what do boxers sniff before the match

It happens, but it’s illegal and not what I’d call a common sight at all - certainly nobody wants to be seen doing it. I’ve cornered both amatuer and professional boxing, kickboxing and MMA fights, and smelling salts are strictly prohibited. Getting caught using smelling salts ringside will get a fighter disqualified, and possibly get his corner team fined and/or disciplined by the state. Referees and state officials are generally watching the corner very closely for rule violations. A few examples:

http://www.uboboxing.com/rules.htm

http://www.nabfnews.com/nabf-mainmenu-87/legal-mainmenu-38/championships-mainmenu-42.html

Texas commission says oxygen OK, no actions taken against referees