Increased pressure on, and investigation of, Iran and North Korea, to get the two nations to halt their nuclear weapons development programs, has uncovered a huge arms smuggling operation between the two countries. Basically, cash strapped North Korea has become a major supplier of cheap weapons to Iran. Weapons manufacturing is one of the major economic activities in North Korea. But there are several sets of international sanctions on Iran and North Korea that restrict the export and import of weapons. North Korea gets around this by using considerable deception to ship the goods to Iran. This usually involves loading the weapons into standard shipping containers, and labeling those containers as carrying something less threatening (farm machinery, or other industrial materials). Then the containers are sent by ship or rail, to Chinese ports (to via Russia, to Russian ports) where bribes are paid to get the containers shifted to another ship, with some new paperwork, and the delivery is made to Iran, or another country (for another switch, if it's believed the Americans are watching from space, or via local agents.)
Iran has been buying ballistic missiles, and components, from North Korea for over two decades. The prices are good, and the missiles are reliable. North Korea has taken the basic Russian SCUD missile (which was developed, with the help of captured German scientists, from the German World War II V-2 ballistic missile) and improved it as the longer range (1,300 kilometers) Nodong missile. The basic SCUD design was also enhanced to produce longer range (up to 600 kilometers) SCUDs. All of these have been sold to foreign buyers like Yemen, Iran and Pakistan. The only big problem the North Koreans have these days is making deliveries.