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In this part, you will write a web server that serves static content.

INSTRUCTIONS TO CANDIDATES
ANSWER ALL QUESTIONS

1 Download Lab

Download the Lab 4 files from Brightspace. The source code will be inside the directory “Lab4/”. You must use the environment from Lab 0 to run and test your code. Next, open a terminal and “cd” into the “Lab4/” directory. Now you are ready to run the lab!

 

2 HTTP Web Page Downloader (4 Points)

There is a very useful program called “wget”. It is a command line tool that you can use to download a web page like this:

$ wget http://www.gnu.org/software/make/manual/make.html

This will download the make manual page, “make.html”, and save it in the current directory. “wget” can do much more (e.g., downloading a whole web site). See the manual for “wget” for more info.

For the first part of this lab, your task is to write a limited version of “wget”, which we will call “http_client”, that can download a single file. You will do all your implementation inside the file “http_client.c” inside the “http_client” directory. To build and run the code, do the following:

First, go inside the “http_client” directory,

$ cd http_client

Next, compile the code,

$ make

Finally, run the code,

$ ./http_client [host] [port number] [filepath]

For example, ./http_client www.gnu.org 80 /software/make/manual/make.html

You must build, run, and test your code on ecegrid using the environment from Lab 0. If your code does not run in that environment, you will not get any credit!

In the above command, you give the components of the URL separately in the command line — (1) the server host, (2) the server port number (which will always be 80 for HTTP), and (3) the file path. The program will download the given file and save it in the current directory. So in the example above, it should produce “make.html” in the current directory. It should overwrite an existing file.

Some useful hints:

1. The program should open a TCP socket connection to the host and port number specified in the command line, and then request the given file using HTTP/1.x protocol.

(See http://www.jmarshall.com/easy/http/ for the details of HTTP/1.x protocol).

2. An HTTP GET request looks like this:

GET /path/file.html HTTP/1.0\r\n [zero or more headers]\r\n [blank line]\r\n

Include the following header in your request:

Host: <the_host_name_you_are_connecting_to>:<port_number>

 

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ECE 46300: Introduction to Computer Communication Networks 2

 

3. The response from the web server will look something like this:

HTTP/1.0 200 OK\r\n

Date: Fri, 31 Dec 2020 23:59:59 GMT\r\n Content-Type: text/html\r\n

Content-Length: 1354\r\n [blank line]\r\n

[file content]

There might be slight variations in the formats of responses from different servers. Go through the document in Step 1 (in particular, this section) to ensure that your parser is robust.

The code ”200” in the first line indicates that the request was successful. If it’s not ”200”, the program should print the first line of the response to the terminal (stdout) and exit.

You will need to extract the file name from file path (for example, extract make.html from file path /software/make/manual/make.html), create a new file with extracted file name in the cur‑ rent directory, and write the received file content into that file.

You should use the “Content-Length” value to figure when to stop receiving data from the web server and close the TCP connection. If the “Content-Length” field is not present in the response header, print the following error message to the terminal (stdout) and exit:

Error: could not download the requested file (file length unknown)

4. Some useful C library functions for parsing—“strchr()”, “strrchr()”, “strtok()”, “strstr()”

5. You program should be able to download any type of file, not just HTML files. Test your code by downloading all the different files on the web site http://www.gnu.org/software/make/manual/.

Use “write()” or “fwrite()” to write to the file in byte chunks. This is important to make your solution work for all different file formats, e.g., non‑ASCII files such as pdf and image files. Functions like “fprintf()” might not work! To verify correctness, also download the file using “wget” and make sure that it exactly matches the file downloaded by your program.

 

3 HTTP Web Server

For the second part of the lab, your task is to write a HTTP web server using sockets interface. This task has two sub‑tasks as described in Sections 3.1 and 3.2 respectively. You will implement both the sub‑tasks in the file“http_server.c” inside the “http_server” directory.

To build the code, run:

$ cd http_server

$ make

You must build, run, and test your code for both Sections 3.1 and 3.2 on ecegrid using the envi‑ ronment from Lab 0. If your code does not run in that environment, you will not get any credit!

 

3.1 Task 1: Serving static contents (7 Points)

In this part, you will write a web server that serves static content. The top level directory (called the “web root”) for your HTML files will be the “Webpage/” directory provided with the lab. The web server will only serve contents inside the web root directory. For testing, you can add/remove/modify contents inside web root. We can also do the same while evaluating your submission.

 

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ECE 46300: Introduction to Computer Communication Networks 3

 

To start the web server, run:

$ ./http_server [SERVPORT] [DBPORT]

The “SERVPORT” argument specifies the port number on which the web server would run, and “DBPORT” argument specifies the port number on which the database server would run (used in Section 3.2).

Choosing the SERVPORT and DBPORT values:

To avoid port number clashes between different lab groups running their code at the same time on the same ecegrid machine, please follow the following convention for choosing the port numbers:

1. Choose “SERVPORT” value as 8000 + Group Number

So, if your group number is 38, choose “SERVPORT” value 8038

If your group has two members, and both of you want to run the code at the same time, then choose the “SERVPORT” values as 8000 + Group Number and 9000 + Group Number respectively.

2. Choose “DBPORT” value as 53000 + Group Number

So, if your group number is 38, choose “DBPORT” value 53038

If your group has two members, and both of you want to run the code at the same time, then choose the “DBPORT” values as 53000 + Group Number and 54000 + Group Number respectively.

The content served by the web server should be accessible through a web browser running on the

ecegrid machine by typing the following URL (assuming the web server is running on port 8888):

http://localhost:8888/path/to/content/relative/to/web/root

For example, URL http://localhost:8888/index.html should display file “Webpage/index.html”.

Writing a web server is not a trivial task. Here is the list of what is expected and what is not expected from your web server:

1. The web server will be iterative, i.e., it will serve client requests one request at a time. The server should close the TCP socket (returned by the “accept()” call) after serving each request. In practice, most web servers are concurrent, i.e., they could serve multiple client requests in parallel using multithreading or multiprocessing (e.g., using “fork()”).

2. The web server will only support the GET method. If a browser sends other methods (POST, HEAD, PUT, for example), the server responds with status code 501. Here is a possible response:

HTTP/1.0 501 Not Implemented r n [blank line] r n

<html><body><h1>501 Not Implemented</h1></body></html>

Note that server adds a little HTML body for the status code and the message. Without this, the browser will display a blank page. This should be done for all status codes except 200.

3. Our server will be strictly HTTP/1.0 server. That is, all responses will say “HTTP/1.0”, and all successful responses will include status code “200 OK”.

The server will accept GET requests that are either HTTP/1.0 or HTTP/1.1 (most browsers these days send HTTP/1.1 requests). But it will always respond with HTTP/1.0. The server should reject any other protocol and/or version, responding with 501 status code.

4. The server should also check that the request URI (the part that comes after GET) starts with “/”. If not, it should respond with “400 Bad Request”.

 

 

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ECE 46300: Introduction to Computer Communication Networks 4

 

5. In addition, the server should make sure that the request URI does not contain “/../” and it does not end with “/..” because allowing “..” in the request URI is a big security risk—the client will be able to fetch a file outside the web root. If true, respond with “400 Bad Request”.

Note: Most modern browsers automatically check for bad URL requests in points 4 and 5, and appropriately format the URL before sending it to the server. So, to test points 4 and 5, you can use your “http_client” instead of the browser (e.g., ./http_client localhost 8888 /../).

6. The server must log each request to terminal (stdout) like this:

128.59.22.109 "GET /index.html HTTP/1.1" 200 OK

It should show the client IP address, the entire request line, and the status code and reason phrase that the server just sent to the browser (Figure 1).

 

 

Figure 1: Sample terminal (stdout) output.

You must log the requests in the exact format as shown in Figure 1. You must not print anything else to the terminal. Violations of these guidelines would result in a 10% grade penalty.

7. If the request URI ends with “/”, the server should treat it as if there were “index.html” ap‑ pended to it. For example, given

http://localhost:8888/

the server will act as if it had been given

http://localhost:8888/index.html

8. If the request URI is a directory, but does not have a “/” at the end, then you should append “index.html” to it.

Use “stat()” function to determine if a path is a directory or a file.

9. The server sends “404 Not Found” if it is unable to open the requested file.

10. For reading the file, use “fread()” or “read()”. You should read the file in chunks and send it to the client as you read each chunk. Do not use “strlen()” to figure the length of buffer that you are sending to the client—“strlen()” will stop counting as soon as it encounters the first NULL character. Instead use the return value from “fread()”/“read()”. The recommended chunk size is 4096 bytes, which is the optimal buffer size for disk I/O for many OS/hardware.

 

 

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ECE 46300: Introduction to Computer Communication Networks 5

 

3.2 Task 2: Serving dynamic contents (4 Points)

In this part, you will add a database service to your web server. Web servers often have to contact a database to serve certain client requests. In this lab, clients can request a cat picture by entering a search string in the textbox displayed on the web page. On getting such a request, the web server will contact the database of cat pictures, and respond with the cat picture requested by the client.

To start the database server, run:

$ ./db_server [DBPORT]

Next, start the web server in a different terminal,

$ ./http_server [SERVPORT] [DBPORT]

The “SERVPORT” and “DBPORT” values must be chosen as described in Section 3.1.

 

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