GR 34
GR 34 (Sentier des Douaniers) is a long-distance coastal footpath that runs along nearly the whole Brittany shoreline, following the old customs-officer tracks and linking Mont-Saint-Michel (Manche) with Saint-Nazaire (Loire-Atlantique). The route length is about 1,700 km (the track is mapped here as ~2,104 km total).
As a purpose-built long-distance hike, it’s designed to be walked in sections: pick a town-to-town segment along the coast, overnight there, then continue by the next marked stage. The route crosses the major coastal areas of Brittany (Manche, Ille-et-Vilaine, Côtes-d’Armor, Finistère, Morbihan, and Loire-Atlantique).
The trail traces the coastline and regularly uses or parallels sea-facing paths; historic customs infrastructure (observation points and buildings tied to coastal surveillance) is part of the route story, since the path origins date to the late 18th century and were formalized under the French customs authority in 1791.
On the ground, GR routes are marked with the standard red-and-white waymarking system. The GR 34 can leave the strict coastline in a few places, with options to stay closer to the sea on coastal alternates marked separately as “sentier côtier” when the main line runs inland between certain coastal/estuary areas.
For planning, GR 34 is a big commitment: the full route is measured in the thousands of kilometers and is typically tackled as a multi-day hike across many stages. Accommodation is generally easier to organize than on remote trails because the route passes through a chain of ports, resort towns, and countryside sections along the coast.
There are multiple widely known highlights along the way, including the Pink Granite Coast (Côte de Granit Rose) in Côtes-d’Armor and the Pointe du Raz area in Finistère, plus the Emerald Coast stretch around Saint-Malo; these anchor popular regional sections for walkers who don’t want the full-through route.
More information: Wikipedia, Walking the GR34 on the coast of Brittany, GR® 34, the customs officers' path - Brittany tourism