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The Web Listener

The Oracle Web Listener is a HyperText Transfer Protocol (HTTP) engine that responds to requests for hypermedia documents from web browsers (clients). This section summarizes the Web Listener's capabilities.

Network Communication

Every Oracle Web Listener process accepts connections from web browsers (clients) on one or more IP address/port combinations using HTTP to encode requests for hypertext documents, and the Transmission Control Protocol/Internet Protocol (TCP/IP) as the underlying connection protocol. Several Web Listener processes may run on single host computer at the same time (see the Oracle Web Listener Administration Form for more information).

Ports

A port is a number that TCP uses to identify a communication channel associated with a specific program. For example, the login program usually accepts login requests on port 49, and the Domain Name Service (DNS) usually accepts name lookup requests on port 53. HTTP engines, such as the Oracle Web Listener, usually accept ordinary connections on port 80 and secure connections on port 443.

Programs must execute with root permissions to access port numbers 1 through 1023, whereas any program can access port numbers 1024 through 65535.

IP Addresses

An Internet Protocol (IP) address is a unique number that identifies exactly one computer on the Internet, although each computer can be represented on the Internet by several addresses. A computer can use different addresses for different Internet functions. For example, a single computer might have one IP address on which it acts as an email routing server, and another address on which it acts as a DNS server.

DNS Host and Domain Names

A fully qualified host name is a unique character string that identifies exactly one computer on the Internet, although each computer can be represented on the Internet by several host names. The Domain Name Service (DNS) maintains a distributed database that maps host names to IP addresses. DNS servers provide Internet computers with lookup services for this database. hal.us.oracle.com is an example of a fully qualified host name.

A domain is a named DNS host name space, such as us.oracle.com. All host names within a domain must be unique. For example, there must be only one host named hal in the us.oracle.com domain.

Secure Connections

The Oracle Web Listener can accept secure connections using the Secure Sockets Layer (SSL), an emerging standard for secure data transmission. (The combination of IP address and port number is called a socket.) See The Secure Sockets Layer for more information on the Oracle WebServer's implementation of SSL.

Clients use a secure version of HTTP called HTTPS to establish secure connections with SSL. To establish a secure connection with a Web Listener process, a request URL must use "https" instead of "http" must specify a port on which the Web Listener has enabled SSL, for example:

https://www.blob.com:443/

Proxies

You can configure an Oracle Web Listener process to act as a proxy server, an HTTP engine running on a firewall machine that allows clients inside the firewall to access web sites outside the firewall. The Oracle Web Listener implements the proxy behavior defined by Luotonen and Altis in World-Wide Web Proxies under the auspices of the World-Wide Web Consortium (W3C).

File Handling

The Oracle Web Listener uses a virtual file system to keep track of the files it makes available to clients. The virtual file system maps the pathnames used in request URLs (Uniform Resource Locators) to the file system maintained by the host machine's operating system.

File Memory Mapping

The Web Listener uses the host operating system's memory mapping capability when opening a file requested by a client so that the Web Listener's virtual address space refers directly to the file's contents. This speeds access, and makes it possible for several clients to share the same copy of the file, which conserves the Web Listener's memory resources.

File Caching

Ordinarily, when the Web Listener opens a requested file, the file remains open and mapped into memory until all clients using the file are finished with it, at which point the Web Listener closes the file and frees the memory mapping associated with it. The Web Listener allows you to specify files to be cached. Cached files are opened when a client requests them, and remain open for the life of the Web Listener process. This optimizes access times for files, such as your home page, that are requested often.

File Protection

The Oracle Web Listener allows you to make secure specific virtual files or directories by assigning authentication and/or restriction schemes to protect them.

Authentication Schemes

When a file or directory is protected by an authentication scheme, a client requesting access to it must provide a user name and password. Authentication schemes allow you to define named groups of user name/password combinations, and named realms that are groups of these groups. You can then assign user, group, and realm names to virtual files and directories, requiring any client requesting access to input one of the specified user name/password combinations.

The Web Listener supports two authentication schemes: basic authentication and digest authentication. Both schemes are identical, except that digest authentication transmits passwords from client to server in an encrypted form called a digest, whereas basic authentication sends unencrypted passwords, making it considerably less secure.

Some older web browsers don't support digest authentication, but for files or directories that require authentication, you should use digest authentication whenever possible.

Restriction Schemes

When a file or directory is protected by a restriction scheme, only a client accessing the Web Listener from a trusted group of host machines may access it. The two restriction schemes that the Web Listener supports are IP-based restriction and Domain-based restriction. IP-based restriction allows you to define groups of trusted hosts identified by IP address, whereas Domain-based restriction allows you to define groups of trusted hosts identified by DNS host or domain name.

File Format Negotiation

The Oracle Web Listener can maintain several versions of a document in different formats, and provide it to clients in the formats they prefer.

Language Formats

For example, if a client requests a document in French, the Web Listener checks to see if it has a French version of the document, and if so, returns that version to the client. Otherwise, the Web Listener returns the version of the document in its own default language (usually English).

MIME Formats

Similarly, a client can request a document of a particular Multipurpose Internet Mail Extensions (MIME) type. If the Web Listener can find a version of the requested file in the requested MIME format, it returns that version to the client. Otherwise, it returns the version of the file that has the smallest size.

Encodings

The Web Listener can also maintain versions of a document that have been encoded by programs such as a compression utilities. For example, if a client can uncompress a document that has been compressed by the gzip program, the Web Listener can return to the client the compressed version of document instead of the uncompressed document, saving transfer time.

Filename Extensions

The Web Listener uses filename extensions to identify a file's format, which can represent language, MIME type, or encoding. For ease of maintenance, it's best to advertise a file to clients only by its base name, allowing clients and server to negotiate formats transparently.

Dynamic Document Generation

Using the Oracle Web Listener, your web site can respond to client requests by generating HTML documents dynamically. This allows you to customize your WebServer's responses.

Like most HTTP engines, the Oracle Web Listener allows clients to use the Common Gateway Interface (CGI) to run programs on the server machine to perform special processing and return data to the client.

The Web Request Broker

Unlike other HTTP engines, the Oracle Web Listener provides an interface called the Oracle Web Request Broker (WRB), which allows clients to run programs on the server machine and return data much more efficiently than CGI allows. To do this, the Web Listener passes requests intended for these programs to the WRB Dispatcher, which maintains a pool of processes to which it can assign the requests. See The Web Request Broker (WRB) for more information.

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This document was last modified at 07:32pm PDT on April 29, 1996.

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