HTTP.SYS

Name

HTTP.SYS -- HTTP Rewrite / Proxy Control File (Optional).

Description

HTTP.SYS is an optional file. If present, it contains REWRITE rules for the HTTP server, and settings for the HTTP proxy server.

URL rewriting modifies the URL's of incoming requests, according to a set of REWRITE rules. This can be used to organise widely-dispersed resources into a logical directory tree.

The resources may be located on the same PC or even on different servers, but can be made to look as if they are all in the same tree on your server. This is done by replacing parts of the requested URL with alternative strings of characters.

The PROXY commands configure XRPi's HTTP proxy to use a "downstream" proxy to reach certain IP address ranges.

File Format

Each command or rule must be on a separate line, and may be up to 255 characters long. Lines beginning with ';' or '#' are ignored, and may be used for comments. Commands and rules are not case sensitive, and their fields must be separated by one or more spaces or tabs. Blank lines are ignored.

The commands and rules are as follows:

	REWRITE <old_string> <new_string>
	PROXY <ipaddr> <port> <subnet_mask>
	PROXYTIMEOUT <secs>

These are discussed in more detail below...

Rewrite

The REWRITE rules have the following syntax:

	REWRITE <old_string> <new_string>

<old_string> is a template which is compared with an equal number of characters from the start of the requested URL when an HTTP request is received. The comparison is case-insensitive.

<new string> is the character sequence which replaces <old_string> if there is a match.

Example: REWRITE /bbs /systems/gb7pzt

This would replace "/bbs" at the start of the URL with "/systems/gb7pzt". Thus a request for "/bbs/msgs.htm" would be rewritten to "/systems/gb7pzt/msgs.htm".

There are no validity checks, so you must be careful not to inadvertently remove / characters from the rewritten string. e.g. the entry "rewrite /bbs/ http://192.168.0.4" would cause "/bbs/index.htm" to rewrite as "http://192.168.0.4index.htm", which will clearly fail.

You can use rewriting to organise your resources into a neat, logical and compact directory tree, regardless of where they are actually located.

<new_string> may also contain a URL, which invisibly invokes the HTTP proxy. For example:

	REWRITE  /bbs  http://192.168.0.4:85

This would replace "/bbs" at the start of the URL with "http://192.168.0.4:85", so that a URL like: /bbs/cgi-bin/menu.pz becomes: http://192.168.0.4:85/cgi-bin/menu.pz

The resulting URL then treated as if it was a proxy request.

When rewriting to proxy another system, the new URL must begin with "http://<address>[:port]" where <address> can be either a hostname or IP address.

	rewrite http://g8pzt.ath.cx/bbs   http://192.168.0.4

Proxy

The PROXY command allows XRPi's HTTP proxy to pass certain traffic to a downstream proxy. The format of the command is:

	PROXY <ipaddr> <port> <subnet_mask>

e.g. "proxy 44.131.91.245 80 255.0.0.0"

<ipaddr> is the IP address of the downstream proxy.

<port> is the TCP port number of the downstream proxy.

<subnet_mask> when combined with the proxy address specifies the range of addresses which are on the same subnet as the proxy. These addressess will bypass the proxy, i.e. XRPi's proxy will connect directly to them instead of via the next proxy.

This kludge allows 44-net systems to use a further proxy to gain a public (Internet) IP address when connecting to non-44 websites. This is necessary because 44-net routing is unreliable at best, i.e. if a 44-net browser tries to connect directly to Google, the reply probably won't get routed back.

For example, imagine a mobile station, consisting of a web browser and XRPi, with an IP/AX25 link to a nearby gateway, but no Internet connection. The IP address used over the radio link is 44.131.91.3. The browser has been configured to use XRPi's HTTP proxy, which means the IP source address for all HTTP traffic from this station is 44.131.91.3.

Via the nearby gateway, whose IP address is 44.131.91.245, the mobile station can happily browse 44-net (amprnet) websites. But when it tries to use Google, the replies aren't routed back.

However, if the local gateway has been set up with the above PROXY command, the 44.x.x.x sites will be connected directly by the mobile XRPi's proxy, whilst the non-44 sites will be passed to the gateway's HTTP proxy, where they will gain a 62.31.206.176 source address, which is reliably routable and doesn't rely on the co-operation of others in the amprnet.

The PROXYTIMEOUT command specifies the maximum time in seconds to wait for a proxied connection. If a connection takes longer than this to establish, an error is returned.

The default is 30 secs, but may need to be longer on a slow radio channel.

Files

If required, HTTP.SYS must be located in the same directory as XRPi.EXE.

See also

HTTP.ACL(8) -- HTTP Proxy Egress Control File
HTTPBAN.SYS(8) -- Blocks Malicious HTTP Requests