Recent changes to Canada's international travel laws have caused confusion for visitors to Canada as well as travelers with connecting flights in Canada, but do not intend to visit Canada.
Effective March 2016, the Government of Canada (IRCC) requires anyone traveling through Canada must apply for an Electronic Travel Authorization (eTA) to fly or transit through a Canadian airport; which costs $7 CAD. According to IRCC, most people need a visa or an Electronic Travel Authorization to travel to Canada; some may only need a valid passport. The only exceptions to needing an eTA are U.S. citizens who can show proper identification at the border.
Canadian dual nationals must travel with a valid Canadian passport. If their plane leaves within 10 days, they must apply for a special eTA to travel with their passport from a visa-exempt country.