Foreigners travelling between Sumdo in Spiti and just east of Spillo in Kinnaur – where the road passes within a few kilometres of Western Tibet – require Inner Line permits, valid for travel through the border districts. Officially you are required to travel in a group of four or more, but in practice that is never enforced – though in most places you officially have to apply as part of a group.
Inner Line permits are valid for fourteen days and available from Shimla, Manali, Kullu, Rampur, Kaza and Rekong Peo. If travelling independently, you’re best off applying at Kaza where you can do the legwork yourself and obtain a permit in an hour or two at no charge. In the other five locations officials normally insist that you can only apply as a group of four through a travel agent, although these agents are very creative in manufacturing the requisite numbers. Wherever you apply, you will need to provide two photographs and photocopies of your passport and visa.
When travelling through restricted areas, you should never take photographs of military installations or sensitive sites such as bridges. Stick to the main route and you should have no problems with officialdom.