Old cast-iron drainpipe can behave unpredictably. Sometimes a small section of a pipe starts to crumble or a joint begins to leak even though most of the pipe is sound.
If a cast-iron pipe is failing at several points, the most economical solution is to replace it with PVC pipe. This is a job for a professional.
Older cast-iron joints were sealed with molten lead, but this is not a danger to your health because only wastewater passes through the pipes.
Support cast-iron pipe
Cast-iron pipe is very heavy and must be held in place with special clamps attached to framing members. Never disturb these clamps; if one weakens, a long section of pipe could come crashing down.
An hour to clean and patch a joint; less time to patch a hole
Wire brush, hammer, cold chisel, putty knife
Cleaning metal, applying putty
Make sure that the pipe is well supported
Repair paste or plumber's epoxy