HTTP/1.1) Says the name of the host that you are trying to contact. Host is useful because www.widgets.com, www.sprockets.com might be hosted on the same machine (virtual hosting). This says which one you are trying to contact.

As an example:

GET /respond.html HTTP/1.1
Host: blanca.uccs.edu
Accept: text/*
Accept: image/gif

Response Phase

An HTTP response has the following format:

Status line
Response header fields
Blank line
Response body

An example status line might be:
```
HTTP/1.1   200   OK
```
In general, one will have a 3-digit status code and a message. The codes come in one of five categories: 1 - information, 2 success, 3 - redirection, 4 - client error, 5 - server error.

An example response header

HTTP/1.1 200 OK
Date: Mon, 28 Aug 2006 19:09:45 GMT
Server: Apache/2.2.2 (Fedora)
Accept-Ranges: bytes
Connection: close
Content-Type: text/html

Message Body

In the above, Date:, Server:, Accept-Ranges:, etc are all response fields. The minimal response consists of just the status line.
Lines are terminated with CRLF for both the request and response

In-Class Exercise

I have already created an account on the class discussion board for you and added you to the class discussion group. Your login is your surname followed by the first two letters of your personal name. For example, for me: pollettch . Your password has been initially set to changeme123 .
I'd like you to go to the Fall 2022 CS 174 Class Discussion and Sign-In.
Next I'd like you to open a shell on your computer and try to determine your IP address on the school LAN. Use the command ipconfig (DOS) or ifconfig (Mac, Linux). Note your machine might have several network interfaces, so choose the wireless one.
Knowing your IP address means you can join the same network on your phone and test how your web apps on your laptop will behave on your phone.
Once you are done, copy and paste your shell session transcript and your answer to the Aug 24 In-Class Exercise Thread.
To get the insurance point, you need to post your solution before the Monday class.

More HTTP

Outline

Introduction

Apache

Example of an .htaccess File to Password Protect a Directory

MIME

HTTP

HTTP Basics

More HTTP

Header Fields

Response Phase

In-Class Exercise