MS RFC 44: Restore URL modification of mapfiles to pre-5.0 levels

Author:Steve Lime
Contact:Steve.Lime at
Last Edited:2008/08/19

MS RFC 31: Loading MapServer Objects from Strings introduced a new syntax for modifying mapfiles via URL. Object parameters could specified together in mapfile snippets making it easier to make changes with far fewer characters. At the same time access to a number of parameters, particularly those that mapfile parsing did no value checks on (mostly strings) were removed for security purposes. In hindsight I underestimated the degree to which that functionality was used by developers. This RFC aims to restore that functionality albeit with security in mind.

Proposed Changes

Presently a few widely modified (and risky) parameters (e.g. layer TEMPLATE and DATA) can be changed via URL if a regular expression (e.g. TEMPLATEPATTERN and DATAPATTERN) is set to validate the incoming value. I propose using the same approach for all un-checked mapfile input. Parameters that represent numbers, colors or have a value domain (e.g. ON/OFF/DEFAULT) are subject to the same checks as when a mapfile is read from disk and as a result should be ok. Those that don’t would require specific validation values be set before input would be allowed. For example, the LAYER VALIDATION block below defines patterns that would be used to validate DATA or FILTER parameter changes. If the appropriate validation key doesn’t exist the value cannot change.

Grouping all validation in a new VALIDATION block will ease use by simplifying key names to match MapServer token names. The block would be valid for MAP, WEB, LAYER and CLASS objects and its core type would be a hashTableObj. The MAP level VALIDATION block would be useful for applying a pattern for all LAYERs or CLASSes (since there is only 1 WEB object there is no need to rely on the MAP object). This would save lots of duplication in cases where a mapfile contains similar layers and the same data validation pattern applies to all. The logic would simply be: look for validation pattern in layer, if not found then look for validation pattern at map level, if not found then no modifications are allowed.

data 'my pattern'
filter 'another pattern'

The validation would only be invoked if the token source is a URL. Mapfile file or string-based processing would be unaffected. An example of how this would work can be seen in mapfile.c near line 2683 with the DATA/DATAPATTERN parameters.

Files Affected

  • maplexer.l: all parameters (a few will never be modifiable, like VALIDATION) will be changed to be recognized in the URL_VARIABLE lexer state; VALIDATION token needs to be added
  • mapfile.h: add VALIDATION token
  • mapfile.c: all non-value checked parameters will require regex validation before changes will be allowed via URL; recognize validation token; write validation hash with mapfile
  • mapserv.c: update code for runtime substitution and qstring validation to check the validation hash as well

A complete list of parameters affected will be attached to this document in the post-implementation notes below.

Mapfile Changes

New VALIDATION token will be recognized.

MapScript Changes

None. MapScript already has a general class for hashTableObj management.

Backwards Compatibility Issues

The parameters DATAPATTERN and TEMPLATE pattern will become deprecated though. The objects in question (LAYER and CLASS) already contain validation blocks that can be used for this.

URL runtime substitution and qstring validation are currently supported through metadata, This would become deprecated as well. The runtime variables and the word “qstring” can be used as keys in the validation block instead.

Post-Implementation Notes

A HowTo will be developed that covers this topic and run-time substitutions.

Voting History

+1 Lime, Woodbridge, Morissette, Assefa