I always use rear racks at the front as what I want is hard to find in the market (low railings and a platform). You will need a rigid fork, ideally with eyelets for mudguards at the dropouts and the fork crown, although there are workarounds.
The important thing is, as above mentioned, that the platform part does not extend too much to the front of the rack, or it will interfere with your head tube. For that I like Tubus and Racktime because they include technical drawings with measurements of all their models and you can check if it will fit even before buying.
This was the set up on my wife's bike for our South America tour last year. Tubus Cosmo rack (very short distance from vertical strut to front of the rack, plus low railing to install panniers a bit lower). At the dropouts I used longer bolts with a few spacers to avoid conflict with the fork legs. At the top the rack is attached to the fork crown using a steel light bracket like this
one, very sturdy. After 6000km in all surfaces with 2 big panniers and a tent on top, not a single issue.
I'm meant to write a short tutorial in my blog with more pictures but I haven't got around to do it.
Best part is that after a trip you rarely need a front rack, but a rear one always comes handy.