For over a decade now Belarus has been trying to develop legitimate defense companies and they have had some success. One such company, TETRAEDR, specializes in upgrading Cold War era Russian air defense systems. There is a market for such services because many countries still rely on these older missile systems and upgrades of existing equipment is a lot cheaper than buying new stuff. One recent satisfied customer is Vietnam, which is having its S-125 (SA-3 Goa) anti-aircraft missile systems upgraded to the S-125-2TM standard. This upgrade makes the missiles more capable in that they are better able to handle jamming and more effective against low-flying targets (especially cruise missiles). The 2TM upgrades enables S-125 batteries to operate independent of a centralized air defense system and be ready for action (or ready to move) more quickly. The upgrades, Vietnam was assured, makes the S-125 more effective against Chinese air attack.
Vietnam is one of many export customers for Russia's 1970s era S-125 surface-to-air missile system. Other known users include Libya, Syria, Venezuela, Burma and Turkmenistan. Some of these countries are buying updated launchers and radars as well. While the basic design of the S-125 is old, it has been frequently updated.
The two stage S-125 missiles weigh nearly a ton, and carry a 59 kg (130 pound) warhead against targets 35 kilometers away (and altitudes as high as 18,000 meters). There is also a smaller missile, weighing closer to half a ton, with a range of 15 kilometers. Having two different size missiles for the same system is a common practice with the Russians (and some other nations as well, like the U.S. Patriot system).
Some users have upgraded or modified their S-125 missiles and radars themselves over the years. The most notable example of this was in Serbia, in 1999, where a missile battery commander used a locally modified S-125s to shoot down a U.S. F-117 stealth aircraft. He did this by using human observers a lot, and his radar rarely. Since the S-125 can be controlled (flown by) a ground operator, once the F-117 was located, an S-125 missile was launched and flown manually to the target. Simple and effective, and largely immune to countermeasures.
This feat gave S-125 sales a shot in the arm and in 2008 the Russians opened a new factory to meet the demand (worth several hundred million dollars). But nations don't buy the inexpensive, and reliable, S-125 because one took down a stealth fighter. No, the missile provides basic air defense against neighbors who don't have high-end air forces. The S-125 provides basic air defense, and keeps aerial smugglers, and secretive users of UAVs, respectfully nervous.