gistfile1.txt

  ...
  
  PerlModule SmartGiant
  PerlOptions +SetupEnv

  RewriteEngine   on

  ProxyPreserveHost On
  RewriteCond %{HTTP_HOST} !^m.solfaktor
  RewriteRule ^/(step1|step1_meta|asset/|services/|layouts/)(.*)$ http://%{HTTP_HOST}.prod.reisegiganten.net/$1$2 [P,L]
  RewriteCond %{QUERY_STRING} tab=flight
  RewriteRule ^.*$ http://%{HTTP_HOST}.prod.reisegiganten.net/flight [P,L]

  ...
  
Example URLs:

http://m.restplass.no.test.r12.no/step1?orig=OSL&dest=BCN&dep=2016-12-10&return=2016-12-10&adt=1&sort=quality
http://www.restplass.no.test.r12.no/
http://www.restplass.no.test.r12.no/?tab=flight
http://www.restplass.no.test.r12.no/?tab=flight
http://www.restplass.no/step1_meta?searchType=RT&depLocCode=NO|CI|OSL&retLocCode=ES|AP|BCN&depDateMem=2016-12-10&retDateMem=2016-12-10&adt=1&chd=0&numberOfDisplayedSegments=2&sc=1&searchCnt=1

checked it quick. i know some calls come from the domains "flyg.solfaktor.se" and "fly.solfaktor.(no/dk)". otherwise you are rock solid :)