the first thing you need to know is if the strainer is metal or pvc. if the strainer is metal than the seal could be rubber, or poured lead even if you have a pvc drain some plumbers still lead pour but if it is rubber the seal will fit snugly around the pipe and the strainer,these have a less likely chance of a leak at this point. If you inspect the inside seal and find it loose or if the pipe has slipped down this could be your problem inspect that first. If you have a pvc strainer it could be glued in or a rubber seal could also have been installed ceak for a crack in the pvc strainer or a loose seal in the rubber. If you need to replace any of the mentioned strainers you will need to get to the under side of the unit you cant just unscrew most strainers because a nut is used to hold the top part in and must be accessed to get to the nut most times the nut is broken or the rubber washer has come loose to fix this you must remove the strainer and use silicone caulk on the top strainer and fasten below withn the nut if you have a metal strainer plumbers putty is ok but plastic is a patrolium prouduct and so is putty , putty will dry out the plastic and make it crack in time after installing the srainer reseal the pipe with either the rubber seaL OR IF IT IS A GLUE TYPE THAN USE A PVC SOLVENT CEMENT TO REINSTALL YOU MAY ALSO NEED A COUPLING BECAUSE IF IT IS A GLUE TYPE STRAINER YOU WILL PROBABLY HAVE TO CUT IT OUT TO REMOVE I.

good LUCK