make sure that the commons: prefix is actually in your
interwiki table
Yes, table wiki_interwiki has it, from
$ grep commons mediawiki-1.11.0/maintenance/interwiki.sql
('commons','http://commons.wikimedia.org/wiki/$1',0),
Also, for easier handling of the interwiki table, try
this extension
Can't I use any of the
$ wc -l < mediawiki-1.11.0/maintenance/interwiki.sql
173
without dabbling in extensions?