ploticus: direct cgi mode

## Direct CGI mode | Welcome Gallery Handbook |
| Manual page for Direct_CGI_mode(PL)

## Direct CGI mode

pl may be invoked directly from URLs in HTML pages, to create plots on the fly.`` Direct CGI mode can produce pseudo-GIF, PNG, JPEG, SVG or SVGZ.`` Caution: there are CGI security concerns that must be fully understood by the programmer before using pl in direct CGI mode.`` Other strategies for on the fly graphing are discussed here.``

To use ploticus in direct CGI mode, copy your pl executable to your cgi-bin (or make a link), and create a ploticus config file (see below).`` The specifics of local CGI bin and file naming conventions vary; see your local admin if having difficulty.`` Then, use a construct such as this in your HTML page:

  `<img src="http://abc.org/cgi-bin/pl?cgi=1&scat1.pl&-png&-scale&0.8& TITLE=Trailer%20Tongue%20Technologies">`

This is equivalent to command line usage:

  `pl scat1.pl -png -scale 0.8 TITLE="Trailer Tongue Technologies"`

`cgi=1` should be the first parameter in your URL construct, as shown in the above example.``

After that, the regular pl command line options and arguments , including those for prefabs , may be given in the usual order. However, to conform to URL syntax, a question mark (?) must separate the program name from the first argument, and ampersand (&) must be used to separate subsequent arguments. Also, arguments should not be enclosed in quotes. Use %20 to represent an embedded space, %26 for an embedded ampersand, etc (%XX may be used to represent any problematic character, where XX is its hex ascii value). Arguments are limited to a length of 250 chars each and may not contain embedded newlines.

## CGI MODE BEHAVIOR

pl automatically goes into CGI mode if it detects the environment variable QUERY_STRING (set by your web server), and there are no conventional command line arguments given. pl behaves this way when in CGI mode:

- a ploticus config file is required, and the config file must contain a `projectroot` setting; other settings are optional. You may want to set the environment variable PLOTICUS_PREFABS via the config file as shown.`` Here is an example of a config file for CGI use:

        projectroot: /home/steve/proj1
        putenv:	     PLOTICUS_PREFABS=/home/steve/ploticus/prefabs

  The config file must have the same name as your pl executable, plus a `.cnf` extension (if your pl executable name ends in `.cgi` or `.exe` these are removed before appending the `.cnf`).`` If pl must be used in several different contexts within the same cgi-bin, you can make soft links to pl, using other names for the links, and have a separate config file corresponding with each one. Examples:

              pl executable name	       use this config file name
          ---------------------------     ---------------------------
      	/data/cgi-bin/pl		/data/cgi-bin/pl.cnf
      	/data/cgi-bin/plproj1		/data/cgi-bin/plproj1.cnf
      	/data/cgi-bin/pl.cgi		/data/cgi-bin/pl.cnf
      	/data/cgi-bin/pl.exe		/data/cgi-bin/pl.cnf

- pl will operate from the `projectroot` directory. All filenames should be expressed relative to `projectroot`. The script file must be below projectroot. Script file names beginning with slash (/) or dot (.), or containing double-dot (..) are prohibited. This restriction does not apply to other files (eg data files).``

- command arguments are extracted from QUERY_STRING which is an environment variable set by your web server before it invokes CGI programs.`` (HTML POST method is not supported).`` If this is not available, pl will exit.``

- default output format is GIF, JPEG, or PNG depending on the pl build; this can also be controlled via the arguments list.``

- an appropriate Content-type header is generated based on output format type. This header as well as image content are written to standard output.``

- output is limited to one image or "page". To do multiple images, temp files will need to be used.``

- pl command line options that are inappropriate in CGI mode, such as -diagfile, -errfile, -viewer, and -o, are disabled.``

- pl -debug automatically writes diagnostic and error info to files, useful in troubleshooting (see below).``

## TROUBLESHOOTING

Use the -debug option in your invoking URL, just after the `cgi=1` argument, eg. `http://abc.com/cgi-bin/pl&cgi=1&-debug&-gif`..`` This causes error message output to be written to `/tmp/plcgi_err` and diagnostic output to be written to `/tmp/plcgi_diag`.`` (If the config file is loaded successfully and a temp directory is specified there, then that directory is used instead of `/tmp`.`` For win32 `c:\temp` is the default.)

If a preliminary error occurs, such as failure to read the config file, an error message is written to `/tmp/plcgi_err` regardless of whether -debug was specified.``

If no error message file is appearing, It is possible that pl was never executed. This could be because your URL path is incorrect, or your web server could not access and/or execute pl because of file permissions.`` To ensure your invoking URL is correct, verify by invoking some other CGI program in the same directory.``

File permissions are important.`` On Unix systems, be sure that your pl executable file mode is readable & executable by world (use command `chmod 755 pl`) so that your web server can execute pl.`` If you can, it may be desirable to set your pl executable file to be setuid and setgid to your own UID and group (`chmod 6755 pl`); then it will be able to see all files that you can see. Otherwise, pl will execute as your web server UID (often `nobody`), and you'll need to ensure that your config file and all other files needed such as script files, data files, and prefabs files are readable by world, so that your web server process can gain access (`chmod 644` filenames).`` If you're using symbolic links, file permissions generally must be set on the actual file, not the link.``

Another thing to try is to simulate CGI invocation from the command line.`` To do this, `cd` to your cgi bin, deploy a new term window, and in the new window manually set the QUERY_STRING environment variable, eg:

  `export QUERY_STRING="scat1.pl&-scale&0.8"`

If you want your CGI to generate PNG you may need to place `plpng` into your cgi-bin and execute it instead of `pl` (this depends on your ploticus build).``

Then, in the new window, invoke pl by hand, with no command line arguments, similarly to this:

  `/data/htdocs/cgi-bin/pl > out`

Any error messages should be visible on screen. (The output file will generally not be displayable because it contains a mime-type header.)

In theory CGI mode should work with PNG, GIF, JPEG and SVG.`` I noticed some flakiness with SVG. If the pl script argument was last and I was generating SVG, MSIE wouldn't display it until I added a trailing `&` to the URL. And, MSIE didn't understand SVGZs when generated in CGI mode.``

## CGI SECURITY

You should be fully familiar with CGI security issues and how they relate to your platform and project, before putting pl into service as a CGI program. Some things to keep in mind:

- Anyone can easily view your HTML, including your invocation of CGI programs.`` It is also very easy for users to submit modified CGI invocations (especially altered arguments).``
- If you pass a data file name (or other filename) to pl via a varvalue argument, use the following within your pl script to guard against hacks on the file name:

          #if @DATAFILE inlike /*,.*,*..*
            #exit
          #endif

- If you build a shell command from anything passed via a varvalue argument, remove shell metacharacters from the variables using the $shellstripchars function .``
- It is possible that undiscovered security holes exist with ploticus in direct CGI mode.`` If you will lose sleep over this, see the discussion on on-the-fly graphics for alternatives.``

Note: A CGI-related security hole was uncovered Feb 8 '02 and affects versions 2.00, 2.01, and 2.02.`` See what's new page to upgrade or apply patch.``
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Copyright Steve Grubb

Markup created by unroff 1.0, `` `` December 10, 2002.