CMU Computer Systems: Network Programming (Part II)

Socket Address Structures

• Generic socket address
  • For address arguments to connect, bind, and accept
  • Necessary only because C did not have generic (void *) pointers when the sockets interface was designed
  • For casting convenience, we adopt the Stevens convention:
    • typedef struct sockaddr SA;
• Internet-specific socket address
  • Must cast (struct sockaddr_int *) to (strcuct sockaddr *) for functions that take socket address arguments

Sockets Helper

• open_clientfd
  • Establish a connection with a server
• open_listenfd
  • Create a listening descriptor that can be used to accept connection requests from clients

Web Server Basics

• Clients and servers communicate using the HyperText Transfer Protocol (HTTP)
  • Client and server establish TCP connection
  • Client requests content
  • Server responds with requested content
  • Client and server close connection (eventually)
• Current version is HTTP/1.1
  • RFC 2616, June, 1999.

Web Content

• Web servers return content to clients
  • content: a sequence of bytes with an associated MIME (Multipurpose Internet Mail Extensions) type
• Example MIME types
  • text/html HTML document
  • text/plain Unformatted text
  • image/gif Binary image encoded in GIF format
  • image/png Binary image encoded in PNG format
  • image/jpeg Binary image encoded in JPEG format

Static and Dynamic Content

• The content returned in HTTP responses can be either static or dynamic
  • Static content: content stored in files and retrieved in response to an HTTP request
    • HTML files, images, audio clips
    • Request identifies which content file
  • Dynamic content: content produced on-the-fly in response to an HTTP request
    • content produced by a program executed by the server on behalf of the client
    • Request identifies file containing executable code
• Bottom line: Web content is associated with a file that managed by the server

URLs and how clients and servers use them

• Unique name for a file: URL (Universal Resource Locator)
• Clients use prefix to infer
  • What kind (protocol) of server to contact (HTTP)
  • Where the server is
  • What port it is listening on
• Servers use suffix to
  • Determine if request is for static or dynamic content
    • No hard and fast rules for this
    • One convention: executables reside in cgi-bin directory
  • Find file on file system
    • Initial "/" in suffix denotes home directory for requested connection
    • Minimal suffix is "/", which server expands to configured domain filename

HTTP Requests

• HTTP request is a request line, followed by zero or more request headers
• Request line: <method> <uri> <version>
  • <method> is one of GET, POST, OPTIONS, HEAD, PUT, DELETE, or TRACE
  • <uri> is typically URL for proxies, URL suffix for servers
    • A URL is a type of URI (Uniform Resource Identifier)
  • <version> is HTTP version of request
• Request headers: <header name>: <header data>
  • Provide additional information to the server

HTTP Responses

• HTTP response is a response line followed by zero or more response headers, possibly followed by content, with blank line ("\r\n") separating headers from content
• Response line:
  • <version> <status code> <status msg>
  • <version> is HTTP version of the response
  • <status code> is numeric status
  • <status msg> is corresponding English text
    • 200 OK Request was handled without error
    • 301 Moved Provide alternate URL
    • 404 Not found Server couldn't find the file
• Response headers: <header name>: <header data>
  • Provide additional information about response
  • Content-Type: MIME type of content in response body
  • Content-Length: Length of content in response body

Tiny Web Server

• Tiny Web server described in text
  • Tiny is a sequential Web server
  • Serves static and dynamic content to real browsers
    • text files, HTML files, GIF, PNG, and JPEG images
  • 239 lines of commented C code
  • Not as complete or robust as a real Web server
    • You can break it with poorly-formed HTTP requests

Tiny Operation

• Accept connection from client
• Read request from client (via connected socket)
• Split into <method> <uri> <version>
  • If method not GET, then return error
• If URI contains "cgi-bin" then serve dynamic content
  • (Would do wrong thing if had file "abcgi-bingo.html"
  • Fork process to execute program
• Otherwise serve static content
  • Copy file to output

Serving Dynamic Content

• The server creates a child process and runs the program identified by the URI in that process

• The arguments are appended to the URI

• Can be encoded directly in a URL typed to a browser or a URL in an HTML link

• Use the environment variable QUERY_STRING to pass the arguments to the child

• The child generates its output on stdout. Server uses dup2 to redirect stdout to its connected socket

• The CGI child must generates those headers